Nicholas Haehn

Turbulent mixing measurements in the Richtmyer-Meshkov instability

The Richtmyer-Meshkov instability is experimentally investigated in a vertical shock tube using a new type of broadband initial condition imposed on an interface between a helium-acetone mixture and argon (A = 0.7). The initial condition is created by first setting up a gravitationally stable stagnation plane between the gases and then injecting the same two gases horizontally at the interface to create a shear layer. The perturbations along the shear layer create a statistically repeatable broadband initial condition. The interface is accelerated by a M = 1.6 planar shock wave, and the development of the ensuing turbulent mixing layer is investigated using planar laser induced fluorescence. By the latest experimental time, 2.1 ms after shock acceleration, the layer is shown to be fully turbulent, surpassing both turbulent transition criteria based on the Reynolds number and shear layer scale. Mixing structures are nearly isotropic by the latest time, as seen by the probability density function of gradien...

Experimental investigation of a twice-shocked spherical density inhomogeneity

Results are presented from a series of experiments investigating the behavior of a twice-shocked spherical density inhomogeneity. The experiments are performed at the Wisconsin Shock Tube Laboratorys (WiSTL) 9?m long vertical shock tube with a square internal cross section, 0.254?m on a side, equipped with a pneumatically retracting bubble injector. Baroclinic vorticity deposition occurs during the interaction of the shock wave with the spherical density inhomogeneity, leading to the formation of a vortex ring. The incident shock wave reflects from the tube end wall and interacts with the translating vortex ring (reshock), resulting in additional vorticity deposition. After reshock, the ambient particle velocity is zero and the subsequent translational motion of the vortex ring is due entirely to the net circulation present. Circulation models are investigated for the reshock scenario and a new model is developed and applied to both once-shocked and reshocked scenarios. Two Atwood numbers (A=0.17,?0.68) and three initial Mach numbers (M=1.35,?2.00,?2.33) are investigated. High-speed cameras at 10?000?fps are used to observe the development of the vortex ring after reshock. An understanding of the shock-induced compression and vortex generation is vital to the future study of turbulent combustion initiated by the shock focusing phenomenon.

Experimental Shock-Initiated Combustion of a Spherical Density Inhomogeneity

A planar shock wave that impulsively accelerates a spherical density inhomogeneity baroclinically deposits vorticity and enhances the mixing between the two fluids resulting in a complex, turbulent flow field. This is known as the classical shockbubble interaction (SBI) and has been a topic of study for several decades [1,2,3,4, 5,6,7,8,9,10,11,12], and closely related the Richtmyer-Meshkov instability (RMI) [13, 14]. While the classical SBI problem concerns a reactively neutral bubble, the present experimental study is the first of its kind in which a spherical bubble filled with a stoichiometric mixture of H2 and O2 diluted with Xe is accelerated by a planar shock wave (1.35 < M < 2.85) in ambient N2, and will be referred to as reactive shock-bubble interaction (RSBI).Experimental results for an inert spherical density inhomogeneity accelerated by a strong incident shock wave (M = 2.8) are compared with a reactive mixture of similar density. When a heavy bubble is shock accelerated in a lighter ambient gas corresponding to a large Atwood number (A > 0), the shock wave at the exterior periphery of the bubble travels faster than the interior transmitted wave, resulting in shock-focusing at the downstream pole of the bubble. The shock wave convergence results in localized temperatures and pressures an order of magnitude higher than the conditions behind the shock wave. If the bubble is composed of a reactive mixture, these localized conditions allow for a controlled, point-source ignition for the combustible mixture within the bubble. The chemical and hydrodynamic coupling is investigated. The reactive mixture is composed of a stoichiometric mixture of H2 and O2 diluted with Xe (30%, 15% and 55% by molar fraction, respectively), corresponding to A = 0.5. For the purpose of comparison, experiments are performed on an inert mixture, where the Atwood number is matched using a combination of Xe and He (58% and 42% by molar fraction, respectively). The experiments are performed at the Wisconsin Shock Tube Laboratory in a 9 m vertical shock tube with a 25.4 £ 25.4 cm 2 cross-section. A pneumatic injector is used to generate a 5 cm diameter soap bubble fllled with the gas mixture. The injector retracts ∞ushly into the side of the tube releasing the bubble into a state of free fall (Ranjan 2005, 2007). Diagnostics are performed using chemiluminescence of the OH i molecule present during the combustion process and planar Mie scattering with a frequency doubled Nd:Yag. Due to an inherently weak signal, the chemiluminescence is captured with an intensifled CCD camera, while the initial conditions are captured with a front-lit, high speed camera.