Articles de revues : « Unsteady computations »

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Ramamurti, Ravi, William C. Sandberg, Rainald Löhner, Jeffrey A. Walker et Mark W. Westneat. « Fluid dynamics of flapping aquatic flight in the bird wrasse:three-dimensional unsteady computations with fin deformation ». Journal of Experimental Biology 205, n^o 19 (1 octobre 2002) : 2997–3008. http://dx.doi.org/10.1242/jeb.205.19.2997.

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SUMMARY Many fishes that swim with the paired pectoral fins use fin-stroke parameters that produce thrust force from lift in a mechanism of underwater flight. These locomotor mechanisms are of interest to behavioral biologists,biomechanics researchers and engineers. In the present study, we performed the first three-dimensional unsteady computations of fish swimming with oscillating and deforming fins. The objective of these computations was to investigate the fluid dynamics of force production associated with the flapping aquatic flight of the bird wrasse Gomphosus varius. For this computational work, we used the geometry of the wrasse and its pectoral fin,and previously measured fin kinematics, as the starting points for computational investigation of three-dimensional (3-D) unsteady fluid dynamics. We performed a 3-D steady computation and a complete set of 3-D quasisteady computations for a range of pectoral fin positions and surface velocities. An unstructured, grid-based, unsteady Navier—Stokes solver with automatic adaptive remeshing was then used to compute the unsteady flow about the wrasse through several complete cycles of pectoral fin oscillation. The shape deformation of the pectoral fin throughout the oscillation was taken from the experimental kinematics. The pressure distribution on the body of the bird wrasse and its pectoral fins was computed and integrated to give body and fin forces which were decomposed into lift and thrust. The velocity field variation on the surface of the wrasse body, on the pectoral fins and in the near-wake was computed throughout the swimming cycle. We compared our computational results for the steady, quasi-steady and unsteady cases with the experimental data on axial and vertical acceleration obtained from the pectoral fin kinematics experiments. These comparisons show that steady state computations are incapable of describing the fluid dynamics of flapping fins. Quasi-steady state computations, with correct incorporation of the experimental kinematics, are useful when determining trends in force production, but do not provide accurate estimates of the magnitudes of the forces produced. By contrast, unsteady computations about the deforming pectoral fins using experimentally measured fin kinematics were found to give excellent agreement, both in the time history of force production throughout the flapping strokes and in the magnitudes of the generated forces.

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Allen, C. B. « Grid adaptation for unsteady flow computations ». Proceedings of the Institution of Mechanical Engineers, Part G : Journal of Aerospace Engineering 211, n^o 4 (1 avril 1997) : 237–50. http://dx.doi.org/10.1243/0954410971532640.

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A grid adaptation procedure suitable for use during unsteady flow computations is described. Transfinite interpolation is used to generate structured grids for the computation of steady and unsteady Euler flows past aerofoils. This technique is well suited to unsteady flows, since instantaneous grid positions and speeds required by the flow solver are available directly from the algebraic mapping. A different approach to grid adaptation is described, wherein adaptation is performed by redistributing the interpolation parameters, instead of the physical grid positions. This results in the adapted grid positions, and hence speeds, still being available algebraically. Grid adaptation during an unsteady computation is performed continuously by imposing an ‘adaptation velocity’ on grid points, thereby applying the adaptation over several time steps and avoiding the interpolation of the solution from one grid to another, which is associated with instantaneous adaptation. For both steady and unsteady flows the adapted grid technique is shown to produce sharper shock resolution for a very small increase in CPU (central processing unit) requirements.

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Wechsler, K., M. Breuer et F. Durst. « Steady and Unsteady Computations of Turbulent Flows Induced by a 4/45° Pitched-Blade Impeller ». Journal of Fluids Engineering 121, n^o 2 (1 juin 1999) : 318–29. http://dx.doi.org/10.1115/1.2822210.

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The present paper summarizes steady and unsteady computations of turbulent flow induced by a pitched-blade turbine (four blades, 45° inclined) in a baffled stirred tank. Mean flow and turbulence characteristics were determined by solving the Reynolds averaged Navier-Stokes equations together with a standard k-ε turbulence model. The round vessel had a diameter of T = 152 mm. The turbine of diameter T/3 was located at a clearance of T/3. The Reynolds number (Re) of the experimental investigation was 7280, and computations were performed at Re = 7280 and Re = 29,000. Techniques of high-performance computing were applied to permit grid sensitivity studies in order to isolate errors resulting from deficiencies of the turbulence model and those resulting from insufficient grid resolution. Both steady and unsteady computations were performed and compared with respect to quality and computational effort. Unsteady computations considered the time-dependent geometry which is caused by the rotation of the impeller within the baffled stirred tank reactor. Steady-state computations also considered neglect the relative motion of impeller and baffles. By solving the governing equations of motion in a rotating frame of reference for the region attached to the impeller, the steady-state approach is able to capture trailing vortices. It is shown that this steady-state computational approach yields numerical results which are in excellent agreement with fully unsteady computations at a fraction of the time and expense for the stirred vessel configuration under consideration.

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Huo, Chao, Peng Lv et Anbang Sun. « Computational study on the aerodynamics of a long-shrouded contra-rotating rotor in hover ». International Journal of Micro Air Vehicles 11 (janvier 2019) : 175682931983368. http://dx.doi.org/10.1177/1756829319833686.

