Rotor Inflow Noise Caused by a Boundary Layer: Inflow Measurements and Noise Predictions

Abstract

A rotor immersed in a thick turbulent boundary layer produces unsteady loading on the blades which generates unwanted noise and vibration. Two point velocity fluctuations were measured in detail to determine the full four-dimensional correlation function of a boundary layer generated over a smooth wall in the Virginia Tech Stability Wind Tunnel. The correlation function reveals anisotropy in the flow dominated by a large scale correlation structure elongated in the streamwise direction and inclined relative to the wall. This correlation function was then evaluated in the blade frame of reference of an idealized 10 bladed rotor partially immersed in the flow. Blade to blade upwash coherence shows significant asymmetry which is a direct result of the anisotropy of the flow. Using a newly developed theory, the correlation function was used to predict the far-field radiated noise from the rotor at various operating and flow conditions. Predictions show the sound field is dominated by the effects of "haystacking" which is further increased with the inclusion of the presence of the wall. Directivity predictions suggest the far-field sound field acts like a monopole/dipole combination.iii

Assessing the influence of inflow turbulence on noise and performance of a tidal turbine using large eddy simulations

“…The turbine advance coefficient (J ¼ p=L ¼ 0:53) is slightly lower than that used by Morton et al [45], where J ¼ 0.7. This may explain the larger increase in level observed, compared to Morton et al [45], who saw a 10 dB difference between the same angles. Hence there is potential for further investigation of the effect of turbine operating condition on noise directivity.…”

“…The reduction in OASPL between 0 and 90 is z16 dB. Morton et al [45] attributed this monopole-like behaviour to increased tip loading at low advance coefficients. The turbine advance coefficient (J ¼ p=L ¼ 0:53) is slightly lower than that used by Morton et al [45], where J ¼ 0.7.…”

“…Morton et al [45] attributed this monopole-like behaviour to increased tip loading at low advance coefficients. The turbine advance coefficient (J ¼ p=L ¼ 0:53) is slightly lower than that used by Morton et al [45], where J ¼ 0.7. This may explain the larger increase in level observed, compared to Morton et al [45], who saw a 10 dB difference between the same angles.…”

Computation of Rotor Noise Generation in Grid Turbulence Using Large-Eddy Simulation

“…They compared the predictions with acoustic measurements in a companion experiment involving a rotor protruding from the trailing edge of a large airfoil, and obtained closer agreement than predictions based on an isotropic turbulence model. Morton et al 10 and Alexander et al 11 conducted experiments with a ten-bladed rotor partially immersed in a thick turbulent boundary layer. They measured detailed four-dimensional space-time correlation functions of the inflow velocity, the sound produced by the rotor over a range of operating conditions, and the turbulence statistics in the rotor wake.…”