Journal articles on the topic 'Steady and unsteady states'
To see the other types of publications on this topic, follow the link: Steady and unsteady states.
Create a spot-on reference in APA, MLA, Chicago, Harvard, and other styles
Consult the top 50 journal articles for your research on the topic 'Steady and unsteady states.'
Next to every source in the list of references, there is an 'Add to bibliography' button. Press on it, and we will generate automatically the bibliographic reference to the chosen work in the citation style you need: APA, MLA, Harvard, Chicago, Vancouver, etc.
You can also download the full text of the academic publication as pdf and read online its abstract whenever available in the metadata.
Browse journal articles on a wide variety of disciplines and organise your bibliography correctly.
1
Ambike, Satyajit, Daniela Mattos, Vladimir M. Zatsiorsky, and Mark L. Latash. "Unsteady steady-states: central causes of unintentional force drift." Experimental Brain Research 234, no. 12 (August 19, 2016): 3597–611. http://dx.doi.org/10.1007/s00221-016-4757-7.
Full text
2
Feng, J., and D. D. Joseph. "The unsteady motion of solid bodies in creeping flows." Journal of Fluid Mechanics 303 (November 25, 1995): 83–102. http://dx.doi.org/10.1017/s0022112095004186.
Full textAbstract:
In treating unsteady particle motions in creeping flows, a quasi-steady approximation is often used, which assumes that the particle's motion is so slow that it is composed of a series of steady states. In each of these states, the fluid is in a steady Stokes flow and the total force and torque on the particle are zero. This paper examines the validity of the quasi-steady method. For simple cases of sedimenting spheres, previous work has shown that neglecting the unsteady forces causes a cumulative error in the trajectory of the spheres. Here we will study the unsteady motion of solid bodies in several more complex flows: the rotation of an ellipsoid in a simple shear flow, the sedimentation of two elliptic cylinders and four circular cylinders in a quiescent fluid and the motion of an elliptic cylinder in a Poiseuille flow in a two-dimensional channel. The motion of the fluid is obtained by direct numerical simulation and the motion of the particles is determined by solving their equations of motion with solid inertia taken into account. Solutions with the unsteady inertia of the fluid included or neglected are compared with the quasi-steady solutions. For some flows, the effects of the solid inertia and the unsteady inertia of the fluid are importanty quantitatively but not qualitatively. In other cases, the character of the particles' motion is changed. In particular, the unsteady effects tend to suppress the periodic oscillations generated by the quasi-steady approximation. Thus, the results of quasi-steady calculatioins are never uniformly valid and can be completely misleading. The conditions under which the unsteady effects at small Reynolds numbers are important are explored and the implications for modelling of suspension flows are addressed.
3
Kori, Hiroshi. "Slow Switching and Broken Cluster State in a Population of Neuronal Oscillators." International Journal of Modern Physics B 17, no. 22n24 (September 30, 2003): 4238–41. http://dx.doi.org/10.1142/s0217979203022246.
Full textAbstract:
We discuss a novel class of cluster states in globally coupled neuronal oscillators. It is well known that steady cluster states such as perfect synchrony and multi-cluster states arise in globally coupled oscillators. However, little has been discussed on unsteady cluster states which often arise in populations of neuronal oscillators. We show three types of unsteady cluster states, i.e. a pair of clusters involving slow switching, three clusters involving slow switching, and a broken single cluster. These states result from saddle connections between different cluster states, and typically arise near a bifurcation point where perfect synchrony loses its stability.
4
de S. Teixeira, Renan, and Leonardo S. de B. Alves. "Minimal gain marching schemes: searching for unstable steady-states with unsteady solvers." Theoretical and Computational Fluid Dynamics 31, no. 5-6 (March 7, 2017): 607–21. http://dx.doi.org/10.1007/s00162-017-0426-0.
Full text
5
Dorney, D. J., and O. P. Sharma. "Evaluation of Flow Field Approximations for Transonic Compressor Stages." Journal of Turbomachinery 119, no. 3 (July 1, 1997): 445–51. http://dx.doi.org/10.1115/1.2841143.
Full textAbstract:
The flow through gas turbine compressors is often characterized by unsteady, transonic, and viscous phenomena. Accurately predicting the behavior of these complex multi-blade-row flows with unsteady rotor–stator interacting Navier–Stokes analyses can require enormous computer resources. In this investigation, several methods for predicting the flow field, losses, and performance quantities associated with axial compressor stages are presented. The methods studied include: (1) the unsteady fully coupled blade row technique, (2) the steady coupled blade row method, (3) the steady single blade row technique, and (4) the loosely coupled blade row method. The analyses have been evaluated in terms of accuracy and efficiency.
6
Stoica, Adrian Mihail, and Marius Stoia-Djeska. "On a New Model Reduction Method for CFD Systems Used for Flow Control Design." Applied Mechanics and Materials 332 (July 2013): 9–14. http://dx.doi.org/10.4028/www.scientific.net/amm.332.9.
Full textAbstract:
The use of active control to get better characteristics of unsteady internal and external flows is the ultimate goal of the research presented in this paper. Usually, unsteady flows are calculated using Euler and/or Navier-Stokes solvers. The efficiency of numerical simulation of an unsteady flow dramatically increases if the unsteady solution is a small perturbation about a steady-state flow, due to disturbances occurring at the boundaries of the flow domain. The main difficulty related to the flow simulation is that any CFD (Computational Fluid Dynamics) technique generates discrete systems with a very large number of states. In order to design an efficient control, the flow solver must be not only accurate and numerically effective, but also it must have a low number of states. The aim of this paper is to present a new method for model reduction of CFD systems using representative governing equations. The focus is on descriptor type systems resulting from the spatial discretization of the CFD governing equations.
7
Zhan, Tianyi, Fengze Sun, Chao Lv, Qian He, Xuan Wang, Kang Xu, Yaoli Zhang, and Liping Cai. "Evaluation of moisture diffusion in lignocellulosic biomass in steady and unsteady states by a dynamic vapor sorption apparatus." Holzforschung 73, no. 12 (November 26, 2019): 1113–19. http://dx.doi.org/10.1515/hf-2019-0063.
Full textAbstract:
Abstract To examine the methodology for determining the moisture diffusion behavior of lignocellulosic biomass in steady and unsteady states (two stages of a sorption isotherm), the diffusion coefficients in the steady and unsteady states (DSS and DUS) were investigated over a range of relative humidity (RH) from 10 to 90% using a dynamic vapor sorption (DVS) apparatus and a specifically designed cell kit. Thin samples with a thickness of 50 μm were prepared from three lignocellulosic biomasses, i.e. poplar, Chinese fir and moso bamboo. Based on Fick’s first and second laws, DSS and DUS were determined. An increase in DSS or DUS was observed with increasing equilibrium moisture content (EMC) or transient status, regardless of the lignocellulosic biomass species. The moisture-dependent DSS of poplar, Chinese fir and moso bamboo was similar to values previously reported. Chinese fir and moso bamboo exhibited the highest and the lowest DSS values, respectively, when the same EMCs were achieved. The results of this study revealed that DSS and DUS of lignocellulosic biomass (even with limited dimensions) could be determined during a sorption isotherm in a wide humidity range. Furthermore, the results are helpful for simulating moisture transport behaviors in the fields of drying, paper packaging and wooden building maintenance.
8
Világi, František, Branislav Knížat, Róbert Olšiak, Marek Mlkvik, Peter Mlynár, František Ridzoň, and František Urban. "Simulation of Steady States of Helium Loop at Long Time Scenarios." MATEC Web of Conferences 328 (2020): 01009. http://dx.doi.org/10.1051/matecconf/202032801009.
Full textAbstract:
The natural circulation helium loop is an experimental facility designed for the research of the possibility of utilizing natural convection cooling for the case of decay heat removal from a fast nuclear reactor. This concept would bring an improved automated safety system for future nuclear power plants operating a gas-cooled reactor. The article presents a new possibility of direct use of energy conservation laws in a 1D simulation of natural circulation loops. The calculation is performed by a triple iteration process, nested into each other. The results of the calculations showed good agreement with the measurements at steady state. A calculation with the proposed model at unsteady state is not yet possible, especially because of the exclusion of heat accumulation into the material.
