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Unsteady Simulation (unsteady + simulation)

Distribution by Scientific Domains
Engineering75%

Selected Abstracts

Parallel simulation of unsteady hovering rotor wakes

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 6 2006
C. B. Allen
Abstract Numerical simulation using low diffusion schemes, for example free-vortex or vorticity transport methods, and theoretical stability analyses have shown the wakes of rotors in hover to be unsteady. This has also been observed in experiments, although the instabilities are not always repeatable. Hovering rotor wake stability is considered here using a finite-volume compressible CFD code. An implicit unsteady, multiblock, multigrid, upwind solver, and structured multiblock grid generator are presented, and applied to lifting rotors in hover. To allow the use of very fine meshes and, hence, better representation of the flow physics, a parallel version of the code has been developed, and parallel performance using upto 1024 CPUs is presented. A four-bladed rotor is considered, and it is demonstrated that once the grid density is sufficient to capture enough turns of the tip vortices, hover exhibits oscillatory behaviour of the wake, even using a steady formulation. An unsteady simulation is then performed, and also shows an unsteady wake. Detailed analysis of the time-accurate wake history shows that three dominant unsteady modes are captured, for this four-bladed case, with frequencies of one, four, and eight times the rotational frequency. A comparison with theoretical stability analysis is also presented. Copyright © 2006 John Wiley & Sons, Ltd. [source]

Parallel solution of lifting rotors in hover and forward flight

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 1 2007
C. B. Allen
Abstract An implicit unsteady, multiblock, multigrid, upwind solver including mesh deformation capability, and structured multiblock grid generator, are presented and applied to lifting rotors in both hover and forward flight. To allow the use of very fine meshes and, hence, better representation of the flow physics, a parallel version of the code has been developed. It is demonstrated that once the grid density is sufficient to capture enough turns of the tip vortices, hover exhibits oscillatory behaviour of the wake, even using a steady formulation. An unsteady simulation is then presented, and detailed analysis of the time-accurate wake history is performed and compared to theoretical predictions. Forward flight simulations are also presented and, again, grid density effects on the wake formation investigated. Parallel performance of the code using up to 1024 CPU's is also presented. Copyright © 2006 John Wiley & Sons, Ltd. [source]

Prediction of unsteady, separated boundary layer over a blunt body for laminar, turbulent, and transitional flow

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 12 2004
D. Scott Holloway
Abstract The focus of this paper is to study the ability of unsteady RANS-based CFD to predict separation over a blunt body for a wide range of Reynolds numbers particularly the ability to capture laminar-to-turbulent transition. A perfect test case to demonstrate this point is the cylinder-in-crossflow for which a comparison between experimental results from the open literature and a series of unsteady simulations is made. Reynolds number based on cylinder diameter is varied from 104 to 107 (subcritical through supercritical flow). Two methods are used to account for the turbulence in the simulations: currently available eddy,viscosity models, including standard and realizable forms of the k,, model; and a newly developed eddy,viscosity model capable of resolving boundary layer transition, which is absolutely necessary for the type and range of flow under consideration. The new model does not require user input or ,empirical' fixes to force transition. For the first time in the open literature, three distinct flow regimes and the drag crisis due to the downstream shift of the separation point are predicted using an eddy,viscosity based model with transition effects. Discrepancies between experimental and computational results are discussed, and difficulties for CFD prediction are highlighted. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Self-sustained oscillations in opposed impinging jets in an enclosure

THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 5 2000
David A. Johnson
Abstract The flow of jets in confining enclosures has significant application in many engineering processes. In particular, the impingement of axisymmetric jets in a confined space has been examined using flow visualization, laser Doppler anemometry, and numerical simulations. Several flow regions were found; stable steady, regular oscillatory, and irregular oscillatory. Initially, a steady flow field existed for all arrangements for Red < ,90 (based on the nozzle diameter d, the fluid kinematic viscosity v and the volumetric flow rate Q through the nozzle (Q = ,d2/4Uavg)) but subsequent increments in the fluid velocity caused a regularly oscillating flow field to emerge. The onset of the oscillations and the upper limit of finite oscillations were found to be a function of the Red, and the nozzle diameter to chamber dimension ratio. Steady numerical simulations predicted the steady flow field well and good agreement was obtained in unsteady simulations of the oscillating flow field. The oscillating flow field is considered to be a class of self-sustaining oscillations where instabilities in the jet shear layer are amplified because of feed back from pressure disturbances in the impingement region. L'écoulement de jets dans des espaces confinés a des applications importantes dans de nombreux precédés d'ingénierie. On a examiné en particulier la collision de jets axisymétriques dans un espace confiné au moyen de la visualisation des écoulements, de l'anémométrie par laser Doppler et de simulations numériques. On a trouvé plusieurs régions d'écoulement : stable-stationnaire, régulier-oscillatoire et irrégulier-oscillatoire. Initialement, il existe un champ d'écoulement stationnaire pour toutes les configurations lorsque Red < ,90 (basé sur le diamètre de tuyère d, la viscosité cinématique du fluide (et le débit volumétrique Q dans la tuyère (Q = ,d24Uavg)); mais une augmentation subséquente de la vitesse du fluide déclenche un champ d'écoulement oscillatoire régulier. On a trouvé que l'apparition des oscillations et la limite supérieure des oscillations finies sont fonction de Red et du rapport entre le diamètre de l'orifice et la dimension de la chambre. Des simulations numériques stationnaires prédisent bien le champ d'écoulement stationnaire et un bon accord est obtenu pour des simulations non stationnaires du champ d'écoulement oscillant. Le champ d'écoulement oscillant est considéré être une catégorie d'oscillations qui se maintiennent seules dans laquelle les instabilités dans la couche de cisaillement des jets sont amplifiés par un effet retour des perturbations de pression dans la région de collision. [source]