Influence of boundary layer flow suction on trailing edge noise generation

Abstract

The effect of uniform inclined flow suction on an equilibrium turbulent boundary layer developing over a flat plate is experimentally investigated for aeroacoustic purposes. Simultaneous measurements of streamwise velocity with hot-wire anemometry, and surface pressure fluctuations using flush-mounted microphones were performed at various locations downstream of the flow control treatment. The paper discusses the effects of flow suction on the turbulent quantities describing the boundary layer, and the associated hydrodynamic pressure field to assess the effects of flow suction on trailing edge noise generation. Two parameters were varied, the inclination of the flow suction velocity (α) and the flow suction severity (σ). The former is the angle of flow suction with respect to the free-stream flow (α = 30 • , 50 • , 70 • and 90 • ), while the latter is the ratio of momentum deficit within the boundary layer to the momentum of flow suction (σ = 2.5 − 9.1). Flow suction reduces the height of the boundary layer, which results in an increase of mean shear. The flow energy content within the boundary layer is reduced by suction with the most significant amount of reduction is observed within the logarithmic region. A moderate energy increase is observed in the buffer layer. These effects strengthen with growing suction severity until σ ≈ 6. Above this point, no further changes are observed in the turbulent quantities. The reduction in the size and energy content of the logarithmic layer is responsible for the reduction of the surface pressure fluctuations at mid-frequencies, while the increased energy content in the buffer layer increases the pressure spectral content at high frequencies. The estimates of the far-field trailing edge noise show that flow suction leads to a noise reduction at mid-frequencies with penalties observed at low and high frequencies. Flow suction at an angle of α = 70 • and σ ≈ 6 exhibits the best performance in reducing the estimated far-field overall noise levels. Above this point, further increasing the flow suction severity does not provide any additional noise reduction benefits.

Uniform flow injection into a turbulent boundary layer for trailing edge noise reduction

“…However, applying flow suction at a rate of σ ≈ 2 (i.e. removing 20% of the boundary layer thickness) was found to reduce the OASPL only by 2 dB for the same observer location 40 . Therefore, at lower flow control severity (σ ≈ 2) inclined flow injection offers a better aeroacoustic performance than flow suction.…”

“…Refs. 40,42,50,51 . The flow control severity relates the momentum deficit of the boundary layer to the momentum of the flow control system.…”

“…Recently, Szőke et al 40 investigated the effects of flow suction on turbulent boundary layer trailing edge noise. Therefore, it would be of interest to compare the aeroacoustic performances of both uniform blowing and uniform suction, and to relate these performances to their required power consumption.…”

“…Most commonly, previous works focused on the use of flow suction from 4, 38-40 , or flow injection into the boundary layer 41,42 . The main drawback of flow suction is that it requires a significant amount of air to be removed from the boundary layer, namely, it was found that removing approximately 60% of the boundary layer thickness provides the best aeroacoustic performance 40,43 . A potential solution to overcome this issue is to employ flow injection into the boundary layer upstream of the trailing edge.…”

Numerical investigation of porous materials for trailing edge noise reduction

“…On the one hand, active methods are usually based on local blowing or boundary layer suction. 6–10 On the other hand, passive devices without energy supply are also promising. These often aim to alter the boundary layer turbulence, for example with boundary layer fences or rails.…”