9
Guan, Xiaofeng, M. S. High, and D. A. Tree. "Viscoelastic Effects in Modeling Web Handling Systems: Unsteady-State Analysis." Journal of Applied Mechanics 65, no. 1 (March 1, 1998): 234–41. http://dx.doi.org/10.1115/1.2789031.
Full textAbstract:
A model to predict the tension in open spans of web handling systems during transient operations has been developed. Governing equations were developed by using the White-Metzner equation to describe the material response in conjunction with mass and force balances. The governing equations were nondimensionalized and solved via the MacCormack predictor/corrector technique. Two dimensionless parameters emerged from the analysis, the Deborah number, De, and the ratio of the viscous stress to the steady state stress, N. The resulting model is the companion to a previously reported model for steady-state operations (Guan et al, 1995). The model was used to predict the behavior of a web handling system during start-up, transition between steady states, and a periodic disturbance. During start-up and transition, systems responded more rapidly at low De. The system response during a periodic disturbance was correlated to De, the frequency, and the magnitude of the disturbance.
10
FOREST, M. GREGORY, and HONG ZHOU. "Unsteady analyses of thermal glass fibre drawing processes." European Journal of Applied Mathematics 12, no. 4 (August 2001): 479–96. http://dx.doi.org/10.1017/s0956792501004557.
Full textAbstract:
Fibre drawing is an important industrial process for synthetic polymers and optical communications. In the manufacture of optical fibres, precise diameter control is critical to waveguide performance, with tolerances in the submicron range that are met through feedback controls on processing conditions. Fluctuations arise from material non-uniformity plague synthetic polymers but not optical silicate fibres which are drawn from a pristine source. The steady drawing process for glass fibres is well-understood (e.g. [11, 12, 20]). The linearized stability of steady solutions, which characterize limits on draw speed versus processing and material properties, is well-understood (e.g. [9, 10, 11]). Feedback is inherently transient, whereby one adjusts processing conditions in real time based on observations of diameter variations. Our goal in this paper is to delineate the degree of sensitivity to transient fluctuations in processing boundary conditions, for thermal glass fibre steady states that are linearly stable. This is the relevant information for identifying potential sources of observed diameter fluctuation, and for designing the boundary controls necessary to alter existing diameter variations. To evaluate the time-dependent final diameter response to boundary fluctuations, we numerically solve the model nonlinear partial differential equations of thermal glass fibre processing. Our model simulations indicate a relative insensitivity to mechanical effects (such as take-up rates, feed-in rates), but strong sensitivity to thermal fluctuations, which typically form a basis for feedback control.
11
Fezai, Salwa, Fakher Oueslati, and Brahim Ben-Beya. "Identification of flow states around three staggered square cylinders at two symmetrical arrangements by a numerical investigation." International Journal of Modern Physics C 31, no. 11 (September 2, 2020): 2050151. http://dx.doi.org/10.1142/s012918312050151x.
Full textAbstract:
The fluid flow over three staggered square cylinders at two symmetrical arrangements has been numerically investigated in this study. The numerical calculations are carried out for several values of the Reynolds number (Re) ranging from 1 to 180. The results are presented in the form of vorticity contours and temporal histories of drag and lift coefficients. Furthermore, the physical parameters, namely, the average drag and lift coefficients and Strouhal number are presented as a function of Re. Two different states of flow are found in this work by systematically varying Re: steady and unsteady states. The transition to unsteady state regime is exhibited via Hopf bifurcation first in the second configuration followed consequently by the first one with critical Reynolds number of Re[Formula: see text] and Re[Formula: see text], respectively. It is observed that the bifurcation point of the steady regime to the unsteady one is very much influenced by the change in the geometry of the obstacle. The unsteady periodic wake is characterized by the Strouhal number, which varies with the Reynolds number and the obstacle geometry. Hence, the values of vortex shedding frequencies are estimated for both the considered configurations. Computations obtained also reveal that the spacing in the wake leads to reducing the pressure and enhancing the fluid flow velocity for both arrangements by monotonically strengthening the Reynolds number value. Furthermore, the drag and lift coefficients are determined, which allowed determining; the best configuration in terms of both lift and drag. It is observed that the drag force is dependent on the obstacle geometry and strengthens while lowering the Reynolds number. On the other hand, an opposite trend of the lift drag evolutions is observed for both configurations and considerably affected by the arrangements shape.
12
Siviglia, A., and M. Toffolon. "Multiple states for flow through a collapsible tube with discontinuities." Journal of Fluid Mechanics 761 (November 14, 2014): 105–22. http://dx.doi.org/10.1017/jfm.2014.635.
Full textAbstract:
AbstractWe study the occurrence of the multiple steady states that flows in a collapsible tube can develop under the effect of: (i) geometrical alterations (e.g. stenosis), (ii) variations of the mechanical properties of the tube wall, or (iii) variations of the external pressure acting on the conduit. Specifically, if the approaching flow is supercritical, two steady flow states are possible in a restricted region of the parameter space: one of these flow states is wholly supercritical while the other produces an elastic jump that is located upstream of the variation. In the latter case the flow undergoes a transition through critical conditions in the modified segment of the conduit. Both states being possible, the actual state is determined by the past history of the system, and the parameter values show a hysteretic behaviour when shifting from one state to the other. First we set up the problem in a theoretical framework assuming stationary conditions, and then we analyse the dynamics numerically in a one-dimensional framework. Theoretical considerations suggest that the existence of multiple states is associated with non-uniqueness of the steady-state solution, which is confirmed by numerical simulations of the fully unsteady problem.
13
Tabiś, Bolesław, Szymon Skoneczny, and Wojciech S. Stryjewski. "Nonlinear Dynamics of a Controlled Stirred Tank Bioreactor With Predator-Prey Relationship." Chemical and Process Engineering 35, no. 3 (September 1, 2014): 349–60. http://dx.doi.org/10.2478/cpe-2014-0026.
Full textAbstract:
Abstract The paper presents the dynamic characteristics of a continuous tank bioreactor for microbiological process, with a developed predator-prey food chain. The presence of the predator microorganism considerably influences the position and stability character of steady-states. There appears to exist a wide range of unstable steady-states and high-amplitude oscillations of state variables. Without automatic control, the system can operate only in unsteady conditions. From technological point of view, this circumstance is unfavorable. It was shown that oscillations can be removed by employing automatic control with continuous P or PI controllers. Moreover, the use of a controller with integrating element causes removal of the predator from the bioreactor. The paper discusses an application of this phenomenon for practical purposes.
14
Korakianitis, T., P. Papagiannidis, and N. E. Vlachopoulos. "Unsteady Flow/Quasi-Steady Heat Transfer Computations on a Turbine Rotor and Comparison With Experiments." Journal of Turbomachinery 124, no. 1 (August 1, 2001): 152–59. http://dx.doi.org/10.1115/1.1405419.
Full textAbstract:
The unsteady flow in stator–rotor interactions affects the structural integrity, aerodynamic performance of the stages, and blade-surface heat transfer. Numerous viscous and inviscid computer programs are used for the prediction of unsteady flows in two-dimensional and three-dimensional stator–rotor interactions. The relative effects of the various components of flow unsteadiness on heat transfer are under investigation. In this paper it is shown that for subsonic cases, the reduced frequency parameter for boundary-layer calculations is about two orders of magnitude smaller than the reduced frequency parameter for the core flow. This means that for typical stator–rotor interactions, the unsteady flow terms are needed to resolve the location of disturbances in the core flow, but in many cases the instantaneous disturbances can be input in steady-flow boundary-layer computations to evaluate boundary-layer effects in a quasi-steady approximation. This hypothesis is tested by comparing computations with experimental data on a turbine rotor for which there are extensive experimental heat transfer data available in the open literature. An unsteady compressible inviscid two-dimensional computer program is used to predict the propagation of the upstream stator disturbances into the downstream rotor passages. The viscous wake (velocity defect) and potential flow (pressure fluctuation) perturbations from the upstream stator are modeled at the computational rotor–inlet boundary. The effects of these interactions on the unsteady rotor flow result in computed instantaneous velocity and pressure fields. The period of the rotor unsteadiness is one stator pitch. The instantaneous velocity fields on the rotor surfaces are input in a steady-flow differential boundary-layer program, which is used to compute the instantaneous heat transfer rate on the rotor blades. The results of these quasi-steady heat-transfer computations are compared with the results of unsteady heat transfer experiments and with the results of previous unsteady heat transfer computations. The unsteady flow fields explain the unsteady amplitudes and phases of the increases and decreases in instantaneous heat transfer rate. It is concluded that the present method is accurate for quantitative predictions of unsteady heat transfer in subsonic turbine flows.
