Research On The Cloud Cavitation Of Hydrofoil And Control Mechanism

Cavitation is one kind of complex multiphase flows. It was first recognized by its negative effect. According to the morphology and physical properties, cavitation can be divided into five types:bubble cavitation, sheet cavitation, cloud cavitation, vortex cavitation (including the tip vortex and hub vortex cavitation,), etc. In these types, sheet cavitation is relatively stable, while other types have a very strong transient feature, along with the development of cavity and bubble collapse process. As the strong unsteady characteristics of cloud cavitation is more harmful, the mechanism of cloud cavitation has been a hot issue in the hydrodynamic field. In the present paper, hydrofoils are chosen for the computational model, and the mechanism of cloud cavitation, also its control mechanism by obstacles placed on the suction surface was simulated. Also cavitating flows in the state of high temperature and pressure have been studied. Based on these researches, some hydrodynamic features of the cavitating flow on hydrofoils are given.Cavitating flow has a relatively large phase density, and in cavitating region, the density is variable; so the solution algorithm is different from the single-phase flow. Based on previous studies, an improved pressure-based algorithm for cavitating flow was proposed. Continuity equation without the material derivative of density was established. Coupled with the momentum equation, an improved pressure correction equation which includes the phase change source was derived. In the present algorithm, the direct coupling of density and pressure was avoided, and the material conservation relation of phase change was determined by the cavitation model. Comparing with the experimental results, the effectiveness of the algorithm was verified.For cloud cavitating flow in normal temperature and pressure state, cavitating flow on2D and3D hydrofoils under different cavitation numbers and attack angles was simulated. The results show that, for the2D cavitating flow the reentrant jet is the main reason to make the sheet cavitation lose its instability and the cloud structure break off; Although under different cavitation numbers, geometric characteristics of the cavitation are different, but the shedding process is experienced same typical state:cavity arises, develops, breaks off, and collapses. Although the basic process of3D cavitating flow on hydrofoils is similar to the2D, it has some different features. The most typical feature of3D cavitating flow is the incoming of U-shaped structure of the shedding cloud which was observed in the experiment by many researchers, and the phenomenon can also be captured in the present simulations.3D cloud cavitation shedding is also due to the emergence and development of the reentrant jet, but the difference with2D is: due to the presence of the pressure gradient formed by the sidewalls, the side-entrant jet comes into being and accelerates the shedding process of the cloud structure, so the3D cloud cavitation is caused by the common effects of reentrant jet and side-entrant jet. In same flow parameters, comparing with the2D cavity, the3D cavity length decrease, while the shedding frequency increases.2D and3D cavitation flow simulations of hydrofoils indicate that the attack angle and cavitation number have a significant impact on cavity geometry. With constant attack angle, when cavitation number decreases, the cavity length and thickness become larger, and cloud cavitation shedding frequency is gradually reduced; with constant cavitation number, when the attack angle increases, same tendency is found. Relation of dimensionless cavity length and parameter σ/2a is approximately linear. For2D cavitating flows, the dimensionless characteristic number which stands for the shedding characteristics of the cloud structure, St (Strouhal Number) are basically the same; while for3D cavitating flows, St changes in the range of0.27to0.36. Meanwhile, for the3D cavitating flows, in the same cavitation number, with the attack angle increases, the U-shaped shedding structure length in spanwise becomes smaller and the thickness becomes larger.The control mechanism of cloud cavitation by obstacles on the suction surface of hydrofoil has been studied by numerical methods. Effects of the height and location, and the shape of the obstacles to inhibit the cavitation cloud are analyzed. The results show that in2D case, the obstacle which was placed on the proper position and had a proper height can control the cloud cavitation effectively. The control mechanism was mainly due to hinder the development of the reentrant jet. It is obviously that the pressure distribution is changed by the obstacle; numerical simulation of the non-cavitating flow show that although the obstacle increases the pressure around the hydrofoil, the effect is not enough to lead the instability of sheet cavitaion.For cavitating flow in high temperature and pressure state, the thermodynamic effect of cavitation flow is consider based on the method in normal temperature and pressure state. Computational results show that the cavity shape in high temperature state is thinner and shorter than in normal state at the same cavitation number, mainly by two reasons:one reason is that as vapor density becomes larger, in the same phase transition quality, the cavity will be smaller; another reason is that the heat absorption in cavitation area reduces the local temperature and the saturated vapor pressure, and thus the cavitation development is suppressed.