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Dr. Gordon Smith starts a research program in Thermoacoustics.
 The newest
faculty member of the physics department is Dr. Gordon Smith, who
brings a new research thrust for the department.
Dr. Smith received his doctorate in Physics from the University of
Mississippi in 2000. Upon graduating, he took a series of Visiting
Assistant Professor positions at Hampden-Sydney College and
Appalachian State University before accepting a tenure-track position
here at WKU. His research specialty is in the niche field of
thermoacoustics. Thermoacoustics is the study of how heat and sound
interplay. In the standard university physics courses, sound behavior
in a resonator is simply described as depending solely on where one is
along the length of the resonator. This neglects an important reality
of gases – namely that they are viscous. We also typically neglect the
thermal effects included when you compress or rarefy a gas.
When these effects are included, though, there is a slight, but
important change in the gas behavior. Near the wall (close enough for
a thermal effect to be seen, but far enough away that viscosity
doesn’t dominate), the oscillating gas trades heat back and forth with
the wall. Outside of this region (or about the remaining 99% of the
system), the gas pretty much behaves as described in the university
physics sequence.
Thermoacoustics takes a small region in the resonator, and replaces
the empty resonator with a porous material that is effectively all
wall, so that the thermoacoustic exchange is effectively utilized.
This material is referred to as a “stack.”
Applying a temperature gradient across this stack creates a prime
mover class of heat engine, resulting in the thermoacoustic generation
of sound. Gas moves toward the warmer end of the stack, expanding and
jostling neighboring gas parcels. Gas moving toward the cooler end
contracts, and also jostles the neighboring parcels. As this is in a
resonator, the random motion is amplified at the resonating frequency
of the system, and the gas rapidly transitions from low-amplitude,
random oscillation to a large amplitude resonant oscillation.
It’s a neat party trick.
The importance of thermoacoustics arises when one considers the
reverse of a prime mover – a refrigerator. Consider an acoustic system
with a stack in the resonator. The acoustics forces the gas back and
forth, oscillating the temperature as it compresses and rarefies.
These temperature fluctuations induce a thermal exchange with the
wall, and the net effect is similar to a bucket brigade model, where
the gas picks up heat from one end of the wall, transports it, and
drops it of on the other end. This results in one end of the stack
cooling off and the other end warming up.
Using a thermoacoustic prime mover to drive a thermoacoustic
refrigerator, one can create a refrigerator that operates with no
moving parts, without environmentally hazardous gases.
The Physics Department is excited to explore this innovative
technology.
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