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This paper aims to investigate the aerodynamics including the global performance and flow characteristics of a long-shrouded contra-rotating rotor by developing a full 3D RANS computation. Through validations by current experiments on the same shrouded contra-rotating rotor, the computation using sliding mesh method and the computational zone with an extended nozzle downstream flow field effectively works; the time-averaged solution of the unsteady computation reveals that more uniform flow presents after the downstream rotor, which implies that the rear rotor rotating at opposite direction greatly compensates and reduces the wake; the unsteady computations further explore the flow field throughout the whole system, along the span and around blade tips. Complex flow patterns including the vortices and their interactions are indicated around the blade roots and tips. For further identifying rotor configurations, the rotor–rotor distance and switching two rotor speeds were studied. The computation reveals that setting the second rotor backwards decreases the wake scale but increases its intensity in the downstream nozzle zone. However, for the effect of switching speeds, computations cannot precisely solve the flow when the rear rotor under the windmill because of the upstream rotor rotating much faster than the other one. All the phenomena from computations well implement the experimental observations.

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Johansen, Stein T., Jiongyang Wu et Wei Shyy. « Filter-based unsteady RANS computations ». International Journal of Heat and Fluid Flow 25, n^o 1 (février 2004) : 10–21. http://dx.doi.org/10.1016/j.ijheatfluidflow.2003.10.005.

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Adami, P., et F. Martelli. « Three-dimensional unsteady investigation of HP turbine stages ». Proceedings of the Institution of Mechanical Engineers, Part A : Journal of Power and Energy 220, n^o 2 (1 mars 2006) : 155–67. http://dx.doi.org/10.1243/095765005x69189.

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This article deals with a three-dimensional unsteady numerical simulation of the unsteady rotor—stator interaction in a HP turbine stage. The numerical approach consists of a computational fluid dynamics (CFD) parallel code, based on an upwind total variation diminishing finite volume approach. The computation has been carried out using a sliding plane approach with hybrid unstructured meshes and a two-equation turbulent closure. The turbine rig under investigation is representative of the first stage of aeronautic gas turbine engines. A brief description of the cascade, the experimental setup, and the measuring technique is provided. Time accurate CFD computations of pressure fluctuations and Nusselt number are discussed against the experimental data.

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De´nos, R., T. Arts, G. Paniagua, V. Michelassi et F. Martelli. « Investigation of the Unsteady Rotor Aerodynamics in a Transonic Turbine Stage ». Journal of Turbomachinery 123, n^o 1 (1 février 2000) : 81–89. http://dx.doi.org/10.1115/1.1314607.

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The paper focuses on the unsteady pressure field measured around the rotor midspan profile of the VKI Brite transonic turbine stage. The understanding of the complex unsteady flow field is supported by a quasi-three-dimensional unsteady Navier–Stokes computation using a k-ω turbulence model and a modified version of the Abu-Ghannam and Shaw correlation for the onset of transition. The agreement between computational and experimental results is satisfactory. They both reveal the dominance of the vane shock in the interaction. For this reason, it is difficult to identify the influence of vane-wake ingestion in the rotor passage from the experimental data. However, the computations allow us to draw some useful conclusions in this respect. The effect of the variation of the rotational speed, the stator–rotor spacing, and the stator trailing edge coolant flow ejection is investigated and the unsteady blade force pattern is analyzed.

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Luo, Da Hai, Chao Yan, Wei Lin Zheng et Wu Yuan. « A New PANS Model for Unsteady Separated Flow Simulations ». Applied Mechanics and Materials 721 (décembre 2014) : 182–86. http://dx.doi.org/10.4028/www.scientific.net/amm.721.182.

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A new Partially Averaged Navier-Stokes (PANS) model is proposed with the aim of simulating unsteady separated flows at reasonable computational expense. The unresolved-to-total ratio of kinetic energy (fk) related to PANS method is taken as a spatially varying and dynamically updating parameter in the computations. Turbulent flow past a backward-facing step is chosen as a test case in an effort to evaluate the model performance. PANS computations are compared to the experimental data and the traditional Detached Eddy Simulations (DES), showing their excellent capability of resolving turbulent fluctuations. Boundary layer shielding technique is also introduced into the PANS approach and effectively improves the computational results.

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Chen, C. P., et M. J. Sheu. « Unsteady transonic computations on porous aerofoils ». AIAA Journal 29, n^o 1 (janvier 1991) : 148–50. http://dx.doi.org/10.2514/3.10557.

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Korakianitis, T., P. Papagiannidis et N. E. Vlachopoulos. « Unsteady Flow/Quasi-Steady Heat Transfer Computations on a Turbine Rotor and Comparison With Experiments ». Journal of Turbomachinery 124, n^o 1 (1 août 2001) : 152–59. http://dx.doi.org/10.1115/1.1405419.

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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.

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Paulon, J., Zhifang Zhang, Pingfang Jia et Jingfei Meng. « Influence of Unsteady Effects on the Measurements in a Transonic Axial Compressor ». Journal of Turbomachinery 114, n^o 3 (1 juillet 1992) : 510–16. http://dx.doi.org/10.1115/1.2929174.

