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1
Simon, Frederick F. "The Use of Transition Region Characteristics to Improve the Numerical Simulation of Heat Transfer in Bypass Transitional Flows." International Journal of Rotating Machinery 2, no. 2 (1995): 93–102. http://dx.doi.org/10.1155/s1023621x95000248.
Повний текст джерелаАнотація:
A method is presented for improving the numerical prediction of bypass transition heat transfer on a flat plate in a high-disturbance environment with zero or favorable pressure gradient. The method utilizes low Reynolds number k-ε turbulence models in combination with the characteristic parameters of the transition region. The parameters representing the characteristics of the transition region used are the intermittency, transition length and turbulent spot properties. An analysis is made of the transition length in terms of turbulent spot variables. The nondimensional spot formation rate, required for the prediction of the transition length, is shown by the analysis to be a function of the spot spreading angle, the dimensionless spot velocity ratio and the dimensionless spot area ratio. The intermittency form of the k-ε equations was derived from conditionally averaged equations which have been shown to be an improvement over global-time-averaged equations for the numerical calculation of the transition region. The numerical predictions are in general good agreement with the experimental data and indicate the potential use of the method in accelerating flows. Turbulence models of the k-ε type are known to underpredict the transition length. The present work demonstrates how incorporating transition region characteristics improves the ability of two-equation turbulence models to simulate bypass transition for flat plates with potential application to turbine vanes and blades.
2
Whitney, Michael M., Daniel L. Codiga, David S. Ullman, Pearse M. McManus, and Ralph Jiorle. "Tidal Cycles in Stratification and Shear and Their Relationship to Gradient Richardson Number and Eddy Viscosity Variations in Estuaries." Journal of Physical Oceanography 42, no. 7 (July 1, 2012): 1124–33. http://dx.doi.org/10.1175/jpo-d-11-0172.1.
Повний текст джерелаАнотація:
Abstract Tidal cycles in stratification and shear lead to tidal variations in mixing in many estuaries. This study 1) defines readily observable dimensionless parameters for establishing the sense and magnitude of gradient Richardson number Ri and eddy viscosity K changes from maximum to minimum stratification during a tidal cycle and 2) calculates where representative estuaries fit in this parameter space. The dimensionless parameters are Ri calculated with tidal-averaged stratification and shear, scaled stratification amplitude, and a scaled shear parameter. The scaled stratification amplitude is approximately the tidal amplitude of stratification divided by the tidal-averaged stratification. The scaled shear parameter depends on the scaled tidal amplitude of shear and the phase difference between the tidal cycles of stratification and shear. Over most of the parameter space, Ri is larger at maximum stratification. If the scaled stratification amplitude falls below a threshold value defined in terms of the scaled shear parameter, shear effects dominate and cause Ri to be greater at minimum stratification. Two-thirds of the selected estuary locations have greater Ri at maximum stratification. The remaining locations are dominated by shear differences and have an atypical mixing cycle with greater Ri at minimum stratification. The Ri and K tidal variations and the position in the parameter space can drastically shift with the spring–neap cycle and spatial variations within an estuary.
3
Pavlenko, Ivan, Oleksandr Liaposhchenko, Marek Ochowiak, Radosław Olszewski, Maryna Demianenko, Oleksandr Starynskyi, Vitalii Ivanov, Vitalii Yanovych, Sylwia Włodarczak, and Michał Doligalski. "Three-Dimensional Mathematical Model of the Liquid Film Downflow on a Vertical Surface." Energies 13, no. 8 (April 15, 2020): 1938. http://dx.doi.org/10.3390/en13081938.
Повний текст джерелаАнотація:
Film downflow from captured liquid without wave formation and its destruction is one of the most important aspects in the development of separation equipment. Consequently, it is necessary to create well-organized liquid draining in areas of captured liquid. Thus, the proposed 3D mathematical model of film downflow allows for the determination of the hydrodynamic parameters of the liquid film flow and the interfacial surface. As a result, it was discovered that the interfacial surface depends on the proposed dimensionless criterion, which includes internal friction stress, channel length, and fluid density. Additionally, equations for determining the averaged film thickness, the averaged velocity vectors over the film thickness, the longitudinal and vertical velocity components, and the initial angle of streamline deviation from the vertical axis were analytically obtained.
4
Moukhtari, Fatima-Ezzahra, and Brice Lecampion. "A semi-infinite hydraulic fracture driven by a shear-thinning fluid." Journal of Fluid Mechanics 838 (January 25, 2018): 573–605. http://dx.doi.org/10.1017/jfm.2017.900.
Повний текст джерелаАнотація:
We use the Carreau rheological model which properly accounts for the shear-thinning behaviour between the low and high shear rate Newtonian limits to investigate the problem of a semi-infinite hydraulic fracture propagating at a constant velocity in an impermeable linearly elastic material. We show that the solution depends on four dimensionless parameters: a dimensionless toughness (function of the fracture velocity, confining stress, material and fluid parameters), a dimensionless transition shear stress (related to both fluid and material behaviour), the fluid shear-thinning index and the ratio between the high and low shear rate viscosities. We solve the complete problem numerically combining a Gauss–Chebyshev method for the discretization of the elasticity equation, the quasi-static fracture propagation condition and a finite difference scheme for the width-averaged lubrication flow. The solution exhibits a complex structure with up to four distinct asymptotic regions as one moves away from the fracture tip: a region governed by the classical linear elastic fracture mechanics behaviour near the tip, a high shear rate viscosity asymptotic and power-law asymptotic region in the intermediate field and a low shear rate viscosity asymptotic far away from the fracture tip. The occurrence and order of magnitude of the extent of these different viscous asymptotic regions are estimated analytically. Our results also quantify how shear thinning drastically reduces the size of the fluid lag compared to a Newtonian fluid. We also investigate simpler rheological models (power law, Ellis) and establish the small domain where they can properly reproduce the response obtained with the complete rheology.
5
Zhao, T. S., and P. Cheng. "Oscillatory Heat Transfer in a Pipe Subjected to a Laminar Reciprocating Flow." Journal of Heat Transfer 118, no. 3 (August 1, 1996): 592–97. http://dx.doi.org/10.1115/1.2822673.
Повний текст джерелаАнотація:
An experimental and numerical study has been carried out for laminar forced convection in a long pipe heated by uniform heat flux and subjected to a reciprocating flow of air. Transient fluid temperature variations in the two mixing chambers connected to both ends of the heated section were measured. These measurements were used as the thermal boundary conditions for the numerical simulation of the hydrodynamically and thermally developing reciprocating flow in the heated pipe. The coupled governing equations for time-dependent convective heat transfer in the fluid flow and conduction in the wall of the heated tube were solved numerically. The numerical results for time-resolved centerline fuid temperature, cycle-averaged wall temperature, and the space-cycle averaged Nusselt number are shown to be in good agreement with the experimental data. Based on the experimental data, a correlation equation is obtained for the cycle-space averaged Nusselt number in terms of appropriate dimensionless parameters for a laminar reciprocating flow of air in a long pipe with constant heat flux.
6
Sharifi, S., M. Sterling, and D. W. Knight. "A novel application of a multi-objective evolutionary algorithm in open channel flow modelling." Journal of Hydroinformatics 11, no. 1 (January 1, 2009): 31–50. http://dx.doi.org/10.2166/hydro.2009.033.
Повний текст джерелаАнотація:
The Shiono and Knight method (SKM) is a simple depth-averaged flow model, based on the RANS equations which can be used to estimate the lateral distributions of depth-averaged velocity and boundary shear stress for flows in straight prismatic channels with the minimum of computational effort. However, in order to apply the SKM, detailed knowledge relating to the lateral variation of the friction factor (f), dimensionless eddy viscosity (λ) and a sink term representing the effects of secondary flow (Γ) are required. In this paper a multi-objective evolutionary algorithm is used to study the lateral variation and value of these parameters for simple trapezoidal channels over a wide range of aspect ratios through the model calibration process. Based on the available experimental data, four objectives are selected and the NSGA-II algorithm is applied to several datasets. The best answer for each set is then selected based on a proposed methodology. Rules relating f, λ and Γ to the wetted parameter ratio (Pb/Pw) for a variety of situations have been developed which provide practical guidance for the engineer on choosing the appropriate parameters in the SKM model.
7
Quinn, Daniel B., George V. Lauder, and Alexander J. Smits. "Scaling the propulsive performance of heaving flexible panels." Journal of Fluid Mechanics 738 (December 5, 2013): 250–67. http://dx.doi.org/10.1017/jfm.2013.597.
Повний текст джерелаАнотація:
AbstractWe present an experimental investigation of flexible panels actuated with heave oscillations at their leading edge. Results are presented from kinematic video analysis, particle image velocimetry, and direct force measurements. Both the trailing edge amplitude and the mode shapes of the panel are found to scale with dimensionless parameters originating from the Euler–Bernoulli beam equation. The time-averaged net thrust increases with heaving frequency, but experiences localized boosts near resonant frequencies where the trailing edge amplitude is maximized. These boosts correspond to local maxima in the propulsive efficiency. For a constant heave amplitude, the time-averaged net thrust coefficient is shown to be a function of Strouhal number over a wide range of conditions. It appears, therefore, that self-propelled swimming (zero net thrust) only occurs over a small range of Strouhal numbers. Under these near-constant Strouhal number conditions, the propulsive economy increases with higher flexibilities and slower swimming speeds.
8
Stone, L. D., and R. J. Goldstein. "Film Cooling Effectiveness Data for Simple Injection Geometries: A Collection of Three-Axis Plots." International Journal of Rotating Machinery 1, no. 1 (1994): 73–81. http://dx.doi.org/10.1155/s1023621x94000072.
