Masters Theses

Date of Award

8-1992

Degree Type

Thesis

Degree Name

Master of Science

Major

Mechanical Engineering

Major Professor

John E. Caruthers

Committee Members

Ahmad Vakili, Roy Schulz

Abstract

Jet engines are tested in facilities (engine test cells) that simulate high altitude conditions. If the natural resonant modes of the facility geometry couple with the jet's resonant modes, there is the potential for extremely high amplitude sound levels. This investigation examines the sensitivity of the coupling mechanism to alterations in the jet's mean flow field and shear layer. Subscale models of a jet engine test cell and three supersonic nozzles (two converging/diverging rectangular nozzles and one convergent conical nozzle) were tested. The model designs were based on a full-scale facility that had experienced very high amplitude sound levels. Tests were conducted for many different operating nozzle pressure ratios and area ratios (for the two rectangular nozzles). The acoustic response to mean flow field alterations was examined in two ways. First, a sharper angled convergent section in one of the rectangular nozzles introduced more non-uniformities and, hence, instabilities into the jet's mean flow field. Second, a series of tests were conducted with the honeycombs (which are located immediately upstream of the nozzle and are used to produce a straighter and more uniform flow) removed. The acoustic response to alterations in the jet's shear layer was examined by running many tests with various boundary layer "trips", designed to thicken the shear layer, constructed along the nozzle surfaces. The rectangular nozzle with the sharper angled convergent section produced higher amplitude sound levels (by 10-15dB) compared with the other rectangular nozzle. However, the amplitudes were still about 20dB lower than those observed at the full-scale facility. Removing the honeycombs generally resulted in higher amplitudes, particularly at an operating pressure ratio of 2.35, where a relatively high amplitude of 150dB at 1250Hz was recorded. These results indicate that a less uniform and, therefore, less stable mean flow field does produce higher amplitude acoustic tones. However, the amplitudes were still significantly lower than those recorded at the full-scale facility. The boundary layer manipulations generally resulted in lower acoustic altitudes. Hence, the higher amplitude tones recorded at the full-scale facility could not be attributed to a thicker shear layer. The findings above indicate a shortcoming in the subscale facility's ability to replicate the acoustic characteristics of the full-scale facility. Since the subscale facility used a cold air jet while the turbines tested in the full-scale facility produce very hot jets, it is suspected that the jet's temperature (particularly, the temperature gradient across the jet's shear layer) is important to the coupling mechanism which produces the high amplitude tones.

Download
Files over 3MB may be slow to open. For best results, right-click and select "save as..."

Share

COinS