Flow dynamics and scalar mixing of transverse jets into crossflows

This research focuses on the simulation of turbulent jets into crossflows under both subsonic and supersonic conditions. The primary objectives are: 1) to establish an efficient numerical framework for treatment of flows at moderate and high Mach numbers; 2) to deepen an understanding of the physical mechanism governing the behavior of transverse jets; 3) to explore the mixing processes in such flows; 4) to study the effects of inlet conditions on flow structures; and 5) to investigate the response of flow dynamics and mixing process to external excitations.;The theoretical formulation applied here is based on three-dimensional conservation equations of mass, momentum, energy and species. The turbulence closure is achieved using a large-eddy-simulation technique. A hybrid scheme combining a lower-dissipation central scheme and a shock-capturing upwind scheme is employed for spatial discretization of the convective terms. Temporal integration is achieved using the Runge-Kutta scheme. The finite-volume approach is used to solve governing equations and associated boundary conditions. The density-based in-house code is paralleled by a domain decomposition method in conjunction with the Message Passing Interface library.;The numerical framework is validated by reproducing the mean flow and turbulent statistics in experiments. The coherent structures and shock waves are captured and their dynamic evolutions are examined according to time-accurate calculations. Findings show the jet shear-layer vortices contribute to the crossflow entrainment in the near field. The hanging vortices break down and account for the early formation of the iv counter-rotating vortex pair. The deflected crossflow separates and induces upright wake vortices. Spectral and proper-orthogonal-decomposition analyses extract shear-layer instability in the near field and suggest transverse jets with current velocity ratios are globally unstable. External low-amplitude excitations have no apparent influence on the flow and mixing fields; moderate and high magnitudes of variations in the crossflow velocity yield strong vorticity generation and subsequent breakdown. The jet core decreases and the gravity center in the mixing field falls, accompanied by an elongated and narrowed jet plume in any transverse planes downstream. In the case of a sonic ethylene jet into supersonic air crossflow, the salient shock structures are presented in the time-averaged field and further elaborated using instantaneous data. The mixing process is closely related to a stretching-tilting-tearing mechanism of shedding eddies. Results also reveal one low-speed, high-temperature zone ahead of the jet with a flame-holding capability, and one subsonic region in the wake providing a potential pathway for the disturbances downstream to travel back to the near field and impact the injection and mixing processes.