On the mechanisms of pressure drop and viscous losses in hydrofoil tip-clearance flows

Local pressure drop and viscous losses due to the tip-clearance flow are key factors in the performance degradation of axial-flow hydraulic machinery. To deepen insight into the underlying mechanisms of pressure drop and viscous losses in the vicinity of the tip-gap, taking a simplified case, the present work studies the tip-clearance flow around a hydrofoil with/without foil tip-edge fillets using very large eddy simulation. A quantitative model for evaluating viscous losses and pressure drop is developed based on a detailed analysis of the conversion between pressure energy, kinetic energy, and internal energy. Results show that the turbulent loss supplied by turbulent kinetic energy (TKE) production accounts for about 90% of the total viscous losses in the tip-gap region, and TKE production is the major contributor to pressure drop. Contributions of the sub-components of the TKE production term to the TKE production of tip-leakage vortices, and the leading derivatives of the velocity for TKE production, have been explored. Additionally, it is found that the hydrofoil edge fillet promotes (suppresses) the TKE production in the tip-leakage vortex (tip-separation vortex), then enhances (weakens) the pressure drop and viscous losses, while the amount of total loss of the tip-clearance flow is not significantly affected.