Co-rotating equal strength vortex pair merging in pressure gradient environment

Experiments were conducted in order to understand the effect of pressure gradient on the merging process of co-rotating equal strength vortex pair. Cross-flow velocity measurements were performed at a series of downstream locations under zero, favorable and adverse pressure gradient conditions. The main finding is that the pressure gradient has a vastly effect on the merging process. As the merging environment changed from favorable pressure gradient (FPG) to adverse pressure gradient (APG), the increasing of the vortex spacing was observed in the first diffusive stage, which lead to a delayed convective stage. However, at the convective stage where the spacing of vortex pair center decreased, the decreasing process gradually speed up from FPG to APG accompanied with a faster vorticity transportation and diffusion process so the merging process is accordingly accelerated. On the other hand, the vortex orientation difference was not as conspicuous as the vortex center separation distance.