Mechanisms of Enhanced Heat Transfer for Oscillating Circular Cylinders

Experiments were conducted to determine the relationship between wake structure and heat transfer for an oscillating circular cylinder in cross-flow. An internally heated cylinder was suspended in a water tunnel and oscillated transverse to the freestream. The cylinder’s heat transfer coefficient was measured over a wide range of oscillation amplitudes and frequencies. By comparing these results to the known wake mode regions in the amplitude-frequency plane, relationships between wake mode and heat transfer were identified. Representative cases were investigated further by using digital particle image thermometry/velocimetry (DPIT/V) to simultaneously measure the temperature and velocity fields in the near-wake. This revealed more detail about the mechanisms of heat transfer enhancement. The dynamics of the vortex formation process, including the trajectories of the vortices during roll-up, are the primary cause of the heat transfer enhancement.

Viscous Forces Acting on Irregularly Oscillating Circular Cylinders and Flat Plates

In the present paper hydrodynamic forces acting on oscillating circular cylinders as well as flat plates perpendicular to the motion in regular and irregular modes due to vortex shedding are experimentally investigated in order to clarify the memory effect or history effect on the hydrodynamic forces. The experimental results for sinusoidal motion show that the drag and added mass coefficients in steady-state condition are significantly different from those of the first swing from the start because of the existence of old vortices generated in the previous cycles around the cylinder. It is found from the experiments in irregular modes that the drag and added mass coefficients significantly vary every swing. It is confirmed that these coefficients during a swing depend on the relative amplitude or the Keulegan-Carpenter number of the previous swings as well as that of the swing under consideration.