The near wall effect of synthetic jets in a laminar boundary layer

An experimental investigation to analyse the qualitative near wall effect of synthetic jets in a laminar boundary layer has been conducted for the purpose of identifying the types of vortical structures likely to have delayed separation on a 2D circular cylinder model described in this paper. In the first instance, dye visualisation of the synthetic jet structures was facilitated in conjunction with a stereoscopic imaging system to provide a unique quasi three-dimensional identification of the vortical structures. Secondly, the impact of synthetic jet structures along the wall was analysed using a thermochromic liquid crystal-based instantaneous convective heat transfer sensing system, whereby the liquid crystals change colour in response to the thermal footprints of a passing flow structure. Of the different vortical structures produced as a result of varying actuator operating and freestream conditions, the footprints of hairpin vortices and stretched vortex rings revealed a marked similarity with the oil flow pattern of a vortex pair interacting with the separation line on the circular cylinder model. For both structures, the role of the suction stroke plays an important part in their formation and subsequent development. Conditions were established For the formation of these structures in non-dimensional parameter space.