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Interaction phenomena between rotor and stator are unavoidable in advanced compressors and their effects increase with the performance of the turbomachines. Until now, it was not possible to quantify the interaction effects, but with the development of three-dimensional unsteady computation codes in a complete stage, it is possible to know, in detail, the flow field through the machine and to make evident and to explain the difficulties encountered in measuring the flow parameters. A study has been conducted in this way at ONERA, on an axial transonic compressor stage. The computations have been made with a simulation of the losses; in this manner, the overall computed and measured performances of the compressor are the same. A detailed analysis of the unsteady computation results makes evident, between rotor and stator, large variations of some parameters of the flow as a function of time, but also as a function of the axial and tangential relative position of steady probes and stator blades. Unsteady measurements made on another transonic machine confirm the indications given by these computations.

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Allen, C. B. « Aeroelastic computations using algebraic grid motion ». Aeronautical Journal 106, n^o 1064 (octobre 2002) : 559–70. http://dx.doi.org/10.1017/s0001924000018182.

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AbstractCoupling an unsteady flow-solver with a structural model offers the opportunity to simulate aeroelastic behaviour of wings and rotor blades. The moving and deforming surfaces resulting from unsteady simulations require deforming meshes during the simulation, and it is common to use simple interpolation of surface displacements and velocities onto the initial undisturbed mesh. However, aeroelastic simulations can result in large displacements and deformations of solid surfaces, and simple interpolation of perturbations results in poor grid quality and possible grid crossover. A new interpolation technique is presented which is still simple in that it is driven solely by surface motion, but represents rotational effects near the solid surface, to maintain grid quality there. Furthermore, the scheme is fully analytic, so is very cheap computationally and results in grid speeds also being available analytically. Results, in terms of unsteady grid motion and flow solution, show the scheme to be effective and efficient.

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Marshall, J. S., et J. R. Grant. « Penetration of a blade into a vortex core : vorticity response and unsteady blade forces ». Journal of Fluid Mechanics 306 (10 janvier 1996) : 83–109. http://dx.doi.org/10.1017/s0022112096001243.

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Numerical calculations are performed for the problem of penetration into a vortex core of a blade travelling normal to the vortex axis, where the plane formed by the blade span and the direction of blade motion coincides with the normal plane of the vortex axis at the point of penetration. The calculations are based on a computational method, applicable for unsteady three-dimensional flow past immersed bodies, in which a collocation solution of the vorticity transport equation is obtained on a set of Lagrangian control points. Differences between this method and other Lagrangian vorticity-based methods in the literature are discussed. Lagrangian methods of this type are particularly attractive for problems of unsteady vortex-body interaction, since control points need only be placed on the surface of the body and in regions of the flow with non-negligible vorticity magnitude. The computations for normal blade-vortex interaction (BVI) are performed for an inviscid fluid and focus on the relationship between the vortex core deformation due to penetration of the blade into the vortex ambient position and the resulting unsteady pressure field and unsteady force acting on the blade. Computations for cases with different vortex circulations are performed, and the accuracy of an approximate formulation using rapid distortion theory is assessed by comparison with the full computational results for unsteady blade force. The force generated from blade penetration into the vortex ambient position is found to be of a comparable magnitude to various other types of unsteady BVI forces, such as that due to cutting of the vortex axial flow.

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Klein, A., Th Lutz, E. Krämer, K. Richter, A. D. Gardner et A. R. M. Altmikus. « Numerical Comparison of Dynamic Stall for Two-Dimensional Airfoils and an Airfoil Model in the DNW–TWG ». Journal of the American Helicopter Society 57, n^o 4 (1 octobre 2012) : 1–13. http://dx.doi.org/10.4050/jahs.57.042007.

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The airfoil sections of helicopter rotors experience a wide range of flow conditions in forward flight from transonic flow on the advancing blade to subsonic flow and high angles of attack on the retreating blade. Most notably, the dynamic stall phenomenon has been a research topic for decades and various models have been introduced to predict the unsteady characteristics of the rotor blade undergoing unsteady separation. The objective of the present paper is to compare two-dimensional (2D) dynamic stall computations, suitable for airfoil design studies considering unsteady characteristics, with computational fluid dynamics simulations of the wind tunnel environment taking into account three dimensionality and wall effects. Differences between experiment and 2D computations can be partly attributed to sidewall effects, which alter the effective angle of attack at the midsection pressure measurement plane. To gain more insight into these effects, investigations are presented, which show the wind tunnel wall boundary layers and separation effects at the sidewall–airfoil junction.

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Cousteix, J. « Three-Dimensional and Unsteady Boundary- Layer Computations ». Annual Review of Fluid Mechanics 18, n^o 1 (janvier 1986) : 173–96. http://dx.doi.org/10.1146/annurev.fl.18.010186.001133.

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Mattsson, Ken, Magnus Svärd, Mark Carpenter et Jan Nordström. « High-order accurate computations for unsteady aerodynamics ». Computers & ; Fluids 36, n^o 3 (mars 2007) : 636–49. http://dx.doi.org/10.1016/j.compfluid.2006.02.004.

