Low Frequency Characteristics 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 = 760. Separation of the incoming turbulent boundary layer is caused by a suction-only boundary condition on the top wall of the test section imposed by an axial fan. The experiments utilize surface pressure measurements and two-dimensional Time-Resolved Particle Image Velocimetry (TR-PIV) to analyze the steady and unsteady characteristics of the PGI-TSB. The time-averaged surface pressure coefficient is compared between the experiment and a three-dimensional Direct Numerical Simulation (DNS) of a similar configuration. The surface pressure coefficients show good agreement between the experiment and the simulation. Unsteady pressure measurements in the experiment show significant energy content at Strouhal numbers of approximately 0.27 and 0.02, which are close to the medium-frequency shear layer shedding and low-frequency ‘breathing’ mode, respectively. Spectral Proper Orthogonal Decomposition (SPOD) of the TR-PIV vector fields reveal high energy content at a very low Strouhal number of, St ≈ 0.01, which is very close to the low-frequency peak in the spectra of the fluctuating surface pressure. The spatio-temporal structures at this low-frequency have a ‘breathing’ behavior. The shear layer shedding mode at St ≈ 0.24 is also revealed via SPOD. A low-order reconstruction of the flow-field using the ‘breathing’ and ‘shedding’ SPOD modes superimposed onto the mean shows low-frequency, large amplitude oscillations of the reversed flow region.