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1
Wynn, A., D. S. Pearson, B. Ganapathisubramani, and P. J. Goulart. "Optimal mode decomposition for unsteady flows." Journal of Fluid Mechanics 733 (September 24, 2013): 473–503. http://dx.doi.org/10.1017/jfm.2013.426.
Повний текст джерелаАнотація:
AbstractA new method, herein referred to as optimal mode decomposition (OMD), of finding a linear model to describe the evolution of a fluid flow is presented. The method estimates the linear dynamics of a high-dimensional system which is first projected onto a subspace of a user-defined fixed rank. An iterative procedure is used to find the optimal combination of linear model and subspace that minimizes the system residual error. The OMD method is shown to be a generalization of dynamic mode decomposition (DMD), in which the subspace is not optimized but rather fixed to be the proper orthogonal decomposition (POD) modes. Furthermore, OMD is shown to provide an approximation to the Koopman modes and eigenvalues of the underlying system. A comparison between OMD and DMD is made using both a synthetic waveform and an experimental data set. The OMD technique is shown to have lower residual errors than DMD and is shown on a synthetic waveform to provide more accurate estimates of the system eigenvalues. This new method can be used with experimental and numerical data to calculate the ‘optimal’ low-order model with a user-defined rank that best captures the system dynamics of unsteady and turbulent flows.
2
Ibraheem AlQadi, Ibraheem AlQadi. "Investigation of Flow Around a Slender Body at High Angles of Attack." journal of King Abdulaziz University Engineering Sciences 30, no. 1 (February 1, 2019): 51–61. http://dx.doi.org/10.4197/eng.30-1.4.
Повний текст джерелаАнотація:
A numerical investigation of flow around a slender body at high angles of attack is presented. Large eddy simulation of the flow around an ogive-cylinder body at high angles of attack is carried out. Asymmetric vortex flow was observed at angles of attack of α = 55◦ and 65◦ . The results showed that the phenomenon is present in the absence of artificial geometrical or flow perturbation. Contrary to the accepted notion that flow asymmetry is due to a convective instability, the development of vortex asymmetry in the absence of perturbations indicates the existence of absolute instability. An investigation of the unsteady flow field was carried out using dynamic mode decomposition. The analysis identified two distinct unsteady modes; high-frequency mode and low-frequency mode. At angle of attack 45◦ the high-frequency mode is dominant in the frontal part of the body and the low-frequency mode is dominant at the rear part. At α = 55◦ , the highfrequency mode is dominant downstream of vortex breakdown. At α = 65◦ , the spectrum shows a wide range of modes. Reconstruction of the dynamical modes shows that the low-frequency mode is associated with the unsteady wake and the high-frequency mode is associated with unsteady shear layer.
3
Vabishchevich, Petr N. "Fundamental mode exact schemes for unsteady problems." Numerical Methods for Partial Differential Equations 34, no. 6 (June 19, 2018): 2301–15. http://dx.doi.org/10.1002/num.22292.
Повний текст джерела
4
Li, Yi-bin, Chang-hong He, and Jian-zhong Li. "Study on Flow Characteristics in Volute of Centrifugal Pump Based on Dynamic Mode Decomposition." Mathematical Problems in Engineering 2019 (April 16, 2019): 1–15. http://dx.doi.org/10.1155/2019/2567659.
Повний текст джерелаАнотація:
To investigate the unsteady flow characteristics and their influence mechanism in the volute of centrifugal pump, the Reynolds time-averaged N-S equation, RNG k-ε turbulence model, and structured grid technique are used to numerically analyze the transient flow-field characteristics inside the centrifugal pump volute. Based on the quantified parameters of flow field in the volute of centrifugal pump, the velocity mode contours and oscillation characteristics of the mid-span section of the volute of centrifugal pump are obtained by dynamic mode decomposition (DMD) for the nominal and low flow-rate condition. The research shows that the first-order average flow mode extracted by DMD is the dominant flow structure in the flow field of the volute. The second-order and third-order modes are the most important oscillation modes causing unsteady flow in the volute, and the characteristic frequency of the two modes is consistent with the blade passing frequency and the 2x blade passing frequency obtained by the fast Fourier transform (FFT). By reconstructing the internal flow field of the volute with the blade passing frequency for the nominal flow-rate condition, the periodic variation of the unsteady flow structure in the volute under this frequency is visually reproduced, which provides some ideas for the study of the unsteady structure in the internal flow field of centrifugal pumps.
5
Corrochano, Adrián, Donnatella Xavier, Philipp Schlatter, Ricardo Vinuesa, and Soledad Le Clainche. "Flow Structures on a Planar Food and Drug Administration (FDA) Nozzle at Low and Intermediate Reynolds Number." Fluids 6, no. 1 (December 24, 2020): 4. http://dx.doi.org/10.3390/fluids6010004.
Повний текст джерелаАнотація:
In this paper, we present a general description of the flow structures inside a two-dimensional Food and Drug Administration (FDA) nozzle. To this aim, we have performed numerical simulations using the numerical code Nek5000. The topology patters of the solution obtained, identify four different flow regimes when the flow is steady, where the symmetry of the flow breaks down. An additional case has been studied at higher Reynolds number, when the flow is unsteady, finding a vortex street distributed along the expansion pipe of the geometry. Linear stability analysis identifies the evolution of two steady and two unsteady modes. The results obtained have been connected with the changes in the topology of the flow. Finally, higher-order dynamic mode decomposition has been applied to identify the main flow structures in the unsteady flow inside the FDA nozzle. The highest-amplitude dynamic mode decomposition (DMD) modes identified by the method model the vortex street in the expansion of the geometry.
6
Hall, K. C., and P. D. Silkowski. "The Influence of Neighboring Blade Rows on the Unsteady Aerodynamic Response of Cascades." Journal of Turbomachinery 119, no. 1 (January 1, 1997): 85–93. http://dx.doi.org/10.1115/1.2841014.
Повний текст джерелаАнотація:
In this paper, we present an analysis of the unsteady aerodynamic response of cascades due to incident gusts (the forced response problem) or blade vibration (the flutter problem) when the cascade is part of a multistage fan, compressor, or turbine. Most current unsteady aerodynamic models assume the cascade to be isolated in an infinitely long duct. This assumption, however, neglects the potentially important influence of neighboring blade rows. We present an elegant and computationally efficient method to model these neighboring blade row effects. In the present method, we model the unsteady aerodynamic response due to so-called spinning modes (pressure and vorticity waves), with each mode corresponding to a different circumferential wave number and frequency. Then, for each mode, we compute the reflection and transmission coefficients for each blade row. These coefficients can be obtained from any of the currently available unsteady linearized aerodynamic models of isolated cascades. A set of linear equations is then constructed that couples together the various spinning modes, and the linear equations are solved via LU decomposition. Numerical results are presented for both the gust response and blade vibration problems. To validate our model, we compare our results to other analytical models, and to a multistage vortex lattice model. We show that the effect of neighboring blade rows on the aerodynamic damping of vibrating cascades is significant, but nevertheless can be modeled with a small number of modes.
7
STEWART, B. E., M. C. THOMPSON, T. LEWEKE, and K. HOURIGAN. "Numerical and experimental studies of the rolling sphere wake." Journal of Fluid Mechanics 643 (January 15, 2010): 137–62. http://dx.doi.org/10.1017/s0022112009992072.