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He, L., et K. Sato. « Numerical Solution of Incompressible Unsteady Flows in Turbomachinery ». Journal of Fluids Engineering 123, n^o 3 (5 avril 2000) : 680–85. http://dx.doi.org/10.1115/1.1383595.

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A three-dimensional incompressible viscous flow solver of the thin-layer Navier-Stokes equations was developed for the unsteady turbomachinery flow computations. The solution algorithm for the unsteady flows combines the dual time stepping technique with the artificial compressibility approach for solving the incompressible unsteady flow governing equations. For time accurate calculations, subiterations are introduced by marching the equations in the pseudo-time to fully recover the incompressible continuity equation at each real time step, accelerated with a multi-grid technique. Computations of test cases show satisfactory agreements with corresponding theoretical and experimental results, demonstrating the validity and applicability of the present method to unsteady incompressible turbomachinery flows.

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Grimberg, Sebastian, et Charbel Farhat. « Fast computation of the wall distance in unsteady Eulerian fluid-structure computations ». International Journal for Numerical Methods in Fluids 89, n^o 4-5 (11 octobre 2018) : 143–61. http://dx.doi.org/10.1002/fld.4686.

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Grace, Sheryl M., S. I. Hariharan et Hafiz M. Atassi. « Direct Computations of Unsteady Flows About Thin Airfoils ». Journal of Computational Acoustics 06, n^o 03 (septembre 1998) : 337–55. http://dx.doi.org/10.1142/s0218396x98000235.

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The intent of this paper is to investigate the effectiveness of direct numerical computation in capturing acoustic fields. It has been the trend to use high-order schemes whenever there is interest in resolving the acoustic field, even though there has never been a complete study of the accuracy of low-order schemes. We revisit a long-standing benchmark problem in aeroacoustics to show the validity of such low-order schemes. The problem we consider is that of a flat-plate airfoil in a non-uniform flow. We show that when the linearized Euler equations are used to model problems in aeroacoustics, a simple second-order numerical scheme is powerful enough to provide accurate aeroacoustic results. We validate this computational approach by comparing both the RMS and instantaneous pressure on the airfoil and in the far field to semianalytic and asymptotic results.

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Shen, Wen Zhong, Jess A. Michelsen et Jens Nørkær Sørensen. « Improved Rhie-Chow Interpolation for Unsteady Flow Computations ». AIAA Journal 39, n^o 12 (décembre 2001) : 2406–9. http://dx.doi.org/10.2514/2.1252.

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Shen, W. Z., J. A. Michelsen et J. N. Sorensen. « Improved Rhie-Chow interpolation for unsteady flow computations ». AIAA Journal 39 (janvier 2001) : 2406–9. http://dx.doi.org/10.2514/3.15042.

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Dubuc, L., F. Cantariti, M. Woodgate, B. Gribben, K. J. Badcock et B. E. Richards. « A grid deformation technique for unsteady flow computations ». International Journal for Numerical Methods in Fluids 32, n^o 3 (15 février 2000) : 285–311. http://dx.doi.org/10.1002/(sici)1097-0363(20000215)32:3<285 ::aid-fld939>3.0.co;2-c.

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Roe, P. L. « Remote boundary conditions for unsteady multidimensional aerodynamic computations ». Computers & ; Fluids 17, n^o 1 (janvier 1989) : 221–31. http://dx.doi.org/10.1016/0045-7930(89)90018-2.

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Cooker, Mark J. « Breaking Water Waves : Unsteady 2D Free-Surface Computations ». PAMM 5, n^o 1 (décembre 2005) : 3–6. http://dx.doi.org/10.1002/pamm.200510002.

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ARTOLI, A. M., A. G. HOEKSTRA et P. M. A. SLOOT. « ACCELERATED LATTICE BGK METHOD FOR UNSTEADY SIMULATIONS THROUGH MACH NUMBER ANNEALING ». International Journal of Modern Physics C 14, n^o 06 (juillet 2003) : 835–45. http://dx.doi.org/10.1142/s012918310300498x.

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We present an adaptation of the lattice BGK method for fast convergence of simulations of laminar time-dependent flows. The technique is an extension to the recent accelerated procedures for steady flow computations. Being based on Mach number annealing, the present technique substantially improves the accuracy and computational efficiency of the standard lattice BGK method for such unsteady flows.

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Van Dommelen, Leon L. « On the Lagrangian description of unsteady boundary-layer separation. Part 2. The spinning sphere ». Journal of Fluid Mechanics 210 (janvier 1990) : 627–45. http://dx.doi.org/10.1017/s0022112090001422.

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A theory to explain the initial stages of unsteady separation has been proposed by Van Dommelen & Cowiey (1990). In the present paper, this theory is verified for the separation process that occurs at the equatorial plane of a sphere or a spheroid which is impulsively spun around an axis of symmetry. A Lagrangian numerical scheme is developed which gives results in good agreement with Eulerian computations, but which is significantly more accurate. This increased accuracy, and a simpler structure to the solution, also allows verification of the Eulerian structure, including the presence of logarithmic terms. Further, while the Eulerian computations broke down at the first occurrence of separation, it is found that the Lagrangian computation can be continued. It is argued that this separated solution does provide useful insight into the further evolution of the separated flow. A remarkable conclusion is that an unseparated vorticity layer at the wall, a familiar feature in unsteady separation processes, disappears in finite time.

