Session S10: Separated Flows: Simulations

Separation Bubble Subject to a Compliant Surface: A Linear Approach

This study aims to build the understanding of the interaction between a separation bubble and compliant surface. As the lighter and active materials are used more in the modern aircraft design, the increasing compliance of the wings also increases the risk of instability. Looking into a canonical problem with controlled separation induced by suction and blowing atop the compliant membrane, we analyze how different physical properties and boundary conditions affect the interaction between the compliant surface and the separation bubble. Assuming small perturbation, a linear model is built to allow fast iteration of the fluid-structure interaction system. The role of surface compliancy on the boundary layer structure is investigated and with the insight from the unsteady dynamics of separation, we present potential controller designs for the compliant wing to suppress or facilitate the separation.   

Vortex formation at the apex of an oblique cone in uniform cross-stream

The present investigation is concentrating on the dynamics of the vortex in the wake region of the cone for high angle of attacks. As the flow built up from the tip toward the base, it is associated with the swept boundary layer, 3D coherent and turbulent structures, asymmetric vortices and no fixed length scale. Based on the base diameter, the study was conducted for moderate to higher Reynolds number with LES and a wall function. Semi-body conformal grid has been generated for higher accuracy and resolution near the body. The tip of the cone has been found very sensitive to the coherent structures formed downstream of the flow. The instantaneous frequencies, wavelengths, turbulent structure, and shedding angles have analyzed in detail to characterize the flow over a cone. The continuous variation of the cone diameter regulate the changes in the shedding frequencies along the span of the cone.   

Effect of corner rounding on the flow around a 5:1 rectangular cylinder. A paradox explained?

The high Reynolds number flow around a rectangular cylinder, having chord-to-depth ratio equal to 5, is the object of the benchmark BARC, collecting several experimental and numerical results. This configuration is characterized, in particular, by flow separation at the upstream corners and reattachment on the cylinder side. A large dispersion was observed in the numerical predictions of the flow features and quantities on the cylinder lateral sides. Sensitivity studies carried out by the BARC contributors were not conclusive and in some cases controversial. In particular, in LES perfomed by different groups it was found that increasing grid resolution or decreasing subgrid-scale dissipation leads to a deterioration of the agreement with the experiments with a too short mean recirculation region on the cylinder side. We show that this paradox can be explained by the fact that the upstream corners in the numerical simulations are perfectly sharp while they have a certain degree of roundness in experiments. Indeed, highly-resolved LES are presented showing that even very small values of the corner curvature radius have a dramatic impact on the numerical solution on the cylinder sides leading to a very good agreement with experimental data.   

RANS simulations of a turbulent separated flow validation test-case.

A new validation bump-geometry for turbulent separation has been analyzed experimentally, in our $3'\times3'$ wind-tunnel, and computationally, by solving the Reynolds-averaged Navier-Stokes (RANS) equations using the Spalart-Allmaras model. The new bump geometry is defined by a Gaussian profile in the streamwise direction and an error function in the spanwise direction, such that the turbulent flow over the bump separates, under the adverse pressure gradient, in the downstream region of the bump. Sensitivities of skin-friction and pressure coefficients, as well as the extent of separation to various geometric and flow parameters (e.g., bump height to length ratio, distance of the bump from the top-wall to length ratio, boundary layer thickness of incoming flow on bottom and top walls) have been analyzed. The study has been performed for a range of Reynolds number varying between $0.59 \times 10^6 \leq Re_L \leq 3.5 \times 10^6$, where L is the streamwise length of the bump. The results obtained by using different turbulence models have also been compared. Initial two-dimensional RANS simulations show discrepancies between the pressure coefficients computed via RANS and the experiments.   

Wall-modeled LES of flow around a prolate spheroid at various angles of attack

We conduct wall-modeled large-eddy simulations (WMLES) of flow around a 6:1 prolate spheroid at various angles of attack (AOA) from $10^\circ$ to $30^\circ$. An equilibrium wall model is used with a coarse mesh deploying 10 to 20 cells within one local boundary layer thickness. The Reynolds number based on the long axis length and the freestream velocity is $4.2\times 10^6$. This flow was extensively examined experimentally by Simpson et al. in the 90s. We examine the pressure coefficients at a few streamwise locations in the azimuthal direction, as well as the locations of flow separation. Our WMLES results agree reasonably well with the previous measurements, and regions of secondary separation are also captured. WMLES provide more detailed flow information than Reynolds Averaged Navier Stokes. In our simulations, a pair of counter-rotating vortices is reproduced. These vortices originate from the leeward surface and extends in the streamwise direction, leading to notable downwash. The resulting wake is non-axisymmetric. Skewness of the wake is measured and compared to predictions of a lift line model.   

High Schmidt Number Washout of Sodium Hypochlorite or a Viscosifying Solute Shielded by Topography.

In many cleaning applications, including washing of fresh-cut produce, surface topography shields soluble contaminants from ``washout.'' Here, we report computations of washout by two-dimensional flow downstream of a backward-facing step, with a contaminant initially confined to a square domain just downstream of the step, with edge length equal to the step height. We consider three cases. In the first, we focus on removal of NaClO for Reynolds numbers (\textit{Re}) of 10 and 100 (based on step height) at a Schmidt number (\textit{Sc}) of 2650. For \textit{Re} $=$ 100, there is a time in the washout process when the maximum concentration shifts from the relatively inaccessible bottom corner behind the step, to a point in the interior of the recirculation zone downstream of the step. The size of the recirculation zone affects washout efficacy, corresponding to a strong \textit{Re} dependence on \textit{Re}. In the second case, we model washout of organic exudate that leaks from freshly cut produce, using data for the viscosifying sugar acid sodium gluconate (NaG) at \textit{Re} $=$ 100 and an infinite-dilution \textit{Sc} of 4165, accounting for the dependence of viscosity on concentration. Finally, we show results for the case in which NaClO is present in the free-stream, and reacts with NaG downstream of the step.   

Lagrangian Flow Separation in External Aerodynamics

Kinematic aspects of flow separation in external aerodynamics are investigated by revealing the initial motion of upwelling fluid material from the wall and its relation to the long-term attracting manifolds in the flow field. With direct numerical simulations of a circular cylinder and a cambered NACA 65(1)-412 airfoil, the location of initial fluid upwelling, the so-called spiking point, is determined from the curvature of advected material lines and from high-order numerical derivatives of the wall-normal velocity. While the short-time kinematics are governed by the formation of a material spike upstream of the zero-skin-friction point, over longer times the trajectories of the fluid tracers are guided by attracting ridges in the finite-time Lyapunov exponents once they leave the vicinity of the wall. The combination of initial fluid upwelling, asymptotic separation line, and attracting Lagrangian Coherent Structures draws a comprehensive picture of the mechanics of flow separation in external aerodynamics.