The effect of freestream turbulence integral length scale on junction flow behavior

Junction flow is a common feature in many flows of practical and natural importance. At the junction of a wall-mounted obstacle and the wall, significant unsteadiness exists due to the presence of the horseshoe vortex. This is a coherent vortex structure which forms in front of the leading edge of the obstacle and is wrapped around the obstacle by the incoming flow. In turbulent flow, the horseshoe vortex experiences highly unsteady and non-periodic large scale motions and breakdown events. This dynamic behavior contributes to significant dynamic pressure loading on the obstacle and augmented heat transfer on the local wall and leading edge surfaces. While recent studies of the horseshoe vortex have suggested that significant interaction may occur between freestream turbulence and the horseshoe vortex, the effect of freestream turbulence length scale on the behavior of the horseshoe vortex has not yet been systematically determined. This study investigates the effect of variation in streamwise integral turbulent length scale on the dynamics of the horseshoe vortex system in front of a symmetric wing for a range of Reynolds numbers, turbulence intensities, and integral length scales. Time-resolved stereo particle image velocimetry (SPIV) measurements show that for the range of parameters studied, unsteadiness in the junction is only affected by integral length scale under the conditions of low body thickness Reynolds number and large turbulence intensity. A vorticity tracking technique is used to show that this effect is due to the greater rate of impingement of large turbulence features at the wing leading edge with increased integral length scale under these conditions.