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Li, Guang Ning, et Min Xu. « Numerical Investigation of Sub-Iteration Criterions for Unsteady Flow Computations with Dual-Time Method ». Applied Mechanics and Materials 432 (septembre 2013) : 189–95. http://dx.doi.org/10.4028/www.scientific.net/amm.432.189.

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The convergence of sub-iteration with the dual-time method is very important for the prediction of unsteady flow field. The influence of sub-iteration step number, criterion of sub-iteration convergence and the choice of physical time step size on the calculation results are discussed by solving of the two-dimensional unsteady Euler equations. A new convergence criterion (named residual criterion) of sub-iteration for unsteady flows is proposed, and the unsteady flow test case AGARD-CT5 is calculated to verify the new criterion. The results show that, with the same criterion of sub-iteration, the results from different physical time step sizes are in agreement with each other. The difference between the experiment data and the numerical results are small, and if the sub-iteration criterion used is reasonable and small enough, the dependence of numerical results of unsteady flows on the physical time step will be decreased as possible. The new criterion of sub-iteration for dual-time step unsteady calculations can be used for engineering problem.

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Ahmed Hussein Hafez, Tamer Heshmat Mohamed Aly Kasem, Basman Elhadidi et Mohamed Madbouly Abdelrahman. « Modelling Three Dimensional Unsteady Turbulent HVAC Induced Flow ». Journal of Advanced Research in Fluid Mechanics and Thermal Sciences 87, n^o 1 (7 septembre 2021) : 76–90. http://dx.doi.org/10.37934/arfmts.87.1.7690.

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A three-dimensional numerical model for HVAC induced flow is presented. The nonlinear set of buoyancy driven incompressible flow equations, augmented with those of energy and turbulence model is solved. Various relevant are discussed. These challenges include avoiding expensive commercial packages, modeling complex boundaries, and capturing near wall gradients. Adaptive time stepping is employed to optimize computational effort. Three-dimensional simulation requirements are addressed using parallel computations. Two-dimensional and three-dimensional results are presented to clarify the model significance. Validation is done using full scale measurements. Good agreement with velocity and temperature profiles are illustrated.

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Li, Yan-Ling, et Abdulnaser I. Sayma. « Computational fluid dynamics simulations of blade damage effect on the performance of a transonic axial compressor near stall ». Proceedings of the Institution of Mechanical Engineers, Part C : Journal of Mechanical Engineering Science 229, n^o 12 (10 octobre 2014) : 2242–60. http://dx.doi.org/10.1177/0954406214553828.

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Gas turbine axial compressor blades may encounter damage during service for various reasons such as damage by debris from casing or foreign objects impacting the blades, typically near the rotor’s tip. This may lead to deterioration of performance and reduction in the surge margin. The damage breaks the cyclic symmetry of the rotor assembly; thus, computational fluid dynamics simulations have to be performed using full annulus compressor assembly. Moreover, downstream boundary conditions are unknown during rotating stall or surge, and simulations become difficult. This paper presents unsteady computational fluid dynamics analyses of compressor performance with tip curl damage. Computations were performed near the stall boundary. The primary objectives are to understand the effect of the damage on the flow behaviour and compressor stability. Computations for the undamaged rotor assembly were also performed as a reference case. A transonic axial compressor rotor was used for the time-accurate numerical unsteady flow simulations, with a variable area nozzle downstream simulating an experimental throttle. Computations were performed at 60% of the rotor design speed. Two different degrees of damage for one blade and multiple damaged blades were investigated. Rotating stall characteristics differ including the number of stall cells, propagation speed and rotating stall cell characteristics. Contrary to expectations, damaged blades with typical degrees of damage do not show noticeable effects on the global compressor performance near stall.

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McBean, Ivan, Kerry Hourigan, Mark Thompson et Feng Liu. « Prediction of Flutter of Turbine Blades in a Transonic Annular Cascade ». Journal of Fluids Engineering 127, n^o 6 (29 mai 2005) : 1053–58. http://dx.doi.org/10.1115/1.2060731.

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A parallel multiblock Navier-Stokes solver with the k‐ω turbulence model is used to solve the unsteady flow through an annular turbine cascade, the transonic Standard Test Case 4, Test 628. Computations are performed on a two- and three-dimensional model of the blade row with either the Euler or the Navier-Stokes flow models. Results are compared to the experimental measurements. Comparisons of the unsteady surface pressure and the aerodynamic damping are made between the three-dimensional, two-dimensional, inviscid, viscous simulations, and experimental data. Differences are found between the stability predictions by the two- and three-dimensional computations, and the Euler and Navier-Stokes computations due to three-dimensionality of the cascade model and the presence of a boundary layer separation, respectively.

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Perumal, D. Arumuga, Gundavarapu V. S. Kumar et Anoop K. Dass. « Numerical Simulation of Viscous Flow over a Square Cylinder Using Lattice Boltzmann Method ». ISRN Mathematical Physics 2012 (23 septembre 2012) : 1–16. http://dx.doi.org/10.5402/2012/630801.

