Martin Skote

Direct numerical simulation of a separated turbulent boundary layer

Direct numerical simulation of two turbulent boundary layer flows has been performed. The boundary layers are both subject to a strong adverse pressure gradient. In one case a separation bubble is created while in the other the boundary layer is everywhere attached. The data from the simulations are used to investigate scaling laws near the wall, a crucial concept in turbulence models. Theoretical work concerning the inner region in a boundary layer under an adverse pressure gradient is reviewed and extended to the case of separation. Excellent agreement between theory and data from the direct numerical simulation is found in the viscous sub-layer, while a qualitative agreement is obtained for the overlap region.

Direct Numerical Simulation of Self-Similar Turbulent Boundary Layers in Adverse Pressure Gradients

Direct numerical simulations of the Navier–Stokes equations have been carried out with the objective of studying turbulent boundary layers in adverse pressure gradients. The boundary layer flows concerned are of the equilibrium type which makes the analysis simpler and the results can be compared with earlier experiments and simulations. This type of turbulent boundary layers also permits an analysis of the equation of motion to predict separation. The linear analysis based on the assumption of asymptotically high Reynolds number gives results that are not applicable to finite Reynolds number flows. A different non-linear approach is presented to obtain a useful relation between the freestream variation and other mean flow parameters. Comparison of turbulent statistics from the zero pressure gradient case and two adverse pressure gradient cases shows the development of an outer peak in the turbulent energy in agreement with experiment. The turbulent flows have also been investigated using a differential Reynolds stress model. Profiles for velocity and turbulence quantities obtained from the direct numerical simulations were used as initial data. The initial transients in the model predictions vanished rapidly. The model predictions are compared with the direct simulations and low Reynolds number effects are investigated.

Varicose instabilities in turbulent boundary layers

An investigation of a model of turbulence generation in the wall region of a turbulent boundary layer is made through direct numerical simulations. The model is based on the varicose instability of a streak. First, a laminar boundary layer disturbed by a continuous blowing through a slot is simulated in order to reproduce and further investigate the results reported from the experiments of Acarlar and Smith [J. Fluid Mech. 175, 43 (1987)]. An isolated streak with an inflectional profile is generated that becomes unstable, resulting in a train of horseshoe vortices. The frequency of the vortex generation is equal to the experimental results. Comparison of the instability characteristics to those predicted through an Orr-Sommerfeld analysis are in good agreement. Second, a direct numerical simulation of a turbulent boundary layer is performed to point out the similarities between the horseshoe vortices in a turbulent and a laminar boundary layer. The characteristics of streaks and the vortical structures surrounding them in a turbulent boundary layer compare well with the model streak. The results of the present study show that one mechanism for the generation of horseshoe vortices in turbulent boundary layers is related to a normal inflectional instability of the streaks.

Reynolds stress budgets in Couette and boundary layer flows

Reynolds stress budgets for both Couette and boundary layer flows are evaluated and presented. Data are taken from direct numerical simulations of rotating and non-rotating plane turbulent Couette flow and turbulent boundary layer with and without adverse pressure gradient. Comparison of the total shear stress for the two types of flows suggests that the Couette case may be regarded as the high Reynolds number limit for the boundary layer flow close to the wall. The limit values of turbulence statistics close to the wall for the boundary layer for increasing Reynolds number approach the corresponding Couette flow values. The direction of rotation is chosen so that it has a stabilizing effect, whereas the adverse pressure gradient is destabilizing. The pressure-strain rate tensor in the Couette flow case is presented for a split into slow, rapid and Stokes terms. Most of the influence from rotation is located to the region close to the wall, and both the slow and rapid parts are affected. The anisotropy for the boundary layer decreases for higher Reynolds number, reflecting the larger separation of scales, and becomes close to that for Couette flow. The adverse pressure gradient has a strong weakening effect on the anisotropy. All of the data presented here are available on the web [36].

Turbulent boundary layer flow subject to streamwise oscillation of spanwise wall-velocity

Direct numerical simulations have been performed to study the effect of a stationary distribution of spanwise wall-velocity that oscillates in the streamwise direction on a turbulent boundary layer. For the first time, a spatially developing flow with this type of forcing is studied. The part of the boundary layer which flows over the alternating wall-velocity section is greatly affected with a drag reduction close to 50% which exhibits an oscillatory distribution with a wavenumber which is twice that of the imposed wall-velocity. The maximum in drag reduction occurs where the wall velocity is at its maximum (or minimum) and the minimum occurs where the wall velocity is zero. Comparisons of the mean spanwise velocity profiles with the analytical solution to the laminar Navier-Stokes equations show very good agreement. The streamwise velocity profile indicates a thickening of the viscous sub-layer when scaled with the local friction velocity and an upward shifting of the logarithmic region when scaled with...