Повний текст джерелаАнотація:
A numerical and experimental investigation is reported for the flow around a rolling sphere when moving adjacent to a plane wall. The dimensionless rotation rate of the sphere is varied from forward to reversed rolling and the resulting wake modes are found to be strongly dependent on the value of this parameter. Results are reported for the Reynolds number range 100 < Re < 350, which has been shown to capture the unsteady transitions in the wake. Over this range of Reynolds number, both steady and unsteady wake modes are observed. As the sphere undergoes forward rolling, the wake displays similarities to the flow behind an isolated sphere in a free stream. As the Reynolds number of the flow increases, hairpin vortices form and are shed over the surface of the sphere. However, for cases with reversed rotation, the wake takes the form of two distinct streamwise vortices that form around the sides of the body. These streamwise structures in the wake undergo a transition to a new unsteady mode as the Reynolds number increases. During the evolution of this unsteady mode, the streamwise vortices form an out-of-phase spiral pair. Four primary wake modes are identified and a very good qualitative agreement is observed between the numerical and experimental results. The numerical simulations also reveal the existence of an additional unsteady mode that is found to be unstable to small perturbations in the flow.
8
Chiang, H. W. D., and S. Fleeter. "Passive Control of Flow-Induced Vibrations by Splitter Blades." Journal of Turbomachinery 116, no. 3 (July 1, 1994): 489–500. http://dx.doi.org/10.1115/1.2929438.
Повний текст джерелаАнотація:
Splitter blades as a passive control technique for flow-induced vibrations are investigated by developing an unsteady aerodynamic model to predict the effect of incorporating splitter blades into the design of an axial flow blade row operating in an incompressible flow field. The splitter blades, positioned circumferentially in the flow passage between two principal blades, introduce aerodynamic and/or combined aerodynamic-structural detuning into the rotor. The unsteady aerodynamic gust response and resulting oscillating cascade unsteady aerodynamics, including steady loading effects, are determined by developing a complete first-order unsteady aerodynamic analysis together with an unsteady aerodynamic influence coefficient technique. The torsion mode flow induced vibrational response of both uniformly spaced tuned rotors and detuned rotors are then predicted by incorporating the unsteady aerodynamic influence coefficients into a single-degree-of-freedom aero-elastic model. This model is then utilized to demonstrate that incorporating splitters into axial flow rotor designs is beneficial with regard to flow induced vibrations.
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Aurahs, L., C. Kasper, M. Kürner, M. G. Rose, S. Staudacher, and J. Gier. "Water flow model turbine flow visualization study of the unsteady interaction of secondary flow vortices with the downstream rotor." Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy 223, no. 6 (July 21, 2009): 677–86. http://dx.doi.org/10.1243/09576509jpe841.
Повний текст джерелаАнотація:
This article presents detailed flow visualization photographs, root mean square processed photography, and computational fluid dynamics (CFD) results of the interaction of the vane passage vortex and horseshoe vortex with the rotor of an axial flow turbine model. Different modes of vortex breakdown behaviour have been experimentally observed inside the rotating passage of the turbine blade. These are spiral vortex mode and bubble mode breakdown. The breakdown mode changes as the vortices are influenced by the periodic pressure field of the rotor. The measurements were taken in a vertical water channel with ink injection for flow visualization. Unsteady CFD analyses have been made with some success in prediction of the unsteady flow structures. In particular, the pre-instability behaviour of the passage vortex in the experiments matches the results of the numerical investigations.
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Wang, Jin-Chun, Xin Fu, Guo-Ping Huang, Shu-Li Hong, and Yuan-Chi Zou. "Application of the Proper Orthogonal Decomposition Method in Analyzing Active Separation Control With Periodic Vibration Wall." International Journal of Turbo & Jet-Engines 36, no. 2 (May 27, 2019): 175–84. http://dx.doi.org/10.1515/tjj-2017-0031.
Повний текст джерелаАнотація:
AbstractThe proper orthogonal decomposition (POD) method is employed to analyze the unsteady flow control mechanism because it is a good approach to decouple the spatial and temporal structures of unsteady flow fields. The results showed that the main effect of the periodic excitation is reallocating the energy of each mode, and selectively strengthening or weakening certain modes. Under proper amplitude and frequency of periodic excitation, the energy in higher modes will be transferred to the first mode and the translation of the modal energy is coming from the reconstructing of spatial flow structures and the ordering of modal evolution characteristics. The best control effect will be achieved when the total energy ratio of the first mode is the highest and the excitation frequency reaches the separation vortex frequency at the same time. In order to quantitatively analyze the order degree of the unsteady flow field, the maximum Lyapunov exponent was introduced. The results showed that with the energy in higher modes transferred to the lower modes, the flow field transfers from a disordered pattern to an ordered one.
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Hochet, Antoine, Alain Colin de Verdière, and Robert Scott. "The Vertical Structure of Large-Scale Unsteady Currents." Journal of Physical Oceanography 45, no. 3 (March 2015): 755–77. http://dx.doi.org/10.1175/jpo-d-14-0077.1.
Повний текст джерелаАнотація:
AbstractA linear model based on the quasigeostrophic equations is constructed in order to predict the vertical structure of Rossby waves and, more broadly, of anomalies resolved by altimeter data, roughly with periods longer than 20 days and with wavelengths larger than 100 km. The subsurface field is reconstructed from sea surface height and climatological stratification. The solution is calculated in periodic rectangular regions with a 3D discrete Fourier transform. The effect of the mean flow on Rossby waves is neglected, which the authors believe is a reasonable approximation for low latitudes. The method used has been tested with an idealized double-gyre simulation [performed with the Miami Isopycnal Coordinate Ocean Model (MICOM)]. The linear model is able to give reasonable predictions of subsurface currents at low latitudes (below approximately 30°) and for relatively weak mean flow. However, the predictions degrade with stronger mean flows and higher latitudes. The subsurface velocities calculated with this model using AVISO altimetric data and velocities from current meters have also been compared. Results show that the model gives reasonably accurate results away from the top and bottom boundaries, side boundaries, and far from western boundary currents. This study found, for the regions where the model is valid, an energy partition of the traditional modes of approximately 68% in the barotropic mode and 25% in the first baroclinic mode. Only 20% of the observed kinetic energy can be attributed to free Rossby waves of long periods that propagate energy to the west.
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Gerolymos, G. A. "Coupled Three-Dimensional Aeroelastic Stability Analysis of Bladed Disks." Journal of Turbomachinery 115, no. 4 (October 1, 1993): 791–99. http://dx.doi.org/10.1115/1.2929317.
Повний текст джерелаАнотація:
In the present work an algorithm for the coupled aeromechanical computation of three-dimensional compressor cascades vibrating in a traveling-wave mode is presented and applied to the determination of aeroelastic stability of a transonic fan rotor. The initial vibratory modes are computed using a finite-element structural analysis code. The unsteady flow field response to blade vibration is estimated by numerical integration of the three-dimensional unsteady Euler equations. Coupling relations are formulated in the frequency domain, using a mode-modification technique, based on modal projection. The vibratory mode is updated at the end of the aerodynamic simulation of each period, and the updated mode is used for the simulation of the next period. A number of results illustrate the method’s potential.
13
Ma, Wei. "Discontinuous Growth Mechanisms of Mode II Crack under High-Speed Impact Conditions." Advanced Materials Research 41-42 (April 2008): 169–73. http://dx.doi.org/10.4028/www.scientific.net/amr.41-42.169.
