Separation control with elliptical air-jet vortex generators

Abstract

The flow organisation of air-jet vortex generators (AJVGs) of elliptical cross sections and their control effectiveness on a 24^{circ }-compression-ramp-induced shock-wave/boundary-layer interaction was analysed on the basis of experiments at M_{infty } = 2.52 and Re_{theta _c} = 8225. We investigated a circular orifice and two elliptical orifices of aspect ratios 0.5 and 2; all characterised by the same hydraulic diameter. Measurements of separation lengths from oil-flow visualisation and PIV reveal that elliptical AJVGs achieve a 25% reduction in total separation length, which constitutes a strong improvement over the 17% reduction achieved with the commonly used circular AJVGs. The jet-induced structures from elliptical AJVGs penetrate on average 25% farther into the boundary layer. However, the lateral spread is limited to a maximum value equal to the inter-jet spacing in the control array, which highlights the onset of jet/jet interactions between adjacent jets in the array. A consequence of these interactions is better flow entrainment for the elliptical cases, as observed in the mean boundary-layer velocity profiles and an improved turbulent mixing (indicated by an increase in Reynolds-shear-stress magnitude).