Date of Award

5-16-2024

Publication Type

Thesis

Degree Name

M.A.Sc.

Department

Mechanical, Automotive, and Materials Engineering

Keywords

Confined Jet;Turbulent Jet;Vortical Structures

Supervisor

Ram Balachandar

Abstract

The turbulent flow resulting from a round jet issuing into an axisymmetric confined chamber was investigated numerically using Large Eddy Simulation. The Reynolds number based on nozzle exit conditions was 3×104. The flow properties of the confined jet were compared to that of a free jet to examine the confinement effect under submerged conditions. To draw a conclusive argument on the effect of the confinement, the mean and turbulent quantities of both flow fields were compared. Proper Orthogonal Decomposition was also performed on the flow field to determine the structures/events with the highest contribution to the total turbulent kinetic energy. Attention was focused on how the confinement modified the coherent structures identified in the near exit region of the jet as these structures play important roles in particle transport and mixing. The results obtained for the confined jet differ substantially from those of the free jet, both in the mean and instantaneous flow fields. These differences are attributed to the confining wall, which introduces a prominent recirculation zone between the shear layers and the confining wall. As a result, the once quiescent ambient became turbulent. The increased turbulence in the flow field produced wider mixing zones in the flow field. The Proper Orthogonal Decomposition results revealed that the contribution of the first mode to the total turbulent kinetic energy in the confined jet was three times higher than that of the free jet. The recirculation zone was identified to be the main contributor to this process. In analyzing the three-dimensional structures using the λ2 criterion, two dominant structural modes were identified in the free jet: ring and helical modes. The eventual breakup of the free jet structures led to the formation of secondary line vortices. In the confined jet, the helical mode was absent, and the turbulent fluid hastened the breakup of the ring vortices. The interaction of the secondary line vortices with the breaking structures led to the formation of new hairpin-like vortices. These structures contributed to the further breakup of the primary ring vortices.

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