Повний текст джерелаАнотація:
A recoverable plate impact testing technology has been used for studying the growth mechanisms of mode II crack. The results show that interactions of microcracks ahead of a crack tip cause the crack growth unsteadily. Failure mode transitions of materials were observed. Based on the observations, a discontinuous crack growth model was established. Analysis shows that the shear crack grows unsteady as the growth speed is between the Rayleigh wave speed cR and the shear wave speed cs; however, when the growth speed approaches 2cs, the crack grows steadily. The transient microcrack growth makes the main crack speed to jump from subsonic to intersonic and the steady growth of all the sub-cracks leads the main crack to grow stably at an intersonic speed.
14
Chen, Hao. "Numerical Calculations on the Unsteady Aerodynamic Force of the Tilt-Rotor Aircraft in Conversion Mode." International Journal of Aerospace Engineering 2019 (December 7, 2019): 1–15. http://dx.doi.org/10.1155/2019/2147068.
Повний текст джерелаАнотація:
A computational method is developed in order to predict the unsteady aerodynamic characteristics of the tilt-rotor aircraft in conversion mode. In this approach, the rotor is modeled as an actuator disk so that the effect of individual blades can be ignored. A novel predictor-corrector-based dynamic mesh method is presented for dealing with extremely large mesh deformation during a conversion process. The dual time-stepping approach and the finite volume scheme are applied to solve the unsteady N-S equation. A parallel algorithm is utilized in this work to improve the computational efficiency. By using the present method, quantitative and qualitative comparisons are made between the aerodynamic coefficients obtained in the quasi-steady fixed conversion mode and the time-accurate continuous transition flight condition. Both two-dimensional (2D) and three-dimensional (3D) computations are carried out. The influence of the tilt modes and the tilt period time on the unsteady aerodynamic forces are also studied. Numerical results demonstrate that the developed method is effective in simulating the aerodynamic characteristics of the tilt-rotor aircraft in conversion mode.
15
Buffum, D. H., and S. Fleeter. "Effect of Wind Tunnel Acoustic Modes on Linear Oscillating Cascade Aerodynamics." Journal of Turbomachinery 116, no. 3 (July 1, 1994): 513–24. http://dx.doi.org/10.1115/1.2929440.
Повний текст джерелаАнотація:
The aerodynamics of a biconvex airfoil cascade oscillating in torsion is investigated using the unsteady aerodynamic influence coefficient technique. For subsonic flow and reduced frequencies as large as 0.9, airfoil surface unsteady pressures resulting from oscillation of one of the airfoils are measured using flush-mounted high-frequency-response pressure transducers. The influence coefficient data are examined in detail and then used to predict the unsteady aerodynamics of a cascade oscillating at various interblade phase angles. These results are correlated with experimental data obtained in the traveling-wave mode of oscillation and linearized analysis predictions. It is found that the unsteady pressure disturbances created by an oscillating airfoil excite wind tunnel acoustic modes, which have detrimental effects on the experimental results. Acoustic treatment is proposed to rectify this problem.
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CHEN, LONG, YIZHAO WU, and JIAN XIA. "AEROELASTIC ANALYSIS OF ROTOR BLADES USING CFD/CSD COUPLING IN HOVER MODE." Modern Physics Letters B 24, no. 13 (May 30, 2010): 1307–10. http://dx.doi.org/10.1142/s0217984910023499.
Повний текст джерелаАнотація:
A computational fluid dynamics (CFD) is coupled with a computational structural dynamics (CSD) to simulate the unsteady rotor flow with aeroelasticity effects. An unstructured upwind Navier-Stokes solver was developed for this simulation, with 2nd order time-accurate dual-time stepping method for temporal discretization and low Mach number preconditioning method. For turbulent flows, both the Spalart-Allmaras and Menter's SST model are available. Mesh deformation is achieved through a fast dynamic grid method called Delaunay graph map method for unsteady flow simulation. The rotor blades are modeled as Hodges & Dowell's nonlinear beams coupled flap-lag-torsion. The rotorcraft computational structural dynamics code employs the 15-dof beam finite element formulation for modeling. The structure code was validated by comparing the natural frequencies of a rotor model with UMARC. The flow and structure codes are coupled tightly with information exchange several times at every time step. A rotor blade model's unsteady flow field in the hover mode is simulated using the coupling method. Effect of blade elasticity with aerodynamic loads was compared with rigid blade.
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Palies, Paul, Milos Ilak, and Robert Cheng. "Transient and limit cycle combustion dynamics analysis of turbulent premixed swirling flames." Journal of Fluid Mechanics 830 (October 5, 2017): 681–707. http://dx.doi.org/10.1017/jfm.2017.575.
Повний текст джерелаАнотація:
Premixed low swirling flames (methane–air and hydrogen–methane–air) are experimentally investigated for three different regimes. Stable, local transient to instability and limit cycle regimes corresponding to three distinct equivalence ratios are considered. Dynamic mode decomposition is applied to the hydrogen–air–methane flame to retrieve the modes frequencies, growth rates and spatial distributions for each regime. The results indicate that a vortical wave propagating along the flame front is associated with the transition from stability to instability. In addition, it is shown that a key effect on stability is the location of the non-oscillating (0 Hz) flame component. The phase-averaged unsteady motion of the flames over one cycle of oscillation shows the vortical wave rolling up the flame front. The Rayleigh index maps are formed to identify the region of driving and damping of the self-sustained oscillation, while the flame transfer function phase leads to the propagation mode of the perturbations along the flame front. The second mechanism identified concerns the swirl number fluctuation induced by the mode conversion. By utilizing hypotheses for the flow field and the flame structure, it is pointed out that those mechanisms are at work for both flames (methane–air and hydrogen–methane–air) and their effects on the unsteady heat release are determined. Both unsteady heat release contributions, the vortical wave induces flame surface fluctuations and swirl number oscillation induces unsteady turbulent burning velocity, are in phase opposition and of similar amplitudes.
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Silkowski, P. D., and K. C. Hall. "1997 Best Paper Award—Structures and Dynamics Committee: A Coupled Mode Analysis of Unsteady Multistage Flows in Turbomachinery." Journal of Turbomachinery 120, no. 3 (July 1, 1998): 410–21. http://dx.doi.org/10.1115/1.2841732.
Повний текст джерелаАнотація:
A computational method is presented for predicting the unsteady aerodynamic response of a vibrating blade row that is part of a multistage turbomachine. Most current unsteady aerodynamic theories model a single blade row isolated in an infinitely long duct. This assumption neglects the potentially important influence of neighboring blade rows. The present “coupled mode” analysis is an elegant and computationally efficient method for modeling neighboring blade row effects. Using this approach, the coupling between blade rows is modeled using a subset of the so-called spinning modes, i.e., pressure, vorticity, and entropy waves, which propagate between the blade rows. The blade rows themselves are represented by reflection and transmission coefficients. These coefficients describe how spinning modes interact with, and are scattered by, a given blade row. The coefficients can be calculated using any standard isolated blade row model; here we use a linearized full potential flow model together with rapid distortion theory to account for incident vortical gusts. The isolated blade row reflection and transmission coefficients, interrow coupling relationships, and appropriate boundary conditions are all assembled into a small sparse linear system of equations that describes the unsteady multistage flow. A number of numerical examples are presented to validate the method and to demonstrate the profound influence of neighboring blade rows on the aerodynamic damping of a cascade of vibrating airfoils.
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Crouch, J. D., A. Garbaruk, and M. Strelets. "Global instability in the onset of transonic-wing buffet." Journal of Fluid Mechanics 881 (October 24, 2019): 3–22. http://dx.doi.org/10.1017/jfm.2019.748.
