Wind and Sea

Heat Transport Processes
Heat Transport Processes in the Wake of Stationary and Oscillating Circular Cylinders
(click on images below to see a larger image)

Tait Pottebaum

(Image at left) Temperature of the wake behind a heated circular cylinder (Re=610) as visualized by Thermochromic Liquid Crystals (TLC) particles. Blue denotes hot; red, cold; green, warm. The hot fluid from the boundary layer form the shear layers which then roll-up into the vortices.

We are developing the technique of Digital Particle Image Velocimetry/Thermometry (DPIV/T) to study the process of heat transport in unsteady flows. DPIV/T allows us to measure simultaneously both the instantaneous 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 as in standard DPIV.

Some of the findings from a study of the heat transport in the near wake of a stationary and oscillating circular cylinder in cross-flow using DPIV/T are:

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. parision of the direction of the phase averaged molecular (green vectors) and incoherent heat flux vectors (blue vectors) in the wake of a stationary cylinder.

The kinetic energy production occurs 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. Besides the previously known increase near the natural vortex shedding frequency for transverse oscillations, there also exist large increase in the heat transfer at certain super-harmonics.

(Image at left) Surface heat transfer versus cylinder oscillation frequency for Re=550.

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. 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.

Stagnant and non-heat convecting fluid near the base of cylinder for stationary case (low heat transfer). The velocity near the base of the cylinder is small as compared to case of oscillation (Image at left).

Vortices scrubbing away fluid near the base of the cylinder for case of cylinder oscillating near the vortex shedding frequency. Note the close roll-up of the vortex and the large velocity near the base of the cylinder as compared to the stationary case (Image at right).

This material is based upon work supported by the National Science Foundation under Grant No. 9417520 and 9903346. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation (NSF).