Cavitation: Modelling and test on a hydrofoil Álvaro Rodríguez Ruiz (1) (Corresponding Author), Hans Christian Soerensen (2), Erik Friis-Madsen (2), Martijn Teela (3), Verónica González de Lena (1) (1) Centro Tecnológico de Componentes, (2) Wave Dragon APS, (3) Tocardo International BV

Abstract For marine power generation, cavitation erosion is a consideration if devices operate at high speed or low pressure. Cavitation erosion degrades the surface of the blades increasing the roughness and hence modifying the performance curves and reducing the design lifetime. This poster introduces ACORN (Advanced Coatings for Offshore Renewable eNergy), an industry-led project supported by the European Commission (FP7/2007-2014, under grant agreement no 605955) to develop new coating technology aimed at lowering the Levelised Cost Of Energy (LCOE) for wave and tidal devices. Three different cavitation-resistant materials have been applied as coating with thermallysprayed technique to reduce the cavitation erosion effect. These coatings resist a combination of seawater corrosion and cavitation very effectively for 10+ year design life. This poster presents results from cavitation testing and CFD modelling of a hydrofoil typically used in tidal turbines.

Objectives This poster aims to present results from cavitation testing and numerical modelling of a hydrofoil typically used in tidal turbines. Methods Tidal devices are at present in a very high technology readiness level (TRL) so optimum working conditions are needed to increase the efficiency. In the case of wave and tidal energy devices which use turbines as method to capture the energy from the water, cavitation is an important challenge. With the use of advanced modelling techniques – Computational Fluid Dynamics (CFD) based – it is able to reasonably predict the cavitation performance on marine renewable energy devices. Due to cavitation is a very complex phenomenon cavitation test are always recommended when a new component is tested. The hydrofoil has been exposed to a wide range of flow speeds and pressure gradients to identify the performance and cavitation inception curves. ANSYS CFX has been used as numerical tool to run different working conditions in order to obtain the most important results from the hydrodynamic point of view. Direct Numerical Simulation (DNS) is a challenge even for the largest computer cluster. Reynolds Averaged Navier-Stokes Simulation (RANS) has been developed as simplified numerical model to address very complex scenarios. Within RANS, SST k-ω (Shear Stress Transport) turbulence model, a two-equation eddy-viscosity model, has become very popular in aerodynamics due to its high accuracy for separation prediction and attached flows and also when boundary layer effects become essential. Turbulence models based on the ω-equation, such as the SST model requires a very fine mesh in the near-wall zone and correspondingly large number of nodes. As a first step, a numerical model of the cavitation tunnel was developed in ANSYS CFX. In order to model adequately the boundary layer, a value of y+=1 was targeted, what means a size of the first element of 3-20 μm and a total number of 26978 cells. Lift and drag curves, cavitation inception and eroded area prediction were the most important results obtained in both, numerical model and cavitation tunnel.

Results and conclusions A assessment between numerical model and cavitation testing was achieved for lift and drag curves, showing differences at larger angles of attack. The cavitation inception results were satisfactory showing a better behaviour in the pressure side of the hydrofoil. A good correspondence between real eroded areas on the hydrofoil (conducted in a cavitation tunnel) and predicted eroded areas in the numerical model was obtained. Conclusions: The flow near the wall is highly affected by roughness (equivalent sand-grain roughness), especially in the boundaries of different stripes. The blockage effect of the hydrofoil over the cavitation tunnel modifies lift and drag curves, so Betz correction is needed. The content of air in the water modifies the cavitation inception. The strongest erosion conditions due to cavitation were achieved for an angle of attack of 5-10º. A CFD model can predict accurately the eroded areas due to cavitation.

AoA = 5°

AoA = 10°

References 1. 2. 3. 4. 5.

http://www.acorn-project.eu/ 1st Workshop, Advanced Coatings for Offshore Renewable Energy (14/10/2015, Santander) Novel coatings to resist corrosion and biofouling, ICOE 2016 Journal Article, Name of Journal http://www.ansys.com/Products/Fluids/ANSYS-CFX

International Conference on Ocean Energy (ICOE) 2016 – Edinburgh – 23-25 February 2016