Numerical analysis of merging supersonic streamwise vortices for enhanced mixing in scramjet combustors

Abstract

In this thesis the merging of streamwise vortices in supersonic flow is investigated numerically, for the application of mixing in scramjet engines. The benefits of using streamwise vorticity to counteract the effect of compressibility on shear layer mixing are well established in literature. Previous work has shown that different interactions between vortices can be used to achieve different outcomes for mixing as well, like increasing the spreading of fuel across the combustor cross section, or increasing turbulence production further. Recent experiments at the University of Texas Arlington have identified several interactions between co-rotating vortices from adjacent vortex pairs that may be used to enhance mixing further.In this work an interaction between merging co-rotating vortices from adjacent counter rotating vortex pairs is investigated. The particular interaction was identified in the experimental campaign , as being useful for mixing because of the large region of vorticity, high strain rates and an increase in average turbulent kinetic energy in the merging vortices it produces. The increase in turbulent kinetic energy indicates this interaction may be used to target turbulent production in specific regions of the flow. RANS simulations with the Spalart-Allmaras turbulence model were able to reproduce vorticity and strain rates similar to that seen in the experiments. The increase in average turbulent kinetic energy was not reproduced but a change in the trend around the same location in the experiments was found, and the turbulent kinetic energy in the middle of the merging vortices increased around the same location. The Spalart-Allmaras model with compressibility correction was found to perform worse than the standard Spalart-Allmaras model for this flow, underpredicting the spreading of vorticity by the merging vortices. The same interaction was investigated at a series of Mach numbers from M=2.5 to M=4.5. The behaviour of the merging vortices is found to be similar for each Mach number when the freestream velocity is taken into account, allowing for the behaviour to be characterised in different stages relative to the rotation of the merging vortices. In this analysis the increase in strain rates and turbulent kinetic energy is linked to a period of rapid rotation of the merging vortices and regions of circulation that can be observed in the rotating reference frame of the merger, known as ghost vortices. This explanation expands on a similar explanation that arose from the University of Texas Arlington experiments. The rapid rotation and ghost vortices behaviour is used to relate the trend in turbulent kinetic energy to stages in the physics of vortex merging defined in literature. The results indicate that this merging interaction is less effective at increasing the strain rates and turbulent kinetic energy with increasing Mach number.

Date of Award	Jul 2024
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Northern Ireland Department for the Economy
Supervisor	Declan Nolan (Supervisor), Rob Watson (Supervisor) & Marco Geron (Supervisor)