Peter R. Voke

Numerical study of bypass transition

A large‐eddy simulation has been performed of a flat‐plate boundary layer undergoing transition to turbulence under free‐stream turbulence at a level of 6%. The properties of the simulated transition match those found experimentally: not only is the position and length of transition in agreement with available data, but the mechanism of transition also appears to correspond closely, since disturbances seen in the laminar layer prior to transition are found in the simulation. Statistical data have been gathered that allow the computation of all terms in the derived equations for the Reynolds stresses at four x stations. Aspects of these balances are presented that allow new insights into the physical mechanisms at work. The importance of the wall‐normal component of free‐stream turbulence and the timing of the redistribution of energy into this component through the fluctuating pressure field are revealed.

Subgrid-scale modelling at low mesh reynolds number

Subgrid-scale models are derived for large-eddy simulations in the limit of low mesh Reynolds number, or, equivalently, resolution approaching that required for full resolution of the simulated turbulent flow. The models are constructed from standard forms of the dissipation spectrum in a manner analogous to that used to derive the classical Smagorinsky-Lilly model from the inertial range spectrum. Practical methods for computing the subgrid-scale eddy viscosity are described, together with examples of the effects of using such models in a real simulation.

Numerical study of heat transfer from an impinging jet

Abstract A computational study of the impingement of a thermally inhomogeneous turbulent jet on a solid plate, using large-eddy simulation, is reported. We investigate the case of a plane jet of water issuing from a plane channel into an enclosed pool and impinging normally on a perspex plate 1.8 jet-widths downstream. It is shown that the dynamics of the turbulence in this particular geometry results in the temperature variations at the plate surface having very high lateral correlation, so that lateral conduction of heat within the plate fails to have any significant effect on the transmission of thermal fluctuations from the fluid into the plate. By this means a simple one-dimensional model of the thermal, interaction between the media may be justified.

Flow Past a Square Cylinder: Test Case LES2

A series of large-eddy simulations of flow past a square cylinder at Re = 21400 have been conducted by seven groups of researchers. A comparative report is presented of the time-average results with a view to establishing any clear advantage of particular subgrid scale modelling approaches, wall treatments, and numerical methods.

Large-eddy simulation of dispersion: comparison between elevated and ground-level sources

Large-eddy simulation (LES) is used to calculate the concentration fluctuations of passive plumes from an elevated source (ES) and a ground-level source (GLS) in a turbulent boundary layer over a rough wall. The mean concentration, relative fluctuations and spectra are found to be in good agreement with the wind-tunnel measurements for both ES and GLS. In particular, the calculated relative fluctuation level for GLS is quite satisfactory, suggesting that the LES is reliable and the calculated instantaneous data can be used for further post-processing. Animations are shown of the meandering of the plumes, which is one of the main features to the numerical simulations. Extreme value theory (EVT), in the form of the generalized Pareto distribution (GPD), is applied to model the upper tail of the probability density function of the concentration time series collected at many typical locations for GLS and ES from both LES and experiments. The relative maxima (defined as maximum concentration normalized by the local mean concentration) and return levels estimated from the numerical data are in good agreement with those from the experimental data. The relative maxima can be larger than 50. The success of the comparisons suggests that we can achieve significant insight into the physics of dispersion in turbulent flows by combining LES and EVT.

Large-Eddy simulation of turbulent flow over a rough surface

A family of wall models is proposed that exhibits moresatisfactory performance than previousmodels for the large-eddy simulation (LES) of the turbulentboundary layer over a rough surface.The time and horizontally averaged statistics such asmean vertical profiles of windvelocity, Reynolds stress, turbulent intensities, turbulentkinetic energy and alsospectra are compared with wind-tunnel experimental data.The purpose of the present study is to obtain simulatedturbulent flows that are comparable with wind-tunnelmeasurements for use as the wind environment for thenumerical prediction by LES of source dispersion in theneutral atmospheric boundary layer.

Large-eddy simulation of turbulent heat transport in enclosed impinging jets

The results of large-eddy simulation (LES) of thermally inhomogeneous jets issuing into an enclosed pool and impinging on a plate are presented. The LES has been performed by strongly conservative linear finite-volume techniques, with a simple subgrid-scale model. Several cases have been simulated with different geometries of the outflows at either side of the plate, or with different fluid properties. The mechanisms by which thermal eddies are formed and transported into the impingement zone have been fully elucidated for one of the simulated cases through graphic and video output of the thermal field. The reason for the very high lateral correlations found in the experiments and confirmed by LES results is explained convincingly in terms of the altered shape of thermal eddies as they convect toward the plate.

Large-eddy Simulation Studies of Bypass Transition

Abstract A large-eddy simulation has been performed of a boundary layer undergoing transition to turbulence owing to the presence of free-stream turbulence. The properties of the simulated transitional boundary layer are very similar to those found experimentally. Damping factors for popular closure models used to predict transition have been extracted from the large-eddy simulation, and are compared with three closure approximations, showing that all models overestimate the damping close to the wall. The results of numerical experiments (in the form of coarse simulations) are also reported, indicating the roles of the three perpendicular components of free-stream turbulence intensity, and of pressure fluctuations, in provoking bypass transition.

Mechanisms and Models of Boundary Layer Receptivity Deduced from Large-Eddy Simulation of By-pass Transition

An analysis has been performed of a large-eddy simulation of a flat-plate boundary layer undergoing by-pass transition due to a high level of free-stream turbulence. Data have been gathered that allow the computation of all terms in the equations for the Reynolds stresses, allowing new insights into the physical mechanisms at work in transiton under turbulence, and suggesting improvements to existing closure models for the process of by-pass transition.

A DYNAMIC SUBGRID-SCALE MODEL FOR LOW-REYNOLDS-NUMBER CHANNEL FLOW

SUMMARY Several issues related to applications of the dynamic subgrid-scale (SGS) model in large-eddy simulation (LES) at low Reynolds number are investigated. A modified formulation of the dynamic model is constructed and its perfoxmance in low-Reynolds-number LES of channel flow is assessed through a comparison of length scales computed respectively by this modified model, the German-Lilly dynamic SGS model and two empirical wall damping functions with optimum model coefficients, which have been successfully used in many simulations of channel flows. Two values of the ratio of filter widths are set for each of the dynamic models. The results have confirmed that the modified dynamic SGS model gives the correct behaviour of the subgrid eddy viscosity in the region of a plane wall to an accuracy that exceeds the best-tuned wall damping function, and almost collapses with the theoretical behaviour of the length scale near the wall without any tuning and adjustment. In addition, the impact of the choice of the ratio of filter widths on the modified dynamic SGS model is found to be much less than with the German-Lilly model.