Three-dimensional flow features in a nominally two-dimensional rectangular cavity

The object of the present paper was to look at the three-dimensional features of a flow in a nominally two-dimensional rectangular cavity having the length-to-depth ratio equal 3, with laminar incoming boundary layer. The motivation was to give a contribution to the understanding of the origin of the spanwise flow structures observed in cavity flows. Particle image velocimetry was used to highlight the flow structure at low Reynolds number in the plane parallel to the cavity external flow and normal to the cavity bottom (vertical plane) and in planes parallel to cavity bottom (horizontal planes). The results demonstrate that the present measurements are qualitatively and quantitatively in agreement with published studies having as object the observation of the cavity flow behavior in the vertical plane, assuming the cavity to be nominally two-dimensional. Results in the planes parallel to the cavity bottom show the three-dimensional nature of the flow, characterized by the discontinuous presence of jet-like flows directed towards the cavity leading edge and towards the cavity trailing edge. The origin and nature of these discontinuous "jets" are examined and discussed in the paper. The flow structures are characterized by a wavelength scaling with the cavity length in proximity of the leading edge and with the cavity depth in the region of the trailing edge. Isolated three-dimensional flow structures consisting of side by side counter-rotating vortices have been also observed in the horizontal planes and tentatively interpreted as secondary flows induced by the instability of the main recirculation bubbl