Повний текст джерелаАнотація:
Global stability analysis is used to analyse the onset of transonic buffet on infinite swept and unswept wings. This high-Reynolds-number flow is governed by the unsteady Reynolds averaged Navier–Stokes equations. The analysis generalizes earlier studies focused on two-dimensional airfoils. For the unswept wing, results show spanwise-periodic stationary modes in addition to the earlier-observed oscillatory mode. The oscillatory mode is nominally two-dimensional with a spanwise wavelength greater than ten wing chords. The stationary modes of instability exist over two bands of spanwise wavelengths centred around an intermediate wavelength of one wing chord, and around a short wavelength of one tenth of a wing chord. The intermediate-wavelength modes have a flow structure characteristic of airfoil buffeting modes, concentrated at the shock and in the shear layer downstream of the shock. The short-wavelength modes are only concentrated in the shear layer downstream of the shock. These stationary modes can lead to spanwise-periodic flow structures for the unswept wing. For the swept wing, these stationary modes become unsteady travelling modes and contribute to the more complex buffeting-flow structures observed on swept wings as compared with unswept wings. The spanwise-wavelength bands of the travelling modes translate to different frequencies, resulting in a broad-banded unsteady response for the swept wing. For a $30^{\circ }$ swept wing, the frequencies associated with the intermediate-wavelength modes are approximately 10 times higher than the swept-wing generalization of the long-wavelength oscillatory mode, and approximately 6 times higher than the long-wavelength mode for the unswept wing. These instability characteristics are in good agreement with experimental observations.
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Buffum, D. H., and S. Fleeter. "The Aerodynamics of an Oscillating Cascade in a Compressible Flow Field." Journal of Turbomachinery 112, no. 4 (October 1, 1990): 759–67. http://dx.doi.org/10.1115/1.2927719.
Повний текст джерелаАнотація:
Fundamental experiments are performed in the NASA Lewis Research Center Transonic Oscillating Cascade Facility to investigate and quantify the aerodynamics of a cascade of bioconvex airfoils executing torsion mode oscillations at realistic reduced frequency values. Both steady and unsteady airfoil surface pressures are measured at two inlet Mach numbers, 0.65 and 0.80. and two incidence angles, 0 and 7 deg, with the harmonic torsional airfoil cascade oscillations at realistic high reduced frequency and unsteady data obtained at several interbladephase angle values. The time-variant pressures are analyzed by means of discrete Fourier transform techniques, with these unique data compared with predictions from a linearized unsteady cascade model. The experimental results indicate that the interblade phase angle has a major effect on the chordwise distributions of the airfoil surface unsteady pressure, with the effects of reduced frequency, incidence angle, and Mach number somewhat less significant.
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Li, Z., and P. Xia. "Aeroelastic modelling and stability analysis of tiltrotor aircraft in conversion flight." Aeronautical Journal 122, no. 1256 (September 12, 2018): 1606–29. http://dx.doi.org/10.1017/aer.2018.93.
Повний текст джерелаАнотація:
ABSTRACTIn conversion flight, the aeroelastic modelling of tiltrotor aircraft needs to consider the unsteady effect of the rotor wake bending due to the rotor tilting. In this paper, the unsteady models of the rotor wake bending and dynamic inflow have been introduced into the aeroelastic modelling of the tiltrotor aircraft in conversion flight by using Hamilton’s generalized principle. The method for solving the aeroelastic stability of tiltrotor aircraft in conversion flight has been established by using the small perturbation theory and the Floquet theory. The influences of unsteady dynamic inflow on trim control inputs and aeroelastic stability of a tiltrotor aircraft in conversion flight were calculated and analysed. The calculation results show that the required collective pitch increases with the pylon tilting forward and the unsteady inflow is trimmed primarily by the lateral cyclic pitch of the rotor. The wake bending unsteady dynamic inflow can obviously reduce the stability of the flapping modes of the rotor, and have no obvious influence on the lag modes of the rotor and the motion modes of the wing. The instability of tiltrotor occurs in the chordwise bending mode of the wing when the pylon tilts to a certain angle in high speed forward flight.
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Zhao, Dan. "Nonlinear Self-Excited Combustion Oscillations of a Premixed Laminar Flame." Applied Mechanics and Materials 110-116 (October 2011): 1150–54. http://dx.doi.org/10.4028/www.scientific.net/amm.110-116.1150.
Повний текст джерелаАнотація:
Self-excited combustion oscillations are caused by a coupling between acoustic waves and unsteady heat release. A premixed laminar flame in a Rijke tube, anchored to a metal gauze, is considered in this work. The flame response to flow disturbances is investigated by developing a nonlinear kinematic model based on the classical-equation, with the assumption of a time-invariant laminar flame speed. Unsteady heat release from the flame is assumed to be caused by its surface variations, which results from the fluctuations of the oncoming flow velocity. The flame is acoustically compact, and its presence causes the mean temperature undergoing a jump, whose effect on the dynamics of the thermo-acoustic system is discussed. Coupling the flame model with a Galerkin series expansion of the acoustic waves present enables the time evolution of the flow disturbances to be calculated. It was found that the model can predict the mode shape and the frequencies of the excited combustion oscillations very well. Moreover, the fundamental mode is found to be the easiest one to be triggered among all acoustic modes. To gain insight about the mode selection and triggering, further numerical investigation is conducted by linearizing the flame model and recasting into the popular formulation.
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Makoveeva, Eugenya V., Dmitri V. Alexandrov, and Alexander A. Ivanov. "On the Theory of Unsteady-State Operation of Bulk Continuous Crystallization." Crystals 12, no. 11 (November 14, 2022): 1634. http://dx.doi.org/10.3390/cryst12111634.
Повний текст джерелаАнотація:
Motivated by an important application in the chemical and pharmaceutical industries, we consider the non-stationary growth of a polydisperse ensemble of crystals in a continuous crystallizer. The mathematical model includes the effects of crystal nucleation and growth, fines dissolution, mass influx and withdrawal of product crystals. The steady- and unsteady-state solutions of kinetic and balance equations are analytically derived. The steady-state solution is found in an explicit form and describes the stationary operation mode maintained by the aforementioned effects. An approximate unsteady-state solution is found in a parametric form and describes a time-dependent crystallization scenario, which tends toward the steady-state mode when time increases. It is shown that the particle-size distribution contains kinks at the points of fines dissolution and product crystal withdrawal. Additionally, our calculations demonstrate that the unsteady-state crystal-size distribution has a bell-shaped profile that blurs with time due to the crystal growth and removal mechanisms. The analytical solutions found are the basis for investigating the dynamic stability of a continuous crystallizer.
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Yentsch, Robert J., and Datta V. Gaitonde. "Unsteady Three-Dimensional Mode Transition Phenomena in a Scramjet Flowpath." Journal of Propulsion and Power 31, no. 1 (January 2015): 104–22. http://dx.doi.org/10.2514/1.b35205.
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Liu, Yong, Hongjuan Ran, and Dezhong Wang. "Research on Unsteady Characteristics of Pump Turbine in Pumping Mode." IOP Conference Series: Earth and Environmental Science 627 (January 7, 2021): 012028. http://dx.doi.org/10.1088/1755-1315/627/1/012028.
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Bardin, Alexey, Vyacheslav Ignatjev, Andrey Orlov, and Sergey Perchenko. "Verifying of reciprocal relations for nonlinear quadripole in unsteady mode." Results in Physics 7 (2017): 665–66. http://dx.doi.org/10.1016/j.rinp.2017.01.029.
