Postdoctoral Scholar, University of California, Los Angeles
Strategic control of jet in crossflow mixing will be the future for innovative propulsion systems
The present experimental studies focus on a flowfield called jets in crossflow (JICF), typically consisting of a jet flow perpendicularly issuing into a crossflow. This flowfield has been studied for many decades, mainly because of its extensive applications in engineering propulsion systems. In particular, achieving the optimal degree of mixing of this flowfield for a given system is crucial in this research field, e.g., enhancing the mixing to reduce the emission of NOx and/or CO for an environmentally-kind aircraft as practically achieved in the GE TAPS combustor, or lowering the mixing to prevent overheating of turbine blades for a secure aircraft. Mixing characteristics of the JICF has been observed to be deeply related to JICF instability and structural characteristics, suggesting the necessity of fundamental studies in the JICF to understand these characteristics in depth. Also, to obtain the optimal degree of mixing for the JICF, an external forcing of the jet fluid has been considered a potentially very effective technology. Hence, I have been exploring instability, structural and mixing characteristics of the JICF with and without external acoustic forcing, using laser diagnostics to visualize the JICF and enable one to quantify the mixing. Our research group at UCLA has found that the optimal degree of mixing could be potentially achieved by applying one or multiple types of external forcing methods (e.g., sinusoidal and square wave forcing), depending on flow conditions of the JICF, leading to significantly varied JICF instability and structural characteristics. For a relatively weak JICF instability, fairly weak sine wave forcing can have an impact on the JICF mixing. However, for a relatively strong JICF instability, fairly strong sine wave forcing or, with feedback control, square wave forcing is required to affect the JICF mixing. For the strong JICF instability, more advanced forcing method, called double-pulse square wave forcing, is also currently developed, which creates two separate puff-like vortex rings and then triggers interactions or collisions of these structures in the jet’s nearfield to more significantly enhance the JICF mixing. All these studies above suggest that strategic control of JICF mixing will be practically feasible by making a connection between the fundamental characteristics of the JICF from this study and practical requirements in engineering systems, potentially an innovative technology for a future aircraft.