Visualization of turbulent flow around a sphere at subcritical reynolds numbers

Abstract

The shear layer evolution and turbulent structure of near-wake behind a sphere at Re=11,000 and 5,300 were investigated using a smoke-wire visualization method. A laminar flow separation was found to occur near the equator. The smooth laminar shear layers appeared to be axisymmetrically stable to the downstream location of about x/d=1.0 at Re=11,000 and x/d=1.7∼1.8 at Re=5,300, respectively. At Re=11,000, the vortex ring-shaped protrusions were observed with the onset of shear layer instability. Moreover, the transition from laminar to turbulence in the separated flow region occurred earlier at the hiher Reynolds number of Re=11,000 than at Re=5,300. The PIV measurements in the streamwise and cross-sectional planes at Re=11,000 clearly revealed the turbulent structures of the sphere wake such as recirculating flow, shear layer instability, vortex roll-up, and small-scale turbulent eddies.