Excitation of parametric instabilities in statically stable and unstable fluid interfaces

Abstract

By enforcing an oscillatory motion on superimposed fluids perpendicular to their interface, one can excite parametric instabilities, i.e., the unstable growth of interface or surface modes. Both the oscillation parameters and the wavelengths of the excited parametric instabilities depend on the viscosity and the surface tension of the fluids. This has been measured for the interface between air and various liquids and compared with theoretical predictions from Troyon and Gruber. The position of the liquids was either normal (air upon liquid) or inverted (liquid superimposed upon air). In the latter case the oscillation parameters had to be limited to those beyond the threshold for dynamic stabilization of the Rayleigh-Taylor instability. The applied frequencies were in the range between 30 and 4000 Hz, the surface tension was varied between 10 and 100 dyn/cm, and the viscosity between 10−2 and 10 P. Typical liquids were mineral oils, water, and gasoline.