The sensitivity of actuator-disc RANS simulations to turbulence length scale assumptions

Abstract

It has previously been shown that ambient turbulence affects the results from computational fluid dynamics (CFD) models when using an actuator disc to simulate marine current turbines. The turbulence parameters are often estimated using an empirical equation that is dependent on a turbulence length scale. In most literature this length scale is commonly highlighted as the authors ‘best guess’ with little scientific reasoning. The work presented here investigates the effects of using different length scales on the development of a flow in a circulating water channel. The results showed that the best agreement is achieved with a length scale of one third the channel depth. The obtained turbulence parameters were then used with an actuator disc model. Agreement with experimental data was initially poor as the velocity deficit was severely under predicted. The addition of a turbulence source at the disc improved the agreement with experimental data significantly. It was found that the length scale of the disc turbulence should be the diameter of the holes used on the porous discs for experiments. However, there were still discrepancies between the experimental and model turbulence intensities. A possible cause of this may be that the turbulence intensity added at the disc was under predicted. Further work is needed to establish if better agreement can be achieved by increasing the turbulence at the disc