Повний текст джерелаАнотація:
A three-axis plot of laterally averaged film cooling effectiveness versus blowing rate and versus dimensionless distance downstream is a sufficiently compact representation of film cooling effectiveness data for injection through a row of holes or two rows of holes to enable easy, global comparisons among a variety of data sets. By plotting data from several film cooling studies conducted at the University of Minnesota, the dominant roles played by density ratio, hole spacing, and surface curvature become apparent. It is also seen that a slot-plate prediction/correlation from two-dimensional film cooling studies makes an excellent standard of comparison for all film cooling studies, even though the more commonly encountered (two-axis) comparisons using standardized nondimensional parameters do not usually suggest this.
9
Lenci, Alessandro, and Vittorio Di Federico. "Shear-Thinning Fluid Flow in Variable-Aperture Channels." Proceedings 48, no. 1 (November 12, 2019): 28. http://dx.doi.org/10.3390/ecws-4-06426.
Повний текст джерелаАнотація:
Non-Newtonian fluid flow in a single fracture is a 3D nonlinear phenomenon that is often averaged across the fracture aperture and described as 2D. To capture key interactions between fluid rheology and spatial heterogeneity, we adopted a simplified geometric model to describe aperture variability, consisting of adjacent one-dimensional channels with constant aperture, each drawn from assigned aperture distribution. The flow rate was then derived under the lubrication approximation for the two limiting cases of an external pressure gradient that was parallel/perpendicular to the channels; these two arrangements provided an upper/lower bound to fracture conductance. Fluid rheology was described via the Prandtl–Eyring shear-thinning model. Novel closed-form results for flow rate and hydraulic aperture were derived and are discussed; different combinations of parameters describing the fluid rheology and variability of the aperture field were considered. In general, flow rate depends, in a nonlinear fashion, on the dimensionless pressure gradient and distribution parameters.
10
Ivchenko, V. O., S. Danilov, B. Sinha, and J. Schröter. "Integral Constraints for Momentum and Energy in Zonal Flows with Parameterized Potential Vorticity Fluxes: Governing Parameters." Journal of Physical Oceanography 44, no. 3 (March 1, 2014): 922–43. http://dx.doi.org/10.1175/jpo-d-13-0173.1.
Повний текст джерелаАнотація:
Abstract Integral constraints for momentum and energy impose restrictions on parameterizations of eddy potential vorticity (PV) fluxes. The impact of these constraints is studied for a wind-forced quasigeostrophic two-layer zonal channel model with variable bottom topography. The presence of a small parameter, given by the ratio of Rossby radius to the width of the channel, makes it possible to find an analytical/asymptotic solution for the zonally and time-averaged flow, given diffusive parameterizations for the eddy PV fluxes. This solution, when substituted in the constraints, leads to nontrivial explicit restrictions on diffusivities. The system is characterized by four dimensionless governing parameters with a clear physical interpretation. The bottom form stress, the major term balancing the external force of wind stress, depends on the governing parameters and fundamentally modifies the restrictions compared to the flat bottom case. While the analytical solution bears an illustrative character, it helps to see certain nontrivial connections in the system that will be useful in the analysis of more complicated models of ocean circulation. A numerical solution supports the analytical study and confirms that the presence of topography strongly modifies the eddy fluxes.
11
Jiang, Yahong, and James B. Grotberg. "Bolus Contaminant Dispersion in Oscillatory Tube Flow With Conductive Walls." Journal of Biomechanical Engineering 115, no. 4A (November 1, 1993): 424–31. http://dx.doi.org/10.1115/1.2895507.
Повний текст джерелаАнотація:
Dispersion of a bolus contaminant in a straight tube with oscillatory flows and conductive walls is solved by using a derivative-expansion method. Using asymptotic methods when small conductance exists, the axial dispersion, as measured by the time-averaged effective diffusivity, increases over the insulated case, as long as the dimensionless frequency (Womersley parameter), α, is smaller than a critical value. When α exceeds this value, axial dispersion is diminished by wall conductance. The functional dependence of this critical α on the system parameters is investigated. We examine the radial wall transport both for total mass and localized flux, which is found to be independent of velocity field, and compute the time-dependent total mass of wall transport and asymptotic Sherwood number for large times as a function of the wall conductance.
12
Kleinstreuer, C., M. Lei, and J. P. Archie. "Flow Input Waveform Effects on the Temporal and Spatial Wall Shear Stress Gradients in a Femoral Graft-Artery Connector." Journal of Biomechanical Engineering 118, no. 4 (November 1, 1996): 506–10. http://dx.doi.org/10.1115/1.2796037.
Повний текст джерелаАнотація:
Employing a validated finite volume code, a computer-aided design of the distal end of a femoral graft-artery junction has been considered to simulate transient three-dimensional blood flow for various flow input waveforms. The study relies on the hypothesis that large sustained wall shear stress gradients play a major role in the rapid recurrence of intimal hyperplasia plus atheroma after bypass surgery, leading to early graft failure. Two new dimensionless parameters have been introduced to correlate flow waveform characteristics with the severity of nonuniform hemodynamics and hence the potential risk for restenosis. The transient and, more importantly, the time-averaged wall shear stress gradient distributions shown, map out the junction areas which are still susceptible to restenosis, especially the toe region. Future geometric modifications will further reduce disturbed flow patterns and hence the probability of graft failure.
13
Helmi, Ahmed M., and Ahmed H. Shehata. "Three-Dimensional Numerical Investigations of the Flow Pattern and Evolution of the Horseshoe Vortex at a Circular Pier during the Development of a Scour Hole." Applied Sciences 11, no. 15 (July 27, 2021): 6898. http://dx.doi.org/10.3390/app11156898.
Повний текст джерелаАнотація:
In the current study, a three-dimensional CFD model is utilized to investigate the variation of the flow structure and bed shear stress at a single cylindrical pier during scour development. The scour development is presented by seven solidified geometries of the scour hole, collected during previous experimental work at different scour stages. Different turbulence models are evaluated and the (k-ω) model is chosen due to its relative accuracy in capturing the flow oscillation and vortex shedding at the pier downstream side with personal computer computational and storage resources. The numerical results are verified against dimensionless parameters from different previous experimental works. This research describes in detail the flow structure and bed shear stress variations through seven stages of the scour hole development. The dimensionless area-averaged circulation coefficient (Ψi) is developed to evaluate the changes in the vortex strength through the scouring process by eliminating the calculation area effect. It was concluded that the circulation in the (Y) direction is the main driving factor in the development of the scour hole more than the circulation in the (X) direction. The ratio between the horseshoe vortex (HV) mean size and the scouring depth (DV/dS) in addition to the location of the maximum bed shear stress are investigated during different stages of the scour development.
14
Jabbal, M., J. Wu, and S. Zhong. "The performance of round synthetic jets in quiescent flow." Aeronautical Journal 110, no. 1108 (June 2006): 385–93. http://dx.doi.org/10.1017/s0001924000001305.
Повний текст джерелаАнотація:
AbstractPIV measurements in the near-field region of a jet flow emanating from a round synthetic jet actuator into quiescent air were conducted over a range of operating conditions. The primary purpose of this work was to investigate the nature of synthetic jets at different operating conditions and to examine the jet flow parameters that dictate the behaviour of synthetic jet actuators. The effects of varying diaphragm displacement and oscillatory frequency for fixed actuator geometry were studied. It was observed that the characteristics of synthetic jets are largely determined by the Reynolds number and stroke length. An increase in the former is observed to increase the strength of consecutive vortex rings that compose a synthetic jet, whereas an increase in the latter results in an increase in relative vortex ring spacing and for further increases in stroke length, shedding of secondary vortices. Correlations were also made between the operating parameters and the performance parameters most effective for flow control and which therefore determine the impact of a synthetic jet on an external flow. Relations of time-averaged dimensionless mass flux, momentum flux and circulation with the jet flow conditions were established and found to widely support an analytical performance prediction model described in this paper. It is anticipated that the experimental data obtained in this study will also contribute towards providing a PIV database for macro-scale synthetic jet actuators.
15
Coman, Melissa Anne, and Mathew Graeme Wells. "Temperature variability in the nearshore benthic boundary layer of Lake Opeongo is due to wind-driven upwelling events." Canadian Journal of Fisheries and Aquatic Sciences 69, no. 2 (February 2012): 282–96. http://dx.doi.org/10.1139/f2011-167.
Повний текст джерелаАнотація:
A major source of temperature variation in nearshore and benthic regions of stratified lakes is due to wind-driven tilting of the thermocline. The maximum displacement of the thermocline depends upon the wind forcing, stratification, and basin morphometry; combined these three-dimensional physical parameters give the dimensionless lake number (LN). We find that the deflection of the thermocline across the length of the basin (Δh*) is correlated to the temporally averaged LN as Δh*/2h1 = 0.37LN–1, where h1 is the depth of the thermocline. We report field observations from Lake Opeongo (Ontario, Canada) showing that during large vertical movements of the thermocline, there are unstable temperature gradients in the benthic layer, as measured by the occurrence of temperature inversions on our thermistor strings. High-frequency waves interacting with the benthic boundary are most likely causing the temperature inversions. Measurements at two shallow slopes of S = 1% and 4%, show that inversions only occur when LN < 5, whereas for S = 8%, the inversions occur only when LN < 2.
16
Bryant, Sarah K., W. Andy Take, and Elisabeth T. Bowman. "Observations of grain-scale interactions and simulation of dry granular flows in a large-scale flume." Canadian Geotechnical Journal 52, no. 5 (May 2015): 638–55. http://dx.doi.org/10.1139/cgj-2013-0425.