15
HILL, REGHAN J., and DONALD L. KOCH. "The transition from steady to weakly turbulent flow in a close-packed ordered array of spheres." Journal of Fluid Mechanics 465 (August 25, 2002): 59–97. http://dx.doi.org/10.1017/s0022112002008947.
Full textAbstract:
The sequence of transitions in going from steady to unsteady chaotic flow in a close-packed face-centred cubic array of spheres is examined using lattice-Boltzmann simulations. The transition to unsteady flow occurs via a supercritical Hopf bifurcation in which only the streamwise component of the spatially averaged velocity fluctuates and certain reflectional symmetries are broken. At larger Reynolds numbers, the cross-stream components of the spatially averaged velocity fluctuate with frequencies that are incommensurate with those of the streamwise component. This transition is accompanied by the breaking of rotational symmetries that persisted through the Hopf bifurcation. The resulting trajectories in the spatially averaged velocity phase space are quasi-periodic. At larger Reynolds numbers, the fluctuations are chaotic, having continuous frequency spectra with no easily identified fundamental frequencies. Visualizations of the unsteady flows in various dynamic states show that vortices are produced in which the velocity and vorticity are closely aligned. With increasing Reynolds number, the geometrical structure of the flow changes from one that is dominated by extension and shear to one in which the streamlines are helical. A mechanism for the dynamics is proposed in which energy is transferred to smaller scales by the dynamic interaction of vortices sustained by the underlying time-averaged flow.
16
Bai, Yuxing, Fanyu Kong, Bin Xia, Fei Zhao, and Yingying Liu. "Effects of Impeller Diameter on High-Speed Rescue Pump." Mathematical Problems in Engineering 2017 (2017): 1–15. http://dx.doi.org/10.1155/2017/1387210.
Full textAbstract:
Impeller diameter is a crucial design parameter of high-speed rescue pumps because it affects the performance and inner flow characteristics of these pumps. In this study, a pump with an impeller diameter of 248 mm was modeled and its performance was tested. Numerical simulations were conducted under steady and unsteady states, in which the sizes of the impeller diameters were designated as 248 mm (original), 235.6 mm (5% trimmed), 223.2 mm (10% trimmed), and 210.8 mm (15% trimmed). ANSYS software was used to test the shear stress transport (SST k-ω) of the four models, and results agreed well with experimental data. Diameter size affected impeller characteristics in both steady and unsteady states. Subsequently, the differences in performance, hydraulic loss, pressure pulsation, and radial force of the impellers were evaluated. In the performance test, the head and efficiency of the pump decreased as impeller diameter was reduced. The result trends are in accordance with the trim law within the acceptable error range. In terms of hydraulic loss, the impeller and diffuser vane components presented opposite trends with flow rate increase. Finally, in terms of pressure pulsation and radial force, the amplitude diminished while periodicity improved as impeller diameter decreased.
17
Rusak, Zvi, Yuxin Zhang, Harry Lee, and Shixiao Wang. "Swirling flow states in finite-length diverging or contracting circular pipes." Journal of Fluid Mechanics 819 (April 27, 2017): 678–712. http://dx.doi.org/10.1017/jfm.2017.179.
Full textAbstract:
The dynamics of inviscid-limit, incompressible and axisymmetric swirling flows in finite-length, diverging or contracting, long circular pipes is studied through global analysis techniques and numerical simulations. The inlet flow is described by the profiles of the circumferential and axial velocity together with a fixed azimuthal vorticity while the outlet flow is characterized by a state with zero radial velocity. A mathematical model that is based on the Squire–Long equation (SLE) is formulated to identify steady-state solutions of the problem with special conditions to describe states with separation zones. The problem is then reduced to the columnar (axially-independent) SLE, with centreline and wall conditions for the solution of the outlet flow streamfunction. The solution of the columnar SLE problem gives rise to the existence of four types of solutions. The SLE problem is then solved numerically using a special procedure to capture states with vortex-breakdown or wall-separation zones. Numerical simulations based on the unsteady vorticity circulation equations are also conducted and show correlation between time-asymptotic states and steady states according to the SLE and the columnar SLE problems. The simulations also shed light on the stability of the various steady states. The uniqueness of steady-state solutions in a certain range of swirl is proven analytically and demonstrated numerically. The computed results provide the bifurcation diagrams of steady states in terms of the incoming swirl ratio and size of pipe divergence or contraction. Critical swirls for the first appearance of the various types of states are identified. The results show that pipe divergence promotes the appearance of vortex-breakdown states at lower levels of the incoming swirl while pipe contraction delays the appearance of vortex breakdown to higher levels of swirl and promotes the formation of wall-separation states.
18
McDonald, N. Robb, and E. R. Johnson. "Gap-Leaping Vortical Currents." Journal of Physical Oceanography 39, no. 10 (October 1, 2009): 2665–74. http://dx.doi.org/10.1175/2009jpo4191.1.
Full textAbstract:
Abstract A one-parameter family of exact solutions describing the bifurcation of a steady two-dimensional current with uniform vorticity near a gap in a thin barrier is found. The unsteady evolution of source-driven flows toward these steady states is studied using a version of contour dynamics, appropriately modified to take into account the presence of a barrier with a single gap. It is shown that some of the steady solutions are realizable as large-time limits of the source-driven flows, although some are not owing to persistent eddy-shedding events in the vicinity of the gap. For the special case when there is zero net flux through the gap, numerical experiments show that the through-gap flux of vortical fluid increases with the width of the gap and that this flux approaches a steady limit with time.
19
Forest, M. Gregory, Panon Phuworawong, Qi Wang, and Ruhai Zhou. "Rheological signatures in limit cycle behaviour of dilute, active, polar liquid crystalline polymers in steady shear." Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 372, no. 2029 (November 28, 2014): 20130362. http://dx.doi.org/10.1098/rsta.2013.0362.
Full textAbstract:
We consider the dilute regime of active suspensions of liquid crystalline polymers (LCPs), addressing issues motivated by our kinetic model and simulations in Forest et al. (Forest et al. 2013 Soft Matter 9 , 5207–5222 ( doi:10.1039/c3sm27736d )). In particular, we report unsteady two-dimensional heterogeneous flow-orientation attractors for pusher nanorod swimmers at dilute concentrations where passive LCP equilibria are isotropic. These numerical limit cycles are analogous to longwave (homogeneous) tumbling and kayaking limit cycles and two-dimensional heterogeneous unsteady attractors of passive LCPs in weak imposed shear, yet these states arise exclusively at semi-dilute concentrations where stable equilibria are nematic. The results in Forest et al. mentioned above compel two studies in the dilute regime that complement recent work of Saintillan & Shelley (Saintillan & Shelley 2013 C. R. Physique 14 , 497–517 ( doi:10.1016/j.crhy.2013.04.001 )): linearized stability analysis of the isotropic state for nanorod pushers and pullers; and an analytical–numerical study of weakly and strongly sheared active polar nanorod suspensions to capture how particle-scale activation affects shear rheology. We find that weakly sheared dilute puller versus pusher suspensions exhibit steady versus unsteady responses, shear thickening versus thinning and positive versus negative first normal stress differences. These results further establish how sheared dilute nanorod pusher suspensions exhibit many of the characteristic features of sheared semi-dilute passive nanorod suspensions.
20
Leamy, Michael J. "On a Perturbation Method for the Analysis of Unsteady Belt-Drive Operation." Journal of Applied Mechanics 72, no. 4 (October 29, 2004): 570–80. http://dx.doi.org/10.1115/1.1940660.
