Abstract:  The fluid flow through a T-shaped pipe bifurcation (with the inlet at the bottom of the "T") is a very familiar occurrence in both natural and man-made systems.  Everyday examples include industrial pipe networks, microfluidic channels, and blood flows near the heart and brain.  Yet, many questions about the flow physics remain, and prior analyses have been rudimentary and qualitative.  This seminar addresses three important questions: 1) How does the flow evolve with Reynolds number?  2) What are the important flow structures?  3) Lastly, where does the flow exhibit dynamical sensitivity?  Much of this research focuses on the relation between vortex breakdown in the outlet pipes and the regions of stability, receptivity, and sensitivity as defined by linear global stability theory.  The vortex breakdown, which occurs above a Reynolds number of 320, gives rise to recirculation regions near the junction; a supercritical Hopf bifurcation first occurs at a Reynolds number of 556.  Regions of growth are concentrated in the outlet pipes, but regions of receptivity to initial conditions and disturbances are confined to compact regions near the walls of the inlet and junction.  Finally, the flow is most sensitive to localized dynamical perturbations in the recirculation regions, which we explain using an inviscid Lagrangian short-wavelength theory.  To the best of our knowledge, this is the most complicated flow for which anyone has observed the relation between sensitivity and recirculation. Bio:  Kevin Chen attended Caltech as an Axline Scholar, where he received a B.S. with Honor in Engineering and Applied Science, with a focus in Aeronautics, in 2009.  At Caltech, he conducted research in experimental and computational fluid dynamics with Mory Gharib, Beverley McKeon, and Tim Colonius.  In 2011, he received an M.A. in Mechanical and Aerospace Engineering from Princeton University, where he is currently a Ph.D. candidate working under Clancy Rowley and Howard Stone.  He expects to receive a Ph.D. at the end of April.  He has received support from the Barry M. Goldwater Scholarship, the DOD NDSEG and NSF GRFP fellowships, and awards from Caltech and Princeton University.  Kevin's primary research interest is the development of feedback flow control, where fluid mechanics intersect with modern control theory, dynamical systems, and computational methods. Host: Francesco Bullo