1,721,107 research outputs found
Transitional separation bubbles and unsteady aspects of aerofoil stall
Full text linkA time-accurate solution method for the coupled potential flow and integral boundary-layer equations is used to study aerofoils near stall, where laboratory experiments have shown high-amplitude low-frequency oscillations. The laminar-turbulent transition model incorporates an absolute instability formulation, which allows the transition process in separation bubbles to be sustained in the absence of upstream disturbances, in agreement with recent direct numerical simulations. The method is demonstrated to capture large scale flow oscillations with Strouhal numbers and amplitudes comparable to experiments. The success of this particular physical model suggests that bubble bursting is primarily due to a potentialflow/ boundary-layer interaction effect, in which relatively simple models of boundary layer transition and turbulence suffice to describe the key phenomena
On the merging of turbulent spots in a supersonic boundary-layer flow
No full textThe complex transition flow physics associated with the merging of turbulent spots in a Mach 2 boundary-layer has been studied using direct numerical simulation. Dynamics of an isolated turbulent spot, merging of laterally displaced spots, and merging of two spots in tandem are considered. The coherent structures associated with the wingtip region of the spot are found to play a major role in destabilising the surrounding laminar fluid. In the merging of laterally displaced spots a strong velocity defect, resulting in unstable inflectional velocity profiles, is observed in the interaction zone. These local inflectional instabilities within the interaction region trigger new large scale coherent structures. During the inline merging, the calmed region behind the tail of the downstream spot is found to suppress the growth of the upstream spot. The upstream spot is ultimately engulfed by the downstream spot
Noise due to unsteady flow past trailing edges
No full textThis paper presents two-dimensional direct numerical simulations (DNS) of noise generated at trailing edges (TE) with zero thickness. The simulations are conducted specifying either no-slip or slip walls in order to investigate viscous effects. In both cases, small amplitude disturbances are introduced close to the inflow boundary that serve as pressure disturbances at the TE. DNS data reveals that the unsteady Kutta condition is not satisfied, irrespective of the wall boundary condition. However, it appears that the validity of the unsteady Kutta condition is not essential for making an accurate prediction of the far field noise. The far field pressure is predicted as a function of the surface pressure difference using a 2-D modification of Amiet's classical theory, and compared with the far field pressure computed directly. Directivity plots provide evidence that the presence of boundary layers and noise generated by an unsteady wake in the no-slip cases lead to smearing of individual lobes, and that the downstream pointing lobes in no-slip wall cases are probably due to nonlinear noise generation in the wake. The simulations are conducted using a high-order accurate numerical method which is free of upwinding, artificial dissipation or any form of explicit filtering, and employs a novel boundary treatment
A constrained vortex model with relevance to helicopter vortex ring state
No full textA planar model of a lifting surface descending into its own wake is constructed with the aim of demonstrating some underlying mechanisms of the ‘vortex ring’ state that may be enteredby a rotary wing aircraft during a vertical descent. The model uses line vortices that are periodically released from a point in space and then allowed to evolve in a constrained manner. For low descent velocities the model reproduces a hover state, where the wake vortices move downwards relative to the lifting plane. A critical descent speed is reached after which the model produces a quasi-periodic shedding of vortex agglomerations. This state is reached via a Hopf bifurcation of the steady state and persists until another critical descent velocity after which a steady windmill brake state is possible, in which vortices travel upwards in a regular manner. Unconstrained simulations reveal a more chaotic vortex pattern, but frequency analysis reveals an underlying structure similar to that shown for the constrained model. Besides offering a qualitative understanding of possible mechanisms of vortex ring state, the analysis suggests some dimensionless parameters that collapse themodel data
A numerical investigation of the compressible mixing layer
No full textThe effect of Mach number on the plane muong layer has been investigated by means of linear stability theory and two- and three-dimensional direct numerical simulations of the compressible Navier-Stokes equations. The objective was to identify the effects of compressibility on a building-block fluid flow, with applications to supersonic mixing and combustion.Results from linear stability theory show that the amplification rate is reduced as Mach number is increased. Above a convective Mach number of 0.6 it is found that three-dimensional waves are more amplified than two-dimensional waves and a simple relation is found to give the orientation of the most amplified waves. It isalso shown that the linear stability theory can be used to predict the mixing layer growth rate as a function of velocity ratio, density ratio and Mach number.Two-dimensional simulations show a strong reduction in growth rate of the two-dimensional motion as Mach number is increased, with more elongated structures forming at high Mach numbers. Shock waves are observed in