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.

Flat surfaces with arrays of wall-normal pores called micro-cavity arrays have shown potential to reduce turbulent skin friction drag. Their designs have been inspired by acoustic liners which are sandwich panels consisting of a honeycomb core, a perforated top plate and a solid back sheet used on aircraft engine nacelles for noise abatement. Research on acoustic liners has paved the way to identify the parameters that are crucial for optimising the design of micro-cavity arrays. Spanwise rectangular grooves, a special case of cavities, have also shown potential to reduce skin friction drag. This thesis aims to validate the reported drag performance of micro-cavity arrays and grooves through experimental methods used in literature and direct force measurements. 

In this research, experimental results show that although pores and grooves are capable of reducing turbulence energy in the boundary layer, there is a significant drag increase with respect to a smooth reference. Based on measured drag performance and past research on cavity flows, it has been argued that the observed drag increase is caused by pressure forces acting on the inner walls of the pores and grooves. As past research on micro-cavity arrays did not perform direct force measurements, the contribution of pressure drag had not been investigated. Pressure drag is expected to be present and perhaps even dominate over skin friction drag reductions. Further research into the flow mechanics inside the pores is required to validate this argument and find a way to minimise the pressure forces. Wall-normal pores could still be an effective turbulent drag reduction technique if arrays are optimised for skin friction drag reductions that surpass pressure drag.

Turbulent drag is the largest source of fuel consumption in aviation and forms a significant contribution to climate warming. Recent numerical studies have suggested the ability of surfaces with streamwise-preferential permeability to reduce turbulent friction, and a theoretical framework behind the working mechanism has been proposed. This work is the first experimental study on this concept in air and in which the permeability requirements for predicted drag reductions are met. Three different physical realisations of streamwise-preferential permeable substrates were manufactured: two seal furs, a substrate with unidirectional fibres, and a 3D-printed structure. They were investigated through wind tunnel experiments using direct force measurements and planar (2D-2C) particle image velocimetry (PIV). Results show an increase in drag for all test specimens and suggest that drag sources other than surface friction are non-negligible. One-point turbulent statistics show an overall increase in turbulence, mainly wall-normal velocity fluctuations enabled by the wall-normal permeability, which results in higher Reynolds stresses. No significant flow modulation in terms of turbulent events or change in coherent structures is observed. The measurements and results are a first of its kind and none of the theoretical framework predictions on drag reduction or flow modulation are found. Based on analytical derivations relating characteristic length scales to permeability, the validity of the framework and its assumptions in experimental settings is questioned. It is thought that there is an inherent mismatch between the pore size assumption and the virtual origin approach taken in the framework. All things considered, it is deemed unlikely that turbulent drag reduction by means of streamwise-preferential permeable surfaces is feasible in experimental settings. Nevertheless, given the relatively small increase in drag for the 3D-printed specimen, streamwise-preferential permeable surfaces might be interesting for other (flow control) purposes involving turbulent boundary layers.