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Description
Various industries such as urban air mobility, wind energy, aerospace, transportation, etc. have been imposed with noise emission regulations by most governments. This has led to an increase in research about noise emission and mitigation strategies. Specifically for the wind industry, larger turbine rotors implemented to increase energy production are also emitting higher sound levels which is a serious concern for the industry.
Additionally, rotor blades experience adverse pressure gradients leading to flow separation, reduced performance and efficiencies, and stall in extreme conditions. To tackle this, a specific type of flow control device known as rod-type vortex generators (RVGs) have been investigated for wind turbine rotors and airfoils. These add-ons have been proven to enhance flow mixing in the boundary layer thus delaying or reducing turbulent flow separation. However, studies on their impact on the emitted noise levels are limited.
In order to gain a fundamental understanding of noise generation and propagation mechanisms, a computational approach – an in-house post-processing tool based on the popular Ffowcs-Williams and Hawkings acoustic analogy (FW–H) is utilized. It is based on the linear, integral solution of the FW-H analogy derived by Farassat known as Farassat’s Formulations. It predicts noise emitted by rotating bodies in subsonic motion (M < 0.8). Based on the surface pressure distribution on the blade surface and the rotor blade kinematics the FW—H code predicts the rotational noise (harmonic noise) and its components in great detail. Additionally, acoustic measurements using a phased microphone array are conducted on a wind turbine airfoil implemented with RVGs.
With the implementation of the RVGs the separation noise gets reduced to the reduction of flow separation area, but the rods also increase pressure fluctuations as they generate streamwise vortices that could potentially increase loading noise. This research focussed on the interplay of the two competing noise mechanisms to assess the acoustic impact of the flow control device.
The numerical investigation of the acoustic impact of the RVGs on the NREL Phase VI wind turbine rotor shows that they have no significant impact on the rotational noise emitted. There is an increase in the noise levels at higher frequencies which is not of major concern for the wind industry. Experimental investigation on the DU96-W-180 airfoil with RVGs show that the rods decrease noise at low frequencies and slightly increase noise at higher frequencies (2 dB with array uncertainty 1 dB).