Повний текст джерелаАнотація:
A series of 30 tests on dry granular flows were performed using a large-scale flume under varying source volumes and basal friction conditions to capture grain-scale interactions and their impact on overall runout behaviour. These grain interactions and ultimately the flow regimes developed were found to be a function of material source volume and boundary roughness. The dimensionless inertial number was computed for each flow, but was found to be of limited utility except perhaps to define a general state (e.g., liquid regime) for the material due to the high slip velocity encountered in the granular flows. Using the depth-averaged “dynamic analysis” numerical model DAN, it was found that a single set of semi-empirically derived frictional parameters (i.e., specific to internal and basal friction conditions) was appropriate for matching the overall mobility of the experimental flows over a range of flow volumes and slope inclinations. However, these angles were found to be lower than those determined from laboratory interface friction tests, highlighting the importance of collisional stress transfer in the basal zone of the dry flowing landslides.
17
Liao, S. M., and T. S. Zhao. "Measurements of Heat Transfer Coefficients From Supercritical Carbon Dioxide Flowing in Horizontal Mini/Micro Channels." Journal of Heat Transfer 124, no. 3 (May 10, 2002): 413–20. http://dx.doi.org/10.1115/1.1423906.
Повний текст джерелаАнотація:
Heat transfer from supercritical carbon dioxide flowing in horizontal mini/micro circular tubes cooled at a constant temperature has been investigated experimentally. Six stainless steel circular tubes having inside-diameters of 0.50 mm, 0.70 mm, 1.10 mm, 1.40 mm, 1.55 mm, and 2.16 mm were tested. Measurements were carried out for the pressures ranging from 74 to 120 bar, the temperatures ranging from 20 to 110°C, and the mass flow rates ranging from 0.02 to 0.2 kg/min. It is found that the buoyancy effect was still significant, although supercritical CO2 was in forced motion through the horizontal tubes at Reynolds numbers up to 105. The experimental results also indicate that the existing correlations developed in the previous studies for large tubes deviate significantly from the experimental data for the present mini/micro tubes. Based on the experimental data, a correlation was developed for the axially averaged Nusselt number in terms of appropriate dimensionless parameters for forced convection of supercritical carbon dioxide in horizontal mini/micro tubes cooled at a constant temperature.
18
Pandey, S. K., and Dharmendra Tripathi. "Influence of Magnetic Field on the Peristaltic Flow of a Viscous Fluid through a Finite-Length Cylindrical Tube." Applied Bionics and Biomechanics 7, no. 3 (2010): 169–76. http://dx.doi.org/10.1155/2010/294517.
Повний текст джерелаАнотація:
The paper presents an analytical investigation of the peristaltic transport of a viscous fluid under the influence of a magnetic field through a tube of finite length in a dimensionless form. The expressions of pressure gradient, volume flow rate, average volume flow rate and local wall shear stress have been obtained. The effects of the transverse magnetic field and electrical conductivity (i.e. the Hartmann number) on the mechanical efficiency of a peristaltic pump have also been studied. The reflux phenomenon is also investigated. It is concluded, on the basis of the pressure distribution along the tubular length and pumping efficiency, that if the transverse magnetic field and the electric conductivity increase, the pumping machinery exerts more pressure for pushing the fluid forward. There is a linear relation between the averaged flow rate and the pressure applied across one wavelength that can restrain the flow due to peristalsis. It is found that there is a particular value of the averaged flow rate corresponding to a particular pressure that does not depend on the Hartmann number. Naming these values ‘critical values’, it is concluded that the pressure required for checking the flow increases with the Hartmann number above the critical value and decreases with it below the critical value. It is also inferred that magneto-hydrodynamic parameters make the fluid more prone to flow reversal. The conclusion applied to oesophageal swallowing reveals that normal water is easier to swallow than saline water. The latter is more prone to flow reversal. A significant difference between the propagation of the integral and non-integral number of waves along the tube is that pressure peaks are identical in the former and different in the latter cases.
19
Lu, Lianshan, and Dong Li. "Determination of Wall Friction Velocity of the Turbulent Boundary Layer by Two-Parameter Nonlinear Curve Fitting." Xibei Gongye Daxue Xuebao/Journal of Northwestern Polytechnical University 38, no. 2 (April 2020): 253–60. http://dx.doi.org/10.1051/jnwpu/20203820253.
Повний текст джерелаАнотація:
Hot-wire anemometer is a fundamental tool for flow field measurement, and has been widely adopted in research of turbulent flow. A turbulent boundary layer was generated by a trip wire and a piece of sandpaper in the low-speed wind tunnel, and the time-averaged velocity profiles at three different streamwise stations in the boundary layer were measured with IFA-300 constant temperature anemometer. Targeting the dimensionless velocity profile model White Law of the turbulent boundary layer, a nonlinear curve fitting MATLAB program for two parameters, which were theoretical original point of wall coordinate y0 and wall friction velocity uτ, was developed. Based on the measured time-averaged velocity profiles, the program was adopted to determine y0 and uτ of the three streamwise stations. It is found that the initial search domain and search step size of y0 and uτ have no effect on the fitting results if the physical solutions are included in the domain. It is also found that the selected fitting velocities data are closely related to the results. The value of the friction velocity with high precision will be obtained by this nonlinear curve fitting method if the highest fitting velocity data point dose not fall into the wake region. The method given in this paper is simple in programming and reliable in fitting results, and it is of practical to obtain the wall friction velocity of the turbulent boundary layer.
20
Hamilton, Chris, and Roman R. Rafikov. "Secular dynamics of binaries in stellar clusters – I. General formulation and dependence on cluster potential." Monthly Notices of the Royal Astronomical Society 488, no. 4 (August 5, 2019): 5489–511. http://dx.doi.org/10.1093/mnras/stz1730.
Повний текст джерелаАнотація:
AbstractOrbital evolution of binary systems in dense stellar clusters is important in a variety of contexts: origin of blue stragglers, progenitors of compact object mergers, millisecond pulsars, and so on. Here we consider the general problem of secular evolution of the orbital elements of a binary system driven by the smooth tidal field of an axisymmetric stellar cluster (globular, nuclear, etc.) in which the binary orbits. We derive a secular Hamiltonian (averaged over both the inner Keplerian orbit of the binary and its outer orbit within the cluster) valid to quadrupole order for an arbitrary cluster potential and explore its characteristics. This doubly averaged ‘tidal’ Hamiltonian depends on just two parameters, which fully absorb the information about the background cluster potential and the binary’s orbit within it: a dimensional parameter A setting the secular time-scale, and a dimensionless parameter Γ which determines the phase portrait of the binary’s inner orbital evolution. We examine the dependence of A and Γ on cluster potential (both spherical and axisymmetric) and on the binary orbit within the cluster. Our theory reproduces known secular results – such as Lidov–Kozai evolution and the effect of the Galactic tide on Oort Cloud comets – in appropriate limits, but is more general. It provides a universal framework for understanding dynamical evolution of various types of binaries driven by the smooth tidal field of any axisymmetric potential. In a companion paper we provide a detailed exploration of the resulting orbital dynamics.
21
Shin, Jong-Hyeon, Jong-Hwi Lee, and Se-Myong Chang. "A Simplified Numerical Model for the Prediction of Wake Interaction in Multiple Wind Turbines." Energies 12, no. 21 (October 29, 2019): 4122. http://dx.doi.org/10.3390/en12214122.
Повний текст джерелаАнотація:
In the design of wind energy farms, the loss of power should be seriously considered for the second wind turbine located inside the wake region of the first one. The rotation of the first wind-front rotor generates a high-vorticity wake with turbulence, and a suitable model is required in computational fluid dynamics (CFD) to predict the deficit of energy of the second turbine for the given configuration. A simplified numerical model based on the classical momentum theory is proposed in this study for multiple wind turbines, which is proposed with a couple of tuning parameters applied to Reynolds-averaged Navier-Stokes (RANS) analysis, resulting in a remarkable reduction of computational load compared with advanced methods, such as large eddy simulation (LES) where two parameters reflect on axial and rotational wake motion, simply tuned with the wind-tunnel test and its corresponding LES result. As a lumped parameter for the figure of merit, we regard the normalized efficiency on the kinetic power output of computational domain, which should be directed to maximize for the optimization of wind farms. The parameter surface is plotted in a dimensionless form versus intervals between turbines, and a simple correlation is obtained for a given hub height of 70% diameter and a fixed rotational speed tuned from the experimental data in a wide range.
22
Abhangi, Nikhilkumar D., and G. M. Deheri. "Numerical Modelling of Squeeze Film Performance between Rotating Transversely Rough Curved Circular Plates under the Presence of a Magnetic Fluid Lubricant." ISRN Mechanical Engineering 2012 (September 18, 2012): 1–9. http://dx.doi.org/10.5402/2012/873481.
Повний текст джерелаАнотація:
An endeavour has been made to study and analyze the behaviour of a magnetic fluid-based squeeze film between curved transversely rough rotating circular plates when the curved upper plate lying along a surface determined by an exponential function approaches the curved lower plate along the surface governed by a secant function. A magnetic fluid is used as the lubricant in the presence of an external magnetic field oblique to the radial axis. The random roughness of the bearing surfaces is characterised by a stochastic random variable with nonzero mean, variance, and skewness. The associated nondimensional averaged Reynolds equation is solved with suitable boundary conditions in dimensionless form to obtain the pressure distribution, leading to the expression for the load carrying capacity. The results establish that the bearing system registers an enhanced performance as compared to that of the bearing system dealing with a conventional lubricant. This investigation proves that albeit the bearing suffers due to transverse surface roughness, there exist sufficient scopes for obtaining a relatively better performance in the case of negatively skewed roughness by properly choosing curvature parameters and the rotation ratio. It is appealing to note that the negative variance further enhances this positive effect.
23
ud-Doula, Asif, Stanley P. Owocki, and Richard H. D. Townsend. "Angular momentum loss and stellar spin-down in magnetic massive stars." Proceedings of the International Astronomical Union 4, S259 (November 2008): 423–24. http://dx.doi.org/10.1017/s174392130903097x.
