Zhiyin Yang

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.

Computational analysis and flow structure of a transitional separated-reattached flow over a surface mounted obstacle and a forward-facing step

Large-eddy simulation (LES) of transitional separating-reattaching flow on a two-dimensional square surface mounted obstacle and a forward facing step has been performed using a dynamic sub-grid scale model. The Reynolds number based on the uniform inlet velocity and the obstacle/step height is 4.5 × 103. The mean LES results for both the obstacle and step flow compare reasonably well with the available experimental and DNS data. The flow structures upstream of the surface-mounted obstacle (referred to hereafter as obstacle) and the forward-facing step (referred to hereafter as FFS) consist of unstable two-dimensional structures and coherent rib-shaped structures. These structures with the aid of 3D streamline visualisation strongly indicate that the upstream separation bubble is a closed one rather than an open one in the sense that there is little evidence to suggest that there is fluid injection from the upstream separation region into the downstream separated region for the two geometries. The spectra and time history for the velocities and pressure fields at locations immediately upstream of the obstacle and FFS (including the recirculation region) were analysed using both the Fourier and wavelet transforms and revealed the unsteady nature of the recirculation region upstream of the obstacle and FFS. The transition process has been elucidated using both 2D and 3D flow visualisation of the flow. In both geometries (obstacle and FFS), the separated boundary layer downstream of the leading edge shows 2D nature and roll-up shortly downstream of the separation line leading to 2D K-H rolls to be shed from the leading edge. Coherent structures such as the λ-shaped and rib-like vortices commonly associated with a flat plate boundary layer and also found in the separated-reattached flow of a blunt leading edge plate aligned horizontally to a flow are not common in the separated-reattached flow over the obstacle and FFS.

Large eddy simulation of fully developed turbulent flow in a rotating pipe

Large eddy simulation (LES) has been applied to study the fully developed turbulent pipe flow, in particular, to examine the effects of swirl driven by the rotating wall of the pipe. Experimental observations have shown that the intensity of turbulence in the rotating pipe decreases gradually with an increase in rotation rate due to the stabilizing effect of the centrifugal force. These experimentally observed phenomena are confirmed numerically using LES by comparing not only mean velocity profiles but also turbulent intensity and Reynolds stresses at two different rotation rates. The performance of two different subgrid scale models, a dynamical model and the usual Smagorinsky model, has also been assessed for the case of fully developed turbulent swirling flow. A brief description of the numerical methods used with an efficient hybrid Fourier multigrid pressure solver is presented. Particular attention has been paid to the numerical treatment of boundary conditions at the centreline.

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.

Numerical Study of a Separated-Reattached Flow on a Blunt Plate

Large-eddy simulation (LES) of transitional separatingreattaching flow on a flat plate with a blunt leading edge has been performed. The Reynolds number based on the uniform inlet velocity and the plate thickness is 6.5103. A dynamic subgrid-scale model is employed in the transitional flow case. The LES results compare reasonably well with the available experimental data. The entire transition process has been visualized by using the LES data, and large-scale vortical structures have been observed at different stages of transition. It is known that different vortex shedding frequencies exist, especially the so-called low-frequency flapping when there is a separation bubble. It is not clear whether all transitional and turbulent separatingreattaching flows have different vortex shedding frequencies. It is also not clear what the working mechanisms are behind the so-called low-frequency flapping as reported widely. These issues are addressed.

Numerical study of the primary instability in a separated boundary layer transition under elevated free-stream turbulence

Numerical studies of laminar-to-turbulent transition in a separation bubble subjected to two free-stream turbulence levels (FST) have been performed using Large-Eddy Simulation (LES). Separation of the laminar boundary layer occurs at a curvature change over a plate with a semi-circular leading edge at Re = 3450 based on the plate thickness and the uniform inlet velocity. A numerical trip is used to produce the targeted free-stream turbulence levels and the decay of free-stream turbulence is well predicted. A dynamic sub-grid-scale model is employed in the current study and a good agreement has been obtained between the LES results and the experimental data. Detailed analysis of the LES data has been carried out to investigate the primary instability mechanism. The flow visualisations and spectral analysis of the separated shear layer reveal that the 2D Kelvin-Helmholtz instability mode, well known to occur at low FST levels, is bypassed at higher levels leading to earlier breakdown to turbulence.

