Large eddy simulations of inflow turbulence noise: application to tidal turbines

Abstract

Marine anthropogenic noise is increasing, along with concern about its impact on the environment. Hence minimising noise within engineering design is important, including in applications such as ships, submarines and turbines. The desire to mitigate noise may also be related to reducing the detectability of certain types of marine craft. Noise reduction typically focuses on rotating machinery such as propellers, due to the high velocity of the blades.

A common source of broadband noise in engineering scenarios is often termed inflow turbulence noise. Resulting from upstream turbulence impinging onto rotor blades, this source typically dominates the low to mid range of the frequency spectrum. This is due to the high turbulence intensity and large length scales present in the inflow turbulence, which exceed those generating competing noise sources.

This thesis uses a library of numerical tools to simulate broadband inflow turbulence noise. Synthetic turbulence is generated numerically within the simulations. Turbulence is resolved on the grid by solving the filtered Navier-Stokes equations. Based on the assumption of incompressible flow, noise sources may be predicted without resolving acoustic waves on the grid. This decoupling of hydrodynamic and acoustic processes means that radiated noise may be estimated using an acoustic analogy.

Validation of two inflow turbulence generators revealed the importance of obtaining the prescribed turbulence statistics, as well as minimising artificial pressure fluctuations. This is used to simulate homogeneous isotropic turbulence impinging onto a foil, allowing acoustic sources to be located. The far-field sound prediction is in good agreement with experimental measurement data for low frequencies. It is then shown that the effect of foil thickness on noise can successfully be predicted using the proposed methodology.

Noise radiation from a tidal turbine is then estimated by fully resolving all turbine blades, both spatially and temporally, in the simulation. A good agreement is seen in comparison to an analytical model, demonstrating that the simulation captures the dominant flow features which affect the acoustic spectrum. These spectral ‘humps’ are a result of turbulence-rotor interaction, which is implicitly included. Full scale noise estimates made from the simulations are then used to inform environmental impact assessment; the turbine hydrodynamic noise is not expected to be an issue in this regard.

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Identifiers

ORCID for Stephen R. Turnock: orcid.org/0000-0001-6288-0400

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