Повний текст джерелаАнотація:
AbstractWe examine the angular momentum loss and associated rotational spin-down for magnetic hot stars with a line-driven stellar wind and a rotation-aligned dipole magnetic field. Our analysis here is based on our previous 2-D numerical MHD simulation study that examines the interplay among wind, field, and rotation as a function of two dimensionless parameters, W(=Vrot/Vorb) and ‘wind magnetic confinement’, η∗ defined below. We compare and contrast the 2-D, time variable angular momentum loss of this dipole model of a hot-star wind with the classical 1-D steady-state analysis by Weber and Davis (WD), who used an idealized monopole field to model the angular momentum loss in the solar wind. Despite the differences, we find that the total angular momentum loss averaged over both solid angle and time follows closely the general WD scaling ~ ṀΩR2A. The key distinction is that for a dipole field Alfvèn radius RA is significantly smaller than for the monopole field WD used in their analyses. This leads to a slower stellar spin-down for the dipole field with typical spin-down times of order 1 Myr for several known magnetic massive stars.
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Dijkstra, Yoeri M., and Henk M. Schuttelaars. "A Unifying Approach to Subtidal Salt Intrusion Modeling in Tidal Estuaries." Journal of Physical Oceanography 51, no. 1 (January 2021): 147–67. http://dx.doi.org/10.1175/jpo-d-20-0006.1.
Повний текст джерелаАнотація:
AbstractThe salinity structure in estuaries is classically described in terms of the salinity structure as well mixed, partially mixed, or salt wedge. The existing knowledge about the processes that result in such salinity structures comes from highly idealized models that are restricted to either well-mixed and partially mixed cases or subtidal salt wedge estuaries. Hence, there is still little knowledge about the processes driving transitions between these different salinity structures and the estuarine parameters at which such a transition is found. As an important step toward a unified description of the dominant processes driving well-mixed, partially mixed, and salt wedge estuaries, a subtidal width-averaged model applicable to all these salinity structures is developed and systematically analyzed. Using our model, we identify four salinity regimes, resulting from different balances of dominant processes. It is shown that each regime is uniquely determined by two dimensionless parameters: an estuarine Froude and Rayleigh number, representing freshwater discharge and tidal mixing, respectively, resulting in a classification of the regimes in terms of these two parameters. Furthermore, analytical expressions to approximate the salt intrusion length in each regime are developed. These expressions are used to illustrate that the salt intrusion length in different regimes responds in a highly different manner to changes in depth and freshwater discharge. As one of the key results, we show that there are only very weak relations between the process-based regime of an estuary and the salt intrusion length and top–bottom stratification. This implies that the salinity structure of an estuary cannot be uniquely matched to a regime.
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Lecampion, Brice, and Dmitry I. Garagash. "Confined flow of suspensions modelled by a frictional rheology." Journal of Fluid Mechanics 759 (October 22, 2014): 197–235. http://dx.doi.org/10.1017/jfm.2014.557.
Повний текст джерелаАнотація:
AbstractWe investigate in detail the problem of confined pressure-driven laminar flow of neutrally buoyant non-Brownian suspensions using a frictional rheology based on the recent proposal of Boyer et al. (Phys. Rev. Lett., vol. 107 (18), 2011, 188301). The friction coefficient (shear stress over particle normal stress) and solid volume fraction are taken as functions of the dimensionless viscous number $I$ defined as the ratio between the fluid shear stress and the particle normal stress. We clarify the contributions of the contact and hydrodynamic interactions on the evolution of the friction coefficient between the dilute and dense regimes reducing the phenomenological constitutive description to three physical parameters. We also propose an extension of this constitutive framework from the flowing regime (bounded by the maximum flowing solid volume fraction) to the fully jammed state (the random close packing limit). We obtain an analytical solution of the fully developed flow in channel and pipe for the frictional suspension rheology. The result can be transposed to dry granular flow upon appropriate redefinition of the dimensionless number $I$. The predictions are in excellent agreement with available experimental results for neutrally buoyant suspensions, when using the values of the constitutive parameters obtained independently from stress-controlled rheological measurements. In particular, the frictional rheology correctly predicts the transition from Poiseuille to plug flow and the associated particles migration with the increase of the entrance solid volume fraction. We also numerically solve for the axial development of the flow from the inlet of the channel/pipe toward the fully developed state. The available experimental data are in good agreement with our numerical predictions, when using an accepted phenomenological description of the relative phase slip obtained independently from batch-settlement experiments. The solution of the axial development of the flow notably provides a quantitative estimation of the entrance length effect in a pipe for suspensions when the continuum assumption is valid. Practically, the latter requires that the predicted width of the central (jammed) plug is wider than one particle diameter. A simple analytical expression for development length, inversely proportional to the gap-averaged diffusivity of a frictional suspension, is shown to encapsulate the numerical solution in the entire range of flow conditions from dilute to dense.
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Ren, Yun, Zuchao Zhu, Denghao Wu, Xiaojun Li, and Lanfang Jiang. "Investigation of flow separation in a centrifugal pump impeller based on improved delayed detached eddy simulation method." Advances in Mechanical Engineering 11, no. 12 (December 2019): 168781401989783. http://dx.doi.org/10.1177/1687814019897832.
Повний текст джерелаАнотація:
The mechanism of flow separation in the impeller of a centrifugal pump with a low specific speed was explored by experimental, numerical, and theoretical methods. A novel delayed Reynolds-averaged Navier–Stokes/large eddy simulation hybrid algorithm combined with a rotation and curvature correction method was developed to calculate the inner flow field of the original pump for the large friction loss in the centrifugal impeller, high adverse pressure gradient, and large blade curvature. Boundary vorticity flux theory was introduced for internal flow diagnosis, and the relative velocity vector near the surface of the blade and the distribution of the dimensionless pressure coefficient was analyzed. The validity of the numerical method was verified, and the location of the backflow area and its flow features were determined. Finally, based on flow diagnosis, the geometric parameters influencing the flow state of the impeller were specifically adjusted to obtain a new design impeller. The results showed that the distribution of the boundary vorticity flux peak values, the skin friction streamline, and near-wall relative velocities improved significantly after the design change. In addition, the flow separation was delayed, the force applied on the blade was improved, the head under the part-load condition was improved, and the hydraulic efficiency was improved over the global flow ranges. It was demonstrated that the delayed Reynolds-averaged Navier–Stokes/large eddy simulation hybrid algorithm was capable to capture the separation flow in a centrifugal pump, and the boundary vorticity flux theory was suitable for the internal flow diagnosis of centrifugal pump.
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Lenci, Alessandro, Sandro Longo, and Vittorio Di Federico. "Shear-Thinning Fluid Flow in Variable-Aperture Channels." Water 12, no. 4 (April 17, 2020): 1152. http://dx.doi.org/10.3390/w12041152.
Повний текст джерелаАнотація:
Non-Newtonian fluid flow in a single fracture is a 3-D nonlinear phenomenon that is often averaged across the fracture aperture and described as 2-D. To capture the key interactions between fluid rheology and spatial heterogeneity, we adopt a simplified geometric model to describe the aperture variability, consisting of adjacent one-dimensional channels with constant aperture, each drawn from an assigned aperture distribution. The flow rate is then derived under the lubrication approximation for the two limiting cases of an external pressure gradient that is parallel/perpendicular to the channels; these two arrangements provide upper and lower bounds to the fracture conductance. The fluid rheology is described by the Prandtl–Eyring shear-thinning model. Novel closed-form results for the flow rate and hydraulic aperture are derived and discussed; different combinations of the parameters that describe the fluid rheology and the variability of the aperture field are considered. The flow rate values are very sensitive to the applied pressure gradient and to the shape of the distribution; in particular, more skewed distribution entails larger values of a dimensionless flow rate. Results for practical applications are compared with those valid for a power-law fluid and show the effects on the fracture flow rate of a shear stress plateau.
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Zeeshan, Ahmed, Moshan Hassan, Rahmat Ellahi, and Muhammad Nawaz. "Shape effect of nanosize particles in unsteady mixed convection flow of nanofluid over disk with entropy generation." Proceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering 231, no. 4 (April 26, 2016): 871–79. http://dx.doi.org/10.1177/0954408916646139.
Повний текст джерелаАнотація:
The aim of this paper is to study the different shapes of nanoparticles on mixed convective steady flow over a rotating disk. For nanofluid, the copper nanoparticles of disk, cylindrical, and spherical shapes of different sizes and water as base fluid are considered. The physical problem is first modeled and then the governing equations are transformed into nonlinear ordinary differential equations. These equations are dimensionless using geometrical and physical flowfield-dependent parameters and solved analytically. A very good agreement is observed between the obtained results of the current study and previously published study in limiting cases. The shape effects on velocity profiles in radial, tangential, axial directions, and temperature distribution are displayed graphically with the reflection of specific range of nanolayer thickness and its conductivity. In addition, irreversibility due to heat and fluid friction is investigated that supports the heat transfer enhancement in renewable energy systems and industrial thermal management. For the analysis of the averaged entropy generation number, the results are shown in pie charts and tablet form. It is evident from the study that proper choice of nanoparticles will be helpful in controlling velocity and heat transfer. It is also observed that irreversibility process can be reduced by using nanoparticles, especially the spherical particles.
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Holub, A. P., A. F. Zubkov, A. A. Masterova, and Y. D. Selyutskiy. "Dynamics of a Wheeled Cart Driven by a Savonius Rotor." Mekhatronika, Avtomatizatsiya, Upravlenie 22, no. 5 (May 17, 2021): 254–61. http://dx.doi.org/10.17587/mau.22.254-261.
