Influence of Roughness Trips on Near- and Far-Field Trailing-Edge Noise

Roughness trips are widely applied in wind-tunnel testing to hasten the laminar-turbulent boundary-layer transition. It is well known that they influence the boundary-layer transition and its development. However, their influence on aeroacoustic quantities (in particular, on the airfoil trailing-edge noise) is not yet fully understood. This paper discusses the relationship between the trip height and the boundary-layer development and radiated trailing-edge noise. Wind-tunnel experiments were performed with zigzag strips and grit elements placed on the surface of a NACA 0012 airfoil. The roughness trip heights k ranged from 17 to 214% of the undisturbed boundary-layer thickness delta(k) at the trip location. The chord-based Reynolds number varied from 1.3 x 10(5) to 3.3 x 10(5). This paper shows that trip heights within 50 to 105% of delta(k) have little influence on the near field and the trailing-edge noise. However, trips with k/delta(k) > 1.05 increase the spectra of velocity and pressure fluctuations in the low-frequency range up to 2.5 and 3 dB, respectively, resulting in an increase of the trailing-edge radiated noise up to 4 dB in a similar frequency range. In summary, this paper shows the importance of carefully defining the roughness trip used to trigger a laminar-turbulent transition in aeroacoustic and aerodynamic measurements.