Date of Award

3-2-2021

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Mechanical, Automotive, and Materials Engineering

Keywords

CFD, Turbulent flow

Supervisor

Ronald Barron

Supervisor

Ram Balachandar

Rights

info:eu-repo/semantics/embargoedAccess

Abstract

Oscillating jets have many practical applications in industry. The self-oscillating behavior of a jet can be observed when the jet emanates into a confined cavity. In this thesis, a step-by-step approach has been followed to investigate important aspects of self-oscillating turbulent jets. The first step focuses on evaluating the characteristics of self-oscillating square and round jets. The jet exits from a submerged round nozzle or a square nozzle with the same hydraulic diameter into a narrow rectangular cross-section cavity at a Reynolds number of 54,000 based on nozzle hydraulic diameter and average jet exit velocity. A numerical investigation of the three-dimensional self-oscillatory fluid structures in the cavity is conducted by solving the unsteady Reynolds-Averaged Navier-Stokes (URANS) equations using a Reynolds stress turbulence model (RSM). Vortex identification using the λ2-criterion method is used to investigate the flow dynamics. The simulations show that the vortex rings initially have the nozzle shape near the nozzle exit and, for a square nozzle, axis-switching occurs at about 0.7 hydraulic diameters downstream. Furthermore, after impact on the walls, the vortex rings are converted into two tornado-like vortices. The decay rates of both types of self-oscillating jets initially show the same trend as free round and square jets but change significantly as the effects of oscillation and confinement begin to dominate. The results show that the spread and decay rates of the self-oscillating square jet are higher, while the self-oscillating round jet has higher turbulence intensities near the jet center. Moreover, the Reynolds stress profiles of both round and square self-oscillating jets are qualitatively similar and show two peaks on either side of the centerline, which convert to mild peaks at distances farther downstream.The second step focuses on the numerical study of self-oscillating twin jets emanating from round and square cross-section nozzles into a narrow rectangular cavity. The flow characteristics are evaluated at nozzle spacing-to-diameter ratios of 2, 3, 4 and 5 at a jet Reynolds number of 27,000. The effects of nozzle spacing on the frequency of oscillation, mean velocity and turbulence features are examined. The results indicate that increasing the spacing does not have much effect on the frequency of oscillations. For a spacing-to-diameter ratio up to four, the two jets merge in the downstream and oscillate as one. At the largest nozzle spacing, the two jets do not merge but oscillate separately across half of the cavity width. Furthermore, as the nozzle spacing is increased, the profiles of Reynolds shear stress demonstrates that the mixing increases in the inner shear layer region. The last part of the thesis focuses on potential cooling applications of self-oscillating jets. The jet exits from a square cross-section nozzle at a Reynolds number of 54,000. The heated devices are attached externally on the front surface of the cavity. A three-dimensional numerical simulation of the flow is conducted by solving the URANS and energy equations with RSM to assess the thermal features of the flow field. The cooling performance of the self-oscillating jet is compared with the channel flow and the wall jet. The results show that the channel flow has the lowest heat transfer. The heat transfer of wall jets increases around the central region, while the heat transfer of self-oscillating jets is higher farther from the central region. Self-oscillating jets can improve heat transfer over a larger area when the heated elements are in a horizontal arrangement, while the wall jet shows a higher performance for a vertical arrangement of elements.

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