Acoustics of Jet Surface Interaction - Scrubbing Noise

Abstract

Concepts envisioned for the future of civil air transport consist of unconventional propulsion systems in the close proximity to the structure or embedded in the airframe. While such integrated systems are intended to shield noise from the community, they also introduce new sources of sound. Sound generation due to interaction of a jet flow past a nearby solid surface is investigated here using the generalized acoustic analogy theory. The analysis applies to the boundary layer noise generated at and near a wall, and excludes the scattered noise component that is produced at the leading or the trailing edge. While compressibility effects are relatively unimportant at very low Mach numbers, frictional heat generation and thermal gradient normal to the surface could play important roles in generation and propagation of sound in high speed jets of practical interest. A general expression is given for the spectral density of the far field sound as governed by the variable density Pridmore-Brown equation. The propagation Green's function is solved numerically for a high aspect-ratio rectangular jet starting with the boundary conditions on the surface and subject to specified mean velocity and temperature profiles between the surface and the observer. It is shown the magnitude of the Green's function decreases with increasing source frequency and/or jet temperature. The phase remains constant for a rigid surface, but varies with source location when subject to an impedance type boundary condition. The Green's function in the absence of the surface, and flight effects are also investigated.

Noise Reduction with Fluidic Inserts in Supersonic Jets Exhausting Over a Simulated Aircraft Carrier Deck

“…The noise produced by jet scrubbing has also been thoroughly researched, most recently by Khavaran [16]. Furthermore, Smith and Miller [17] showed the ground plane affects the mean flow and turbulent statistics within the jet plume and is dependent upon the distance between the jet centerline and ground surface.…”

Jet Noise in Airframe Integration and Shielding

“…Early assessments of the HWB noted limited potential for noise reduction, specifically a Boeing version of an HWB called the "blended wing body" of Liebeck et al [20]. This configuration had the engine exhaust positioned aft of the trailing edge, making shielding of the aft-radiated engine noise sources impossible, unless the engines were moved at least some limited distance upstream on the airframe [21]. Also, various future aircraft design concepts consider over-the-wing engine configurations, in which the wing or any other airframe structures can be used as a shield against engine noise (i.e., [22]).…”

A proposed wavy shield for suppression of supersonic jet noise utilizing reflections

“…Several researchers have investigated the engine-airframe interactions. For the subsonic case, on the numerical side, Khavaran 3 looked at the jet-surface interactions with focus on the scrubbing noise. Ramamurti et al.…”