Putting a sound stop to convection

The phenomenon of dynamic stabilization can be demonstrated with an inverted pendulum: If the pivot point vibrates fast enough and strongly enough, the pendulum aligns with the vibration direction and can stably stand upside down, even at an angle, seeming to defy gravity. Physicists Greg Swift and Scott Backhaus (Los Alamos National Laboratory) looked at an analogous situation with gas in a so-called pulse tube that has one end much hotter than the other. Colder gas is denser and therefore sinks below the hotter gas; a vertical tube with the cold end down is like an undisturbed pendulum with the heavy bob at the bottom. However, raise the cold end above the hot end and convection sets in—the cold gas falls due to gravity and the hot gas rises in a natural convective flow. Such orientation-dependent effects are undesirable for cryogenic thermoacoustic pulse-tube refrigerators, like the commercial one shown here, in which the gas is used to transmit acoustic power but not heat. (For more on thermoacoustics, see Physics Today, July 1995, page 22.) Swift and Backhaus found that suppression of convection when these refrigerators run at high enough frequency and amplitude is related to the well-understood stabilization of the inverted pendulum. Although their experiments and theoretical analysis are beginning to unravel the essentially nonlinear physics at the core of the system, many mysteries remain, including the actual role of the oscillating pressure. (G. W. Swift, S. Backhaus, J. Acoust. Soc. Am. 126, 2273, 2009.) —Stephen G. Benka