Investigation of Low Frequency Unsteadiness in the Turbulent Separation Bubble Produced by a Wall Mounted Hump

The low frequency unsteadiness in a turbulent separation bubble (TSB) produced by a wall-mounted hump is investigated with pressure measurements and particle image velocimetry (PIV) at a free-stream velocity of 30m/s and chord based Reynolds number of 1 · 106. Proper orthogonal decomposition (POD) of velocity fields in the streamwise/wall-normal plane shows a dominant first mode containing approximately 1/3 of the total turbulent kinetic energy. Comparison of the first mode to time resolved pressure fluctuations indicates a connection to a low frequency contraction and expansion ("breathing") of the TSB. Additionally, POD of the velocity fields in three spanwise/wall-normal planes within the separation bubble show large periodic structures and mode pairs suggesting meandering structures in the spanwise direction. Low order reconstruction confirms spanwise undulations similar to peak/valley formations with a wavelength of λz = 0.4c. The cumulative energy of the first four POD modes in the spanwise planes reaches energy levels comparable to that of the first mode in the streamwise plane. The temporal frequency of the observed structures is investigated by spectral proper decomposition (SPOD) of time resolved PIV data. The SPOD confirms the low frequency character of the structures observed via POD, with a similar wavelength at a Strouhal number based on bubble length of St = 0.018.