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This work is concerned with the lattice Boltzmann computation of two-dimensional incompressible viscous flow past a square cylinder confined in a channel. It is known that the nature of the flow past cylindrical obstacles is very complex. In the present work, computations are carried out both for steady and unsteady flows using lattice Boltzmann method. Effects of Reynolds number, blockage ratio, and channel length are studied in detail. As good care has been taken to include appropriate measures in the computational method, these results enjoy good credibility. To sum up, the present study reveals many interesting features of square cylinder problem and demonstrates the capability of the lattice Boltzmann method to capture these features.

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Murthy, Durbha V., et Krishna Rao V. Kaza. « Semianalytical technique for sensitivity analysis of unsteady aerodynamic computations ». Journal of Aircraft 28, n^o 8 (août 1991) : 481–88. http://dx.doi.org/10.2514/3.46052.

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Kandil, Osama A., et H. Andrew Chuang. « Unsteady inviscid and viscous computations for vortex-dominated flows ». Journal of Aircraft 27, n^o 5 (mai 1990) : 387–88. http://dx.doi.org/10.2514/3.25287.

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Ko, Sungho, et W. J. McCroskey. « Computations of Unsteady Separating Flows over an Oscillating Airfoil ». AIAA Journal 35, n^o 7 (juillet 1997) : 1235–38. http://dx.doi.org/10.2514/2.226.

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Kebbie-Anthony, Abu, Nail A. Gumerov, Sergio Preidikman, Balakumar Balachandran et Shapour Azarm. « Fast Multipole Accelerated Unsteady Vortex Lattice Method Based Computations ». Journal of Aerospace Information Systems 16, n^o 6 (juin 2019) : 237–48. http://dx.doi.org/10.2514/1.i010690.

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MANDAL, J. C., et J. BALLMANN. « UNSTEADY FLOW COMPUTATIONS OVER MOVING BODY USING DYNAMIC MESHES ». International Journal of Computational Methods 01, n^o 03 (décembre 2004) : 507–18. http://dx.doi.org/10.1142/s0219876204000253.

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An efficient implicit unstructured grid algorithm for solving unsteady inviscid compressible flows over moving body employing an Arbitrary Lagrangian Eulerian formulation is presented. In the present formulation, the time discretization is performed using a second-order accurate 3-point time integration scheme and the upwind-biased space discretization using second-order accurate finite volume formulation with Venkatakrishnan limiter. The face-velocities of the control volumes are computed using Geometric Conservation Laws. The nonlinear system arising from the implicit formulation is solved using an ILU preconditioned Newton–Krylov iteration at every time step. The computed results for two test cases involving harmonically oscillating NACA0012 airfoil are presented in order to demonstrate the efficacy of the present solver.

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Bousquet, J. M., et P. Gardarein. « Improvements on computations of high speed propeller unsteady aerodynamics ». Aerospace Science and Technology 7, n^o 6 (septembre 2003) : 465–72. http://dx.doi.org/10.1016/s1270-9638(03)00046-4.

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Ko, Sungho, et W. J. McGrosley. « Computations of unsteady separating flows over an oscillating airfoil ». AIAA Journal 35 (janvier 1997) : 1235–38. http://dx.doi.org/10.2514/3.13657.

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Zhao, Yu, Guoyu Wang et Biao Huang. « A cavitation model for computations of unsteady cavitating flows ». Acta Mechanica Sinica 32, n^o 2 (14 août 2015) : 273–83. http://dx.doi.org/10.1007/s10409-015-0455-0.

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Petit, Olivier, et Håkan Nilsson. « Numerical Investigations of Unsteady Flow in a Centrifugal Pump with a Vaned Diffuser ». International Journal of Rotating Machinery 2013 (2013) : 1–14. http://dx.doi.org/10.1155/2013/961580.

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Computational fluid dynamics (CFD) analyses were made to study the unsteady three-dimensional turbulence in the ERCOFTAC centrifugal pump test case. The simulations were carried out using the OpenFOAM Open Source CFD software. The test case consists of an unshrouded centrifugal impeller with seven blades and a radial vaned diffuser with 12 vanes. A large number of measurements are available in the radial gap between the impeller and the diffuse, making this case ideal for validating numerical methods. Results of steady and unsteady calculations of the flow in the pump are compared with the experimental ones, and four different turbulent models are analyzed. The steady simulation uses the frozen rotor concept, while the unsteady simulation uses a fully resolved sliding grid approach. The comparisons show that the unsteady numerical results accurately predict the unsteadiness of the flow, demonstrating the validity and applicability of that methodology for unsteady incompressible turbomachinery flow computations. The steady approach is less accurate, with an unphysical advection of the impeller wakes, but accurate enough for a crude approximation. The different turbulence models predict the flow at the same level of accuracy, with slightly different results.

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Chen, Ming Zhou, et Qi Dou Zhou. « Numerical Simulation of Fluctuating Propeller Forces and Comparison with Experimental Data ». Applied Mechanics and Materials 105-107 (septembre 2011) : 518–22. http://dx.doi.org/10.4028/www.scientific.net/amm.105-107.518.

