Near wake structure behind a 3D square prism with the aspect ratio of 2.7 in a shallow boundary layer flow

Three dimensional structures of unsteady flows behind a square prism with aspect ratio H/D=2.7 in a shallow boundary layer flow are investigated by two dimensional and three dimensional stereoscopic PIV techniques synchronizing with velocity measurement by a hot wire anemometer. According to the conditional PIV image sampling technique conducted by using the velocity signal measured outside the separated shear layer, it becomes clear that an arch-type vortex is formed behind the prism throughout a cycle of the Kلrmلn vortex formation and shedding. As the vortex grows and moves downstream, the vortex line stretches in a stream-wise direction near the tip of the vortex which does not move and stays still just behind the prism throughout a cycle. According to POD analysis, the wake fluctuates very slowly with a period of approximately 6 times of that of the Kلrmلn vortex shedding.