The propulsive effect of thermal waves

The use of thermal waves as a potential propulsion method has been investigated. The test flow system involves two horizontal parallel plates, one fixed and exposed to thermal waves and the other allowed to move. The propulsive effect is quantified using the velocity of this plate. It is shown that, in general, the plate moves opposite to the wave direction, but its characteristics depend on whether the waves have subcritical or supercritical amplitudes. In the case of subcritical amplitudes, the plate velocity varies proportionally to the square of the wave amplitude, and its most effective wavelength corresponds to the wave number α≈2. In the case of supercritical amplitudes, the system response is affected by nonlinear thermal streaming, which produces faster plate movement. The plate velocity variations as a function of the wave velocity exhibit hysteresis with respect to changing wave direction; multiple solutions are possible for nominally identical flow conditions, with some of the solution branches terminating at limit points.