Flow regimes in a trapped vortex cell

This paper presents results of an experimental investigation on the flow in a trapped vortex cell embedded into a flat plate and interacting with a zero pressure-gradient boundary layer. The objective of the work is to describe the flow features and elucidate some of the governing physical mechanisms, in light of recent investigations on flow separation control using vortex cells. Hot-wire velocity measurements of the shear layer bounding the cell and of the boundary layers upstream and downstream are reported, together with spectral and correlation analyses of wall-pressure fluctuation measurements. Smoke flow visualisations are reported to provide qualitative insight into some relevant features of the internal flow, namely a large-scale flow unsteadiness and possible mechanisms driving the rotation of the vortex core. Results are presented for two very dierent regimes: a low Reynolds number case where the incoming boundary layer is laminar and its momentum thickness is small compared to the cell opening, and a moderately high Reynolds number case, where the incoming boundary layer is turbulent and the ratio between the momentum thickness and the opening length is signicantly larger than in the rst case. Implications of the present findings to flow control application of trapped vortex cells are also discussed.