Повний текст джерелаАнотація:
Savonius rotor is one of relatively wide-spread type of wind turbines. The rotation rate of this rotor is considerably lower than that of horizontal axis wind turbines and Darrieus wind turbines. However, it starts rotating at small wind speeds, doesn’t require any additional devices to ensure its re-orientation in case of change of the wind direction, and generates a rather large torque. Therefore, it is suitable for use as a drive in different mechanical and electromechanical systems. In the present paper, dynamics of rectilinear motion of a wheeled cart is studied, which is driven by Savonius rotor installed on it. It is assumed that the wind makes a certain constant angle with the line, along which the cart moves. The aerodynamic load upon the rotor is described with an empirical model, in the context of which the aerodynamic characteristics (aerodynamic torque, drag and lateral force coefficients) are represented as Fourier series with respect to the rotor revolution angle, the coefficients of the series being functions of the rotor tip speed ratio (dimensionless angular speed). Experiments were performed in the subsonic wind tunnel of the Institute of Mechanics of Lomonosov Moscow State University intended to measure aerodynamic characteristics of the rotor at different wind speeds and rotor angular speeds. Based on experimental data, functions were proposed that approximately describe the dependence of the above mentioned coefficients upon the tip speed ratio. The obtained dependences were used for analysis of dynamics of the cart driven by the Savonius rotor. The equations of motion are averaged with respect to the angle of revolution of rotor. Steady solutions of this averaged system are studied. It is shown that, for certain values of parameters, there exist two attracting steady motions corresponding to different directions of the cart velocity. Cart dynamics in the context of the full system of equations of motion is compared with its dynamics in the context of the averaged system.
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Berk, Tim, Guillaume Gomit, and Bharathram Ganapathisubramani. "Vectoring of parallel synthetic jets: a parametric study." Journal of Fluid Mechanics 804 (September 9, 2016): 467–89. http://dx.doi.org/10.1017/jfm.2016.559.
Повний текст джерелаАнотація:
The vectoring of a pair of parallel synthetic jets can be described using five dimensionless parameters: the aspect ratio of the slots, the Strouhal number, the Reynolds number, the phase difference between the jets and the spacing between the slots. In the present study, the influence of the latter four on the vectoring behaviour of the jets is examined experimentally, using particle image velocimetry. Time-averaged velocity maps are used to give a qualitative description of the variations in vectoring for a parametric sweep of each of the four parameters independently. A diverse set of vectoring behaviour is observed in which the resulting jet can be merged or bifurcated and either vectored towards the actuator leading in phase or the actuator lagging in phase. Three performance metrics are defined to give a quantitative description of the vectoring behaviour: the included angle between bifurcated branches, the vectoring angle of the total flow and the normalized momentum flux of the flow. Using these metrics, the influence of changes in the Strouhal number, Reynolds number, phase difference and spacing are quantified. Phase-locked maps of the swirling strength are used to track vortex pairs. Vortex trajectories are used to define three Strouhal number regimes for the vectoring behaviour. In the first regime, vectoring behaviour is dominated by the pinch-off time, which is written as function of Strouhal number only. In the second regime, the pinch-off time is invariant and the vectoring behaviour slightly changes with Strouhal number. In the third regime, given by the formation criterion, no synthetic jet is formed. Vortex positions at a single phase, shortly after creation of the lagging vortex pair, are used to propose a vectoring mechanism. This vectoring mechanism explains the observed qualitative and quantitative variations for all four parameters.
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Isabey, D., H. K. Chang, C. Delpuech, A. Harf, and C. Hatzfeld. "Dependence of central airway resistance on frequency and tidal volume: a model study." Journal of Applied Physiology 61, no. 1 (July 1, 1986): 113–26. http://dx.doi.org/10.1152/jappl.1986.61.1.113.
Повний текст джерелаАнотація:
The resistance of a hollow cast of human central airways was measured during true sinusoidal airflow oscillations over a wide range of frequencies (0.5–40 Hz) and for various flow amplitudes up to 8 l/s. Pressure and flow were measured in the trachea with high-performance transducers, digitized and averaged over 100 cycles. Data were studied at two points in the flow cycle: at peak inspiratory and expiratory flows and in the two neighborhoods around zero flow where airway resistance (Rv approximately equal to o) was taken as the average slope of the pressure-flow (P-V) curve in each zone. When data obtained near peak flow were plotted in terms of dimensionless pressure drop vs. peak Reynolds number (Rem) and compared with steady-state data, we found no difference up to 2 Hz as previously reported (Isabey and Chang, J. Appl. Physiol. 51: 1338–1348, 1981), a slight decay in pressure drop between 4 and 8 Hz, a frequency-dependent increase in peak flow resistance at high frequencies (10–40 Hz) governed by the Strouhal number alpha 2/Rem beyond alpha 2/Rem = 0.5. On the other hand RV approximately equal to o was found to increase relative to steady state as local acceleration increases, e.g., as peak flow increases at a fixed frequency; this differs from the classical linear theory of oscillatory flow in a long straight tube. To explain these results, we had to use, as in our previous study, an alternative expression for the Strouhal number, i.e., epsilon = L X A X (dV/dt)/V2 (where L and A are the length and cross-sectional area of the trachea and V is a constant flow range over which resistance around flow reversal was computed), which accurately reflects the ratio of local acceleration [d(V/A)/dt)] to convective acceleration [(V/A)2/L] in developing branching flow. Finally, to delineate the regions of dominance of each of the dimensionless parameters, we compiled frequency-tidal volume diagrams for peak flows as well as for reversal. Epsilon, which is negligible near peak flows, appeared to govern the oscillatory P-V relationship near flow reversal in a transitional region of the diagram located between regions of steadiness, or moderate unsteadiness, and a region of dominant unsteadiness governed by alpha.
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Adinoyi, Ayo Samuel, Normah Mohd-Ghazali, and Shuhaimi Mansor. "Impact of Adjacent Building on Outdoor Ventilation around a Layout of Two Buildings." Applied Mechanics and Materials 819 (January 2016): 250–56. http://dx.doi.org/10.4028/www.scientific.net/amm.819.250.
Повний текст джерелаАнотація:
The outdoor air ventilation impact of a taller building in different configurations of a layout of two adjacent buildings is presented in this paper. The critical parameters investigated are the separation distance (S) between the buildings and the ratio of height of downwind building to that of the building upwind, herein referred to as building height ratio (HR). The aim is to explore intermediate spacing distances which may engender acceptable ventilation around the buildings. A three-dimensional (3-D) numerical simulation employing the Computational Fluid Dynamics technique which adopts the Reynolds-Averaged Navier-Stokes equation and the realizable k-ε turbulence model was used to study the turbulent flow field around the full-scale two-building configurations. Results show that velocity ratio generally increases with height ratio, indicating that more air motion is induced at the pedestrian level as the height of the downwind building increases. For each of the height ratios, there is a spacing distance at which the velocity ratio is highest. The spacing distances at which the maximum velocity ratio occurs for the various height ratios are proposed. The dimensionless air exchange rate generally increases with height ratio, indicating that greater quantity of air from within the cavity between the buildings is exchanged with air from outside the cavity, which should result in better air quality. The findings of the study demonstrate the importance of incorporating wind data of an urban area in formulating guidelines for layout of buildings.
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Volino, Ralph J. "Separation Control on Low-Pressure Turbine Airfoils Using Synthetic Vortex Generator Jets." Journal of Turbomachinery 125, no. 4 (October 1, 2003): 765–77. http://dx.doi.org/10.1115/1.1626686.
Повний текст джерелаАнотація:
Oscillating vortex generator jets have been used to control boundary layer separation from the suction side of a low-pressure turbine airfoil. A low Reynolds number (Re=25,000) case with low free-stream turbulence has been investigated with detailed measurements including profiles of mean and fluctuating velocity and turbulent shear stress. Ensemble averaged profiles are computed for times within the jet pulsing cycle, and integral parameters and local skin friction coefficients are computed from these profiles. The jets are injected into the mainflow at a compound angle through a spanwise row of holes in the suction surface. Preliminary tests showed that the jets were effective over a wide range of frequencies and amplitudes. Detailed tests were conducted with a maximum blowing ratio of 4.7 and a dimensionless oscillation frequency of 0.65. The outward pulse from the jets in each oscillation cycle causes a disturbance to move down the airfoil surface. The leading and trailing edge celerities for the disturbance match those expected for a turbulent spot. The disturbance is followed by a calmed region. Following the calmed region, the boundary layer does separate, but the separation bubble remains very thin. Results are compared to an uncontrolled baseline case in which the boundary layer separated and did not reattach, and a case controlled passively with a rectangular bar on the suction surface. The comparison indicates that losses will be substantially lower with the jets than in the baseline or passively controlled cases.
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Nwasokwa, O. N. "Mechanism of mechanical alternans in ischemia-reperfusion: role of deficient relaxation of the strong twitch." American Journal of Physiology-Heart and Circulatory Physiology 269, no. 1 (July 1, 1995): H169—H175. http://dx.doi.org/10.1152/ajpheart.1995.269.1.h169.
Повний текст джерелаАнотація:
We tested the hypothesis that impaired and incomplete relaxation of the strong twitch of mechanical alternans causes the peak force deficit (PFD) of the weak twitch and that, by decreasing the relaxation deficit (RD) of the strong twitch, dobutamine would diminish the PFD. We studied isometric twitches of the in situ blood-perfused canine papillary muscle (n = 8). To produce mechanical alternans, we paced the heart at 110-155 beats/min and decreased mean coronary perfusion pressure (MCPP) stepwise to produce ischemia and then increased it to produce reperfusion. We measured the RD and PFD and fit each curve of isometric force [F(t)] with the relation F(t) = F0 + C(t/A)Be1-(t/A)B, where F0 is force at twitch onset, to obtain the parameters A, B, and C. B is a dimensionless index of myocardial relaxation; it decreases with impaired (delayed) relaxation. At each MCPP, we averaged B for the strong and weak twitches. The PFD showed a positive correlation with the RD. At each MCPP, mean B was lower for the strong twitch than for the weak twitch, indicating impaired relaxation of the strong twitch. Dobutamine increased B from 1.83 +/- 0.14 to 2.12 +/- 0.16 (P = 0.00002) in the strong twitch and decreased B from 4.15 +/- 2.42 to 2.19 +/- 0.18 (P = 0.05) in the weak twitch. Dobutamine thus equalized the relaxation of the strong and weak twitches. Consequently it decreased the RD from 2.57 +/- 2.14 to 0.16 +/- 0.24 g (P = 0.01) and the PFD from 5.50 +/- 3.67 to 1.04 +/- 1.15 g (P = 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)
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Vassallo, Daniel, Raghavendra Krishnamurthy, Robert Menke, and Harindra J. S. Fernando. "Observations of Stably Stratified Flow through a Microscale Gap." Journal of the Atmospheric Sciences 78, no. 1 (January 2021): 189–208. http://dx.doi.org/10.1175/jas-d-20-0087.1.
