Unsteady laminar mixed convection heat transfer from a horizontal isothermal cylinder in contra-flow: Buoyancy and wall proximity effects on the flow response and wake structure

Particle image velocimetry (PIV) measurements are carried out in an experimental investigation of laminar opposing mixed convection to assess the thermal effects on the wake of an isothermal circular cylinder placed horizontally and confined inside a vertical closed-loop downward rectangular water channel. The buoyancy effect on the flow distributions are revealed for flow conditions with Reynolds number based on cylinder diameter of Re = 170, blockage ratio, D/H = 0.287, aspect ratio, L/D = 6.97 and values of the buoyancy parameter (Richardson number) in the range −1 ⩽ Ri ⩽ 5. In this work, flow distributions are presented in the form of mean and instantaneous contours of velocity and vorticity. To elucidate the effects of the lateral wall proximity effect and cylinder aspect ratio, separation angle, wake structure behind the cylinder, recirculation bubble length, time traces of velocity fluctuation, Strouhal number and vortex shedding modes are obtained as a function of the Richardson number. The results reported herein demonstrate how the flow structure and vortex shedding pattern are significantly modified by the wall confinement and thermal effects. In addition, our measurements show that for assisted buoyancy (Ri = −1), the breakdown of the Kármán vortex street takes place and vortex shedding is completely suppressed.