Turbulent boundary layer trailing edge noise reduction by using a perforated add-on

Abstract

Turbulent boundary layer trailing edge
noise is becoming an increasingly important problem in wind turbine
applications. The wind turbine rotor diameter is expected to keep growing bigger
in the future and trailing edge noise is known to scale with the relative rotor
tip velocity to the power 5-6. One effective and yet relatively novel method to
reduce this type of noise is the addition of perforated add-on devices to the
rear end of the rotor blades. The perforated add-ons have been extensively
tested experimentally in recent years. The experimental studies have been
instrumental in developing design rules but currently, the precise mechanisms
behind this noise reduction are not yet fully understood. The project objective
is to obtain insights into the aero-acoustic flow physics responsible for the
far-field noise reduction for perforated add-on trailing edge noise problems. To
accomplish this, the first high-fidelity numerical LBM study in which the
geometry of the per- forations is fully resolved is performed. The simulation
is performed with the commercial Lattice Boltzmann method-based solver,
PowerFLOW. A scaled 2.5D airfoil simulation is performed with an inflow of 20
[m/s] in lifting conditions. Three different airfoil add-on configurations are simulated.
One baseline, one with a homogenous perforation spacing, and one geometry where
this spacing is decreasing in the downstream direction. The simulation results
are validated against experiments and a noise reduction of about 3 − 4 [dB] is
found for in the lower frequency range Stc < 10. A distribution of sound
sources was found over the add-on for all frequencies. The frequencies up to
Stc = 10 show signs of destructive acoustic interference contributing to the
observed noise reduction. Besides this phenomenon, two additional mechanisms
were identified as contributing to the observed noise reduction. The second is
the milder ∆p jump at the trailing edge caused by communicating boundary layers
on both sides of the add-on. A good indication of this milder pressure jump is
the correlation coefficient Rpp between two points on opposite sides of the add-on
close to the edge. No development or growth of the zone of the correlation
coefficient was found over the add-on except for the vicinity close to the
trailing edge. A mean vertical upward flow is observed, flowing out of and
extending above the perforations. This is believed to be relevant for the
achieved noise reduction. The effect possibly has simultaneous positive and
negative conflicting contributions to noise reduction. Lastly, it was concluded
that the spectra of the homogeneous and linear perforated geometries showed too
much similarity, therefore it was difficult to couple discrepancies in the
near-field between the two cases to observations of the far-field spectra.