Marc Gohlke

On experimental sensitivity analysis of the turbulent wake from an axisymmetric blunt trailing edge

The sensitivity to local disturbances of the turbulent wake over a 3D blunt body with an axisymmetric detachment is investigated at Re = 2.1·10 4. The flow presents a favored m = 2 azimuthal symmetry set by two wings. The instantaneous wake is measured either above or below the plane containing the wings but leads a statistical symmetric wake. Topology shifts are random but occur mostly after a large number of global mode periods. The statistical symmetry is highly sensitive to any asymmetric disturbance. As a consequence, depending on its position a small control cylinder in the close wake fixes the wake to one asymmetric topology affecting shedding activity and drag. The effect of an axisymmetric perturbation (m = 0) on flow topology and dynamics is also studied ; it induces significant drag reductions and global mode modifications when acting on mixing layers. Whatever the disturbance, the sensitivity of the wake seems concentrated into the mixing layers and may depend more on their local turbulent characteristics than on the inviscid dynamics of vorticity.

Bi-stability in the turbulent wake past parallelepiped bodies with various aspect ratios and wall effects

The turbulent wake past parallelepiped bodies with a rectangular blunt trailing edge of height H and width W is investigated in wall proximity: various aspect ratios H* = H/W ∈ [0.51, 1.63] and ground clearances C* = C/W ∈ [0, 1.00] are explored at a Reynolds number of 4.5 × 104 based on the body width W. Base pressure measurements and particle image velocimetry show that the close wake often undergoes antisymmetric instabilities that can be either in the lateral direction (parallel to the wall) or in the vertical direction (normal to the wall). The instantaneous wake presents preferred states with high degrees of asymmetry; in some configurations, topology shifts are observed after long time scales Tl ∼ 103W/U0 leading to bistable behaviors. The effect of ground proximity is thoroughly studied for H* = 0.74 corresponding to the reference Ahmed geometry and for a case with a height dimension larger than its width H* = 1/0.74 = 1.34. When C* > 0.08, it is found that the Ahmed body is bistable in the latera...

Thorough analysis of vortical structures in the flow around a yawed bluff body

In the present study numerical investigations are used as a complementary approach to analyse large coherent structures in the flow around a bluff-body under cross-wind. The results from the numerical simulation are validated through wind-tunnel experiments and used to analyse unresolved observations from these experimental investigations. This underlines the duality of these two approaches in the analysis of the aerodynamics of a bluff-body. Vortical structures in a three-dimensional, asymmetric and turbulent wake are detected through the use of different vortex detection schemes (vorticity, Q, λ2 and Γ3), permitting to isolate flow structures. The vortex properties of two main lateral vortices are described and discussed on the basis of envelopes detected by the λ2 and the Γ3 criterion. Although the flow is highly three-dimensional and complex, the vortex properties are quite simple and show mechanisms like vortex stretching and bursting. These two criteria lead qualitatively to the same results; quantitative differences are due to the envelope diameter and are therefore observed only on integral flow quantities.

Effect of unsteady separation on an automotive bluff-body in cross-wind

This paper is treating the dynamic effects of cross-wind on a 3D bluff-body. It is shown that contrary to the mean flow, it is not the upper vortex that predominates the forces, but the flow generated and piloted by the front lee-strut wake. It is made responsible for the appearance of a frequency peak in the bluff-body force spectra. Furthermore it is shown that the intensity as well as the frequency of this peak depend on the yawing angle. Experimental and numerical results are used to analyse the flow dynamics.