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Unsteady CFD method based on realizable k-ε model is used for predicting unsteady forces of propeller working in non-uniform wake. First, CFD computations with different mesh scales were conducted at the propeller design condition, the results show that mesh refinement changed the results little. Then unsteady CFD simulation with different time step intervals was conducted for determining suitable time step interval, the results show that it is suitable for propeller rotating 3° per step. Based on the chosen mesh and time step interval, unsteady CFD simulation of propeller P4118 was conducted in 3-cycle and 4-cycle inflow, the unsteady thrust, torque and horizontal force agree well with experimental data, the results show that CFD method has good accuracy in predicting unsteady propeller forces.

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Chen, Hao. « Numerical Calculations on the Unsteady Aerodynamic Force of the Tilt-Rotor Aircraft in Conversion Mode ». International Journal of Aerospace Engineering 2019 (7 décembre 2019) : 1–15. http://dx.doi.org/10.1155/2019/2147068.

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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.

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Suzen, Y. B., et P. G. Huang. « Numerical Simulation of Unsteady Wake/Blade Interactions in Low-Pressure Turbine Flows Using an Intermittency Transport Equation ». Journal of Turbomachinery 127, n^o 3 (1 mars 2004) : 431–44. http://dx.doi.org/10.1115/1.1860375.

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An extensive computational investigation of the effects of unsteady wake/blade interactions on transition and separation in low-pressure turbines has been performed by numerical simulations of two recent sets of experiments using an intermittency transport equation. The experiments considered have been performed by Kaszeta and Simon and Stieger in order to investigate the effects of periodically passing wakes on laminar-to-turbulent transition and separation in low-pressure turbines. The test sections were designed to simulate unsteady wakes in turbine engines for studying their effects on boundary layers and separated flow regions over the suction surface. The numerical simulations of the unsteady wake/blade interaction experiments have been performed using an intermittency transport model. The intermittent behavior of the transitional flows is taken into account and incorporated into computations by modifying the eddy viscosity, with the intermittency factor. Turbulent quantities are predicted by using Menter’s two-equation turbulence model (SST). The intermittency factor is obtained from the transport equation model, which can produce both the experimentally observed streamwise variation of intermittency and a realistic profile in the cross-stream direction. Computational results are compared to the experiments. Overall, general trends are captured and prediction capabilities of the intermittency transport model for simulations of unsteady wake/blade interaction flowfields are demonstrated.

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Karlson, Matthew, Bogdan G. Nita et Ashwin Vaidya. « Numerical Computations of Vortex Formation Length in Flow Past an Elliptical Cylinder ». Fluids 5, n^o 3 (10 septembre 2020) : 157. http://dx.doi.org/10.3390/fluids5030157.

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We examine two dimensional properties of vortex shedding past elliptical cylinders through numerical simulations. Specifically, we investigate the vortex formation length in the Reynolds number regime 10 to 100 for elliptical bodies of aspect ratio in the range 0.4 to 1.4. Our computations reveal that in the steady flow regime, the change in the vortex length follows a linear profile with respect to the Reynolds number, while in the unsteady regime, the time averaged vortex length decreases in an exponential manner with increasing Reynolds number. The transition in profile is used to identify the critical Reynolds number which marks the bifurcation of the Karman vortex from steady symmetric to the unsteady, asymmetric configuration. Additionally, relationships between the vortex length and aspect ratio are also explored. The work presented here is an example of a module that can be used in a project based learning course on computational fluid dynamics.

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Zhao, Yu, Guoyu Wang et Biao Huang. « A curvature correction turbulent model for computations of cloud cavitating flows ». Engineering Computations 33, n^o 1 (7 mars 2016) : 202–16. http://dx.doi.org/10.1108/ec-01-2015-0026.

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Purpose – The purpose of this paper is to assess the predictive capability of the streamline curvature correction model (CCM) and investigate the unsteady vortex behavior of the cloud cavitating flows around a hydrofoil. Design/methodology/approach – The design of the paper is based on introducing the curvature correction method to the original k-ε model. Calculations of unsteady cloud cavitating flows around a Clark-Y hydrofoil are performed using both the CCM and the baseline model. Findings – Compared with the baseline model, better agreements are observed between the predictions of the CCM model and experimental data, especially the cavity shedding process. Based on the computations, it is demonstrated that streamline curvature correction of the CCM model can effectively decrease predicted turbulence kinetic energy and eddy viscosity in cavity shedding region. This leads to the better prediction for the recirculation zone located downstream of the attached cavity, and dynamics of this recirculation zone contribute to the formation and development of the re-entrant jet. Originality/value – The authors apply streamline curvature correction to the calculations of unsteady cloud cavitating flows and discuss the interactions between the cavitation unsteadiness and vortex structures to get an insight of the correction mechanics.

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Busby, J., D. Sondak, B. Staubach et R. Davis. « Deterministic Stress Modeling of Hot Gas Segregation in a Turbine ». Journal of Turbomachinery 122, n^o 1 (1 février 1999) : 62–67. http://dx.doi.org/10.1115/1.555428.

