STATISTICAL PREDICTION OF UNSTEADY WATER JET FLOW BY CFD

摘要

In the preceding paper at AICFM-7, two of the authors have investigated the energy structure in a free water jet flow, and the possibility of predicting by CFD the jet geometry in Pelton turbines. Since the free water jet in air is unsteady by nature, its numerical geometry was fluctuating with time. We introduced the statistical treatment to the fluctuating numerical analysis. Based on a coupled numerical simulation of the conduit flow in a nozzle and the unsteady jet flow under the specified environmental air velocity, the unsteady characteristics of the jet flow was investigated statistically. After deciding the sampling interval and the number of samplings, the radius of water jet at the relevant sections was obtained by the distribution of kinetic energy along the radial coordinate for the every sampling data. In order to sharpen the resolution of the interface between the water and air, the volume of fluid was used as a weight for the kinetic energies in the grid cells. From the time-averaged jet geometry thus acquired, the important design parameters of the jet contraction position X_0, its radius R_0 and velocity C_0 as well as the expansion rate of jet radius k_(Rj) were predicted numerically.