Investigation of the three-dimensional dynamics of a turbulent separation bubble by means of volumetric velocity field measurements

A turbulent separation bubble (TSB) is formed when a turbulent boundary layer detaches from the surface and the resulting separated shear-layer reattaches further downstream. Planar velocity field measurements inside the TSB symmetry plane have shown in past studies that the dynamics of such flows are characterized by low-frequency fluctuations of the streamwise TSB extent. The objective in this study is to advance our knowledge regarding the time-dependent behaviour by identifying the three-dimensional representation of the low-frequency motion. To this end, volumetric particle tracking velocimetry measurements are conducted in an open-return blowdown wind tunnel (Re_θ ≈ 2000) where flow separation is produced in a backward-facing ramp test section with 600 mm span. Three-dimensional velocity field measurements are performed by supplying helium-filled soap bubbles to the flow and tracking them through space-time. The measurement data are assimilated using a physics-informed network that is constrained by the Navier-Stokes equations, and the dynamical analysis is based on spectral proper orthogonal decomposition. The latter returns multiple large-scale regions along the spanwise direction that capture anti-correlated streamwise velocity fluctuations at low frequency. This suggests that the instantaneous state of TSB expansion observed in the symmetry plane is accompanied by an increase in velocity near the side walls of the test section, and (vice versa) by a reduction for the contracted TSB. The findings of this study support the hypothesis that the low-frequency motion, now observed in several experimental configurations, is a fully three-dimensional phenomenon.