Development and Application of Particle Image Velocimetry in High-Speed Separated Flow: Two-Dimensional Base Cavities
Molezzi, Michael Joseph
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https://hdl.handle.net/2142/72239
Description
Title
Development and Application of Particle Image Velocimetry in High-Speed Separated Flow: Two-Dimensional Base Cavities
Author(s)
Molezzi, Michael Joseph
Issue Date
1993
Doctoral Committee Chair(s)
Dutton, J. Craig
Department of Study
Mechancial Engineering
Discipline
Mechanical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Aerospace
Engineering, Mechanical
Abstract
A new particle image velocimetry (PIV) system has been developed to obtain two-dimensional instantaneous velocity data over a planar region in high-speed separated flows for the quantitative analysis of turbulent and unsteady flow structures. This PIV system is the first of its type to incorporate sub-micron seed particles and birefringent image shifting for the resolution of flow velocity in separated regions. The system was also developed with improved in-plane spatial resolution over previous high-speed flow PIV applications by using a 1.0 mm$\sp2$ interrogation region (in flowfield dimensions) for each independent velocity measurement. The system has been proven in preliminary experiments using a simple low-speed round jet flow and had been validated for accuracy with both known-displacement simulated PIV photographs and uniform flow experiments at Mach 0.5 (170 m/s) for comparison to pressure and laser Doppler velocimeter (LDV) data.
The PIV system was also used in a study of the near-wake structure of a two-dimensional base in subsonic flow. This application was chosen in order to determine the fluid dynamic mechanism of the observed base drag reduction in the presence of a base cavity. Experiments were done over a range of freestream Mach numbers up to 0.8, including local flowfield velocities up to 300 m/s. Effects of the base cavity on the von Karman vortex street wake were found to be related to the expansion and diffusion of vortices near the cavity, although the effects are of small magnitude and no significant change of the vortex formation location or path was observed. The base cavity effects are also less significant at higher freestream velocities due to the formation of vortices further downstream from the base. The base cavity drag reduction was found to be mainly due to the displacement of the base surface to a location upstream of the low-pressure wake vortices, with only a slight modification in the vortex street itself.
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