Vortex Dynamics in Synthetic Jets Impinging onto Walls

ABSTRACT

A series of experimental studies are conducted for synthetic jets impinging onto porous walls via laser induced fluorescence (LIF) and particle image velocimetry (PIV). For the case of synthetic jet vortex rings impinging onto a solid wall, it is illustrated that a large-scale spiral vortex is generated in the near wall flow field at small stroke lengths. The secondary vortex induced by the impinging primary vortex ring plays a key role in the formation of this large-scale vortex. As for the impingement onto permeable porous walls, it is shown that the mechanism of vorticity cancellation plays an important role in the formation of downstream transmitted vortex ring; while at a high Reynolds number, the K–H instability within the finger-type jets plays a negative role in the generation of the transmitted vortex ring. A similarity parameter is proposed to quantitatively characterize the dynamical features of the synthetic jet/porous wall interaction. Physically, this similarity parameter reflects the relative importance of the forces due to the momentum relative to the viscous forces, and the drag forces associated with the porous wall geometry, which provides a better understanding on the mechanism of vortex/wall interaction.

Experimental Investigation on Rising Bubbles with/without Liquid Crossflow

ABSTRACT

Bubble flow has attracted significant attention from both industry and academia. It can be generated by injecting air into a liquid and has various practical applications, such as in mass transfer enhancement, underwater environment changing, etc. In this talk, we will present some experimental studies on rising bubbles both with and without liquid crossflow. In the experiments without crossflow, we used both planar PIV and Tomographic PIV to quantitatively acquire the bubble morphology and induced flow fields. Both the effects of gas flow rate (GFR) and orifice spacing on rising bubbles have been investigated and discussed, with special attention paid on the mechanisms of path instability of rising bubble. It was found that the bubble-induced wake shedding plays a key role in triggering the path instability. In addition, the bubble deflection direction caused by the path instability is closely related to the sequence of wake vortices shedding: bubbles turn left with a clockwise vortex shedding first and right with a counterclockwise one. In experiments with crossflow, we examined how crossflow velocity and GFR influence the bubble formation and rising behavior. An increase in GFR results in a larger bubble diameter and faster rising speed. Conversely, an increase in crossflow velocity causes bubbles to detach earlier from the orifice, leading to a reduced bubble diameter and slower rising speed. A theoretical model based on dynamic force equilibrium had been developed to explain these experimental observation. It is believed that our findings could provide some foundational references for modeling two-phase flows and also provide some guidance for the practical application of bubbles.

Professor Xu Yang received his B.S. in Engineering Mechanics from Beihang University in 2011, and his Ph.D. in Fluid Mechanics from Beihang University in 2017. From 2015 to 2016, he conducted the joint PhD project in George Washington University, and worked as a visiting scholar in Pusan National University in 2018. Since 2024, he was promoted as the full professor in Beihang University. His research interests included technique developments for experimental fluid mechanics, bubble & bubbly flow, vortex dynamics, heat transfer using impinging jets. He won the National Postdoctoral Program for Innovative Talents in China in 2017 and the Excellent Youth Project of Natural Science Foundation of China in 2023. In recent 5 years, he has published more 20 journal papers in such as JFM, Exp. Fluids, etc. He serves as deputy secretary general of Beijing Mechanics Society and the fellow of visualization committee of Chinese Aerodynamics Society. He is also the editorial board member of FDMP, and youth editor of Aerodynamic Research & Experiment.