ABSTRACT
In this talk, we will discuss nonlinear instabilities in two different fluid systems. The hydrodynamic system is a low-density axisymmetric jet undergoing a Hopf bifurcation to limit-cycle oscillations. While this bifurcation has been reported to be supercritical in the literature, we show experimentally that it can also be subcritical, revealing the existence of higher-order nonlinearities in the system. We also propose a universal Reynolds number scaling for predicting the onset of global instability in low-density jets. By applying noise to the jet in its unconditionally stable regime, we identify and characterize coherence resonance in the system. In the second system, we study the aeroelastic instability of a cyber-physically mounted pitching wing in a water tunnel. We report two distinct modes of flow-induced oscillations, namely a structural mode, which occurs via a subcritical bifurcation, associated with high wing inertia; and a hydrodynamic mode, which occurs via a supercritical bifurcation, associated with low wing inertia. We characterize these two oscillation modes by analyzing the corresponding amplitude, frequency, force and flow response of the wing, and then use an energy approach to explain the existence of these two modes. Lastly, the nonlinear fluid damping of pitching wings in the absence of a free-stream flow will be discussed.

BIOSKETCH
Yuanhang Zhu is a Ph.D. Candidate in the School of Engineering at Brown University. His research interests include fluid-structure interactions, vortex dynamics and nonlinear dynamics in fluid systems. He is currently working with Prof. Kenny Breuer on flow-induced oscillations and leading-edge vortex dynamics of cyber-physically mounted pitching wings. Prior to Brown, Yuanhang obtained his M.Phil. and B.Eng. in Mechanical Engineering from the Hong Kong University of Science and Technology, where he worked with Prof. Larry Li on the nonlinear hydrodynamic instability in low-density jets.

For more information, please contact Prof. Ann Karagozian at ark@ucla.edu.
All faculty, students, researchers, and guests are welcome to this event.