Full textAbstract:
A perturbation method is presented for use in analyzing unsteady belt-drive operation. The method relies on the important assumption that for operating states close to steady operation, the friction state (i.e., whether the belt is creeping or sticking at any location on the pulley) is similar to that of the well-known steady solution in which a lone stick arc precedes a lone slip arc (Johnson, K. L., 1985, Contact Mechanics, Cambridge U.P., London, Chap. 8; Smith, D. P., 1999, Tribol. Int., 31(8), pp. 465–477). This assumption, however, is not used to determine the friction force distribution, and, in fact, the friction forces in the stick zone are found to be nonzero, in direct contrast to the steady solution. The perturbation analysis is used to derive expressions for the span tensions, the pulley tension distributions, the contact forces between the belt and the pulleys, and the angular velocity of the driven pulleys. Validity criteria are developed which determine bounds on the operation state for which the assumed friction state is upheld. Verification of response quantities from the perturbation solution is accomplished through comparison to quantities predicted by an in-house dynamic finite element model and excellent agreement is found. Additionally, the finite element model is used to verify the key assumption that a lone slip arc precedes a lone stick arc.
21
Akimoto, Koichi, Toshitaka Uchino, Kohei Nakano, Eriko Yasunaga, Shinichiro Kuroki, and Daisuke Hamanaka. "Comparison between Fixed and Flexible Chamber Methods for Measuring Respiratory Rate under Steady and Unsteady States." Engei Gakkai zasshi 69, no. 5 (2000): 646–52. http://dx.doi.org/10.2503/jjshs.69.646.
Full text
22
HAMED, M., and J. M. FLORYAN. "Marangoni convection. Part 1. A cavity with differentially heated sidewalls." Journal of Fluid Mechanics 405 (February 25, 2000): 79–110. http://dx.doi.org/10.1017/s002211209900734x.
Full textAbstract:
Marangoni convection in a cavity with differentially heated sidewalls has been investigated. The analysis includes the complete effects of interface deformation. The results determined for large Biot and zero Marangoni (zero Prandtl) numbers show that steady convection may exist for Reynolds numbers Re larger than, and for capillary numbers Ca and cavity lengths L smaller than, certain critical values. The main factor limiting the existence of steady convection involves the interface becoming tangential to the hot wall at the contact point (tangency condition). Unsteady analysis shows that the tangency condition defines the limit point for the system; its violation is most likely to lead to the formation of a dry spot at the hot wall. The critical values of Re, Ca, and L are mutually dependent and change with the heating rate (they reach a minimum for instantaneous heating). For a certain range of parameters, multiple (i.e. steady and oscillatory) states are possible. The oscillatory state has a form consisting of the steady mode with a simple harmonic sloshing motion superposed on it. A reduction in the heating rate permits heating of the liquid without triggering the oscillatory state. Transition between the steady and the oscillatory states involves a nonlinear instability process.
23
GELFGAT, A. YU, P. Z. BAR-YOSEPH, and A. L. YARIN. "Stability of multiple steady states of convection in laterally heated cavities." Journal of Fluid Mechanics 388 (June 10, 1999): 315–34. http://dx.doi.org/10.1017/s0022112099004796.
Full textAbstract:
A parametric study of multiple steady states, their stability, onset of oscillatory instability, and some supercritical unsteady regimes of convective flow of a Boussinesq fluid in laterally heated rectangular cavities is presented. Cavities with four no-slip boundaries, isothermal vertical and perfectly insulated horizontal boundaries are considered. Four distinct branches of steady-state flows are found for this configuration. A complete study of stability of each branch is performed for the aspect ratio A (length/height) of the cavity varying continuously from 1 to 11 and for two fixed values of the Prandtl number: Pr = 0 and Pr = 0.015. The results are represented as stability diagrams showing the critical parameters (critical Grashof number and the frequency at the onset of the oscillatory instability) corresponding to transitions from steady to oscillatory states, appearance of multi-roll states, merging of multiple states and backwards transitions from multi-roll to single-roll states. For better comparison with the existing experimental data, an additional stability study for varying Prandtl number (0.015 [les ] Pr [les ] 0.03) and fixed value of the aspect ratio A = 4 was carried out. It was shown that the dependence of the critical Grashof number on the aspect ratio and the Prandtl number is very complicated and a very detailed parametric study is required to reproduce it correctly. Comparison with the available experimental data for A = 4 shows that the results of a two-dimensional stability analysis are in good agreement with the experimental results if the width ratio (width/height) of the experimental container is sufficiently large. The study is carried out numerically with the use of two independent numerical approaches based on the global Galerkin and finite-volume methods.
24
FINNIGAN, T. D., and G. N. IVEY. "Submaximal exchange between a convectively forced basin and a large reservoir." Journal of Fluid Mechanics 378 (January 10, 1999): 357–78. http://dx.doi.org/10.1017/s0022112098003437.
Full textAbstract:
If a sill-enclosed basin, connected to a large reservoir, is suddenly subjected to a de-stabilizing surface buoyancy flux, it will first mix vertically by turbulent convection before the resulting lateral buoyancy gradient generates a horizontal exchange flow across the sill. We present a study which examines the unsteady adjustment of such a basin under continued steady forcing. It is shown, through theoretical development and laboratory experimentation, that two consecutive unsteady regimes characterized by different dynamic balances are traversed as the flow approaches a steady state.Once established the exchange flow is controlled at the sill crest where it is hydraulically critical. In the absence of a lateral contraction, the single control at the sill crest allows a range of submaximal exchange states with the flow at the sill being dependent not only on the forcing and geometrical parameters but also on mixing conditions within the basin which are, in turn, dependent on the sill exchange. The sill–basin system is therefore strongly coupled although it remains isolated from the external reservoir conditions by a region of internally supercritical flow. Results from the laboratory experiments are used to demonstrate the link between the forcing and the exchange flow at the sill. Steady-state measurements of the interior mean velocity and buoyancy fields are also compared with previous analytical models.
25
BLACKBURN, H. M., and J. M. LOPEZ. "Modulated rotating waves in an enclosed swirling flow." Journal of Fluid Mechanics 465 (August 25, 2002): 33–58. http://dx.doi.org/10.1017/s0022112002008893.
Full textAbstract:
The loss of axisymmetry in a swirling flow that is generated inside an enclosed cylindrical container by the steady rotation of one endwall is examined numerically. The two dimensionless parameters that govern these flows are the cylinder aspect ratio and a Reynolds number associated with the rotation of the endwall. This study deals with a fixed aspect ratio, height/radius = 2.5. At low Reynolds numbers the basic flow is steady and axisymmetric; as the Reynolds number increases the basic state develops a double recirculation zone on the axis, so-called vortex breakdown bubbles. On further increase in the Reynolds number the flow becomes unsteady through a supercritical Hopf bifurcation but remains axisymmetric. After the onset of unsteadiness, another two unsteady axisymmetric solution branches appear with further increase in Reynolds number, each with its own temporal characteristic: one is periodic and the other is quasi-periodic with a very low frequency modulation. Solutions on these additional branches are unstable to three-dimensional perturbations, leading to nonlinear modulated rotating wave states, but with the flow still dominated by the corresponding underlying axisymmetric mode. A study of the flow behaviour on and bifurcations between these solution branches is presented, both for axisymmetric and for fully three-dimensional flows. The presence of modulated rotating waves alters the structure of the bifurcation diagram and gives rise to its own dynamics, such as a truncated cascade of period doublings of very-low-frequency modulated states.
26
HAMMAMI, FAYÇAL, NADER BEN-CHEIKH, ANTONIO CAMPO, BRAHIM BEN-BEYA, and TAIEB LILI. "PREDICTION OF UNSTEADY STATES IN LID-DRIVEN CAVITIES FILLED WITH AN INCOMPRESSIBLE VISCOUS FLUID." International Journal of Modern Physics C 23, no. 04 (April 2012): 1250030. http://dx.doi.org/10.1142/s0129183112500301.
