The purpose of the work is the discovery and the study of peculiarities of the dynamics of a hydro-mechanical system under the presence of periodic influences possessing no predominant direction in space under the performance a straightforward motion by the liquid part of the system.

The methods. The analytical investigational methods for the equations of solid bodies motion and boundary problems for Navier–Stokes equation are used.

The results. A new problem on the hydro-mechanical system motion under external nonstationary influences is formulated and solved. The system consists of a viscous liquid and three solid bodies that are a wall, a plate and a membrane which is permeable for the liquid. The wall fulfils a prescribed motion. The plate and the membrane are free. Important new circumstances in the formulation of the problem in particular are the possibikity for the liquid to realize only a straightforward motion and that the motion of the membrane [s not prescribed.

The conclusion. The obtained results can be used in particular in a scientific search of ways to control hydro-mechanical systems.