Повний текст джерелаАнотація:
AbstractThis paper reports the findings of a comprehensive field investigation on flow through a mountain gap subject to a range of stably stratified environmental conditions. This study was embedded within the Perdigão field campaign, which was conducted in a region of parallel double-ridge topography with ridge-normal wind climatology. One of the ridges has a well-defined gap (col) at the top, and an array of in situ and remote sensors, including a novel triple Doppler lidar system, was deployed around it. The experimental design was mostly guided by previous numerical and theoretical studies conducted with an idealized configuration where a flow (with characteristic velocity U0 and buoyancy frequency N) approaches normal to a mountain of height h with a gap at its crest, for which the governing parameters are the dimensionless mountain height G = Nh/U0 and various gap aspect ratios. Modified forms of G were proposed to account for real-world atmospheric variability, and the results are discussed in terms of a gap-averaged value Gc. The nature of gap flow was highly dependent on Gc, wherein a nearly neutral flow regime (Gc < 1), a transitional mountain wave regime [Gc ~ O(1)], and a gap-jetting regime [Gc > O(1)] were identified. The measurements were in broad agreement with previous numerical and theoretical studies on a single ridge with a gap or double-ridge topography, although details vary. This is the first-ever detailed field study reported on microscale [O(100) m] gap flows, and it provides useful data and insights for future theoretical and numerical studies.
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Rodriguez-Lucas, Luis, Chen Ning, Marcelo Fajardo-Pruna, and Yugui Yang. "Study of Vortex Systems as a Method to Weakening the Urban Heat Islands within the Financial District in Large Cities." Sustainability 13, no. 23 (November 29, 2021): 13206. http://dx.doi.org/10.3390/su132313206.
Повний текст джерелаАнотація:
This paper presents a new concept called the urban vortex system (UVS). The UVS couples a vortex generator (V.G.) that produces updraft by artificial vortex and a vortex stability zone (VSZ) consisting of an assembly of four buildings acting as a chimney. Through this system, a stable, upward vortex flow can be generated. The Reynolds Averaged Navier–Stokes (RANS) simulation was carried out to investigate the flow field in the UVS. The Renormalized Group (RNG) k–ε turbulent model was selected to solve the complex turbulent flow. Validation of the numerical results was achieved by making a comparison with the large-size experimental model. The results reported that a steady-state vortex could be formed when a vapor-air mixture at 2 m/s and 450 K enters the vortex generator. This vortex presented a maximum negative central pressure of −6.81 Pa and a maximum velocity of 5.47 (m/s). Finally, the similarity method found four dimensionless parameters, which allowed all the flow characteristics to be transported on a large scale. The proposed large-scale UVS application is predicted to be capable, with have a maximum power of 2 M.W., a specific work of 3 kJ/kg, buildings 200-m high, and the ability to generate winds of 6.1 m/s (20 km/h) at 200 m up to winds of 1.5 m/s (5 km/h) at 400 m. These winds would cause the rupture of the gas capsule of the heat island phenomenon. Therefore, the city would balance its temperature with that of the surrounding rural areas.
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Zhang, You-Sheng, Wei-Tao Bi, Fazle Hussain, and Zhen-Su She. "A generalized Reynolds analogy for compressible wall-bounded turbulent flows." Journal of Fluid Mechanics 739 (December 20, 2013): 392–420. http://dx.doi.org/10.1017/jfm.2013.620.
Повний текст джерелаАнотація:
AbstractA generalized Reynolds analogy (GRA) is proposed for compressible wall-bounded turbulent flows (CWTFs) and validated by direct numerical simulations. By introducing a general recovery factor, a similarity between the Reynolds-averaged momentum and energy equations is established for the canonical CWTFs (i.e. pipes, channels, and flat-plate boundary layers that meet the quasi-one-dimensional flow approximation), independent of Prandtl number, wall temperature, Mach number, Reynolds number, and pressure gradient. This similarity and the relationships between temperature and velocity fields constitute the GRA. The GRA relationship between the mean temperature and the mean velocity takes the same quadratic form as Walz’s equation, with the adiabatic recovery factor replaced by the general recovery factor, and extends the validity of the latter to diabatic compressible turbulent boundary layers and channel/pipe flows. It also derives Duan & Martín’s (J. Fluid Mech., vol. 684, 2011, pp. 25–59) empirical relation for flows at different physical conditions (wall temperature, Mach number, enthalpy condition, surface catalysis, etc.). Several key parameters besides the general recovery factor emerge in the GRA. An effective turbulent Prandtl number is shown to be the reason for the parabolic profile of mean temperature versus mean velocity, and it approximates unity in the fully turbulent region. A dimensionless wall temperature, that we call the diabatic parameter, characterizes the wall-temperature effects in diabatic flows. The GRA also extends the analysis to the fluctuation fields. It recovers the modified strong Reynolds analogy proposed by Huang, Coleman & Bradshaw (J. Fluid Mech., vol. 305, 1995, pp. 185–218) and explains the variation of the temperature–velocity correlation coefficient with wall temperature. Thus, the GRA unveils a generalized similarity principle behind the complex nonlinear coupling between the thermal and velocity fields of CWTFs.
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Zierenberg, Jennifer R., Hideki Fujioka, Ronald B. Hirschl, Robert H. Bartlett, and James B. Grotberg. "Pulsatile Blood Flow and Oxygen Transport Past a Circular Cylinder." Journal of Biomechanical Engineering 129, no. 2 (April 1, 2007): 202–15. http://dx.doi.org/10.1115/1.2485961.
Повний текст джерелаАнотація:
The fundamental study of blood flow past a circular cylinder filled with an oxygen source is investigated as a building block for an artificial lung. The Casson constitutive equation is used to describe the shear-thinning and yield stress properties of blood. The presence of hemoglobin is also considered. Far from the cylinder, a pulsatile blood flow in the x direction is prescribed, represented by a time periodic (sinusoidal) component superimposed on a steady velocity. The dimensionless parameters of interest for the characterization of the flow and transport are the steady Reynolds number (Re), Womersley parameter (α), pulsation amplitude (A), and the Schmidt number (Sc). The Hill equation is used to describe the saturation curve of hemoglobin with oxygen. Two different feed-gas mixtures were considered: pure O2 and air. The flow and concentration fields were computed for Re=5, 10, and 40, 0≤A≤0.75, α=0.25, 0.4, and Schmidt number, Sc=1000. The Casson fluid properties result in reduced recirculations (when present) downstream of the cylinder as compared to a Newtonian fluid. These vortices oscillate in size and strength as A and α are varied. Hemoglobin enhances mass transport and is especially important for an air feed which is dominated by oxyhemoglobin dispersion near the cylinder. For a pure O2 feed, oxygen transport in the plasma dominates near the cylinder. Maximum oxygen transport is achieved by operating near steady flow (small A) for both feed-gas mixtures. The time averaged Sherwood number, Sh̿, is found to be largely influenced by the steady Reynolds number, increasing as Re increases and decreasing with A. Little change is observed with varying α for the ranges investigated. The effect of pulsatility on Sh̿ is greater at larger Re. Increasing Re aids transport, but yields a higher cylinder drag force and shear stresses on the cylinder surface which are potentially undesirable.
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Bruni, G., R. Reinoso, N. C. van de Giesen, F. H. L. R. Clemens, and J. A. E. ten Veldhuis. "On the sensitivity of urban hydrodynamic modelling to rainfall spatial and temporal resolution." Hydrology and Earth System Sciences Discussions 11, no. 6 (June 6, 2014): 5991–6033. http://dx.doi.org/10.5194/hessd-11-5991-2014.
Повний текст джерелаАнотація:
Abstract. Cities are increasingly vulnerable to floods generated by intense rainfall, because of their high degree of imperviousness, implementation of infrastructures, and changes in precipitation patterns due to climate change. Accurate information of convective storm characteristics at high spatial and temporal resolution is a crucial input for urban hydrological models to be able to simulate fast runoff processes and enhance flood prediction. In this paper, a detailed study of the sensitivity of urban hydrological response to high resolution radar rainfall was conducted. Rainfall rates derived from X-band dual polarimetric weather radar for four rainstorms were used as input into a detailed hydrodynamic sewer model for an urban catchment in Rotterdam, the Netherlands. Dimensionless parameters were derived to compare results between different storm conditions and to describe the effect of rainfall spatial resolution in relation to storm and hydrodynamic model properties: rainfall sampling number (rainfall resolution vs. storm size), catchment sampling number (rainfall resolution vs. catchment size), runoff and sewer sampling number (rainfall resolution vs. runoff and sewer model resolution respectively). Results show catchment smearing effect for rainfall resolution approaching half the catchment size, i.e. for catchments sampling numbers greater than 0.5 averaged rainfall volumes decrease about 20%. Moreover, deviations in maximum water depths, form 10 to 30% depending on the storm, occur for rainfall resolution close to storm size, describing storm smearing effect due to rainfall coarsening. Model results also show the sensitivity of modelled runoff peaks and maximum water depths to the resolution of the runoff areas and sewer density respectively. Sensitivity to temporal resolution of rainfall input seems low compared to spatial resolution, for the storms analysed in this study. Findings are in agreement with previous studies on natural catchments, thus the sampling numbers seem to be promising as an approach to describe sensitivity of hydrological response to rainfall variability for intra-urban catchments and local convective storms. More storms and different urban catchments of varying characteristics need to be analysed in order to validate these findings.
