Water Tunnel Experiments on Three Dimensional Separation Bubbles on a Flat Plate

Abstract

Three-dimensional laminar separation bubbles generated on a at plate by a threedimensional displacement body were investigated in a water tunnel for a Reynolds number range of ReD = 2; 000 to ReD = 30; 000 based on displacement body thickness, D. The separation bubble was generated on a at plate to exclude complex surface curvature e ects. Boundary layer suction was applied on the surface of the displacement body in order to prevent ow separation from the displacement body. Several displacement bodies with di erent aspect ratios ( = 0:5 3) were used to allow for a variation of the spanwise extent of the imposed pressure gradient. The in uence of the pressure gradient on the separation bubble was also investigated by changing the distance of the displacement body from the at plate. Velocity vector eld measurements using a Particle Image Velocimetry (PIV) system as well as dye ow visualizations were employed for investigating the physical mechanisms governing the dynamics of the three-dimensional separation bubble. With these techniques we were able to identify the reverse ow region in the bubble and obtained limiting streamline patterns. The topologies of the separation bubbles are documented for di erent Reynolds numbers and di erent pressure gradients. For a Reynolds number of ReD = 5; 000 and an aspect ratio of = 0:5 the separation bubble and the wake were found to be steady without any shedding. For higher Reynolds numbers the bubble was found to shed vortical structures which possibly resulted from an inviscid Kelvin-Helmholtz instability. These instabilities of the separated shear layer in combination with a slowly varying reverse ow magnitude were found to likely be the cause of a so-called bubble breathing which manifests itself as an intermittent change of the dimensions and intensity of the separation bubble. The streamline pattern of the three-dimensional separated region for a higher aspect ratio ( = 2) was found to be similar to that of low aspect ratio wings at large angles of attack (close to stall).