Thomas P. Lloyd

MODELLING TECHNIQUES FOR UNDERWATER NOISE GENERATED BY TIDAL TURBINES IN SHALLOW WATERS

The modelling of underwater noise sources and their potential impact on the marine environment is considered, focusing on tidal turbines in shallow water. The requirement for device noise prediction as part of environmental impact assessment is outlined and the limited amount of measurement data and modelling research identified. Following the identification of potential noise sources, the dominant flowgenerated sources are modelled using empirical techniques. The predicted sound pressure level due to inflow turbulence for a typical turbine is estimated to give third-octave-bandwidth pressure levels of 119 dB re 1 ?Pa at 20 metres from the turbine at individual frequencies. This preliminary estimate reveals that this noise source alone is not expected to cause permanent or temporary threshold shift in the marine animals studied.

Using an inflow turbulence generator for leading edge noise predictions

Inflow turbulence noise is often the dominant noise mechanism in turbomachines. It has been shown that the sound pressure level is related to the intensity and integral length scale of the turbulence. We utilise a methodology for generating turbulence with prescribed intensity and length scales within a detached eddy simulation. This is applied to a case of homogeneous isotropic turbulence impinging on a non-symmetric aerofoil at high Reynolds number (2.1×105). The sound pressure level is estimated using Curle’s compact acoustic analogy, and compared to experimental data and analytical estimates. The intensity of the inflow turbulence is higher than expected, though it exhibits approximately homogeneous and isotropic characteristics. While the general shape of the predicted noise spectrum is correct, the magnitude differs from the experimental results by up to 17 dB. Reasons for this are elaborated, and improved predictions based on a flat plate are presented.