Effect of Surface Roughness Geometry on Boundary-Layer Transition and Far-Field Noise

Abstract

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Influence of Surface Roughness Geometry on Trailing Edge Wall Pressure Fluctuations and Noise

“…Wind-turbine and gas-turbine blades erode due to particle impact, resulting in the formation of surface roughness close to the leading edge [11,12]. Previous research has shown that the type of surface roughness and its height have a direct influence on the transition onset and the turbulent boundary layer development [10,[13][14][15][16][17], consequently also affecting the trailing edge noise. Braslow [17] stated that for two-dimensional surface roughness, e.g., zigzag strips, spots of turbulence begin to move forward of the natural position of transition when a critical value of Reynolds number based on the trip height is reached.…”

“…However, there was also a large spanwise coherence in the flow, which they attributed to the surface roughness elements. Ye [15] measured the transition point of the boundary layer by means of infrared thermography, observing that the transition location for a zigzag strip of 0.3 mm high moves closer to the strip as the free-stream velocity increases. This occurs because for higher velocities, the boundary layer becomes thinner, making the roughness height more comparable with the boundary layer thickness.…”

“…A critical parameter in determining which roughness height ( ) to use is its ratio with the undisturbed boundary layer thickness at the trip location ( ) [10,13]. When / is too small it does not introduce enough instabilities in the boundary layer to develop a turbulent flow at the TE [15] and a too high ratio changes the boundary layer characteristics [10,13]. A common practice in wind tunnel tests is to perform velocity sweeps with the same roughness height, as performed by Ye et al [15].…”

“…When / is too small it does not introduce enough instabilities in the boundary layer to develop a turbulent flow at the TE [15] and a too high ratio changes the boundary layer characteristics [10,13]. A common practice in wind tunnel tests is to perform velocity sweeps with the same roughness height, as performed by Ye et al [15]. In these cases, the boundary layer becomes thinner as the velocity increases, with the roughness height becoming much higher than the boundary layer thickness, see Fig.…”

“…They associated this noise increase to an increase of the boundary layer turbulence intensity at the trailing edge due to the roughness, which was observed from numerical simulations. Ye et al [15] performed far-field measurements on a NACA 0012 with grits and zigzag strip of 0.3 mm height for velocities from 16 m/s to 35 m/s. They observed that the far-field noise levels for different velocities were closely related to the tripping conditions and to the unsteady flow features developed in the transition process.…”