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Landel, Julien R., C. P. Caulfield, and Andrew W. Woods. "Streamwise dispersion and mixing in quasi-two-dimensional steady turbulent jets." Journal of Fluid Mechanics 711 (September 12, 2012): 212–58. http://dx.doi.org/10.1017/jfm.2012.388.
Повний текст джерелаАнотація:
AbstractWe investigate experimentally and theoretically the streamwise transport and dispersion properties of steady quasi-two-dimensional plane turbulent jets discharged vertically from a slot of width $d$ into a fluid confined between two relatively close rigid boundaries with gap $W\ensuremath{\sim} O(d)$. We model the evolution in time and space of the concentration of passive tracers released in these jets using a one-dimensional time-dependent effective advection–diffusion equation. We make a mixing length hypothesis to model the streamwise turbulent eddy diffusivity such that it scales like $b(z){ \overline{w} }_{m} (z)$, where $z$ is the streamwise coordinate, $b$ is the jet width, ${ \overline{w} }_{m} $ is the maximum time-averaged vertical velocity. Under these assumptions, the effective advection–diffusion equation for $\phi (z, t)$, the horizontal integral of the ensemble-averaged concentration, is of the form ${\partial }_{t} \phi + {K}_{a} {\text{} {M}_{0} \text{} }^{1/ 2} {\partial }_{z} \left(\phi / {z}^{1/ 2} \right)= {K}_{d} {\text{} {M}_{0} \text{} }^{1/ 2} {\partial }_{z} \left({z}^{1/ 2} {\partial }_{z} \phi \right)$, where $t$ is time, ${K}_{a} $ (the advection parameter) and ${K}_{d} $ (the dispersion parameter) are empirical dimensionless parameters which quantify the importance of advection and dispersion, respectively, and ${M}_{0} $ is the source momentum flux. We find analytical solutions to this equation for $\phi $ in the cases of a constant-flux release and an instantaneous finite-volume release. We also give an integral formulation for the more general case of a time-dependent release, which we solve analytically when tracers are released at a constant flux over a finite period of time. From our experimental results, whose concentration distributions agree with the model, we find that ${K}_{a} = 1. 65\pm 0. 10$ and ${K}_{d} = 0. 09\pm 0. 02$, for both finite-volume releases and constant-flux releases using either dye or virtual passive tracers. The experiments also show that streamwise dispersion increases in time as ${t}^{2/ 3} $. As a result, in the case of finite-volume releases more than 50 % of the total volume of tracers is transported ahead of the purely advective front (i.e. the front location of the tracer distribution if all dispersion mechanisms are ignored and considering a ‘top-hat’ mean velocity profile in the jet); and in the case of constant-flux releases, at each instant in time, approximately 10 % of the total volume of tracers is transported ahead of the advective front.
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Liu, Yuwei, Fuhao Ye, Weizheng Zhang, and Shujun Zhou. "Heat transfer behavior caused by temperature difference in reciprocating sliding contact." Science Progress 103, no. 1 (January 2020): 003685041989956. http://dx.doi.org/10.1177/0036850419899560.
Повний текст джерелаАнотація:
In order to study the heat transfer characteristics between two rough surfaces of two contacting blocks with different bulk temperatures and sliding reciprocating motion, a two-dimensional heat transfer model was used to analyze the dimensionless average heat flux, considering thermal contact conductance. The results of a series simulations were presented, covering a wide range of operating parameters including dimensionless amplitude [Formula: see text], dimensionless frequency [Formula: see text], and measurements of interface conductance [Formula: see text]. The results show that the dimensionless average heat flux increases with the increase of dimensionless frequency and amplitude, and the dimensionless average heat flux rises sharply in the low range of [Formula: see text] and approaches to a steady state approximation when [Formula: see text] and [Formula: see text].
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K., Kalidasan, R. Velkennedy, Jan Taler, Dawid Taler, Pawel Oclon, and Rajesh Kanna P. "Numerical study of air convection in a rectangular enclosure with two isothermal blocks and oscillating bottom wall temperature." International Journal of Numerical Methods for Heat & Fluid Flow 28, no. 1 (January 2, 2018): 103–17. http://dx.doi.org/10.1108/hff-03-2017-0125.
Повний текст джерелаАнотація:
Purpose This study aims to perform a numerical study of air convection in a rectangular enclosure with two isothermal blocks and oscillating bottom wall temperature under laminar flow conditions. The geometry of the enclosure contains two isothermal blocks placed equidistant along the streamwise direction. The top wall is assumed to be cold (low temperature). The bottom wall temperature is either kept as constant or sinusoidally varied with time. The vertical walls are considered as adiabatic. The flow is diagonally upwards and assisted by the buoyancy force. The inlet is positioned at the bottom of the left wall, and the outlet is placed at the top of the right wall. The parameters considered in this paper are Rayleigh number (104-106), Prantdl number (0.71), amplitude of temperature oscillation (0-0.5) and the period (0.2). The effects of these parameters on heat transfer and fluid flow inside the open cavity are studied. The periodic results of fluid flow are illustrated with streamlines and the heat transfer is represented by isotherms and time-averaged Nusselt number. By virtue of increasing buoyancy, the heat transfer accelerates with an increase in the Rayleigh number. Also, the heat transfer is intensive with an increase in the bottom wall temperature. Design/methodology/approach The momentum and energy equations are solved simultaneously. The energy equation (3) is initially solved using the alternating direction implicit (ADI) method. The results of the energy equation are updated into the vorticity equation. The unsteady vorticity transport equation is also solved using the ADI method. Dimensionless time step equal to 0.01 is used for high Ra (105 and 106) and 0.001 is used for low Ra (104). Convergence criteria of 10−5 is used during the vorticity, stream function and temperature calculations, as the sum of error should be very small. Findings Numerical study of air convection in a rectangular enclosure with two isothermal blocks and oscillating bottom wall temperature is performed under laminar flow condition. The effect of the isothermal blocks on the heat transfer is analyzed for different Rayleigh numbers and the following conclusions are arrived. The hydrodynamic blockage effect is subdued by the isothermal heating of square blocks. Based on the streamline diagrams, it is found that the formation of vortices is greatly influenced by the Rayleigh number when all the walls are exposed to a constant wall temperature. The influence of amplitude on the heat transfer is remarkable on the wall exposed to oscillating temperature and is subtle on the opposite static cold wall. The heat transfer increases with an increase in the Rayleigh number and temperature. Research limitations/implications Flow is assumed to be two-dimensional and laminar subject to oscillatory boundary condition. The present investigation aims to study natural convection inside the cavity filled with air whose bottom wall is subject to time-variant temperature. The buoyancy is further intensified through two isothermal square blocks placed equidistant along the streamwise direction at mid-height. Originality/value The authors have developed a CFD solver to simulate the situation. Effect of Rayleigh number subject to oscillatory thermal boundary condition is simulated. Streamline contour and isotherm contour are presented. Local and average Nusselt numbers are presented.
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Ozan, Ayse Yuksel, George Constantinescu, and Andrew J. Hogg. "Lock-exchange gravity currents propagating in a channel containing an array of obstacles." Journal of Fluid Mechanics 765 (January 26, 2015): 544–75. http://dx.doi.org/10.1017/jfm.2014.735.
Повний текст джерелаАнотація:
AbstractLarge eddy simulation (LES) is used to investigate the evolution of Boussinesq gravity currents propagating through a channel of height $H$ containing a staggered array of identical cylinders of square cross-section and edge length $D$. The cylinders are positioned with their axes horizontal and perpendicular to the (streamwise) direction along which the lock-exchange flow develops. The effects of the volume fraction of solids, ${\it\phi}$, the Reynolds number and geometrical parameters describing the array of obstacles on the structure of the lock-exchange flow, total drag force acting on the gravity current, front velocity and global energy budget are analysed. Simulation results show that the currents rapidly transition to a state in which the extra resistance provided by the cylinders strongly retards the motion and dominates the dissipative processes. A shallow layer model is also formulated and similarity solutions for the motion are found in the regime where the driving buoyancy forces are balanced by the drag arising from the interaction with the cylinders. The numerical simulations and this shallow layer model show that low-Reynolds-number currents transition to a drag-dominated regime in which the resistance is linearly proportional to the flow speed and, consequently, the front velocity, $U_{f}$, is proportional to $t^{-1/2}$, where $t$ is the time measured starting at the gate release time. By contrast, high-Reynolds-number currents, for which the cylinder Reynolds number is sufficiently high that the drag coefficient for most of the cylinders can be considered constant, transition first to a quadratic drag-dominated regime in which the front speed determined from the simulations is given by $U_{f}\sim t^{-0.25}$, before undergoing a subsequent transition to the aforementioned linear drag regime in which $U_{f}\sim t^{-1/2}$. Meanwhile, away from the front, the depth-averaged gravity current velocity is proportional to $t^{-1/3}$, a result that is in agreement with the shallow water model. It is suggested that the difference between these two is due to mixing processes, which are shown to be significant in the numerical simulations, especially close to the front of the motion. Direct estimation of the drag coefficient $C_{D}$ from the numerical simulations shows that the combined drag parameter for the porous medium, ${\it\Gamma}_{D}=C_{D}{\it\phi}(H/D)/(1-{\it\phi})$, is the key dimensionless grouping of variables that determines the speed of propagation of the current within arrays with different $C_{D},{\it\phi}$ and $D/H$.
