Investigation on the wake flow instability behind isolated roughness elements on the forebody of a blunt generic reentry capsule

Abstract

Numerical results on modal disturbance growth in the wake §ow downstream of a roughness element submerged in the boundary layer of a typical reentry capsule at an angle of attack are presented. Laminar base §ow computations were conducted for di¨erent roughness heights and planform shapes. The modal instability characteristics of the wake §ow were studied by spatial two-dimensional (2D) eigenvalue analysis. For all cases considered, the varicose wake modes are most ampli¦ed in terms of maximum N -factors, with the cylindrical roughness element being the most e¨ective shape.

Numerical Investigation of Roughness Effects on Transition on Spherical Capsules

“…Thus, immediate transition can be excluded. 18,48 The possible presence of modal as well as non-modal growth mechanisms in the roughness wake is investigated in Sec. V.B.…”

“…The code used to perform LST-2D has already been validated and tested in the case of wake flow instability behind isolated roughness elements. 17,18 No modal instability could be found in the boundary layer downstream of the roughness patch.…”

“…8 Numerical studies on modal disturbance growth in the wake of discrete roughness elements at unit Reynolds number conditions of the experiment showed that roughness element heights well above the mean roughness of the IR coating are required for the onset of a noteworthy modal disturbance growth in the wake flow. 17,18 For further studies of the distributed surface-roughness effects, a specifically designed micron-sized surface-roughness patch of 20 mm × 20 mm was fabricated and placed at the center of the capsule forebody. 16 The patch consists of uniformly spaced rectangular micron-sized roughness elements.…”

Stability Analysis of the Boundary Layer Developing on a Flat Plate with Discrete Roughness Elements in Hypersonic Flow

“…It can be considered to be a subset of a bigger domain which includes the complete flat plate. This approach has already been applied for the same analysis in the case of a cuboidal roughness element [22] and has been used in similar studies by other authors with successful results [9,16]. Avoiding the leading edge induced shock wave helps to reduce the computational effort needed to obtain a converged solution while adding flexibility to test different inflow conditions.…”

“…In all the cases analyzed, the dominant wake instability modes were found to be varicose (even) and sinuous (odd) deformations of the low-velocity streak that characterizes the wake flow structure. Moving to a more practical configuration, Theiss et al [16,17] performed 2D and PSE-3D stability computations in the wake behind different roughness elements located on the heat shield of a reentry capsule in a Mach 5.9 freestream. The varicose wake modes was found to be the one undergoing a higher amplification, with the cylindrical roughness element being the most effective shape.…”