Thesis
Experimental investigation of entrainment ratio in the near exit region of jets
Washington State University
Master of Science (MS), Washington State University
2011
Handle:
https://hdl.handle.net/2376/103485
Abstract
The behavior of volcanic ash plumes is highly dependent on entrainment ratio, which is a ratio of the radial mass inflow of air into a jet to its axial mass flow. This ratio is relatively constant under many conditions, and its magnitude is a factor in determining whether a volcanic column becomes buoyant and rises as a Plinian column or collapses to form a pyroclastic flow. Under fully-developed, similar conditions, the value of this ratio is approximately 0.065 for neutrally buoyant-jets and 0.09 for buoyant jets. However, volcanic eruptions generally involve overpressured exit flows and supersonic speeds downstream, and these factors are likely to impact the rate of air entrainment. High fluid pressures also affect exit fluid density, which can also impact entrainment. A small-scale laboratory model of a high-speed jet was developed, and particle image velocimetry was used to study velocity fields both along the jet centerline and in the ambient atmosphere. The experiments consider exit speeds ranging from low subsonic levels up to sonic velocity, with exit pressures up to 2.93 times atmospheric. Multiple gases also were considered, resulting in a range of densities varying by an order of magnitude. The resulting instantaneous velocity fields were integrated to determine the entrainment ratio. As overpressured conditions increased, the entrainment was significantly reduced downstream of the exit, with a mass flow on the order of 50% lower than for pressure-balanced conditions. This may have significant impact on analytical models of volcanic plume development, as the expected plume growth behavior may be overpredicted. Further downstream, the entrainment does increase towards levels previously modeled. The responses were analyzed to determine an appropriate functional fit for entrainment, resulting in an empirical measure of entrainment ratio in terms of overpressure ratio, axial location, and jet density.
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Details
- Title
- Experimental investigation of entrainment ratio in the near exit region of jets
- Creators
- Farhad Saffaraval
- Contributors
- Stephen A. Solovitz (Degree Supervisor)
- Awarding Institution
- Washington State University
- Academic Unit
- Electrical Engineering and Computer Science, School of
- Theses and Dissertations
- Master of Science (MS), Washington State University
- Publisher
- Washington State University; Pullman, Wash. :
- Identifiers
- 99900525017801842
- Language
- English
- Resource Type
- Thesis