Date of Award

Spring 1975

Document Type

Thesis

Department

Mechanical & Aerospace Engineering

Program/Concentration

Mechanical Engineering

Committee Director

Robert L. Ash

Committee Member

I. E. Beckwith

Committee Member

Gene Goglia

Call Number for Print

Special Collections; LD4331.E56H37

Abstract

It has been well established that the test section flow of conventional supersonic and hypersonic wind tunnels contains significant levels of disturbances. These disturbances are mainly caused by the radiation of aerodynamic noise from the turbulent boundary layer on the nozzle walls. This noise level has been shown to have an important effect on boundary layer transition measured on models in tunnels. The purpose of this thesis is to present results of a coordinated experimental and theoretical study of a sound shield concept which aims to provide a means of noise reduction in .the test section of supersonic wind tunnels at high Reynolds numbers. The model used in the investigation consists of a planar array of circular rods aligned with the flow, with adjustable gaps between them for boundary layer: removal by suction, i.e., laminar flow control. One of the basic requirements of the present sound shield concept is to achieve sonic cross flow through the gaps in order to prevent lee-side flow disturbances from penetrating back into the shielded region. Tests have been conducted at Mach 6 over a local unit Reynolds number range from about 1.2 x 106 to 13.5 x 106 per fool Measurements of heat transfer, static pressure, and sound levels are made to establish the transition characteristics of the boundary layer on the rod array and the sound shielding effectiveness.

For a gap-to-rod diameter ratio of 0.16, the flow is laminar over the entire model at a maximum local Reynolds number based on model length of about 15 x _106 which occurs at a unit Reynolds number of about 7.5 x 106 per foot. At this unit Reynolds number the transition Reynolds number on a conventional flat plate in the same wind tunnel is 2 x 106. Transition Reynolds number on the suction model decreases with decreasing gap spacing and suction but the model still provides a significant increase in transition Reynolds number over that for a flat plate. The characteristics of the boundary layer flow on the rods and the transition behavior indicates that for large gaps the circular rods function as isolated swept cylinders. Hence, transition is expected to be essentially independent of rod length and therefore should be primarily a function of Reynolds number based on rod diameter for a given gap spacing and leading edge configuration.

For a gap-to-rod diameter ratio of 0.16, reduction in sound levels within the semi-shielded region of the model were about 90 percent (20 dB attenuation) of the maximum theoretical reduction for an ideal, finite, planar shield. Reductions in gap spacing and suction caused reductions in the sound attenuation. It is concluded that a cylindrical shroud utilizing the slotted wall concept with boundary layer suction can provide significant reductions of disturbance levels in supersonic wind tunnels at high unit Reynolds numbers.

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DOI

10.25777/bh1g-h150

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