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.

Large-eddy simulation of buoyancy-driven natural ventilation in an enclosure with a point heat source

Rising buoyant plumes from a point heat source in a naturally ventilated enclosure have been investigated using large-eddy simulation (LES). The aim of the work is to assess the performance and the accuracy of LES for modelling buoyancy-driven displacement ventilation of an enclosure and to shed more light on the transitional behaviour of the plume and the coherent structures involved. The Smagorinsky sub-grid scale model is used for the unresolved small-scale turbulence. The Rayleigh number, Ra is chosen to be in the range where spatial transition from laminar to turbulent flow takes place (Ra = 1.5 × 109). The plume properties (source strength and rate of spread) as well as the ventilation properties (stratification height and temperature of stratified layer) estimated using the theory of Linden et al. are found to agree reasonably well with the LES results. The variation of the plume width with height indicates a linear variation of the entrainment coefficient rather than a constant value used by Linden et al. for a fully turbulent thermal plume. Flow visualisation revealed the nature of the large-scale coherent structures involved in the transition to turbulence in the plume. The most excited modes observed in the velocity, pressure and temperature fields spectra correspond to Strouhal number in the range 0.3 ≤ St ≤ 0.55 which is in agreement with those observed by Zhou et al. for a turbulent forced plume. Excited modes less than thisvalue (St = 0.2) were observed and may be due to low-frequency motions felt throughout the flow.

Aerodynamics of battle damaged finite aspect ratio wings

Wind-tunnel tests have been carried out on a battle-damaged NACA 641412 half-wing aspect ratio of 8.2. The simulated gunfire damage had a diameter of 0.2 wing chord and was located at midchord and at one of two spanwise locations. Tests were carried out at a Reynolds number of 5:5 105. Compared with an undamaged wing, the damage resulted in reduced lift, increased drag and a positive increase in pitching moment at zero lift. Moving the damage to near the tip reduced the magnitude of these effects. Using the static pressure difference between the upper and lower surfaces of the undamaged wing allowed the data from the present study to be successfully compared with previously published drag and lift data for a two-dimensional damaged airfoil. Tests on wings with aspect ratios of 6.2 and 10.3 produced similar trends in the aerodynamic characteristics and showed that the use of static pressure difference was equally effective in allowing comparisons with two-dimensional data.

Large Eddy Simulation of Isothermal Confined Swirling Flow with Recirculation

ABSTRACT The present paper reports progress in applying Large Eddy Simulation (LES) techniques to the analysis of a flow problem relevant to Lean-Premixed-Prevapourised (LPP) combustor designs currently under investigation. Such flows combine high levels of swirl with flow confinement geometries that induce a central recirculation zone (CRZ). The turbulent mixing processes in these flows are difficult to capture accurately using RANS statistical turbulence models. In addition, the required precise control of fuel/air ratio means that detailed knowledge of the unsteady fluctuating behaviour is required, hence the interest in the ability of LES methods for flow simulation. Details are presented of the particular LES algorithm adopted, together with two benchmark problems (one including swirl) that have been used for code validation purposes. The multi-block structured mesh LES method is then applied to the isothermal flow in a typical LPP pre-mixer geometry. The instantaneous behaviour predicted when the calculations have reached a statistically stationary state is analysed by examination of time histories at selected points. These show evidence of extremely large-scale oscillations of the CRZ not obvious in the time-averaged flow pattern. Some indications of swirl precession are also observed. Time-mean data evaluated by averaging the LES predictions show good agreement with measurements and an improved level of fidelity compared to RANS eddy viscosity predictions. This encourages further use of the LES approach for this type of flow, for example by extension to scalar mixing predictions.