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Simulation of unsteady viscous turbomachinery flowfields is presently impractical as a design tool due to the long run times required. Designers rely predominantly on steady-state simulations, but these simulations do not account for some of the important unsteady flow physics. Unsteady flow effects can be modeled as source terms in the steady flow equations. These source terms, referred to as Lumped Deterministic Stresses (LDS), can be used to drive steady flow solution procedures to reproduce the time-average of an unsteady flow solution. The goal of this work is to investigate the feasibility of using inviscid lumped deterministic stresses to model unsteady combustion hot streak migration effects on the turbine blade tip and outer air seal heat loads. The LDS model is obtained from an unsteady inviscid calculation. The inviscid LDS model is then used with a steady viscous computation to simulate the time-averaged viscous solution. The feasibility of the inviscid LDS model is demonstrated on a single-stage, three-dimensional, vane-blade turbine with a hot streak entering the vane passage at midpitch and midspan. The steady viscous solution with the LDS model is compared to the time-averaged viscous, steady viscous, and time-averaged inviscid computations. The LDS model reproduces the time-averaged viscous temperature distribution on the outer air seal to within 2.3 percent, while the steady viscous has an error of 8.4 percent, and the time-averaged inviscid calculation has an error of 17.2 percent. The solution using the LDS model is obtained at a cost in CPU time that is 26 percent of that required for a time-averaged viscous computation. [S0889-504X(00)00601-2]

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Kumar, K. Siva, et Sharanappa V. Sajjan. « Unsteady Flow past a Combined Pitching and Plunging Aerofoil Using an Implicit RANS Solver ». Applied Mechanics and Materials 110-116 (octobre 2011) : 3481–88. http://dx.doi.org/10.4028/www.scientific.net/amm.110-116.3481.

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Unsteady Reynolds-averaged Navier-Stokes (RANS) computations are presented for low Mach number flow past a combined pitching and plunging NACA 0012 aerofoil. The Implicit RANS solver used for obtaining time-accurate solutions is based on a finite volume nodal point spatial discretization scheme with dual time stepping. The aim is to validate the unsteady solver for flapping motion of the aerofoil. Results are presented in the form of aerodynamic coefficients and compared with available literature, thus demonstrating the capability of the solver to provide useful unsteady input data for aeroelastic and aeroacoustic analysis.

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Medic, G., et P. A. Durbin. « Unsteady Effects on Trailing Edge Cooling ». Journal of Heat Transfer 127, n^o 4 (30 mars 2005) : 388–92. http://dx.doi.org/10.1115/1.1860565.

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It is shown how natural and forced unsteadiness play a major role in turbine blade trailing edge cooling flows. Reynolds averaged simulations are presented for a surface jet in coflow, resembling the geometry of the pressure side breakout on a turbine blade. Steady computations show very effective cooling; however, when natural—or even moreso, forced—unsteadiness is allowed, the adiabatic effectiveness decreases substantially. Streamwise vortices in the mean flow are found to be the cause of the increased heat transfer.

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Otoguro, Yuto, Hiroki Mochizuki, Kenji Takizawa et Tayfun E. Tezduyar. « Space–Time Variational Multiscale Isogeometric Analysis of a tsunami-shelter vertical-axis wind turbine ». Computational Mechanics 66, n^o 6 (31 août 2020) : 1443–60. http://dx.doi.org/10.1007/s00466-020-01910-5.

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AbstractWe present computational flow analysis of a vertical-axis wind turbine (VAWT) that has been proposed to also serve as a tsunami shelter. In addition to the three-blade rotor, the turbine has four support columns at the periphery. The columns support the turbine rotor and the shelter. Computational challenges encountered in flow analysis of wind turbines in general include accurate representation of the turbine geometry, multiscale unsteady flow, and moving-boundary flow associated with the rotor motion. The tsunami-shelter VAWT, because of its rather high geometric complexity, poses the additional challenge of reaching high accuracy in turbine-geometry representation and flow solution when the geometry is so complex. We address the challenges with a space–time (ST) computational method that integrates three special ST methods around the core, ST Variational Multiscale (ST-VMS) method, and mesh generation and improvement methods. The three special methods are the ST Slip Interface (ST-SI) method, ST Isogeometric Analysis (ST-IGA), and the ST/NURBS Mesh Update Method (STNMUM). The ST-discretization feature of the integrated method provides higher-order accuracy compared to standard discretization methods. The VMS feature addresses the computational challenges associated with the multiscale nature of the unsteady flow. The moving-mesh feature of the ST framework enables high-resolution computation near the blades. The ST-SI enables moving-mesh computation of the spinning rotor. The mesh covering the rotor spins with it, and the SI between the spinning mesh and the rest of the mesh accurately connects the two sides of the solution. The ST-IGA enables more accurate representation of the blade and other turbine geometries and increased accuracy in the flow solution. The STNMUM enables exact representation of the mesh rotation. A general-purpose NURBS mesh generation method makes it easier to deal with the complex turbine geometry. The quality of the mesh generated with this method is improved with a mesh relaxation method based on fiber-reinforced hyperelasticity and optimized zero-stress state. We present computations for the 2D and 3D cases. The computations show the effectiveness of our ST and mesh generation and relaxation methods in flow analysis of the tsunami-shelter VAWT.

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Winter, Maximilian, et Christian Breitsamter. « Neurofuzzy-Model-Based Unsteady Aerodynamic Computations Across Varying Freestream Conditions ». AIAA Journal 54, n^o 9 (septembre 2016) : 2705–20. http://dx.doi.org/10.2514/1.j054892.

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