CAVITATION AND TURBULENCE MODELLING FOR VALVE FLOWS - AN APPLICATION TO A PILOT STAGE OF A SERVO VALVE

摘要

Standard Computational Fluid Dynamic (CFD) techniques are widely used in the design of hydraulic valves for optimising the valve performance and reducing the production effort. They calculate the turbulent flow and predict cavitation. Unfortunately, the currently used models are often inadequate and out of date to catch the complexity of these phenomena such as the transient interaction between cavitation and turbulence. Advanced computational methods have been developed and applied to other engineering branches. Despite this fact, they face many difficulties to be employed in hydraulics. In this paper, a first step is taken towards the usage of these cutting edge CFD methods for hydraulic valves. At first, the different challenges for a CFD code to simulate valve flows are highlighted. A novel computational approach is then presented. It combines a Large Eddy Simulation (LES) model for the turbulence modelling as well as a Full Cavitation Model (FCM). The LES technique explicitly resolves the large turbulence scales while the smaller ones are modelled. The FCM not only predicts vapour but also gas cavitation, which plays a vital role in hydraulic fluids. This method is tested to simulate the flow in a pilot stage of a jet-pipe servo-valve. The test case is presented and the different boundary conditions used for the simulations are given. The results of the simulation are compared with experimental results showing a good agreement. A comparison between the LES model and the standard two-equation turbulence model shows the advantages of the LES approach. Finally, the transient features of the flow are highlighted in terms of velocity oscillation.