The research at The Ohio State University’s Gas Dynamics and Turbulence Laboratory (GDTL) is based on understanding flow physics and control of high-speed and high Reynolds number flows of interest in propulsion and aerodynamic applications. A primary focus of the GDTL is in free shear layers, which are present in many flows of interest in applications. This class of flows, which develops away from surfaces that would impose a no-slip boundary condition, is ubiquitous in practical applications. They include, for example, jets, cavity flows, wakes behind vehicles, and separated flows over solid surfaces. The existence of large-scale structures in turbulent flows in general, and free shear flows in particular, have been known for a long time. This came to focus with two seminal discoveries in free shear flows, which occurred in the 1960s and 70s. The first discovery was the finding that free shear flows, which have vorticity distribution that contains a maximum (or a velocity distribution with an inflection point), are unstable to small perturbations over a wide range of frequencies. This instability is called the Kelvin-Helmholtz instability or the inviscid instability at sufficiently high Reynolds numbers. The second discovery was the existence of coherent large-scale structures in free shear layers, even in very high Reynolds number flows.