Wake Flow Instability Studies Behind Discrete Roughness Elements on a Generic Re-Entry Capsule

Numerical and experimental results on disturbance growth in the wake flow downstream of a roughness element submerged in the boundary layer of a typical re-entry capsule at an angle of attack are presented. Laminar basic flow computations were conducted at Mach 5.9 freestream conditions of the Hypersonic Ludwieg Tube Braunschweig (HLB) for different roughness heights, widths and planform shapes. The modal instability characteristics of the wake flow were studied by spatial two-dimensional eigenvalue analysis and three-dimensional parabolized stability equations. For all cases considered, the varicose wake modes are most amplified in terms of maximum N-factors. A comparison of the disturbance kinetic energy production terms reveals that the wall-normal shear is mainly responsible for the dominant instability modes. A flush-mounted high frequency pressure transducer (PCB) is used to measure the wall-pressure fluctuation of the instability modes behind a cylindrical element at various Reynolds numbers. A comparison of the spectral distributions of pressure fluctuations with and without roughness clearly shows the appearance of distinct spectral humps.