Graduation Semester and Year

Summer 2024

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Aerospace Engineering

Department

Mechanical and Aerospace Engineering

First Advisor

Dr. Frank K. Lu

Second Advisor

Dr. Liwei Zhang

Abstract

Flow through curved diffusers is complicated due to the possibility of flow separation and secondary flows. Vortex generators (VGs) are a class of passive flow separation control devices used to improve the flow quality in curved diffusers by suppressing flow separation. However, the literature review demands an in-depth investigation on the physical mechanisms related to flow separation control. Thus, this study aims to improve the understanding of the underlying flow physics associated with VG-induced flow separation control in confined flows. The chosen geometry for this study is a well-documented asymmetric diffuser which exhibits a mild flow separation.

The theoretical formulation of this work is based on the three-dimensional, incompressible form of mass and momentum conservation equations, numerically solved across the computational domain using a finite-volume approach. The turbulence closure is achieved using two approaches: 1) SST k-w based Reynolds-averaged Navier-Stokes (RANS) simulations and 2) dynamic Smagorinsky-Lilly model-based large eddy simulations (LES).

Firstly, a performance evaluation study was conducted using RANS approach. To suppress flow separation, improve pressure recovery and reduce distortion, different VG configurations were deployed upstream of the diffuser inlet. Analysis of skin friction lines in the downstream flowfield revealed various topological features, which helped identify separation and reattachment locations and determine the separation region size. The ramp-type VGs reduced the separation zone with increased total drag. While the upwash vanes consistently underperformed in distortion index across all cases, the downwash vanes with larger size decreased the distortion index without impacting total drag.

A pair of vanes that has demonstrated high performance was selected for further analysis using LES. Turbulence events at several probe locations close to the curved wall and the vertical centerline were analyzed using the normalized joint probability density functions. Under adverse pressure gradients, increased ejection and sweep events were detected near the trailing vortices. The heightened sweep events enhances momentum redistribution and energizes the near-wall flow to prevent flow separation. The trailing vortices from the VGs were visualized using iso-surfaces of the Q-criterion and streamlines on transverse planes. The downwash induced by the trailing vortices energized the near-wall flow, demonstrating a notable reduction in the separation region. By tracking the vortex size at various streamwise locations, a drastic increase in vortex size was detected, suggesting the onset of vortex breakdown. The breakdown location exhibited increased ejection and sweep events, velocity fluctuations, Reynolds stress, and turbulent kinetic energy (TKE) production. The peak TKE production shifted transversely and spanwise toward the high-momentum regions, aiding in the suppression of flow separation through enhanced momentum redistribution.

Keywords

Flow separation control, Vortex generators, Quadrant analysis, Vortex breakdown

Disciplines

Aerodynamics and Fluid Mechanics | Aerospace Engineering | Automotive Engineering | Heat Transfer, Combustion | Hydraulic Engineering | Mechanical Engineering | Propulsion and Power

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