Synchronization of Vortex Shedding and Heat Transfer Enhancement Over a Heated Cylinder Oscillating With Small Amplitude in Streamwise Direction

Experiments are performed to study the flow structure and heat transfer over a heated cylinder oscillating radially with small amplitude in streamwise direction. Both flow visualization using a smoke wire in the upstream and the local heat transfer measurements based on wall temperatures around the cylinder were made. The excitation frequencies of the cylinder are selected at $Fe/Fn=0,$ 0.5, 1, 1.5, 2, 2.5, and 3. The oscillation amplitude selected is less than a threshold value of $A/D=0.06$ where synchronization of vortex shedding with the cylinder excitation was not expected. However, experiments indicate that synchronization still occurs which stimulates a great interest to study its enhancement in the heat transfer. Synchronization occurred at $Fe/Fn=2$ is antisymmetric vortex formation while synchronization at $Fe/Fn=2.5$ and 3 is symmetric type. The forward motion (advancing into the cross flow) of the cylinder during one cycle of oscillation has an effect to suppress the instability and the vortex formation. This leads to the occurrence of a smaller and symmetric vortex formation and a less enhancement of heat transfer than the case of antisymmetric type $Fe/Fn=2.$ For excitations at lower frequencies $Fe/Fn⩽1.5,$ all the vortex formations occurred are mostly antisymmetric. The dominant mode of the instability in the shear layer is actually the natural shedding frequency $Fn$ of the vortex. A closer excitation frequency to $2Fn$ causes a greater enhancement in the heat transfer. During the experiments, the Reynolds numbers varies from 1600 to 3200, the dimensionless amplitude $A/D$ from 0.048 to 0.016.