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Lomanowski, B., M. Dunne, N. Vianello, S. Aleiferis, M. Brix, J. Canik, I. S. Carvalho, et al. "Experimental study on the role of the target electron temperature as a key parameter linking recycling to plasma performance in JET-ILW*." Nuclear Fusion 62, no. 6 (April 13, 2022): 066030. http://dx.doi.org/10.1088/1741-4326/ac5668.
Повний текст джерелаАнотація:
Abstract Changes in global and edge plasma parameters (H 98(y,2), dimensionless collisionality ν *, core density peaking, separatrix density n e,sep) with variations in the D2 fueling rate and divertor configuration are unified into a single trend when mapped to ⟨T e,ot⟩, the spatially averaged spectroscopically derived outer target electron temperature. Dedicated JET with the ITER-like wall (JET-ILW) experiments in combination with an extended JET-ILW database of unseeded low-triangularity H-mode plasmas spanning a wide range of D2 fueling rates, I p, B t and heating power have demonstrated the importance of ⟨T e,ot⟩ as a key physics parameter linking the recycling particle source and detachment with plasma performance. The remarkably robust H 98(y,2) trend with ⟨T e,ot⟩ is connected to a strong inverse correlation between ⟨T e,ot⟩, n e,sep and ν *, thus directly linking changes in the divertor recycling moderated by ⟨T e,ot⟩ with the previously established relationship between ν *, core density peaking and core pressure resulting in a degradation in core plasma performance with decreasing ⟨T e,ot⟩ (increasing ν *). A strong inverse correlation between the separatrix to pedestal density ratio, n e,sep/n e,ped, and ⟨T e,ot⟩ is also established, with the rise in n e,sep/n e,ped saturating at ⟨T e,ot⟩ > 10 eV. A strong reduction in H 98(y,2) is observed as ⟨T e,ot⟩ is driven from 30 to 10 eV via additional D2 gas fueling, while the divertor remains attached. Consequently, the pronounced performance degradation in attached divertor conditions has implications for impurity seeding radiative divertor scenarios, in which H 98(y,2) is already low (∼0.7) before impurities are injected into the plasma since moderate gas fueling rates are required to promote high divertor neutral pressure. A favorable pedestal pressure, p e,ped, dependence on I p has also been observed, with an overall increase in p e,ped at I p = 3.4 MA as ⟨T e,ot⟩ is driven down from attached to high-recycling divertor conditions. In contrast, p e,ped is reduced with decreasing ⟨T e,ot⟩ in the lower I p branches. Further work is needed to (i) clarify the potential role of edge opacity on the observed favorable pedestal pressure I p scaling; as well as to (ii) project the global and edge plasma performance trends with ⟨T e,ot⟩ to reactor-scale devices to improve predictive capability of the coupling between recycling and confined plasma fueling in what are foreseen to be more opaque edge plasma conditions.
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Rezvaya, Kseniya, Irina Tynyanova, and Оleksandr Kosorukov. "APPLICATION OF THE METHOD OF AVERAGE DIMENSIONLESS PARAMETERS FOR DETERMINING THE OPTIMAL OPERATING MODE OF A HIGH-PRESSURE REVERSIBLE HYDRAULIC MACHINE." Bulletin of the National Technical University "KhPI". Series: Hydraulic machines and hydraulic units, no. 2 (December 26, 2019): 82–88. http://dx.doi.org/10.20998/2411-3441.2019.2.10.
Повний текст джерела
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Lua, A. C., and R. F. Boucher. "Magnetic Filtration of Fine Particles from Gas Streams." Proceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering 207, no. 2 (August 1993): 109–22. http://dx.doi.org/10.1243/pime_proc_1993_207_214_02.
Повний текст джерелаАнотація:
Dust particles from the basic oxygen furnace steelmaking process were removed from air streams using a laboratory high-gradient magnetic filter. Particle sizes and number concentrations were determined by an optical particle spectrometer and were found to be largely in the submicrometre range. Dust loadability increased greatly when a magnetic field was applied and showed no deterioration in filter performance, even though the matrix had captured 10 times its own volume of dust. Particle penetrations of 1 per cent and less were achieved for submicrometre particles down to an optically measurable 0.24 μm diameter. Varying only one parameter at a time has isolated the individual effects of filter length, magnetic field, matrix packing fraction, gas velocity and wire size and aspect ratio. The dimensionless groups in an idealized single-wire model for high-gradient magnetic filtration are not adequate for a real filter. Other dimensionless parameters, viz. average dimensionless wire separation and cumulative dimensionless wire blockage, arise and are found to be important.
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Yu, Xin Qi, Qing Gang Liu, Hui Qin Gao, and Jia Hui Yu. "Liquid Film Performance Analysis of the Mechanical Seals with a Laser-Textured Micro-Pore Face." Advanced Materials Research 139-141 (October 2010): 418–21. http://dx.doi.org/10.4028/www.scientific.net/amr.139-141.418.
Повний текст джерелаАнотація:
A theoretical model has been developed for the mechanical seals with a laser-textured porous seal face. By means of variable dimensionless steps, parametric analysis has been performed to obtain dimensionless liquid film pressure by the finite difference method. Liquid film pressure profiles over the pore column have be achieved by the computer program MATLAB. It is found from calculating results that average liquid film pressure increases with increase of the rotational speed, liquid viscosity and reduction of liquid film thickness. Hydrodynamic effect produced by micro-pores decreases with increase of fluid pressure. In addition, the effect of the pore area density and depth over diameter ratio on the liquid film pressure is very significant. Optimum values of these parameters can maximize the average liquid film pressure.
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Averbukh, M. "Improved dimensionless nomograms approach in the electric drives and power electronics courses." International Journal of Electrical Engineering & Education 56, no. 1 (May 23, 2018): 38–50. http://dx.doi.org/10.1177/0020720918776459.
Повний текст джерелаАнотація:
Parameter estimation of the nonlinear electronic schemes, including inductances, capacitances, resistances, and switches represents a nontrivial problem in the “power electronics” and “electrical drives” courses. Difficulties could be observed during solutions of electrical circuits, which describe the behavior of electric motors in drives, and various electronic appliances are being used in power electronics. This situation is a result of complicated analytical approaches aiming to solve nonlinear ordinary differential equations describing the occurring processes. A typical student has a significant problem in attaining the analytical results. Various methods of linearization nonlinear elements permitting to obtain roughly analytical solutions and simplified solution were applied over the past. Nowadays, coarsely obtained results are not acceptable as a rule. As a result, smart simulation based on PSIM, MATLAB Simulink, and WOLFRAM Mathematica giving excellent opportunity for accurate answers are recommended. All software programs represent undoubtedly important and extremely accurate approaches. However, only numerical results are provided and are not capable of solution’s generalization. It seems that in the Power Electronics and Electrical Drives courses, a wide submission could obtain methods of numerical nomograms with dimensionless representation of input–output parameters. Dimensionless approach allows significant diminishing of a number of decisive parameters and simplifies calculation, whereas keeping acceptable precision of results, as well as a possibility of outcome’s generalization and representation variables trends in a wide range of input parameters. The likely objects of analysis may be, for example, output characteristics of DC motors fed by controllable and uncontrollable n-phase rectifiers, resistive rectifier losses and motor’s efficiency, average and root mean square currents or voltages in electronic circuits, total harmonic distortion, and others. Long-time practice of these approaches approved their usefulness, productivity, and helpfulness.
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García-Ros, Gonzalo, Iván Alhama, and Manuel Cánovas. "Use of discriminated nondimensionalization in the search of universal solutions for 2-D rectangular and cylindrical consolidation problems." Open Geosciences 10, no. 1 (June 21, 2018): 209–21. http://dx.doi.org/10.1515/geo-2018-0016.
Повний текст джерелаАнотація:
Abstract The solution to the 2-D consolidation problem, both for rectangular and cylindrical domains, has been widely studied in the scientific literature, reporting the most precise solutions in the form of analytical expressions difficult to handle for the engineer due to the high number of parameters involved. In this paper, after introducing a precise definition of the characteristic time, both this magnitude and the average degree of consolidation are obtained in terms of the least number of dimensionless groups that rule the problem. To do this, the groups are firstly derived from the dimensionless governing equations deduced from the mathematical model, following a discriminated nondimensionalization procedure which provides new groups that cannot be obtained by classical nondimensionalization. By a large number of numerical simulations, the dependences of the characteristic time and the average degree of consolidation on the new dimensionless groups have allowed to represent these unknowns graphically in the form of universal curves. This allows these quantities to be read with the least mathematical effort. A case study is solved to demonstrate the reliability and accuracy of the results.
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Ji, Jinghu, Yonghong Fu, and Qinsheng Bi. "The influence of partially textured slider with orientation ellipse dimples on the behavior of hydrodynamic lubrication." Industrial Lubrication and Tribology 66, no. 2 (March 4, 2014): 161–67. http://dx.doi.org/10.1108/ilt-11-2011-0087.
Повний текст джерелаАнотація:
Purpose – The purpose of this paper is to investigate a partially textured slider of infinite width with orientation ellipse dimples in liquid application. Design/methodology/approach – In this paper, the pressure distribution of lubrication between a partially textured slider and a smooth sliding slider is calculated by the multi-grid method. For the same dimple area, the influence of the ellipse dimple with geometric parameters, and distribution and orientation on the hydrodynamic lubrication is evaluated in terms of the dimensionless average pressure for a given set of operating parameters. Findings – In the present work, the magnitude of the dimensionless average pressure seems proportional to the slender ratio. Consequently, the slender ratio may be chosen as large as possible based on fabrication techniques. The longer axes of ellipse dimples placed parallel to the direction of sliding always show the better hydrodynamic effect. Furthermore, the results show that the ellipse dimples can greatly improve hydrodynamic effect of partially surface textured slider of infinite width by proper design of these texturing parameters. Originality/value – This paper develops a partial surface texturing infinitely width slider with orientation ellipse dimples for improving hydrodynamic lubrication.