The formation of harmonics at frequencies below a monochromatic pump frequency in a system of capillary-gravity waves on surfaces of liquid hydrogen and superfluid helium in a turbulent regime is studied experimentally. By choosing the spectral characteristics of the exciting force and the resolution in the spectrum of the surface oscillations, it is possible to create conditions for low-frequency wave generation by changing the boundaries of the experimental cell. For certain monochromatic pump frequencies, low-frequency harmonics are observed on liquid hydrogen surfaces only in a rectangular cell. Energy transfer to the low-frequency subharmonics, as well as to high-frequency harmonics, is caused by three-wave decay processes. An inverse cascade develops on superfluid helium surfaces in a cylindrical cell as a result of three-wave decay processes, with about 90% of the energy concentrated in the inverse cascade.

The results of studies on instability of the charged and neutral surfaces of liquid hydrogen and helium are discussed. The charged surface of a fluid loses stability in an electric field, when its intensity reaches a critical value, and the neutral surface becomes unstable when the flow rate of the fluid under the surface reaches a certain critical value. In both cases, the surface is affected by an additional pressure which is responsible for developing the instability.