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Long, Yuexiao, Huaxing Li, Xuanshi Meng, Jia Li, and Zhengchao Xiang. "Structure optimization of the AC-SDBD plasma actuator under duty-cycle mode." Modern Physics Letters B 32, no. 26 (September 20, 2018): 1850315. http://dx.doi.org/10.1142/s0217984918503153.
Повний текст джерелаАнотація:
Alternating current dielectric barrier discharge plasma actuators driven by steady and unsteady mode were experimentally optimized in a static atmosphere. The purpose of this optimization is to enhance the effective controllability of flow control. Electrical properties were evaluated using the measured voltage, current and power consumption data. The dielectric barrier with different materials was tested and the aerodynamic characteristics were identified by particle image velocimetry and electronic force balance. Meanwhile, the duty-cycle technique was applied to operate the actuator in unsteady mode. The dynamic characteristics of induced flow were analyzed by processing the results with the phase-locked method. The development of induced flow structure at different frequencies was compared. Results showed that the plasma actuator with 4 mm-thick Teflon dielectric barrier induced the maximum force and velocity of 75 mN/m and 5.6 m/s, respectively. The discharge frequency has little effect on the control authority at the kilohertz level. The dimensionless area of the induced flow is about [Formula: see text] under steady actuation. The phase-locked results confirm that the scale and strength of the induced vortex vary with the duty-cycle frequencies. The effectiveness of unsteady flow control can be explained as the promotion of the boundary layer and the mainstream.
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SCHRADER, LARS-UVE, LUCA BRANDT, and DAN S. HENNINGSON. "Receptivity mechanisms in three-dimensional boundary-layer flows." Journal of Fluid Mechanics 618 (January 10, 2009): 209–41. http://dx.doi.org/10.1017/s0022112008004345.
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Receptivity in three-dimensional boundary-layer flow to localized surface roughness and free-stream vorticity is studied. A boundary layer of Falkner–Skan–Cooke type with favourable pressure gradient is considered to model the flow slightly downstream of a swept-wing leading edge. In this region, stationary and travelling crossflow instability dominates over other instability types. Three scenarios are investigated: the presence of low-amplitude chordwise localized, spanwise periodic roughness elements on the plate, the impingement of a weak vortical free-stream mode on the boundary layer and the combination of both disturbance sources. Three receptivity mechanisms are identified: steady receptivity to roughness, unsteady receptivity to free-stream vorticity and unsteady receptivity to vortical modes scattered at the roughness. Both roughness and vortical modes provide efficient direct receptivity mechanisms for stationary and travelling crossflow instabilities. We find that stationary crossflow modes dominate for free-stream turbulence below a level of about 0.5%, whereas higher turbulence levels will promote the unsteady receptivity mechanism. Under the assumption of small amplitudes of the roughness and the free-stream disturbance, the unsteady receptivity process due to scattering of free-stream vorticity at the roughness has been found to give small initial disturbance amplitudes in comparison to the direct mechanism for free-stream modes. However, in many environments free-stream vorticity and roughness may excite interacting unstable stationary and travelling crossflow waves. This nonlinear process may rapidly lead to large disturbance amplitudes and promote transition to turbulence.
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Zhang, Kai, and AJ Wang. "Analysis of dynamic stall flow field of high bypass fan rotor based on airworthiness certification." Proceedings of the Institution of Mechanical Engineers, Part G: Journal of Aerospace Engineering 234, no. 11 (April 16, 2020): 1757–69. http://dx.doi.org/10.1177/0954410020919281.
Повний текст джерелаАнотація:
In order to ensure flight safety, the stall test is one of the most important steps in the airworthiness certification phase of civil aircraft. The twisted-swept fan is one of the most important components of the high bypass ratio engine. The unsteady flow field of the fan rotor stall condition is obtained by numerical simulation. At the same time, the time series flow field data of the stall condition flow field is acquired. The modal analysis of the unsteady flow field at stall condition was performed using the dynamic mode decomposition and proper orthogonal decomposition methods. Through modal identification of a large number of unsteady flow field data, the eigenvalues and corresponding modal information about the unsteady flow field change process are obtained. Finally, the evolution process of the unsteady flow field of the fan rotor under stall condition is visually demonstrated, and the coherent structures of different scales in the complex flow field under stall condition are revealed.
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Kishore, V. Ratna, S. Minaev, M. Akram, and Sudarshan Kumar. "Dynamics of premixed methane/air mixtures in a heated microchannel with different wall temperature gradients." RSC Advances 7, no. 4 (2017): 2066–73. http://dx.doi.org/10.1039/c6ra27582f.
Повний текст джерелаАнотація:
The unsteady flame propagation mode (FREI) is affected by the wall temperature gradient. As the temperature gradient approaches zero, the mixture ignites at its auto-ignition temperature, frequency decreases and this leads to extinction of FREI mode.
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MORSE, T. L., and C. H. K. WILLIAMSON. "Steady, unsteady and transient vortex-induced vibration predicted using controlled motion data." Journal of Fluid Mechanics 649 (April 13, 2010): 429–51. http://dx.doi.org/10.1017/s002211200999379x.
Повний текст джерелаАнотація:
In this study, we represent transient and unsteady dynamics of a cylinder undergoing vortex-induced vibration, by employing measurements of the fluid forces for a body controlled to vibrate sinusoidally, transverse to a free stream. We generate very high-resolution contour plots of fluid force in the plane of normalized amplitude and wavelength of controlled oscillation. These contours have been used with an equation of motion to predict the steady-state response of an elastically mounted body. The principal motivation with the present study is to extend this approach to the case where a freely vibrating cylinder exhibits transient or unsteady vibration, through the use of a simple quasi-steady model. In the model, we use equations which define how the amplitude and frequency will change in time, although the instantaneous forces are taken to be those measured under steady-state conditions (the quasi-steady approximation), employing our high-resolution contour plots.The resolution of our force contours has enabled us to define mode regime boundaries with precision, in the amplitude–wavelength plane. Across these mode boundaries, there are discontinuous changes in the fluid force measurements. Predictions of free vibration on either side of the boundaries yield distinct response branches. Using the quasi-steady model, we are able to characterize the nature of the transition which occurs between the upper and lower amplitude response branches. This regime of vibration is of practical significance as it represents conditions under which peak resonant response is found in these systems. For higher mass ratios (m* > 10), our approach predicts that there will be an intermittent switching between branches, as the vortex-formation mode switches between the classical 2P mode and a ‘2POVERLAP’ mode. Interestingly, for low mass ratios (m* ~ 1), there exists a whole regime of normalized flow velocities, where steady-state vibration cannot occur. However, if one employs the quasi-steady model, we discover that the cylinder can indeed oscillate, but only with non-periodic fluctuations in amplitude and frequency. The character of the amplitude response from the model is close to what is found in free vibration experiments. For very low mass ratios (m* < 0.36 in this study), this regime of unsteady vibration response will extend all the way to infinite normalized velocity.
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Krishnappan, B. G. "Modelling of settling and flocculation of fine sediments in still water." Canadian Journal of Civil Engineering 17, no. 5 (October 1, 1990): 763–70. http://dx.doi.org/10.1139/l90-089.
