2008

Passive Porous Treatment for Reducing Flap Side-Edge Noise

Advantages include broadband noise reduction with no aerodynamic-lift penalty.

A passive porous treatment has been proposed as a means of suppressing noise generated by the airflow around the side edges of partial-span flaps on airplane wings when the flaps are extended in a high-lift configuration. The treatment proposed here does not incur any aerodynamic penalties and could easily be retrofit to existing airplanes. The treatment could also be applied to reduce noise generated by turbomachinery, including wind turbines. Innovative aspects of the proposed treatment include a minimum treatment area and physics-based procedure for treatment design. The efficacy of the treatment was confirmed during wind-tunnel experiments at NASA Ames, wherein the porous treatment was applied to a minute surface area in the vicinity of a flap edge on a 26-percent model of Boeing 777-200 wing.

The flap side-edge noise constitutes a significant portion of the overall airframe noise during descent and landing of an aircraft. The acoustically relevant flow features at typical flap side edges consist of free shear layers, the roll-up of these layers to form multiple vortices, merging of vortices, and, at high flap deflections, breakdown of these vortices. Because of their unsteadiness and their proximity to flap side-edge surface, these features can contribute to the noise radiated from the flap side edges. To be effective, any treatment for reducing the flap side-edge noise must eliminate, reduce, or alter the vortex initiation regime and the intensity of the vortex roll up and/or breakdown process near the side edge of the flap.

According to the proposal, small, carefully selected areas in the flap-tip regions of each flap would be rendered porous by use of materials similar to those used for wall cooling of turbine blades or the materials used towards acoustical treatment of aircraft-engine ducts (see figure). Porosity at the tips would provide a means of communication between the flow over the lower, side, and upper surfaces near the edge of the flap and, hence, modify the vortex structures near the tip.

Unlike side-edge fences that have been investigated for reduction of flap side-edge noise, the proposed treatment would not incur extra weight and is not likely to accrue drag penalty during the cruise phase of the flight. Unlike the porous tip treatments considered previously in a cut-and-try approach, the proposed porous tip treatment is based on comprehensive analysis of the acoustically relevant features of the flow field and, consequently, would be amenable to optimization. The airflow around the side edges of the flaps can be simulated using computational fluid dynamics (CFD), and results of CFD simulations can be combined with simplified mathematical models of candidate porous treatments to analyze the effectiveness of the treatment in a specific application. Minimization of the amount of area that must be treated in order to reduce the flap side-edge noise to an acceptable degree could be an integral part of the design optimization process.

This work was done by Meelan M. Choudhari and Mehdi R. Khorrami of Langley Research Center.

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