Heat and fluid flow around a cylinder in wake

The flow and heat transfer topology of an isothermal cylinder (diameter D) in the wake of another smaller (diameter d) cylinder is numerically investigated at a Reynolds number of 200 for a cylinder spacing ratio L/d = 5.5. The focus is given on how the size of the upstream cylinder influences the flow and heat transfer topology around the downstream cylinder when the diameter ratio d/D is varied from 0.24 to 1.0. The upstream-cylinder shear layers for d/D < 0.4 reattach on the downstream cylinder and shed vortices behind it. They for 0.4 ≤ d/D ≤ 1.0, on the other hand, shed vortices in the gap, and the wake of the downstream cylinder features a primary vortex street followed by a secondary vortex street. The primary street evolves into the secondary street through merging and isolated-shedding processes. The heat transfer enhances when d/D is decreased from 1.0 to 0.4, and it declines for a further decrease in d/D. The increase in heat transfer is attributed to an increased shear layer velocity, reduced wake recirculation size, and enhanced recirculation in wake bubble. A novel tertiary frequency in heat transfer fluctuation is identified. The correlation between the flow and heat transfer fluctuation at the tertiary frequency is explicatively done through first Fourier transform.