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EXPERIMENTAL INVESTIGATION OF TURBULENT WAKE BEHIND A SPHERE AT A SUBCRITICAL REYNOLDS NUMBER

Young II Jang
Department of Aerospace and Mechanical Engineering, Korea Air Force Academy P.O.Box 335-2, Cheongwon, Choongbuk, 363-849, Korea

Sang Joon Lee
Department of Mechanical Engineering Advanced Fluids Engineering Research Center Pohang University of Science and Technology San31, Hyoja-dong, Pohang, 790-784, Republic of Korea

Abstract

The flow characteristics of near-wake and turbulent flow over a sphere at a subcritical flow regime were experimentally investigated. The qualitative and quantitative results obtained from smoke-wire visualization, PIV measurements and POD(proper orthogonal decomposition) modal analysis were used to get detailed flow information such as laminar to turbulent transition, vortex formation, vortex shedding, shear-layer instability, wavy structure of wake and coherent structures of the sphere wake. The detailed turbulent structure of sphere wake such as recirculating flow, shear layer instability, vortex roll-up, and small-scale turbulent eddies were clearly visualized. In the streamwise center plane, mean velocity field had two large-scale recirculation vortices, of which the recirculation length was about x/d = 1.05. In the cross-sectional planes, the instantaneous vorticity fields revealed an unsteady wavy structure of the sphere wake. The regions of large values of turbulence statistics such as turbulent intensities and turbulent kinetic energy were closely related to the onset of shear layer instability in the near-wake behind the sphere model. That is, the location of the local maximum turbulence statistics both in the streamwise and cross-sectional planes corresponded to the onset location of shear layer instability observed in the visualized flow fields at x/d = 1.0~1.2. The relative contribution of the POD mode 1, 2 and 3 in eigenvalues was 26, 11, and 8 %, respectively. The velocity fields for the POD mode 1 at the near-wake region of x/d = 0.7−1.4 in the cross-sectional planes showed similar pattern with those of time-averaged mean velocity fields. In addition, the unique pattern of sweeping flow in the region from x/d = 1.5 to x/d = 2.0 showed the wavy structure of the sphere wake along the streamwise direction. This implies that the wavy pattern is the dominant coherent structure of the sphere wake. These experimental results on sphere near-wake would contribute to the fundamental understanding of the turbulent wake flow behind a sphere and provide useful data for validating numerical prediction at a subcritical flow regime.