Simulations of the linear plasma synthetic jet actuator utilizing a modified Suzen-Huang model

The linear plasma synthetic jet actuator (L-PSJA) is a unique form of flow control device which harnesses the interaction of induced flows from two linear plasma actuators to form an upward jet. Since each injection can be manipulated in intensity, the synthetic jet has thrust vectoring properties. Our study simulates the L-PSJA by utilizing a modified Suzen-Huang (S-H) model that accounts for drift and diffusive properties in the surface charge. The results of the present model show that the centreline velocity is closer to the experimental values found in literature as compared to the default form of S-H modelling. Thrust vectoring simulations were also performed to demonstrate the feasibility of flow directional variation in the L-PSJA.

Direct numerical simulation of a turbulent boundary layer over an oscillating wall

Direct Numerical Simulations have been performed to study the effect of a partially oscillating wall on the turbulent boundary layer. Even though the Reynolds number is three times lower than in previous experimental investigations, many of the characteristic flow features are confirmed. The drag reduction is of the same magnitude as for higher Reynolds number flows, and the spatial development follows closely earlier experimental findings. The reduction of Reynolds shear stress is more pronounced than the decrease in streamwise and normal velocity fluctuations. In addition, comparisons are made with earlier numerical studies of channel flow. The sensitivity of the Reynolds shear stress on the time allowed for statistics collection is scrutinised, and the discrepancy in results from earlier experiments are, thus, explained.

Boundary Condition Modifications of the Suzen-Huang Plasma Actuator Model

The accuracy of the Suzen-Huang (S-H) model is improved by altering the boundary condition of the dielectric surface above the lower electrode. For the equation governing the electric field, we introduce a ‘dielectric shielding’ condition at the same region, which results in a spread of the electric field strength along the dielectric surface. For the equation governing the surface charge density, we introduce boundary conditions that modify the behavior of the charge density variable in the S-H model. The conditions represent a fitting procedure by adding the features of propagation and dissipation in a one-dimensional Fokker-Plank equation. The equation is initiated by a normal distribution function centered at the leading edge of the lower electrode. These modifications improved model results by about 50% when comparing the maximum induced velocity value with experimental results. Furthermore, charge density growth is propagating in a similar manner to that obtained by charge transport models.

Effects of the scalar parameters in the Suzen‐Huang model on plasma actuator characteristics

Purpose – For the past decade, plasma actuators have been identified as a subset in the realm of active flow control devices. As research into plasma actuators continues to mature, computational modelling is needed to complement the investigation of the actuators. This paper seeks to address these issues.Design/methodology/approach – In this study, the Suzen‐Huang model is chosen because of its ability to simulate both the charge density and Lorentz body force. Its advantages and limitations have been identified with a parametric study of two constants used in the modelling: the Debye length (λD) and the maximum charge density value (ρc* ). By varying the two scalars, the effects of charge density, body force and induced velocity are examined.Findings – The results show that the non‐dimensionalised body force (Fb*) is nonlinearly dependent on Debye length. However, a linear variation of Fb* is observed with increasing values of maximum charge density. The optimized form of the Suzen‐Huang model shows bett...

Study of lift enhancing mechanisms via comparison of two distinct flapping patterns in the dragonfly sympetrum flaveolum

The computational fluid dynamic model of a live-sized dragonfly (Sympetrum flaveolum) hindwing is simulated according to the in-flight flapping motions measured in kinematic experiments. The flapping motion of the simulated wing is accomplished by dynamically re-gridding the wing-fluid mesh according to the established kinematic model for each flapping pattern. Comparisons between two distinct flapping patterns (double figure-eight and simple figure-eight) are studied via analysis of the aerodynamic forces and flow field structures. The result shows that additional lift is generated during supination and upstroke for the double figure-eight pattern, while maximum thrust is generated during pronation for the simple figure-eight pattern. In addition, through our comparisons of the different kinematics, we are able to reveal the mechanism behind the leading edge vortex stabilization prior to supination and the kinematic movement responsible for additional lift generation during supination. By increasing the ...