INSIGHT INTO PRESSURE DROP AND VISCOUS LOSSES IN TIP-LEAKAGE FLOW BETWEEN A HYDROFOIL AND STATIONARY ENDWALL

Viscous losses and local pressure drop due to tip-clearance flow are the primary factors for efficiency decline and tip-clearance cavitation in axial-flow hydraulic machinery. In this paper, the tip-clearance flow between a NACA0009 hydrofoil and a stationary endwall is investigated using very large eddy simulation, with the aim of exploring the viscous loss properties and the underlying mechanism of pressure drop in the tip-gap region. A quantitative model for the evaluation of viscous losses has been proposed based on the analysis of mean-flow kinetic energy conversion and transport, and the viscous losses and pressure drop associated with the tip-clearance flow are extensively discussed. Gross features of the tip separation vortex (TSV), tip-leakage vortex (TLV), and induced vortex (IV) have been revealed by investigating the mean-flow fields. The production of turbulent kinetic energy (TKE) is found to be the dominant contributor to pressure drop in the TSV, while pressure drop in the TLV is mainly affected by TKE production as well as the convection and transport of mean-flow kinetic energy. In the tip-clearance region, the dissipation of TKE is the main contributor to the viscous losses, accounting for 91.2% of the total losses. The flow structures in the tip gap region have different influences on TKE production. It shows that the shear flow close to the suction surface of the hydrofoil mainly generates the TKE component \left\langle \overline u' u' \right\rangle, while the tip-clearance vortices mainly generate the components \left\langle \overline v' v' \right\rangle and \left\langle \overline w' w' \right\rangle. The analysis of the mechanism of TKE production indicates that the TKE production term component P_vw is the dominant contributor to TKE production in both TLV and TSV, suggesting that reducing the spanwise derivative of the pitchwise velocity \partial \left\langle \bar v \right\rangle \mathord\left/ \vphantom \partial \left\langle \bar v \right\rangle \partial z \right. \partial z in the TSV and TLV is a potential way to reduce TKE production, and then alleviate the viscous losses associated with turbulent dissipation in the tip-clearance region. The findings provide a reference for tip-clearance flow control.