Full textAbstract:
In this work, a numerical study devoted to the two-dimensional and three-dimensional flow of a viscous, incompressible fluid inside a lid-driven cavity is undertaking. All transport equations are solved using the finite volume formulation on a staggered grid system and multi-grid acceleration. Quantitative aspects of two and three-dimensional flows in a lid-driven cavity for Reynolds number Re = 1000 show good agreement with benchmark results. An analysis of the flow evolution demonstrates that, with increments in Re beyond a certain critical value Rec, the steady flow becomes unstable and bifurcates into unsteady flow. It is observed that the transition from steadiness to unsteadiness follows the classical Hopf bifurcation. The time-dependent velocity distribution is studied in detail and the critical Reynolds number is localized for both 2D and 3D cases. Benchmark solutions for 2D and 3D lid-driven cavity flows are performed for Re = 1500 and 6000.
27
Wang, Hao, Jianhua Wang, and Yanming Liu. "Implementation of Artificial Compressibility Method for Steady and Unsteady Incompressible Flows Based on Finite Volume Method of Unstructured Grids." Journal of Physics: Conference Series 2381, no. 1 (December 1, 2022): 012087. http://dx.doi.org/10.1088/1742-6596/2381/1/012087.
Full textAbstract:
Abstract The development of a new computational fluid dynamics solver for simulating incompressible flows is described. Unlike traditional pressure-based solvers, the artificial compressibility method simulates both steady and unsteady states. Some classical numerical examples are used to validate the solvers. These findings show that the artificial compressibility method’s LES/DNS and steady flow simulations are competitive with the best previously reported methods. The following characteristics make this paper unique: (1) We implement the artificial compression method in OpenFOAM for the first time and show the detailed mathematical process of the algorithm so that readers can easily reproduce and improve it; (2) steady flow simulation case, the convergence time of the present method is 52% of the pressure base solver; (3) to accelerate the convergence rate of each time step to the incompressible state, the L-stable Singly Diagonal Implicit Runge-Kutta method is introduced into the pseudo-time advance, which enlarges the limit of maximum Courant number to accelerate convergence.
28
Dottori, F., M. L. V. Martina, and E. Todini. "A dynamic rating curve approach to indirect discharge measurement." Hydrology and Earth System Sciences Discussions 6, no. 1 (February 23, 2009): 859–96. http://dx.doi.org/10.5194/hessd-6-859-2009.
Full textAbstract:
Abstract. The operational measurement of discharge in medium and large rivers is mostly based on indirect approaches by converting water stages into discharge on the basis of steady-flow rating curves. Unfortunately, under unsteady flow conditions, this approach does not guarantee accurate estimation of the discharge due, on the one hand, to the underlying steady state assumptions and, on the other hand, to the required extrapolation of the rating curve beyond the range of actual measurements used for its derivation. Historically, several formulas were proposed to correct the steady-state discharge value and to approximate the unsteady-flow stage-discharge relationship on the basis of water level measurements taken at a single cross section, where a steady state rating curve is available. However, most of them are either over-simplified or based on approximations that prevented their generalisation. Moreover all the mentioned formulas have been rarely tested on cases where their use becomes essential, namely under unsteady-flow conditions characterised by wide loop rating curves. In the present work, a new approach, based on simultaneous stage measurements at two adjacent cross sections, is introduced and compared to the approaches described in the literature. The comparison has been carried out on channels with constant or spatially variable geometry under a wide range of flood wave and river bed slope conditions. The results clearly show the improvement in the discharge estimation and the reduction of estimation errors obtainable using the proposed approach.
29
Kovářík, Karel, and Juraj Mužík. "The Modified Local Boundary Knots Method for Solution of the Two-Dimensional Advection–Diffusion Equation." Mathematics 10, no. 20 (October 18, 2022): 3855. http://dx.doi.org/10.3390/math10203855.
Full textAbstract:
This paper deals with a new modification of the local boundary knots method (LBKM), which will allow the irregular node distribution and the arbitrary shape of the solution domain. Unlike previous localizations, it has no requirements on the number of nodes in the support or on the number of virtual points. Owing to the limited number of virtual points, the condition number of boundary knots matrix remains relatively low. The article contains the derivation of the relations of the method for steady and unsteady states and shows its effectiveness in three control examples.
30
LUCHTENBURG, DIRK M., BERT GÜNTHER, BERND R. NOACK, RUDIBERT KING, and GILEAD TADMOR. "A generalized mean-field model of the natural and high-frequency actuated flow around a high-lift configuration." Journal of Fluid Mechanics 623 (March 6, 2009): 283–316. http://dx.doi.org/10.1017/s0022112008004965.
Full textAbstract:
A low-dimensional Galerkin model is proposed for the flow around a high-lift configuration, describing natural vortex shedding, the high-frequency actuated flow with increased lift and transients between both states. The form of the dynamical system has been derived from a generalized mean-field consideration. Steady state and transient URANS (unsteady Reynolds-averaged Navier–Stokes) simulation data are employed to derive the expansion modes and to calibrate the system parameters. The model identifies the mean field as the mediator between the high-frequency actuation and the low-frequency natural shedding instability.
31
Hall, Philip, P. Balakumar, and D. Papageorgiu. "On a class of unsteady three-dimensional Navier—Stokes solutions relevant to rotating disc flows: threshold amplitudes and finite-time singularities." Journal of Fluid Mechanics 238 (May 1992): 297–323. http://dx.doi.org/10.1017/s0022112092001721.
Full textAbstract:
A class of ‘exact’ steady and unsteady solutions of the Navier—Stokes equations in cylindrical polar coordinates is given. The flows correspond to the motion induced by an infinite disc rotating in the (x, y)-plane with constant angular velocity about the z-axis in a fluid occupying a semi-infinite region which, at large distances from the disc, has velocity field proportional to (x, — y,O) with respect to a Cartesian coordinate system. It is shown that when the rate of rotation is large Kármán's exact solution for a disc rotating in an otherwise motionless fluid is recovered. In the limit of zero rotation rate a particular form of Howarth's exact solution for three-dimensional stagnation-point flow is obtained. The unsteady form of the partial differential system describing this class of flow may be generalized to time-periodic flows. In addition the unsteady equations are shown to describe a strongly nonlinear instability of Kármán's rotating disc flow. It is shown that sufficiently large perturbations lead to a finite-time breakdown of that flow whilst smaller disturbances decay to zero. If the stagnation point flow at infinity is sufficiently strong the steady basic states become linearly unstable. In fact there is then a continuous spectrum of unstable eigenvalues of the stability equations but, if the initial-value problem is considered, it is found that, at large values of time, the continuous spectrum leads to a velocity field growing exponentially in time with an amplitude decaying algebraically in time.
32
HEWITT, R. E., P. W. DUCK, and M. R. FOSTER. "Steady boundary-layer solutions for a swirling stratified fluid in a rotating cone." Journal of Fluid Mechanics 384 (April 10, 1999): 339–74. http://dx.doi.org/10.1017/s0022112099004255.
Full textAbstract:
We consider a set of nonlinear boundary-layer equations that have been derived by Duck, Foster & Hewitt (1997a, DFH), for the swirling flow of a linearly stratified fluid in a conical container. In contrast to the unsteady analysis of DFH, we restrict attention to steady solutions and extend the previous discussion further by allowing the container to both co-rotate and counter-rotate relative to the contained swirling fluid. The system is governed by three parameters, which are essentially non-dimensional measures of the rotation, stratification and a Schmidt number. Some of the properties of this system are related (in some cases rather subtly) to those found in the swirling flow of a homogeneous fluid above an infinite rotating disk; however, the introduction of buoyancy effects with a sloping boundary leads to other (new) behaviours. A general description of the steady solutions to this system proves to be rather complicated and shows many interesting features, including non-uniqueness, singular solutions and bifurcation phenomena.We present a broad description of the steady states with particular emphasis on boundaries in parameter space beyond which steady states cannot be continued.A natural extension of this work (motivated by recent experimental results) is to investigate the possibility of solution branches corresponding to non-axisymmetric boundary-layer states appearing as bifurcations of the axisymmetric solutions. In an Appendix we give details of an exact, non-axisymmetric solution to the Navier–Stokes equations (with axisymmetric boundary conditions) corresponding to the flow of homogeneous fluid above a rotating disk.
