Masters Theses

Date of Award

12-2006

Degree Type

Thesis

Degree Name

Master of Science

Major

Aerospace Engineering

Major Professor

Ahmad Vakili

Committee Members

Basil Antar, Christian Parigger

Abstract

A systematic study was performed of the flow phenomena in an axisymmetric cavity. Axial flow Reynolds numbers ranged from 100,000 to 530,000 for cavity L/D range of 0.25 to 1.65. Working medium for this study was water reflecting that only incompressible domain of cavity flow was covered in this study. Previous results of axisymmetric flow investigations were reproduced and extended to lower L/D values, with the modified test article. Introduction of swirl in the cavity flow was studied in detail for its effects on the enhancement or suppression of cavity modes that have been measured.

The axial apparatus tested here recreated the results of that of the original apparatus. The values of the maximum peaks fell within the range of Helmholtz resonator values at that specific velocity, for a given L/D at and below L/D = 1.05, at the same flow conditions.

At and above moderate swirl numbers (S > 0.4), first mode of oscillation was completely suppressed. There was also evidence of a flattening of the remaining trend at higher swirl numbers (S > 0.6).

The range of maximum amplitudes experienced in this study was also in agreement with earlier work.

Overall, a trend of an increase of the frequency with an increase in swirl was seen in all tests. Generally, the range of greatest increase was for 47.23 ft/s axial flow, with the addition of 38.56 ft/s to 54.54 ft/s of tangential flow. The frequency increase was attributed to the effects of swirl through the range of 0.3 < S < 0.6. Over this range, the magnitude of the peak oscillation was seen to decrease markedly up to the point that the spectrum depicted no peaks, only a nominal level of broadband noise. This resulted in the conclusion that swirl effect existed only within a certain range of swirl numbers, and that the high swirl caused the elimination of certain cavity generated pressure oscillations.

With the introduction of swirl, through tangential injection, the frequency of peak oscillations in axisymmetric cavity flow can be controlled to a limited extend. The primary region of control lies in the moderate swirl number region of 0.3 < S < 0.6.

The presence of helical modes in the flow was explored. Aliasing of high frequency acoustical tones is presented with peaks noted at the calculated values demonstrating the potential for the presence of helical modes in the flow. Helical modes were present at higher swirl velocities, with a trend approaching near agreement with theory.

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