Повний текст джерелаАнотація:
A numerical model to predict settling behaviour of fine sediment mixtures in a stagnant water column is described. Both single-grain settling mode and the floc settling mode are considered. The single-grain settling mode is analyzed by solving the unsteady, one-dimensional diffusion–advection equation numerically and the floc settling mode is examined by solving a coagulation equation expressed as a discrete equation in logarithmic radius space considering the differential settling as the only collision mechanism. The model results are compared with the laboratory experimental data of K. Kranck for both modes of sediment settling. The agreement between the model predictions and the experimental data is good. The model can be used as a basis for such practical applications as predicting sedimentation rates in reservoirs and settling basins. Key words: sediment mixtures, settling, flocculation, advection, diffusion, floc size distribution, primary particles.
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Chen, Guanbin, and Wenli Chen. "Experimental Investigation and Validation on Suppressing the Unsteady Aerodynamic Force and Flow Structure of Single Box Girder by Trailing Edge Jets." Applied Sciences 12, no. 3 (January 18, 2022): 967. http://dx.doi.org/10.3390/app12030967.
Повний текст джерелаАнотація:
In the present investigation, a wind tunnel experiment was performed to evaluate the effectiveness of the trailing edge jets control scheme to mitigate the unsteady aerodynamic force and flow structure of a single box girder (SBG) model. The flow control scheme uses four isolated circular holes for forming the jet flow to modify the periodic vortex shedding behind the SBG model and then alleviate the fluctuation of the aerodynamic force acting on the test model. The Reynolds number is calculated as 2.08 × 104 based on the incoming velocity and the height of the test model. A digital pressure measurement system was utilized to obtain and record the surface pressure that was distributed around the SBG model. The surface pressure results show that the fluctuating amplitude of the aerodynamic forces was attenuated in the controlled case at a specific range of the non-dimensional jet momentum coefficient. The Strouhal number of the controlled case also deviates from that of the original SBG model. Except for the pressure measurement experiment, a high-resolution digital particle image velocimetry system was applied to investigate the detailed flow structure behind the SBG model to uncover the unsteady vortex motion process from the SBG model with and without the trailing edge jets flow control. As the jet flow blows into the wake, the alternating vortex shedding mode is switched into a symmetrical shedding mode and the width of the wake flow is narrowed. The proper orthogonal decomposition was used to identify the energy of the different modes and obtain its corresponding flow structures. Moreover, the linear stability analysis of the flow field behind the SBG model shows that the scheme of trailing edge jets can dramatically suppress the area of unsteady flow.
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Guo, Zhengfei, and Markus J. Kloker. "Control of crossflow-vortex-induced transition by unsteady control vortices." Journal of Fluid Mechanics 871 (May 22, 2019): 427–49. http://dx.doi.org/10.1017/jfm.2019.288.
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The fundamental mechanisms of a hitherto unstudied approach to control the crossflow-induced transition in a three-dimensional boundary layer employing unsteady control vortices are investigated by means of direct numerical simulations. Using a spanwise row of blowing/suction or volume-force actuators, subcritical travelling crossflow vortex modes are excited to impose a stabilizing (upstream) flow deformation (UFD). Volume forcing mimics the effects of alternating current plasma actuators driven by a low-frequency sinusoidal signal. In this case the axes of the actuators are aligned with the wave crests of the desired travelling mode to maximize receptivity and abate the influence of other unwanted, misaligned modes. The resulting travelling crossflow vortices generate a beneficial mean-flow distortion reducing the amplification rate of naturally occurring steady or unsteady crossflow modes without invoking significant secondary instabilities. It is found that the stabilizing effect achieved by travelling control modes is somewhat weaker than that achieved by the steady modes in the classical UFD method. However, the energy requirements for unsteady-UFD plasma actuators would be significantly lower than for steady UFD because the approach makes full use of the inherent unsteadiness of the plasma-induced volume force with alternating-current-driven actuators. Also, the input control amplitude can be lower since unsteady crossflow vortex modes grow stronger in the flow.
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Shatrov, M. G., A. L. Yakovenko, I. V. Alekseev, S. N. Bogdanov, and A. O. Glazkov. "Modeling of diesel structure-borne noise in an unsteady operating mode." IOP Conference Series: Materials Science and Engineering 1159, no. 1 (June 1, 2021): 012065. http://dx.doi.org/10.1088/1757-899x/1159/1/012065.
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Dai, Yao, and Keh-Chih Hwang. "Unsteady growth of mode III crack in elastic perfectly-plastic material." Engineering Fracture Mechanics 31, no. 6 (January 1988): 1027–34. http://dx.doi.org/10.1016/0013-7944(88)90213-5.
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Li, Yangxi, Yiqun Hou, Ben Zhang, Xuan Zou, David Johnson, Fan Wan, Chaoyan Zhou, Yao Jin, and Xiaotao Shi. "The Kinematics and Dynamics of Schizopygopsis malacanthus Swimming during Ucrit Testing." Animals 12, no. 20 (October 19, 2022): 2844. http://dx.doi.org/10.3390/ani12202844.
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The swimming kinematics (how fish move) and dynamics (how forces effect movement) of Schizopygopsis malacanthus were investigated during the determination of by stepped velocity testing. A video tracking program was used to record and analyze the motion of five test fish in a Brett-type flume during each velocity step. The findings fell into three groups: (1) Even when flow was uniform, fish did not swim steadily, with speeds fluctuating by 2.2% to 8.4% during steady swimming. The proportion of unsteady swimming time increased with water velocity, and defining steady and unsteady swimming statistically, in terms of the definition of standard deviation of instantaneous displacements, may have higher accuracy. (2) In steady swimming, the forward velocity and acceleration of fish were correlated with body length (p < 0.05), but in unsteady swimming the correlations were not significant. The maximum swimming speed (1.504 m/s) and acceleration (16.54 m/s2) occurred during unsteady swimming, but these measurements may not be definitive because of tank space constraints on fish movement and the passive behavior of the test fish with respect to acceleration. (3) Burst-coast swimming in still water, investigated by previous scholars as an energy conserving behavior, is not the same as the gait transition from steady to unsteady swimming in flowing water. In this study, the axial force of fish swimming in the unsteady mode was significantly higher (×1.2~1.6) than in the steady mode, as was the energy consumed (×1.27~3.33). Thus, gait transition increases, rather than decreases, energy consumption. Our characterization of the kinematics and dynamics of fish swimming provides important new information to consider when indices of swimming ability from controlled tank testing are applied to fish passage design.
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Sun, Tiezhi, Qingmo Xie, Li Zou, Hao Wang, and Chang Xu. "Numerical Investigation of Unsteady Cavitation Dynamics over a NACA66 Hydrofoil near a Free Surface." Journal of Marine Science and Engineering 8, no. 5 (May 11, 2020): 341. http://dx.doi.org/10.3390/jmse8050341.
Повний текст джерелаАнотація:
Cavitation is a typical and unavoidable phenomenon for small waterline ships and high-speed vehicles. It creates a highly complex multiphase flow near the free surface and is primarily represented by the free surface-cavitation interaction. In this paper, the large-eddy method and Schnerr-Sauer cavitation model are combined to address the effects of a free surface on the cavitation dynamics of a NACA66 hydrofoil. The numerical method is validated by comparing the cavitation morphology and pressure with available experimental data. The results show that the presence of a free surface affects the cavitation evolution and hydrodynamic load characteristics. Compared with the non-free surface case for the same cavitation number, the free surface suppresses the cavitation intensity and increases the frequency of cavitation shedding. Furthermore, an improved dynamic mode decomposition method is applied to investigate the unsteady cavitation flow features. The results show a correlation between the characteristic mode and the flow state. Meanwhile, the presence of a free surface is found to reduce the energy content in each order mode and results in smaller scale of the coherent structure in higher-order modes. Moreover, with increasing distance from the hydrofoil to the free surface, the cavitation intensity increases, as well as the average lift and drag coefficients. In particular, significant free-surface unsteady fluctuations are observed in the wake region.
