Our research focuses on a variety of problems in high-speed flow, including hypersonic turbulence transition, shock-wave boundary-layer interactions, fluid-structure interactions, scramjet unstart phenomena, and meteorics. We also have some interest in low-speed flows, such as the role of wing deformation in dragonfly kinematics.
Transition to turbulence of the boundary layers on a hypersonic vehicle can lead to highly elevated surface heating as well as enhanced frictional drag; understanding the transition process is thus vitally important for the design of such vehicles. On slender axisymmetric or planar bodies, the dominant boundary-layer instability mechanism at hypersonic speeds is typically what is known as the second or Mack mode, which may be thought of as acoustic waves that become trapped within the boundary layer. Studies of the second mode are made difficult by the high frequencies involved - typically several hundreds of kHz. Experimental investigations have shown that it is possible to visualize second-mode wave packets using high-speed schlieren images (see below) and derive valuable information regarding the propagation and breakdown characteristics of these disturbances. Our recent work at the Air Force's AEDC Tunnel 9 explores optical and pressure measurements of boundary-layer transition at Mach 10 and 14.
Shock-Wave Boundary-Layer Interactions
Shock-wave boundary-layer interactions are a complex flow phenomenon occuring in both external and internal supersonic flows. Shock-induced separation of turbulent boundary-layers causes a recirculation region that can result in highly unsteady flow features, dramatically increased thermal loading, and coupling of low-frequency pressure oscillations to vehicle structural modes. While the exact mechanism giving rise to the low-frequency unsteadiness has yet to be identified, experimental and computational efforts have made progress to determine that upstream flowfield features are better correlated to shock motion than is the inflow boundary-layer. We are conducting experiments in a compression corner configuration to investigate both the dominant flow mechanisms and the intense fluid-structure interactions that can develop.
Supersonic Combustion and Propulsion
Scramjets (supersonic combustion ramjets) are a theoretically attractive alternative to rockets for high-speed powered flight. There are numerous challenges, however, associated with bringing a practical scramjet-powered aircraft into operation. One important factor that can restrict scramjet design is engine unstart, i.e., the disgorging of the inlet shock system. Recent studies have investigated the combustor behavior associated with the onset of unstart in a generic scramjet engine. Detailed observations of the formation and propagation of the choking shock train (see below images) has provided a much improved understanding of the linked fluid-combustor phenomena involved, and have shown that it may be possible to stabilize the shock train downstream of injection, leading to increased combustion efficiency and combustor performance.
Dragonfly Wing Dynamics
Dragonflies, the apex predator of the insect biosphere, are highly competent flyers and among the only insects possessing tandem wings that can be independently actuated. Fossils of primordial dragonflies reveal that their general stucture has been modified only slightly over the ages, indicating that they are essentially evolutionarily optimized. Thus, by characterizing their kinematics, we seek to understand the aerodynamic advantages that make dragonflies such a persistent species. Their large, transparent wings allow for marker tracking, so, in subjecting the dragonflies to both wind-off and gusting conditions, we attempt to perform full reconstructions of position, orientation, and wing deformation data.
Hypersonic waveriders are a conceptual vehicle that take advantage of a leading-edge-attached shock to improve the lift-to-drag ratio and consequently increase performance. Waveriders can be rapidly generated by procedurally streamline-tracing an exact flowfield, allowing the lift-to-drag ratio to be computed easily and making them an attractive option for parametric optimization studies. Current work endeavors to implement position and orientation estimation of free-flight waveriders used as test articles in a hypersonic wind tunnel.