This project involves the study of low density non-reactive and reactive (burning) jets injected into crossflow, a topic applicable to improved efficiency and reduced emissions for a range of energy and propulsion systems. The ability to control the trajectory, spread, mixing, reaction completion, and temperature or density field associated with such jets is the overall goal of these studies. The present approach is primarily experimental, involving optical diagnostics as a means of interrogating and analyzing the instabilities in the flowfield as a means of developing methods for their control. This work will help to advance our ability to design and develop highly efficient future engines for stationary power plants, airbreathing propulsion systems, and rocket propulsion systems.
The intellectual merits of this research involve detailed explorations of the mechanisms underlying convective to absolute shear layer instability transition for low density non-reactive and reactive transverse jets. For both equidensity and low density jets in crossflow, there is significant evidence to confirm that the shear layer becomes globally unstable or self-excited; such evidence includes the initiation of strong single-tone disturbances that do not vary along the jet's shear layer, the inhibition of subharmonic initiation, lock-in to applied frequencies only at very large forcing amplitudes and/or at frequencies close to the fundamental, and evidence of a Hopf bifurcation. We are able to take advantage of the knowledge of these instabilities in applying strategic forcing to the transverse jet, whereby relatively weak sinusoidal jet excitation can enhance the convectively unstable jet, but strong square wave forcing at specific temporal pulse widths is required to control the absolutely unstable transverse jet. Yet the specific mechanisms for this transition, a quantification of altered mixing states during different excitation conditions, and the behavior of transverse jets in the presence of a chemical reaction are all significant unknowns, with substantial practical relevance.
These experimental studies employ laser diagnostics, including particle image velocimetry (PIV) simultaneous to planar laser-induced fluorescence (PLIF) imaging of seeded acetone, in order to quantify 3D velocity and vorticity fields as well as scalar transport processes in transverse jets. Building on these explorations will be quantification of mixing associated with the non-reactive flow (equidensity as well as low density jets) via acetone PLIF, as a means of determining flow and excitation conditions to optimize jet-crossflow mixing. Closed loop control methods are being developed for the optimization of jet behavior and response to upset conditions, using non-intrusive sensing whose measurements may be related to global flow characteristics. With respect to the reactive jet in crossflow, OH* chemiluminescence imaging will be used to determine flame characteristics, instabilities, and flow characteristics. These fundamental explorations will be critical to the ability to predict and control such instabilities in a range of engineering systems.
The broader impacts of the proposed studies go well beyond the practical applications of the transverse jet and the usual "graduate level research and education". Our track record in such impacts has included undergraduate research training as well as outreach to and laboratory experiences for local high school students and underrepresented students at other universities. In the present project, undergraduate researchers will continue to be trained and employed in our laboratory, and outreach presentations and demonstrations will continue to be provided for local high school and potentially middle school and elementary school students, with special focus on students at public schools. Opportunities for local high school students to gain laboratory experiences in the summer will be provided. Cutbacks in funding by the state of California to the public schools- MESA (Mathematics Engineering Science Achievement) programs has made it even more important for universities such as UCLA to extend outreach programs to public middle schools and high schools as a means of encouraging the development of future engineers and scientists.