33
GUROV, SERGEY V., and LEV V. UTKIN. "FUZZY RELIABILITY OF GRACEFULLY DEGRADING SYSTEMS WITH THE COMPOSITE SOFTWARE." International Journal of Reliability, Quality and Safety Engineering 03, no. 02 (June 1996): 153–65. http://dx.doi.org/10.1142/s0218539396000119.
Full textAbstract:
In this paper, we focus on the transient and steady-state reliability behavior of a computer system whose software component as a program executes an infinite loop to provide control function and consists of several subprograms sequentially executed during iterations of the loop. The performance degradation of the system is due to the hardware failure occurrence. For reducing a large number of the system performance levels, we assume that the system success and failure are characterized by the fuzzy success and failure states with the defined membership functions. The system failure behavior is fully characterized in the context of probability measures. All random variables, such as times to hardware failures and mission times of subprograms, are arbitrarily distributed. In fact, we solve two problems: (1) reliability analysing under condition of many states and (2) reducing the state reliability measures to fuzzy success and failure reliability characteristics. The systems of integral and linear equations are used to obtain the unsteady and steady-state reliability characteristics, respectively. A numerical example is provided to illustrate and analyze the results.
34
Lee, Kyutae, Ali R. Firoozfar, and Marian Muste. "Technical Note: Monitoring of unsteady open channel flows using the continuous slope-area method." Hydrology and Earth System Sciences 21, no. 3 (March 30, 2017): 1863–74. http://dx.doi.org/10.5194/hess-21-1863-2017.
Full textAbstract:
Abstract. The advent of low-cost pressure transducers capable of directly measuring water surface elevation enables continuous measurements of dynamic water surface slopes. This opens up a new possibility of dynamically monitoring unsteady flows (i.e., hysteresis) during the course of flood wave propagation. Hysteresis in this context refers to a looped stage–discharge rating caused by unsteadiness of flows. Hysteresis is monitored in this study using a continuous slope area (CSA) method, which uses Manning's equation to calculate unsteady discharges based on continuously measured water surface slopes. In the rising stage, water surface slopes become steeper than a steady water surface slope, resulting in higher discharges than steady-based discharges, while the trends are reversed in the falling stage. The CSA method applied to Clear Creek near Oxford (Iowa, USA) estimates the maximum differences of peak discharges by 30–40 %, while it shows sound agreements for a low to medium range of discharges against USGS steady-based records. The primary cause of these differences is the use of a single channel bed slope in deriving Manning's roughness coefficients. The use of a single channel bed slope (conceptually equal to the water surface slopes at every stage in uniform flow conditions) causes substantial errors in estimating the channel roughness, specifically at high stages, because non-uniformities of natural channels result in varying (non-uniform) steady water surface slopes at each stage. While the CSA method is promising for dynamically tracking unsteady water surface slopes and flows in natural streams, more studies are still needed to increase the accuracy of the CSA method in future research.
35
Halama, Jan, Tony Arts, and Jaroslav Fořt. "Numerical solution of steady and unsteady transonic flow in turbine cascades and stages." Computers & Fluids 33, no. 5-6 (June 2004): 729–40. http://dx.doi.org/10.1016/j.compfluid.2003.05.003.
Full text
36
SHTERN, VLADIMIR, and FAZLE HUSSAIN. "Instabilities of conical flows causing steady bifurcations." Journal of Fluid Mechanics 366 (July 10, 1998): 33–85. http://dx.doi.org/10.1017/s002211209800891x.
Full textAbstract:
A new stability approach is developed for a wide class of strongly non-parallel axisymmetric flows of a viscous incompressible fluid. This approach encompasses all conical flows, and all steady and weakly unsteady disturbances, while prior studies were limited to specific flows and particular disturbances. A specially derived form of the Navier–Stokes equations allows the exact reduction of the linear stability problem to a system of ordinary differential equations. We found that disturbances originating at the boundaries of a similarity region cause a variety of steady bifurcations. Consideration of the still fluid allows disturbances to be classified into inner, outer and global modes, depending on the boundary conditions perturbed. Then we identify and study modes which cause bifurcation as the Reynolds number increases. The study provides improved understanding of (a) azimuthal symmetry breaking, (b) genesis of swirl, (c) onset of heat convection, (d) hydromagnetic dynamo, (e) hysteretic transitions, and (f) jump flow separation. We also discover and analyse two new bifurcations: (g) fold catastrophes and (h) appearance of radial oscillations in swirl-free jets. The stability analysis reveals that bifurcations (a), (c) and (f) are caused by inner perturbations, bifurcations (b), (d), (e) and (g) by outer perturbations, and bifurcation (h) by global perturbations. We deduce amplitude equations to describe the nonlinear spatiotemporal development of disturbances near the critical Reynolds numbers for (b) and (g). Disturbances switching between the basic and secondary steady states are found to grow monotonically with time.
37
Kim, H. J., and P. A. Durbin. "Investigation of the flow between a pair of circular cylinders in the flopping regime." Journal of Fluid Mechanics 196 (November 1988): 431–48. http://dx.doi.org/10.1017/s0022112088002769.
Full textAbstract:
The wakes of a pair of circular cylinders are grossly unsteady when the cylinders are separated in a direction normal to the approaching flow by less than one cylinder diameter. The wakes flop randomly between two asymmetric states. The time-scale for the flopping is several orders of magnitude longer than the timescale of vortex shedding, and also several orders of magnitude longer than the timescale for instability of the separating shear layers. When a splitter plate is positioned suitably on the centreline of the cylinders, the flopping can be stopped and the flow made to assume either of the asymmetric states, or a symmetric steady state. For a range of plate positions a new, periodic oscillation occurs. Acoustic excitation can also destroy the flopping mean flow, replacing it by a symmetric flow.
38
CETINER, O., and D. ROCKWELL. "Streamwise oscillations of a cylinder in steady current. Part 2. Free-surface effects on vortex formation and loading." Journal of Fluid Mechanics 427 (January 25, 2001): 29–59. http://dx.doi.org/10.1017/s0022112000002226.
Full textAbstract:
A cylinder in a steady current beneath a free surface is subjected to oscillations in the streamwise direction. Techniques of high-image-density particle image velocimetry and instantaneous force measurement provide the relationship between the instantaneous, global flow patterns and the unsteady loading on the cylinder.The existence of locked-on states for the fully submerged cylinder is addressed in the companion study of Cetiner & Rockwell (2001). The present investigation shows that it is possible to generate distinctly different locked-on states of vortex formation, provided the cylinder is located immediately beneath the free surface. As a consequence, the time-dependent transverse force is phase-locked to the cylinder motion. In the event that a finite gap exists between the cylinder and the free surface, however, instantaneous, jet-like flow through the gap acts to destabilize such locked-on states. Lissajous representations of the forces demonstrate the degree of phase-locking or, in some cases, a loss of lock-on and associated phase drift. Moreover, the degree of submergence of the cylinder beneath the free surface has remarkable consequences for the magnitudes of positive and negative spikes of the time-dependent force signatures, as well as the averaged spectra of the transverse force. In turn, these alterations of the unsteady transverse force are accompanied by substantial changes of the averaged in-line and transverse forces.Vortex systems can exist at locations both upstream and downstream of the cylinder. They are due to vorticity from the cylinder surface and/or the free surface. The space–time development of the entire system of vorticity concentrations is interpreted in terms of the time histories of the relative velocity of the cylinder and the instantaneous forces on the cylinder. In turn, these features of the vorticity field are related to critical points near the free surface, deduced from topologies of the corresponding velocity and streamline patterns. Despite the fact that changes in the patterns of vorticity and the corresponding topologies occur in conjunction with large fluctuations of the transverse force coefficient, the dimensionless strength of the vortices is below the threshold for which distinguishable, localized deformations of the free surface occur.
39
Dottori, F., M. L. V. Martina, and E. Todini. "A dynamic rating curve approach to indirect discharge measurement." Hydrology and Earth System Sciences 13, no. 6 (June 19, 2009): 847–63. http://dx.doi.org/10.5194/hess-13-847-2009.