two-dimensional simulations above a convective Mach number of 0.7. The supersonic modes of instability, which are the only two-dimensional unstable modes at high Mach numbers, are shown to be radiating and vortical, but have very low growth rates.Three-dimensional simulations with random initial conditions confirm the linear stability result that oblique waves become the most amplified waves at high Mach numbers, with no evidence for any other modes of instability. Simulations beginning with a two-dimensional wave and a pair of equal and opposite oblique waves show a change in the evolved large-scale structure as Mach number is increased. Above a convective Mach number of 0.6 the oblique modes have most of the energy in the developed structure, and above a convective Mach number of 1 the two-dimensional instability wave has little effect on flow structure. Similar organized structure was found in a simulation with random initial conditions. No shock waves were found in the three-dimensional simulations, even at convective Mach numbers above 1
Introduction to direct numerical simulation
No full textContents
Introduction B. E. Launder and N. D. Sandham; Part I. Physical and Numerical Techniques: 1. Linear and non-linear eddy viscosity models T. B. Gatski; 2. Second-moment turbulence closure modelling K. Hanjalic and Jakirilic Suad; 3. Closure modelling near the two-component limit T. J. Craft and B. Launder; 4. The elliptic relaxation method P. A. Durbin and B. A. Patterson; 5. Numerical aspects of applying second-moment closure to complex flows M. Leschziner and F.-S. Lien; 6. Modelling heat transfer in near-wall flows Y. Nagano; 7. Introduction to direct numerical simulation N. D. Sandham; 8. Introduction to large-eddy simulation of turbulent flows J. Fröhlich and W. Rodi; 9. Two-point closure strategies C. Cambon; 10. Introduction to pdf approaches in turbulence modelling D. Roekaerts; Part II. Flow Types and Processes and Strategies for Modelling Them: 11. Modelling of separated and impinging flows T. J. Craft; 12. Large eddy simulation of the flow past bluff bodies W. Rodi; 13. LES modelling of industrial flows D. Laurence; 14. Application of TCL modelling to stratified flows T. J. Craft and B. E. Launder; 15. Higher-moment diffusion in stable stratification B. Ilyushin; 16. DNS of by-pass transition P. A. Durbin, R. Jacobs and X. Wu; 17. By-pass transition using conventional closures A. M. Savill; 18. New strategies in modelling by-pass transition A. M. Savill; 19. Compressible, high-speed flows S. Barre, J.-P. Bonnet, T. B. Gatski and N. D. Sandham; 20. Closure strategies for reacting flows W. P. Jones; 21. Pdf strategies for reacting flows D. Roekaerts; 22. TRANS approach to convection in unstably stratified layers K. Hanjalic and S. Kenjeres; 23. Use of higher moments to construct pdfs in stratified flows B. Ilyushin; 24. DNS of separation bubbles G. N. Coleman and N. D. Sandham; 25. Is LES ready for complex flows? B. J. Geurts and A. Leonard; 26. Further developments in two-point closure C. Cambon
A review of progress on direct and large-eddy simulation
No full textA review is given of direct and large-eddy simulation which is intended
to summarize state-of-the-art research presented during the 1999 Isaac Newton Institute Programme on Turbulence. Introductions to the techniques are given, along with examples of applications across a range of turbulent and transitional flow problems. Recent developments in techniques for large-eddy simulation are highlighted. The discussion includes a summary of the guidelines proposed during the
programme for practical large-eddy simulations and comments on problem areas that require further research
A note on the structure of the acoustic field emitted by a wave packet
No full textThe sound field created by wave packets has been subject to rigorous experimental, analytical and numerical research in the last two decades. It has been argued that the wave packet can model a large scale structure in the form of an instability wave, initially growing, then saturating and decaying. These structures were found to be dominant sound producers in the mixing region of forced jets [1, 2]. Several analytical models for analyzing the emitted sound by these structures have been offered over the years. Ffowcs Williams and Kempton [3] used Lighthill's analogy to determine the magnitude of the radiated sound. Tam and Burton [4, 5] used a linear stability analysis for calculating the sound field emitted by a slowly expanding supersonic shear layer and Crighton and Huerre [6] looked at the appearance of superdirectivity in the sound field of low subsonic shear layers. The purpose of this study is to produce simple analytical approximations for the sound field, which the previous studies seem to lack. There is a need for such approximations not just for the physical understanding of the sound field but also as a design and validation tool for computational and experimental aeroacoustics. The model suggested by Crighton and Huerre [5] seems to be the most appropriate for this aim. It treats the problem as a boundary value problem where the effect of the wave packet comes from the boundary condition. They have already succeeded in estimating the penetration distance of the transition region between the near field and the far field of a superdirective sound field of a low subsonic Gaussian wave packet. This study will give simple approximations for the near field and far field pressure for a subsonic and a supersonic wave packet. It will investigate the effect of the convective Mach number and different wave packet shapes on the penetration distance of the transition region in the transverse and longitudinal directions
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