Three-Dimensional Nature of Low-Frequency Unsteadiness in a Turbulent Separation Bubble

Three-dimensional behavior of low-frequency unsteadiness in the incompressible turbulent separation bubble (TSB) produced by a wall-mounted hump is investigated using time-resolved planar and stereoscopic particle image velocimetry measurements at several planes across the separated region. The aspect ratio (wind tunnel width/separation length, L-z,L-WT/L-b=4.4) provides nominally two-dimensional flow for more than half of the span wise extent of the test section. Analysis in the stream wise/wall-normal plane along the center of the test section shows low-frequency (St<0.1) large-scale motion of the separated region. The flow field contains features of both geometry-induced and pressure-gradient-induced separation, but unsteady dynamics produce dominant frequencies closer to geometry-induced TSBs (St approximate to 0.1) compared to purely pressure-gradient-induced TSBs (St approximate to 0.01). Measurements along the span wise direction and parallel to the initial shear layer development show strong evidence that the low-frequency motion is inherently three-dimensional, providing an additional dimension to the understanding of the flapping/breathing typically observed in planar stream wise/wall-normal measurements. Spectral proper orthogonal decomposition and low-order modeling identify span wise undulations with wavelengths of the order of L-b and frequencies of St<0.1. The three-dimensional behavior causes a peak/valley formation along the span and a more localized expansion/contraction, leading to only small variations in the integral volume of the TSB.