A Study of Heat Transport Processes in the Wake of a Stationary and Oscillating Circular Cylinder Using Digital Particle Image Velocimetry/Thermometry

Citation

Park, Han G. (1998) A Study of Heat Transport Processes in the Wake of a Stationary and Oscillating Circular Cylinder Using Digital Particle Image Velocimetry/Thermometry. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/C9KN-RQ12. https://resolver.caltech.edu/CaltechETD:etd-04132004-150955

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. An experimental investigation is carried out on the processes of heat transfer associated with a heated circular cylinder in crossflow. Two studies are made. First, a study of the transport of heat in the near wake (x/D<5) of a stationary and transversely oscillated cylinder is made at Reynolds number of 610. Second, a study is made of the surface heat transfer from a cylinder which is undergoing forced oscillations in the transverse direction. The studies are made using the technique of Digital Particle Image Velocimetry/Thermometry (DPIV/T) which allows simultaneous measurements of both the velocity and temperature fields of the flow. The temperature is measured by seeding the flow with thermochromic liquid crystal (TLC) particles which change their reflected wavelength as function of temperature. By calibrating reflected wavelength versus temperature using a color multi-CCD camera, the local temperature of the flow may be deduced. The velocity is measured by using the same particles as Lagrangian flow tracers, and local velocity or displacement of the flow may be measured by cross-correlating two sequential images. A limitation of DPIV/T, which is the low level of precision (5% - 20% of the temperature span of TLC particles), may be overcome by a process in which the temperature at a given location is computed by averaging the temperatures of the particles within a specified sampling window. This process increases the precision to 2% - 10%. In the study of the heat transport in the near wake, the velocity and temperature measurements obtained from DPIV/T are decomposed into their mean, coherent, and incoherent components using the triple decomposition. It is found that the heat from the cylinder is transported down the wake mostly by the mean heat flux and is laterally transported out of the wake by the coherent and the incoherent heat fluxes. In examining the direction of the turbulent heat flux vectors, the vectors are found not to be co-linear with the gradient of mean temperature. This misalignment implies that the gradient transport models are inappropriate for modeling the turbulent heat transport in the near wake of a circular cylinder. In examining the production of turbulence, it is found that that kinetic energy fluctuations are produced in the saddle regions (regions where the fluid is being stretched in one direction and compressed in another) while the temperature fluctuations are produced at the edges of center regions (regions where the fluid is rotating), i.e., the edges of the vortex cores. From the study of the heat convection from a cylinder as function of forced oscillation frequency [...] and amplitudes (A/D=0.1, 0.2), it is found that besides the previously known increase near the natural vortex shedding frequency, there also exists a large increase in the heat transfer at approximately three times this frequency for A/D=0.1. For A/D=0.2, there exist large increases at roughly two and three times the natural vortex shedding frequency. From a DPIV/T study, it is found that the wake pattern becomes synchronized with the mechanical oscillation of the cylinder at these frequencies where the heat transfer increases significantly. At the frequencies corresponding to roughly two and three times the unforced vortex shedding frequency, the wake pattern may become synchronized by processes of period doubling and tripling with respect to the cylinder oscillation period, respectively. The increase in the heat transfer rate is found to correlate with the distance at which vortices roll-up behind the cylinder. The distance is observed to decrease sharply at the frequencies corresponding to a sharp increase in the heat transfer. Therefore, the near wake is found to play a critical role in the heat transfer from the surface of a circular cylinder, and the cause of the increase in heat transfer is believed to the removal of the stagnant and low heat convecting fluid at the base of the cylinder during the roll-up of the vortices.

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