Numerical simulations of the tip leakage vortex in a waterjet pump using a Reynolds averaged approach

Abstract

Waterjet pumps are widely used among maritime transport vehicles, such as high-speed vessels. Many experiments have been performed to analyze the complex flow patterns present in these axial pumps, such as the tip leakage vortex (TLV) [1, 2]. Experiments are time and resource consuming, which is pushing the use of computational methods in order to provide accurate and faster results. The aim of this project is to provide a good prediction of the TLV structures using Reynolds Averaged Navier Stokes models (RANS) within steady and unsteady conditions by validating with experimental data provided in Li et. al [1]. The studies include Multiple Reference Frame (MRF) approach for steady mode and Sliding Mesh (SM) for unsteady mode. The 𝑘 − 𝜔 𝑆𝑆𝑇 turbulence model is used as a baseline for both configurations although two additional variations will be tested: 𝑘 − 𝜔 𝑆𝑆𝑇 with Curvature Correction (CC) for the steady mode and the 𝑘 − 𝜔 𝑆𝑆𝑇 Scale Adaptative Simulation (SAS) for unsteady mode. Concerning mesh resolution, 40 cells in the radial direction covering the tip gap are sufficient to obtain good agreement with experiment. Additionally, the optimum divergence scheme is the limitedLinear scheme as it provides qualitative good agreement. Using the resulting mesh and the chosen scheme, unsteady simulations showed to improve the agreement with experimental data compared to steady simulations. Also, the CC and SAS turbulence models proved to be less adequate for TLV prediction in comparison with the 𝑘 − 𝜔 𝑆𝑆𝑇 model. Additionally, some parametrical studies are performed in steady mode including the Boundary Layer (BL) thickness analysis and blade tip shape effect on the TLV. It is concluded that there is no qualitative difference for different inlet BL thickness. On the other hand, blade shape highly influences the TLV. Rounded blade tip edges increase the tip clearance flow leading to a stronger TLV with a core further into the core flow passage. This is related to a reduction in the recirculation flow in the tip clearance.