Investigation Of Unsteady Supercavitating Flow Behind Bodies

The supercavity technology can significantly avoid the viscous drag between water and the body moving in water, and for some case the drag of supercavitating body is possible to reach the values comparable as in the air. It is necessary to keep a steady supercavity's shape, for reducing the drag when the bodies moving in the unsteady flow filed, and the necessity for controlling and adjusting the unsteady supercavity's shapes comes up. Therefore, it is necessary to study the principles as to unsteady supercavity.A numerical discretization method solving the singular integral equations is employed for studying the problems of unsymmetrical hydrofoil with steady supercavity. Under the frame of potential flow theory, with the help of method of integro-differential equation, unsteady supercavities of the slender symmetrical wedge and slender cone are studied here respectively, with the unsteady caused by the angles of the wedge and cone being changed and the angles of them being changed in the as sinusoidal impulse, cosine impulse and uniformly increasing or decreasing ways, and consequently, the histories as to the shapes of the corresponding unsteady supercavities were acquired. With the bodies being the same as above, the unsteady supercavities induced by the variational cavitation numbers were studied too. A time lag property is being presented, comparing with the quasi-steady supercavities.Employing the method of integro-differential equation, the unsteady supercavities induced by the variational motions of the symmetrical wedge and slender cone were studied, with the variational moving velocities being sinusoidal or cosine impulse changing and uniformly increasing or decreasing. With acquiring the shape and length of the unsteady supercavities induced by the variational motion, and comparing with the quasi-steady supercavities, the former ones exhibit time lag, and the time lag augmented as the impulse frequency or acceleration being increased.Considering the influence of the gravity, and as for the problems as to vertical supercavities of the thin wedge and slender cone, the length of the water-entry supercavity is longer than the water-yielding supercavity, if neglecting the gravity and being the same of the cavitation number on the level of the separation point as to the supercavity and cavitator. Being the same of the incoming folw's velocity, the cavitation number of ventilation supercavity is smaller than the nature supercavity's, another presentation with the same meaning is, the smaller velocity is needed for ventilation ones when keeping the cavitation number being the same, and consequently, the former ones'Frouder number is smaller, that is to say, the gravity influence the ventilation ones much more.The critical Froude number as to water-entry supercavity has been searched, with the help of principle of independent expansion. With that tool, the unsteady supercavities were studied respectively, induced by variational moving disc and size-varying disc. Corresponding to various motion conditions, the unsteady supercaivties shape were obtained, and curves about the time and length of the unsteady and quasi-steady supercavitatis were also acquired; and the results exhibit that the shape and length's change of the unsteady supercavities are lag in time to the quasi-steady ones. Some elementary analysis as to the supercavity's stability has been done, with the help of linear theory.