Nonlinear random waves in fluid, and the main mechanism of their excitation

To describe the problem of the random nonlinear waves in fluid, we must know, exactly or approximately, how occurs the process of the vortex separation. For this it is conveniently to use models based on physical considerations and (or) some experimental data. The main attention in this review will be attended to random waves, emerging, for example, at stall flutter. Such waves often appear in fluid, and they are the main cause of many disasters in seas and oceans. As a rule, stall flutter is connected with the pulling phenomenon, and observed in systems with two and (or) more degrees of freedom. In principle, in such systems both approximately one-frequency (synchronous) mode, and many-frequency (asynchronous) modes (when each mode oscillates with its natural frequency) are possible. But in the case of the pulling phenomenon only one-frequency mode, corresponding to its natural frequency (see [1]) is stable. Unlike to usual turbulence stall flutter is a self-oscillatory process. The feedback in this process appears due to interaction between the fluid and the streamline body. It should be noted that wave motions in fluid can be of very complex character. In last years a great interest appears to waves of an anomalously high amplitude – so called freak-waves, and rogue-waves. We assume that the main cause of such waves is also vortex separation.