VORTEX IDENTIFICATION TECHNOLOGY AND ITS APPLICATION IN THE WAKE FIELD OF MARINE PROPELLER

Vortex identification is a very important problem of fluid and flow, in order to find a reasonable method of vortex identification in the wake of marine propeller, this paper studies the theory of six kinds of vortex recognition technology combined with practice, in which analytic solutions of both Burgers vortex and Lamb-Oseen vortex are also used for necessary explanation. The advantages and disadvantages of various vortex identification methods are discussed in detail at the angle of theory and application. The local low-pressure criterion is intuitive, but it is obviously insufficient after considering viscous and unsteady effects. The path line or streamline criterion obviously cannot satisfy Galileo invariance, which will cause confusion in vortex identification. The magnitude of vorticity criterion needs to specify its threshold value, which has certain uncertainty, and can also incorrectly identify vortex sheets that are not vortices. The complex eigenvalue of the velocity gradient tensor will also have an unrecognized region. The second invariant criterion of the velocity gradient tensor and the second eigenvalue criterion of the symmetric tensor can better identify the vortex core, and these two criteria are sometimes equivalent. The numerical simulation of propeller wake is implemented on the open source software OpenFOAM platform. The large eddy model is modeled by a local dynamic equation, which is better than the dynamic Smagorinsky model to a certain extent. The results of numerical experiment show that, for the vortex identification in the marine propeller wake, the second invariant criterion of the velocity gradient tensor is consistent with the second eigenvalue criterion of the symmetric tensor. However, the local minimum pressure criterion, streamline or path line criterion, vorticity magnitude criterion and complex eigenvalue criterion of velocity gradient tensor have some defects, which are not suitable for vortex identification in the wake of marine propeller.