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)
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
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.
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.
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
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
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).