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Polivyanchuk, A. "MATHEMATICAL MODELING OF INFLUENCE THE OPERATION MODE DIESEL ENGINE ON THE CONTENT OF PARTICULATE MATTER IN EXHAUST GASES." Municipal economy of cities 3, no. 156 (July 1, 2020): 62–68. http://dx.doi.org/10.33042/2522-1809-2020-3-156-62-68.
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The work is devoted to solving an urgent scientific and practical task – establishing mathematical models that describe the effect of operating modes of diesel engines on the content in the exhaust gases of a dangerous pollutant – particulate matter (PM). The purpose of the work was to create and study the accuracy and practical suitability of the calculation method estimating concentrations and emissions with exhaust gases PM by means of mathematical modeling of the influence on them of parameters that determine steady state and unsteady diesel operation modes. The studies were carried out on the basis of the motor stand of a 4ChN12/14 autotractor diesel equipped with a partial-flow system for diluting EG with air – MKT-2 microtunnels. Measurements mass and vol-ume concentrations – cpt (g/kg) and Cpt (g/mn3), mass – PTmass (g/h) and specific – PTp (g/kWh) PM emissions was carried out by the gravimetric method with errors of ± 3 .. 10% in accordance with the requirements of regulatory documents – ISO8178 standard, UNECE Rules R-49, R-96, etc. As parameters which determine the mode of opera-tion of the diesel engine, were considered: during the study steady-state modes – the number of revolutions of the engine crankshaft (n, min-1) and the load (L,%); in the study of unsteady modes – the parameters n and L and the rate of change over time – Δn/Δt and ΔL/Δt. A dependence has been established for indirectly determining the mass concentration of PM in the exhaust gases at steady and unsteady diesel operating modes, which are charac-terized by a duration of 10 ... 30 s and ranges of vari-ation of the parameters n, L, Δn/Δt and ΔL/Δt, given in di-mensionless form: 0.4 ... 0.8, 0.3 ... 1.0, -0.2 ... 0.2 and -0.35 ... 0.35, respectively. The deviation of the calculated and experimental data when using this dependence is ± 0.005-0.006 g/kg, which is comparable with the sensitivity limit of MKT-2. Keywords: diesel, exhaust gases, particulate matter, concentration, test mode, mathematical model, accuracy.
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Saveliev, Anatoly Petrovich, Vladimir Stepanovich Shkrabak, Roman Vladimirovich Shkrabak, Sergey Viktorovich Glotov, and Svetlana Anatolyevna Enaleeva. "The rationale for the loading modes of diesel engines for the diagnosis at unsteady load." Agrarian Scientific Journal, no. 11 (November 23, 2020): 125–29. http://dx.doi.org/10.28983/asj.y2020i11pp125-129.
Повний текст джерелаАнотація:
To increase the reliability of diagnosing tractor engines, a dynamic loading mode is proposed under bench conditions. The loading modes are substantiated from the point of view of diagnostics of the input action and the most informative mode of engine operation.
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Hu, Jiawei, Yangang Wang, Hanru Liu, Weixiong Chen, and Yong Xu. "Comparative Study on Modal Decomposition Methods of Unsteady Separated Flow in Compressor Cascade." Xibei Gongye Daxue Xuebao/Journal of Northwestern Polytechnical University 38, no. 1 (February 2020): 121–29. http://dx.doi.org/10.1051/jnwpu/20203810121.
Повний текст джерелаАнотація:
The present work investigated the vortex structure and fluctuation frequency characteristics generated by boundary layer separation of a high-load compressor cascade using modal decomposition methods. The dominant modes and dynamic behaviors of unsteady flow in the cascade were obtained, and the differences of three modal decomposition methods (Proper Orthogonal Decomposition, Dynamic Mode Decomposition and Spectral Proper Orthogonal Decomposition) in feature recognition of cascade flow were discussed. The results show that:(1) The POD method can accurately extract the dominant spatial structure of the flow field, but the modal coefficients are multi-frequency coupled, which makes the dominant modal characteristics of cascade flow unclear. (2) The standard DMD method can obtain the spatial-temporal single frequency mode of cascade flow, as well as their growth rates and frequencies. However, this method is likely to capture the suboptimal mode of large amplitude with large attenuation rate, and fails to obtain the high-frequency coherent structure, which makes it impossible to obtain the dominant feature with limited mode number. (3) The SPOD method, based on spectral characteristics, can obtain spatial and temporal single frequency modes, and there is no modal screening problem. The use of spectral estimation method (SPOD) reduces the sensitivity to numerical noise. This method can obtain the low-rank behavior of cascade flow, which is helpful to understand cascade flow mechanisms. Therefore, SPOD method is more advantageous for the modal analysis of unsteady separated flow in high-load compressor cascade.
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Kielb, R. E., and J. K. Ramsey. "Flutter of a Fan Blade in Supersonic Axial Flow." Journal of Turbomachinery 111, no. 4 (October 1, 1989): 462–67. http://dx.doi.org/10.1115/1.3262294.
Повний текст джерелаАнотація:
An application of a simple aeroelastic model to an advanced supersonic axial flow fan is presented. Lane’s cascade theory is used to determine the unsteady aerodynamic loads. Parametric studies are performed to determine the effects of mode coupling, Mach number, damping, pitching axis location, solidity, stagger angle, and mistuning. The results show that supersonic axial flow fan and compressor blades are susceptible to a strong torsional mode flutter having critical reduced velocities that can be less than one.
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LUO, X. Y., Z. X. CAI, W. G. LI, and T. J. PEDLEY. "The cascade structure of linear instability in collapsible channel flows." Journal of Fluid Mechanics 600 (March 26, 2008): 45–76. http://dx.doi.org/10.1017/s0022112008000293.
Повний текст джерелаАнотація:
This paper studies the unsteady behaviour and linear stability of the flow in a collapsible channel using a fluid–beam model. The solid mechanics is analysed in a plane strain configuration, in which the principal stretch is defined with a zero initial strain. Two approaches are employed: unsteady numerical simulations solving the nonlinear fully coupled fluid–structure interaction problem; and the corresponding linearized eigenvalue approach solving the Orr–Sommerfeld equations modified by the beam. The two approaches give good agreement with each other in predicting the frequencies and growth rates of the perturbation modes, close to the neutral curves. For a given Reynolds number in the range of 200–600, a cascade of instabilities is discovered as the wall stiffness (or effective tension) is reduced. Under small perturbation to steady solutions for the same Reynolds number, the system loses stability by passing through a succession of unstable zones, with mode number increasing as the wall stiffness is decreased. It is found that this cascade structure can, in principle, be extended to many modes, depending on the parameters. A puzzling ‘tongue’ shaped stable zone in the wall stiffness–Re space turns out to be the zone sandwiched by the mode-2 and mode-3 instabilities. Self-excited oscillations dominated by modes 2–4 are found near their corresponding neutral curves. These modes can also interact and form period-doubling oscillations. Extensive comparisons of the results with existing analytical models are made, and a physical explanation for the cascade structure is proposed.
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Kobayashi, H. "Annular Cascade Study of Low Back-Pressure Supersonic Fan Blade Flutter." Journal of Turbomachinery 112, no. 4 (October 1, 1990): 768–77. http://dx.doi.org/10.1115/1.2927720.
