Abstract-
The convective heat transfer coefficient of a circular cylinder in cross-flow can be significantly increased by transverse oscillations. This heat transfer enhancement has previously been correlated with changes in the vortex roll-up process in the near wake. However, the mechanism of these changes and how they affect heat transfer are not well understood.
A series of experiments were carried out in order to understand the relationship between wake structure and heat transfer for a transversely oscillating circular cylinder in cross-flow and to explore the dynamics of the vortex formation processes in the wake. The cylinder's heat transfer coefficient was measured over a wide range of oscillation amplitudes and frequencies, and the results were compared to established relationships between oscillation conditions and wake structure. Digital particle image thermometry/velocimetry (DPIT/V) was used to measure the temperature and velocity fields in the near wake for a limited set of cases chosen to be representative of the variety of wake structures that exist for this type of flow. The experiments were carried out in a water tunnel at a Reynolds number of 690.
It was found that wake structure and heat transfer both significantly effect one another. The wake mode, a classification system for the number and sign of vortices shed in each oscillation period, is directly related to the observed heat transfer enhancement. The dynamics of the vortex formation process, including the movement of the vortices during roll-up and the influence of previously shed vortices, provide the explanation for this relationship. The cylinder's transverse velocity is shown to influence the heat transfer by affecting the circulation of the wake vortices and the portion of the cylinder surface affected by the vortices.
A new phenomenon was discovered in which the wake structure switches back and forth between distinct wake modes. This does not occur for unheated cylinder wakes. Temperature induced variations in the boundary layer viscosity were identified as the mechanism of this mode-switching. This discovery underscores the role of viscosity in determining wake mode and may lead to an improved understanding of vortex formation and pinch-off processes for wakes in general.
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