The relationship between near-wake structure and heat transfer for an oscillating circular cylinder in cross-flow

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 process in the wake. The cylinder's heat transfer coefficient was determined for oscillation amplitudes up to 1.5 cylinder diameters and oscillation frequencies up to 5 times the stationary cylinder natural shedding frequency. Digital particle image thermometry/velocimetry (DPIT/V) was used to measure the temperature and velocity fields in the near-wake for a subset of cases. The experiments were carried out in a water tunnel at a Reynolds number of 690.; Previously observed heat transfer enhancement at frequencies near the natural shedding frequency and its harmonics were shown to be limited to amplitudes of less than about 0.5 cylinder diameters. In addition, several new heat transfer effects were observed. The wake mode, a label indicating the number and type 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 trajectories of vortices during roll-up, explain this relationship. The streamwise spacing between shed vortices also affects heat transfer coefficient, possibly by influencing entrainment of freestream temperature fluid by the forming vortices, thereby affecting the temperature gradient at the cylinder base. The cylinder's transverse velocity was shown to influence the heat transfer by affecting the circulation of the wake vortices. Also, the cylinder's transverse velocity varies the orientation of the wake with respect to the freestream flow, thereby spreading the main source of heat transfer enhancement—the vortices near the cylinder base—over a larger portion of the cylinder surface.; A new phenomenon was discovered in which the wake structure switches back and forth between distinct wake modes. Temperature induced variations in the fluid viscosity are believed to be the cause of this mode-switching. It is hypothesized that the viscosity variations change the vorticity and kinetic energy fluxes into the wake, thereby changing the wake mode and the heat transfer coefficient.