Aeroacoustic and aerodynamic characteristics of the turbulent boundary layer encountering a large obstacle are experimentally investigated in this paper. Two-dimensional obstacles with a square and a semi-circular cross-section mounted on a flat plate are studied in wind tunnel tests, with particular interests in the shear layer characteristics, wall pressure fluctuations, and far-field noise induced by the obstacles. Synchronized measurements of the far-field noise and the wall pressure fluctuations were conducted using microphone arrays in the far-field and flush-mounted in the plate, respectively. Additionally, the streamwise and wall-normal velocity fluctuations behind the obstacle were measured using the X-wire probe. The measured velocity profiles, spectra, and wall pressure spectra are compared, showing that the rectangular obstacle has a significant impact on both the turbulent flow and far-field noise. The large-scale vortical structures shed from the obstacles can be identified in the wall pressure spectra, the streamwise velocity spectra, and the wall pressure coherence analysis. Within the shear layer, the pairing of vortices occurs and the frequency of the broadband peak in the velocity spectra decreases as the shear layer grows downstream. Further eddy convective velocities of large-scale vortical structures inside the shear layer were analyzed based on the wall pressure fluctuations.

This paper presents an experimental investigation on improving the noise reduction capability of the trailing edge serrations using vortex generators. Experiments were conducted on a 150 mm chord flat plate at four angle of attack between 0° and 9° and flow speeds of 20-50 m/s. The Reynolds number based on the chord length ranges from 2.1×105 to 5×105. Vane type vortex generators with various heights of h/δ = 0.16-0.25 were placed along with the serration roots, where δ is the boundary layer thickness at the trailing edge without the vortex generators. The aeroacoustic measurements were made with a phased microphone array, and the aerodynamic measurement near the trailing edge was measured using a hot-wire. Additionally, the lift and drag forces were measured by load cells. The source integrated noise spectra showed a slight reduction in the trailing edge noise over a broadband frequency range. The generation of streamwise vortices by vortex generators can be identified in the wake measurement. These streamwise vortices were further found to counteract the cross-flow generated at serration roots, hence improving the performance of the serration. Moreover, the presence of low-profile vortex generators was found to have an insignificant effect on the lift and drag coefficients.

No abstract

This paper reports an experimental study on the aerodynamic noise generated by a two-dimensional large obstacle in a turbulent boundary layer. Square and triangular obstacles with varying heights of h/δ=0.48,0.8,1.2,1.6, and 2 (where δ is the boundary layer thickness measured without the obstacle present) are tested at various flow speeds ranging from 20 to 50 m/s. The Reynolds number based on step height and free stream velocity ranged between 7500 and 79 000. A linear microphone array is arranged aside to measure the sound radiation in the spanwise direction. It is suggested that both square and triangular obstructions can lead to a broadband source with a dipole-type directivity. A consistent increase in the noise strength is observed with obstacle height and flow speed. The underlying noise source is revealed by comparing the acoustic spectra of different obstacle geometries. The low-frequency noise is contributed by the turbulence modification due to the flow impingement onto the obstacle, while the high-frequency sound is mainly caused by the diffraction of hydrodynamic pressure by the sharp leading edge.