Low Frequency Dynamics of a Pressure-Gradient Induced Turbulent Separation Bubble*

A pressure-gradient induced (PGI) turbulent separation bubble (TSB) is experimentally investigated on an elliptic leading edge flat-plate model at a Reynolds number based on the momentum thickness of the incoming turbulent boundary layer of approximately Re_Θ0 = 640. Separation of the incoming turbulent boundary layer is caused by a suction-only boundary condition from a small cutout on the top wall of the test section imposed by an axial fan. Surface oil flow visualization is used to capture the evolution and dimensions of the PGI-TSB, highlighting the relevant features of the surface flow field. Unsteady pressure measurements in the experiment show significant energy at Strouhal numbers (St_Lsep= fL_sep/U₀) of approximately 0.47 and 0.007 which is close to what is documented in the literature to be associated to the high frequency shear-layer shedding and low frequency ‘breathing’ mode of a PGI-TSB, respectively. Synchronous low-speed PIV (non-time resolved) combined with time-resolved unsteady surface pressure measurements near the mid-span of the flat plate model are acquired to utilize the spectral analysis modal method (DOI: 10.1007/s00348-020-03057-8) to determine the dynamically coherent modes associated to the two characteristic frequencies of the PGI-TSB.