Performance Prediction of Cavitating Hydrofoil Sections

Abstract

Hydrofoils can under certain circumstances cause a phase change from liquid water to water vapor. This phenomenon is called cavitation and is caused by the low pressure over the hydrofoil when the vessel exceeds a certain velocity though the water. Cavitation can take different forms depending the angle of attack α and the flow conditions which are characterized with the cavitation number σ . The different types of cavitation can have different effects on flow and loads.
Computational Fluid Dynamics (CFD) is widely used in aero and hydrodynamic design, with (U)RANS most commonly used for CFD in the industry due to its relatively low computational cost while providing sufficiently accurate results. Cavitation models in URANS simulations need a multiphase framework in order to model the liquid/vapor interface of the cavitation bubbles. The Schnerr-Sauer cavitation model uses simplified bubble dynamics equations for relatively fast calculation while providing accurate results. In current project, a Volume of Fluid method is used the Schnerr-Sauer cavitation model is used with URANS CFD simulations to improve and enhance the behaviour and performance prediction of hydrofoils sections under cavitating conditions. Given the industrial context of this project, the simulations are conducted using constrained computational resources.
Validation is performed for a non-cavitating test case using a NACA-0012 section, followed by validation for a cavitating test case using a modified NACA-66 section. Mesh convergence studies have been performed, turbulence models have been compared and the turbulent viscosity has been modified. The final set-up uses the SST turbulence model with a modified turbulent viscosity exponent n = 2.3.
To assess the flow behavior and hydrofoil section performance under cavitating conditions, a comparison is made in CFD using cavitation models, relative to the current practice. This study shows that the lift and drag results for a simulation without cavitation model are underestimated compared to the simulation with cavitation model in conditions of stable cavitation. For conditions with unstable cavitation, strong unsteady disturbed flow and loads are found that are not captured by the simulation without cavitation model. The transition from stable to unstable cavitation is studied by investigating cavitation bubble length and its
corresponding fluctuation as a function of the stability parameter
ps = σ/2(α−α0) . The conditions found for the transition from stable to unstable cavitation are consistent with reference value at about ps = 4. The inception of stable cavitation is found at about ps = 7 which is considered to be more optimized to delay the formation of cavitation compared to the NACA-0012 at ps = 8.5.
The lift, drag and performance polars are studied for several values of σ . The lift and drag polars for lowerσ , i.e. higher cavitation rate, show a stronger increase in both lift and drag for stable cavitation cases. The performance (or Lift over Drag) is slightly increased at α = 4◦ for σ = 1.2 and 1. For higher angles of attack, the increase in drag surpasses the increase in lift and the performance decreases. These finding only hold the stable cavitation cases (α < 8◦ for all tested σ , and α = 6◦ for σ = 1) since the unstable cavitation results are
inconclusive.
The main limitation of the set-up developed in the current project is that the predictions show significant discrepancies in capturing the unstable bubble shedding characteristics, with respect to the reference data. As a result, the cloud cavitation shedding frequency is not accurately captured, resulting in an inadequate representation of vibrations and loading due to cloud cavitation.