Full textAbstract:
Abstract. The operational measurement of discharge in medium and large rivers is mostly based on indirect approaches by converting water stages into discharge on the basis of steady-flow rating curves. Unfortunately, under unsteady flow conditions, this approach does not guarantee accurate estimation of the discharge due, on the one hand, to the underlying steady state assumptions and, on the other hand, to the required extrapolation of the rating curve beyond the range of actual measurements used for its derivation. Historically, several formulae were proposed to correct the steady-state discharge value and to approximate the unsteady-flow stage-discharge relationship. In the majority of these methods, the correction is made on the basis of water level measurements taken at a single cross section where a steady state rating curve is available, while other methods explicitly account for the water surface slope using stage measurements in two reference sections. However, most of the formulae available in literature are either over-simplified or based on approximations that prevent their generalisation. Moreover they have been rarely tested on cases where their use becomes essential, namely under unsteady-flow conditions characterised by wide loop rating curves. In the present work, an original approach, based on simultaneous stage measurements at two adjacent cross sections, is introduced and compared to the approaches described in the literature. The most relevant feature is that the proposed procedure allows for the application of the full dynamic flow equations without restrictive hypotheses. The comparison has been carried out on channels with constant or spatially variable geometry under a wide range of flood wave and river bed slope conditions. The results clearly show the improvement in the discharge estimation and the reduction of estimation errors obtainable using the proposed approach.
40
HEIL, MATTHIAS, and SARAH L. WATERS. "How rapidly oscillating collapsible tubes extract energy from a viscous mean flow." Journal of Fluid Mechanics 601 (April 25, 2008): 199–227. http://dx.doi.org/10.1017/s0022112008000463.
Full textAbstract:
We present a combined theoretical and computational analysis of three-dimensional unsteady finite-Reynolds-number flows in collapsible tubes whose walls perform prescribed high-frequency oscillations which resemble those typically observed in experiments with a Starling resistor. Following an analysis of the flow fields, we investigate the system's overall energy budget and establish the critical Reynolds number, Recrit, at which the wall begins to extract energy from the flow. We conjecture that Recrit corresponds to the Reynolds number beyond which collapsible tubes are capable of performing sustained self-excited oscillations. Our computations suggest a simple functional relationship between Recrit and the system parameters, and we present a scaling argument to explain this observation. Finally, we demonstrate that, within the framework of the instability mechanism analysed here, self-excited oscillations of collapsible tubes are much more likely to develop from steady-state configurations in which the tube is buckled non-axisymmetrically, rather than from axisymmetric steady states, which is in agreement with experimental observations.
41
Fezai, Salwa, Fakher Oueslati, Nader Ben-Cheikh, and Brahim Ben-Beya. "Prediction of wake structure and aerodynamic characteristics of flow around square cylinders at different arrangements." International Journal of Modern Physics C 30, no. 02n03 (February 2019): 1950015. http://dx.doi.org/10.1142/s0129183119500153.
Full textAbstract:
Two-dimensional incompressible fluid flows around square cylinders at different arrangements have been numerically analyzed in the present work. The calculations are carried out for several values of Reynolds number (Re) ranging from 20 to 180. The results are presented in the form of vorticity contours and temporal histories of drag and lift coefficients. Besides, the physical parameters, namely, the average drag and lift coefficients and Strouhal number, are evaluated as a function of Re. Two different states of flow are predicted in the current investigation by systematically varying Re for steady and unsteady regimes. Vortex shedding is studied at different arrangements of the square cylinders allowing the investigation of three possible configurations. Special attention is paid to compute the drag and lift forces acting on the different obstacles, which allowed determining the optimal configuration in terms of both drags and lifts. The unsteady periodic wake is characterized by the Strouhal number, which varies with the Reynolds number and the obstacle geometry. The values of vortex shedding frequencies are consequently calculated in this study.
42
Karamanis, N., R. F. Martinez-Botas, and C. C. Su. "Mixed Flow Turbines: Inlet and Exit Flow Under Steady and Pulsating Conditions." Journal of Turbomachinery 123, no. 2 (February 1, 2000): 359–71. http://dx.doi.org/10.1115/1.1354141.
Full textAbstract:
The performance and detailed flow characteristics of a high pressure ratio mixed flow turbine has been investigated under steady and pulsating flow conditions. The rotor has been designed to have a nominal constant incidence (based on free vortex flow in the volute) and it is for use in an automotive high speed diesel turbocharger. The results indicated a departure from the quasi-steady analysis commonly used in turbocharger turbine design. The pulsations from the engine have been followed through the inlet pipe and around the volute; the pulse has been shown to propagate close to the speed of sound and not according to the bulk flow velocity as stated by some researchers. The flow entering and exiting the blades has been quantified by a laser Doppler velocimetry system. The measurements were performed at a plane 3.0 mm ahead of the rotor leading edge and 9.5 mm behind the rotor trailing edge. The turbine test conditions corresponded to the peak efficiency point at 29,400 and 41,300 rpm. The results were resolved in a blade-to-blade sense to examine in greater detail the nature of the flow at turbocharger representative conditions. A correlation between the combined effects of incidence and exit flow angle with the isentropic efficiency has been shown. The unsteady flow characteristics have been investigated at two flow pulse frequencies, corresponding to internal combustion engine speeds of 1600 and 2400 rpm. Four measurement planes have been investigated: one in the pipe feeding the volute, two in the volute (40 deg and 130 deg downstream of the tongue) and one at the exit of the turbine. The pulse propagation at these planes has been investigated; the effect of the different planes on the evaluation of the unsteady isentropic efficiency is shown to be significant. Overall, the unsteady performance efficiency results indicated a significant departure from the corresponding steady performance, in accordance with the inlet and exit flow measurements.
43
Jha, B. K., C. T. Babila, and S. Isa. "Unsteady Hartmann Two-Phase Flow: The Riemann-Sum Approximation Approach." International Journal of Applied Mechanics and Engineering 21, no. 4 (December 1, 2016): 891–904. http://dx.doi.org/10.1515/ijame-2016-0053.
Full textAbstract:
Abstract We consider the time dependent Hartmann flow of a conducting fluid in a channel formed by two horizontal parallel plates of infinite extent, there being a layer of a non-conducting fluid between the conducting fluid and the upper channel wall. The flow formation of conducting and non-conducting fluids is coupled by equating the velocity and shear stress at the interface. The unsteady flow formation inside the channel is caused by a sudden change in the pressure gradient. The relevant partial differential equations capturing the present physical situation are transformed into ordinary differential equations using the Laplace transform technique. The ordinary differential equations are then solved analytically and the Riemann-sum approximation method is used to invert the Laplace domain into time domain. The solution obtained is validated by comparisons with the closed form solutions obtained for steady states which have been derived separately and also by the implicit finite difference method. The variation of velocity, mass flow rate and skin-friction on both plates for various physical parameters involved in the problem are reported and discussed with the help of line graphs. It was found that the effect of changes of the electric load parameter is to aid or oppose the flow as compared to the short-circuited case.
44
Bons, Jeffrey P., Rolf Sondergaard, and Richard B. Rivir. "Turbine Separation Control Using Pulsed Vortex Generator Jets." Journal of Turbomachinery 123, no. 2 (February 1, 2000): 198–206. http://dx.doi.org/10.1115/1.1350410.
Full textAbstract:
The application of pulsed vortex generator jets to control separation on the suction surface of a low-pressure turbine blade is reported. Blade Reynolds numbers in the experimental, linear turbine cascade match those for high-altitude aircraft engines and aft stages of industrial turbine engines with elevated turbine inlet temperatures. The vortex generator jets have a 30 deg pitch and a 90 deg skew to the free-stream direction. Jet flow oscillations up to 100 Hz are produced using a high-frequency solenoid feed valve. Results are compared to steady blowing at jet blowing ratios less than 4 and at two chordwise positions upstream of the nominal separation zone. Results show that pulsed vortex generator jets produce a bulk flow effect comparable to that of steady jets with an order of magnitude less massflow. Boundary layer traverses and blade static pressure distributions show that separation is almost completely eliminated with the application of unsteady blowing. Reductions of over 50 percent in the wake loss profile of the controlled blade were measured. Experimental evidence suggests that the mechanism for unsteady control lies in the starting and ending transitions of the pulsing cycle rather than the injected jet stream itself. Boundary layer spectra support this conclusion and highlight significant differences between the steady and unsteady control techniques. The pulsed vortex generator jets are effective at both chordwise injection locations tested (45 and 63 percent axial chord) covering a substantial portion of the blade suction surface. This insensitivity to injection location bodes well for practical application of pulsed VGJ control where the separation location may not be accurately known a priori.
