INFLUENCE OF CAVITATION ON TURBULENT SEPARATED FLOW

Abstract

Attention was focused on the interaction between vortex and cavitation as a basic study on modeling unsteady turbulent flows with cavitation. First, separated shear layer with cavitation were directly simulated. Cavitation caused by spanwise and streamwise vortices, which are typical features in high shear layer, was represented by a simple model. Experimentally observed tendencies in the location of vortex formation, the frequency of vortex shedding and the intensity of Reynolds stresses were reasonably reproduced. Influence of cavitation on a typical example of streamwise vortex was observed. It was found that the vorticity in the core of Burgers type vortex was significantly reduced. To establish a description of it, we used an artificially maintained Burgers vortex. As a result, an assumption of constant circulation along a closed circle expanding due to sudden development of cavity could represent the modification of the vortex. INTRODUCTION Flows in hydro-machineries are affected by various types of cavitation. Since 1990’s, several methods have been proposed for the numerical simulation of flow fields including unsteady cavitation. Attached (sheet) cavitation and cloud (bubble) cavitation have been reasonably reproduced. However, the influence of turbulence has not been taken into account completely in previous simulations although most of cavitating flows are turbulent. This causes inaccuracy in predicting the cavitation inception because the local minimum of pressure is thought to be corresponding to the core of turbulence vortices. On the other hand, the effect of cavitation on turbulence has usually been omitted. Therefore, to establish the computational method for turbulent cavitating flows, the interaction between cavitation and turbulence vortices should be correctly modeled. The aim of our study is to address the modeling strategies considering two questions. (1) How are fine-scale vortices in turbulence related to cavitation inception? (2) How does the cavitation modify turbulent vortices and turbulence statistics? Since both are interactive phenomena, they must be analyzed by two-way methodology. One of typical and appropriate examples for our objective is the separated flow in the wake of a thin fence in a two-dimensional channel. Iyer and Cessio (2002) visualized various types of vortex cavitation and they reported the turbulence statistics in the wake region. In the first half of this report, we show the results of the direct numerical simulation with cavitation model, which was developed by Okita and Kajishima (2002). Our result is compared with experimental observation by Iyer and Cessio (2002) qualitatively, because the flow configuration was simplified in our simulation. Then, we investigate the relationship between cavitation and vortical structure; namely, primary (spanwise) vortices and the secondary (streamwise) vortices. In the second half, the interaction between a Burgers vortex and cavitation is directly simulated in an idealized situation. One reason is that streamwise vortices in a shear layer, simulated in the former part, has a profile of Burgers vortex. Moreover, the finest scale eddies in fully developed turbulence has been found to be Burgers type. A Burgers vortex in a uniform stream is given at the inlet cross section of the computational domain for this purpose. Reducing the cavitation number, the structure of vortex decaying by cavitation inception is analyzed. Then we give a phenomenological model that represents the vortical structure during this process. OUTLINE OF COMPUTATION The procedure including cavitation model and numerical method should fit in with the spatio-temporal scale of unsteady motion of vortices in the turbulent shear layer. In