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1. Vortex Shedding and Noise Radiation from a Slat Trailing Edge

Vortex shedding from a slat trailing edge
Vortex shedding from a slat trailing edge

Vortex shedding and the associated noise radiation from a trailing edge were experimentally investigated for a leading-edge slat of a multi-element airfoil at stowed-chord Reynolds number Re < 5.9×105.  A particular focus was on the competition between the instability of  the slat boundary-layer excited by acoustic feedback and the absolute instability of the wake. Periodic vortex shedding was observed to occur from the slat trailing-edge at the Reynolds numbers examined. For Re < 1.9×105, the vortex shedding is governed by the absolute instability of the laminar wake of the slat without any distinct tonal noise radiation. For Re > 2.1×105, however, acoustic feedback becomes pronounced between the trailing-edge noise and boundary-layer instability-waves on the suction-surface, so that multiple spectral peaks appear both in the velocity fluctuations and sound pressure.  At and around the Reynolds number for the first appearance of tonal noise, Re = 1.9×105, both of the instability modes coexist. Beyond Re = 2.1×105, the boundary-layer instability waves excited by the acoustic feedback evolve into high-intensity vortices before reaching the trailing edge and suppress the absolute instability of the wake through diminishing the reversed flow region in the wake. [Makiya, S., Inasawa, A. and Asai, M., AIAA J., 48, 2 (2010) 502-509]

2. Generation of Trailing-Edge Noise of Airfoil at Low Reynolds Numbers

Vortex shedding and noise radiation from a wing trailing-edge
Vortex shedding and noise radiation
from a wing trailing-edge

In order to clarify generation mechanism of trailing-edge noise of wing, flow around NACA0012 airfoil is investigated experimentally at low Reynolds numbers less than 4.3×105. The flow instability on the suction side together with feed-back mechanism between receptivity point of disturbance and sound generation found to generate discrete noise at the Reynolds number of order of 104, while no instability wave leading to vortex roll-up is observed on the pressure side. With increasing the Reynolds number, however, transition to turbulence occurs on the suction side and the instability of flow over the pressure side plays an important role for the generation of discrete tone. It is also found that such discrete tone generation is suppressed by forcing the flow to be turbulent state.  [Inasawa, Ninomiya and Asai, AIAA J. 51, 7 (2013) 1695-1702]