Families of two-dimensional, unsteady shock-induced vortical flows are simulated numerically The flows
consist of one or more regions of light gas, surrounded by heavy gas, being overtaken by a normal shock wave
The interaction of the density gradient at each light/heavy interface with the pressure gradient from the shock
wave generates vorticity This causes the light gas regions to roll up into one or more counter-rotating vortex
pairs, which stir and mix the light and heavy gases The mixing is characterized by an asymptotic stretching rate
The effects of shock strength, light/heavy gas density ratio, and geometry on the mixing are investigated These
two-dimensional, unsteady flows are analogous to three-dimensional, steady flows that may be used in
SCRAMJET combustors demanding rapid and efficient mixing of fuel and oxidizer For such applications, 1)
the fuel injectors should be elongated in the direction of the shock; 2) multiple smaller injectors are preferable
to a single larger injector; 3) injectors should be arranged in groups of closely spaced pairs, rather than
uniformly; and 4) multiple shock waves should be utilized, if possible

/pdf/applications-of-shock-induced-mixing-to-supersonic-259vlhanh6.pdf

Applications of Shock-Induced Mixing to Supersonic Combustion

Scramjet combustor fuel injection and mixing enhancement techniques are reviewed. The injection techniques include hole injection from the combustor wall, slot injection parallel to the flow, and injection from struts and the rear of ramps. Three new advanced mixing techniques are presented. The first is a combustor, curved so that buoyancy forces will aid in the penetration of the fuel across the combustor. The second is pulsation of the fuel injectors to increase penetration and mixing. A fluidic technique, a modified Hartmann-Sprenger tube, is identified as a strong candidate to generate the pulsations. The third is the injection behind pylons to allow deep penetration into the air stream. This technique is likely to produce high base pressures on the injector structure, particularly if base burning is encouraged. Curved or slanted pylons can be used to increase the recovery of fuel jet momentum. The potential of the new mixing techniques to increase scramjet engine performance is assessed.

Advanced Injection and Mixing Techniques for Scramjet Combustors

An experimental and computational investigation of a
contoured wall fuel injector is presented. The injector
was aimed at enabling shock-enhanced mixing for the
supersonic combustion ramjet engines currently envisioned for applications on hypersonic vehicles. Three-dimensional flow field surveys, and temporally resolved planar Rayleigh scattering measurements are presented for Mach 1.7 helium injection into Mach 6 air. These experimental data are compared directly with a three-dimensional Navier-Stokes simulation of the flow about the injector array. Two dominant axial vorticity sources are identified and characterized. The axial vorticity produced strong convective mixing of the injectant with the freestream. Shock-impingement was particularly effective as it assured seeding of baroclinic vorticity directly on the helium/air interface. The vorticity coalesced into a counter-rotating vortex pair of a sense which produced migration of the helium away from the wall. The influences of spatial averaging on the representation of the flow field as well as the importance of the fluctuating component of the flow in producing molecularly-mixed fluid are addressed.

/pdf/an-investigation-of-a-contoured-wall-injector-for-4n49rv1vsf.pdf

An investigation of a contoured wall injector for hypervelocity mixing augmentation

Rayleigh-Taylor and Richtmyer-Meshkov instabilities: A journey through scales

We report on experiments to measure the shock-induced growth of sinusoidal perturbations on thick interfaces separating two gases of different densities. The results show that the growth rates are reduced as the interface thickness is increased. A model that accounts for the growth rate reduction caused by the presence of a finite density gradient on the interface is proposed and good agreement is obtained with the experimental results.

/pdf/experiments-on-the-richtmyer-meshkov-instability-single-5zbe1268hu.pdf

Experiments on the Richtmyer-Meshkov instability: single-scale perturbations on a continuous interface

A parametric study of a class of contoured wall fuel injectors is presented. The injectors were aimed at enabling shock-enhanced mixing for the supersonic combustion ramjet engines currently envisioned for applications on hypersonic vehicles. Short combustor residence time, a requirement for fuel injection parallel to the freestream, and strong sensitivity of overall vehicle performance to combustion efficiency motivated the investigation. Several salient parametric dependencies were investigated. Injector performance was evaluated in terms of mixing, losses, jet penetration and heating considerations. A large portion of the research involved a series of tests conducted at the NASA Langley High - Reynolds Number Mach 6 Wind-Tunnel. Helium was used as an injectant gas to simulate hydrogen fuel. The parameters investigated include injector spacing, boundary layer height, and injectant to freestream pressure and velocity ratios. Conclusions concerning injector performance and parameter dependencies are supported by extensive three-dimensional flow field surveys as well as data from a variety of flow visualization techniques including Rayleigh scattering, Schlieren, spark-shadowgraph, and surface oil flow. As an adjunct to these experiments, a three-dimensional Navier-Stokes solver was used to conduct a parametric study which closely tracked the experimental effort. The results of these investigations strongly complemented the experimental work. Use of the code also allowed research beyond the fairly rigid bounds of the experimental test matrix. These studies included both basic investigations of shock-enhanced mixing on generic injectors, and applied efforts such as combining film-cooling with the contoured wall injectors. Location of an oblique shock at the base of the injection plane was found to be a loss-effective method for enhancing hypervelocity mixing through baroclinic generation of vorticity and subsequent convection and diffusion. Injector performance was strongly dependent on the displacement effect of the hypersonic boundary layer which acted to modify the effective wall geometry. Strong dependence on injectant to freestream pressure ratio was also displayed. Mixing enhancement related to interaction of the unsteady component of the boundary layer with both steady and unsteady components of the flow field was found to be secondary, as were effects due to variation in mean shear between the injectant and the freestream in the exit plane.

Ian Anton Waitz

Papers

An Investigation of Contoured Wall Injectors for Hypervelocity Mixing Augmentation