45
Ma, Yukun, Qingchen Yao, Junhao Wang, Shuo Sun, and Zhanlin Qiu. "Time-Dependent Creep Constitutive Model of Roadway Surrounding Rock Based on Creep Parameters." Geofluids 2022 (May 21, 2022): 1–11. http://dx.doi.org/10.1155/2022/7981192.
Full textAbstract:
In order to better study the creep deformation characteristics of rock under different stresses, the generalized Burgers model was used as the basic model after analyzing the creep characteristics of rock and its relationship with stress and strain, and its application scope was verified. Based on the damage mechanics theory, a viscoplastic body considering aging damage was established, which was connected with the generalized Burgers model in series. A new nonlinear creep constitutive model was obtained, which was extended to three dimensions. In addition, the relationship between model parameters and damage variables was established by introducing damage variables that considered damage effects. The unsteady creep constitutive model of rock is obtained, and the correctness and rationality of the model are verified by test data. The results show that this model not only accurately reflects the creep characteristics of attenuation and steady creep stage but also overcomes the defect that the generalized Burgers model is difficult to describe the accelerated creep. Considering the deterioration of creep parameters with time, the creep damage process of rock under different stress states can be better described, which provides a new idea for establishing unsteady creep model and determining model parameters.
46
Kitoh, Aya, Kazuaki Sugawara, Hiroyuki Yoshikawa, and Terukazu Ota. "Expansion Ratio Effects on Three-Dimensional Separated Flow and Heat Transfer Around Backward-Facing Steps." Journal of Heat Transfer 129, no. 9 (September 28, 2006): 1141–55. http://dx.doi.org/10.1115/1.2739619.
Full textAbstract:
Direct numerical simulation methodology clarified the three-dimensional separated flow and heat transfer around three backward-facing steps in a rectangular channel, especially effects of channel expansion ratio ER upon them. ER treated in the present study was 1.5, 2.0, and 3.0 under a step aspect ratio of 36.0. The Reynolds number Re based on the mean velocity at inlet and the step height was varied from 300 to 1000. The present numerical results for ER=2.0 were found to be in very good agreement with the previous experimental and numerical ones in the present Reynolds number range for both the steady and unsteady flow states. The time averaged reattachment length on the center line increases with a decrease of ER. The flow became unsteady at RE=700, 600, and 500 for ER=1.5, 2.0, and 3.0, respectively, accompanying the remarkable increase of the three-dimensionality of the flow and temperature fields in spite of a very large step aspect ratio of 36.0. The Nusselt number increases in the reattachment flow region, in the neighborhood of the sidewalls, and also in the far downstream depending on both Re and ER.
47
Dritschel, David G. "A general theory for two-dimensional vortex interactions." Journal of Fluid Mechanics 293 (June 25, 1995): 269–303. http://dx.doi.org/10.1017/s0022112095001716.
Full textAbstract:
A general theory for two-dimensional vortex interactions is developed from the observation that, under slowly changing external influences, an individual vortex evolves through a series of equilibrium states until such a state proves unstable. Once an unstable equilibrium state is reached, a relatively fast unsteady evolution ensues, typically involving another nearby vortex. During this fast unsteady evolution, a fraction of the original coherent circulation is lost to filamentary debris, and, remarkably, the flow reorganizes into a set of quasi-steady stable vortices.The simplifying feature of the proposed theory is its use of adiabatic steadiness and marginal stability to determine the shapes and separation distance of vortices on the brink of an inelastic interaction. As a result, the parameter space for the inelastic interaction of nearby vortices is greatly reduced. In the case of two vortex patches, which is the focus of the present work, inelastic interactions depend only on a single parameter: the area ratio of the two vortices (taking the vorticity magnitude inside each to be equal). Without invoking adiabatic steadiness and marginal stability, one would have to contend with the additional parameters of vortex separation and shape, and the latter is actually an infinitude of parameters.
48
Castillo, Mariano E. Gurfinkel, and Anthony T. Patera. "Three-dimensional ribbing instability in symmetric forward-roll film-coating processes." Journal of Fluid Mechanics 335 (March 25, 1997): 323–59. http://dx.doi.org/10.1017/s0022112096004600.
Full textAbstract:
We propose a framework for interpreting the formation, evolution, and spatial persistence of ribbing in coating processes, and present companion ‘quantifying’ parallel spectral element simulations of fully nonlinear unsteady three-dimensional free-surface symmetric forward-roll film-coating fluid flows. The framework couples, by means of a transition region, two well-understood phenomena: the ‘viscous fingering’ instability of a splitting meniscus; and the levelling of viscous films under the effect of surface tension. The transition-region length, Lt, is on the order of the coating film thickness, while the downstream extent of the levelling region – the distance over which ribs persist – Lt, depends on the fluid properties, the flow conditions, and the wavenumber content of the nonlinear meniscus rib profile. Numerical results are presented for the evolution of the coating flow from perturbed unstable two-dimensional steady states to three-dimensional saturated ribbed states for several representative supercritical capillary numbers, Ca, and spanwise periodicity lengths, b. Nonlinear state selection is briefly discussed.
49
Martínez-Aranda, S., J. Murillo, and P. García-Navarro. "A robust two-dimensional model for highly sediment-laden unsteady flows of variable density over movable beds." Journal of Hydroinformatics 22, no. 5 (July 2, 2020): 1138–60. http://dx.doi.org/10.2166/hydro.2020.027.
Full textAbstract:
Abstract In this work, a novel 2D depth-integrated numerical model for highly sediment-laden shallow flows over non-uniform erodible beds is presented, including variable density and exchange between the bed layer and the water–sediment mixture flow. The system of equations is formed by the 2D conservation equations for the mass and momentum of the mixture, the mass conservation equation for the different sediment size-classes transported in the flow and the bed evolution equation. The depth-averaged mixture density varies according to the volumetric concentration of the different sediment size-classes that can be incorporated from the bed to the flow and transported as suspended materials. The rheological behaviour of the flow is directly controlled by the properties of the mixture. A new x-split augmented Roe (xA-Roe) scheme is derived to solve the coupled flow and suspended solid-phase equations in both structured and unstructured meshes. The numerical scheme is defined to properly include density variations and momentum source terms, retaining a well-balanced flux formulation in steady states and the correct treatment of the wet–dry fronts. The numerical scheme is assessed with steady and transient cases involving highly sediment-laden flows, demonstrating its accuracy, stability and robustness in the presence of complex bed topography, wetting–drying fronts and rapid morphological changes.
50
Sheng-nan, Li, Wang Jing-lin, Yang Le, and ZhangShang-tian. "Classification of helicopter’s typical flight state based on threshold." IOP Conference Series: Materials Science and Engineering 1207, no. 1 (November 1, 2021): 012024. http://dx.doi.org/10.1088/1757-899x/1207/1/012024.
Full textAbstract:
Abstract Dividing the 37 flying state of a certain line number helicopter. Firstly, dividing the helicopter rotation and single-engine flight. Secondly, performing preliminary state division for the remaining samlpes, the specific division of yaw angle, helicopter flight altitude and indicated air speed are different states, the least squares polynomial method is used for smoothing respectively. Calculating the extreme value of each parameter data, with the difference value of the extreme value of the parameter data being less than 10 as the limiting condition, dividing the original data segment into non-turning, level flight and steady speed state. The remaining sampling points are in the state of unsteady turning and non-level flight. Taking the difference value 0 as the limiting condition, further divide the non-steady speed and non-level flight state. Dividing the state of turning and non-turning, level flight, ascent and descent, steady speed, increase speed and deceleration state, which is the preliminary division state. Finally, dividing the near-ground and non-near-ground, classifying the helicopter status according to the height threshold, and analyze the accuracy of the classification results. The results show that this method is versatile, can quickly divide helicopters with different flight complexity, and has high accuracy.