Повний текст джерелаАнотація:
Low back-pressure supersonic fan blade flutter in the torsional mode was examined using a controlled-oscillating annular cascade test facility. Precise data of unsteady aerodynamic forces generated by shock wave movement, due to blade oscillation, and the previously measured data of chordwise distributions of unsteady aerodynamic forces acting on an oscillating blade, were joined and, then, the nature of cascade flutter was evaluated. These unsteady aerodynamic forces were measured by direct and indirect pressure measuring methods. Our experiments covered a range of reduced frequencies based on a semichord from 0.0375 to 0.547, six interblade phase angles, and inlet flow velocities from subsonic to supersonic flow. The occurrence of unstalled cascade flutter in relation to reduced frequency, interblade phase angle, and inlet flow velocity was clarified, including the role of unsteady aerodynamic blade surface forces on flutter. Reduced frequency of the flutter boundary increased greatly when the blade suction surface flow became transonic flow. Interblade phase angles that caused flutter were in the range from 40 to 160 deg for flow fields ranging from high subsonic to supersonic. Shock wave movement due to blade oscillation generated markedly large unsteady aerodynamic forces which stimulated blade oscillation.
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Couzinet, Anthony, Laurent Gros, and Daniel Pierrat. "Characteristics of Centrifugal Pumps Working in Direct or Reverse Mode: Focus on the Unsteady Radial Thrust." International Journal of Rotating Machinery 2013 (2013): 1–11. http://dx.doi.org/10.1155/2013/279049.
Повний текст джерелаАнотація:
Experimental and numerical investigations have been carried out to study the behaviour of a centrifugal pump operating in direct mode or turbine mode. First of all, the complete characteristics (head, power, and efficiency) were measured experimentally using a specific test loop. The numerical data obtained from a CFD study performed with the ANSYS CFX software and based on steady state and unsteady approaches were compared to the experimental results. The representation in the 4 operating quadrants shows the various operating zones where the head is always positive. Then, the unsteady radial forces were analysed from transient computations. The results obtained for the pump operation are consistent with the literature and extended to the nonnormal operating conditions, namely, for very high flowrate values. The evolution of the radial load during turbine operation is presented for various partial flow operating points.
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Zahra, Ragoub, Lagha Mohand, and Dilmi Smain. "Classical and Fuzzy Sliding Mode Control for a Nonlinear Aeroelastic System with Unsteady Aerodynamic Model." International Journal of Computing and Digital Systems 09, no. 6 (November 1, 2020): 1099–109. http://dx.doi.org/10.12785/ijcds/090608.
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Khanal, B., K. Knowles, and A. J. Saddington. "Computational study of flowfield characteristics in cavities with stores." Aeronautical Journal 115, no. 1173 (November 2011): 669–81. http://dx.doi.org/10.1017/s0001924000006394.
Повний текст джерелаАнотація:
Abstract In this paper, the results of computational studies on the unsteady flow features in three-dimensional empty cavities and cavities with a representative store are presented. Flow simulations with a turbulence model based on a hybrid method, which behaves as a standard Reynolds-averaged Navier-Stokes (RANS) model within the attached boundary layer and as a Large-Eddy Simulation LES sub-grid scale model in the rest of the flow (commonly known as Detached-Eddy Simulation (DES)) are used in this study. The time-mean flow study showed the presence of three-dimensional effects inside the cavities. The mean flowfield visualisation also clearly showed the presence of a pair of ‘tornado-like’ vortices in the upstream half of the cavity which merge to a single, large recirculation further downstream. Visualisation for the cavity-with-store case revealed that the mean flowfield was effectively divided into two halves with significant reduction of the spanwise flow across the cavity width. In the unsteady flow study, near-field acoustic spectra were computed for the empty cavity and cavity-with-store cases. Study of unsteady pressure spectra for the cavity-with-store case found the presence of many peaks and the corresponding mode frequencies were found to agree well with the Rossiter modes. The blockage effect of store and strut on the spanwise flow is thought to have reduced the interaction, and subsequent non-linear coupling, between the Rossiter modes. This may be the reason for the co-existence of multiple modes without the coupling among them.
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Chebair, A. El, and A. K. Misra. "On the Dynamics and Stability of Cylindrical Shells Conveying Inviscid or Viscous Fluid in Internal or Annular Flow." Journal of Pressure Vessel Technology 113, no. 3 (August 1, 1991): 409–17. http://dx.doi.org/10.1115/1.2928775.
Повний текст джерелаАнотація:
This paper investigates theoretically for the first time the dynamical behavior and stability of a simply supported shell located coaxially in a rigid cylindrical conduit. The fluid flow is incompressible and the fluid forces consist of two parts: (i) steady viscous forces which represent the effects of upstream pressurization of the flow; (ii) unsteady forces which could be inviscid or viscous. The inviscid forces were derived by linearized potential flow theory, while the viscous ones were derived by means of the Navier-Stokes equations. Shell motion is described by the modified Flu¨gge’s shell equations. The Fourier transform technique is employed to formulate the problem. First, the system is subjected only to the unsteady inviscid forces. It is found that increasing either the internal or the annular flow velocity induces buckling, followed by coupled mode flutter. When both steady viscous and unsteady inviscid forces are applied, for internal flow, the system becomes stabilized; while for annular flow, the system loses stability at much lower velocities. Second, the system is only subjected to the unsteady viscous forces. Calculations are only performed for the internal flow case. The results are compared to those of inviscid theory. It is found that the effects of unsteady viscous forces on the stability of the system are very close to those of unsteady inviscid forces.
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Huang, Wen Tao, Qi Zhou, Fu Xin Wang, Hong Liu, and Jun Qi Wu. "Measurements and Visualizations of the Unsteady Flow Field on a Freely Falling Plate." Advanced Materials Research 718-720 (July 2013): 1049–54. http://dx.doi.org/10.4028/www.scientific.net/amr.718-720.1049.
Повний текст джерелаАнотація:
Freely falling of a plate contains very complicated unsteady characteristics. In the past decades, most of the studies focused on the mode and trajectoryofa freely falling plate. In this paper, we use PIV to study the instantaneous flow structure and analyze the force exerted on the plate and the influence of the flow on the motion of the plate. As the Reynolds number increases, the flow structure will be changed from the stable mode to the chaotic mode,and it accompanies with a mode switching of the freely falling plate. However, there is no necessarilycorresponding with the stable flow structure, even for the freely falling plate with a stable mode.
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Feszty, D., K. J. Badcock, and B. E. Richards. "Utilising CFD in the investigation of high-speed unsteady spiked body flows." Aeronautical Journal 106, no. 1058 (April 2002): 161–74. http://dx.doi.org/10.1017/s0001924000012963.
Повний текст джерелаАнотація:
AbstractUnsteady spiked body flows were simulated by a second order time-accurate CFD method. Laminar, axisymmetric flow was considered at Mach 2.21 and Mach 6 freestreams and Reynolds’ numbers based on the blunt body diameter of 0.12 million and 0.13 million, respectively. A spiked forward facing cylinder with spike lengths between LID = 1.00 and LID = 2.40 was used as the model geometry. Following the numerical method’s verification, the individual flow modes of oscillation and pulsation were examined. The frequency of the events was found in good agreement with the experiment, while the pressure amplitudes were overpredicted in the Mach 6 cases. Analysis of the numerical results showed that the oscillation flow mode was driven by a viscous mechanism, whereas the pulsation by an inviscid one. The hysteresis phenomenon in the transition between the two flow modes was predicted qualitatively.