R. M. Gehre

The transverse field Richtmyer-Meshkov instability in magnetohydrodynamics

The magnetohydrodynamic Richtmyer-Meshkov instability is investigated for the case where the initial magnetic field is unperturbed and aligned with the mean interface location. For this initial condition, the magnetic field lines penetrate the perturbed density interface, forbidding a tangential velocity jump and therefore the presence of a vortex sheet. Through simulation, we find that the vorticity distribution present on the interface immediately after the shock acceleration breaks up into waves traveling parallel and anti-parallel to the magnetic field, which transport the vorticity. The interference of these waves as they propagate causes the perturbation amplitude of the interface to oscillate in time. This interface behavior is accurately predicted over a broad range of parameters by an incompressible linearized model derived presently by solving the corresponding impulse driven, linearized initial value problem. Our use of an equilibrium initial condition results in interface motion produced solely by the impulsive acceleration. Nonlinear compressible simulations are used to investigate the behavior of the transverse field magnetohydrodynamic Richtmyer-Meshkov instability, and the performance of the incompressible model, over a range of shock strengths, magnetic field strengths, perturbation amplitudes and Atwood numbers.

Computational Investigation of Thermal Nonequilibrium Effects in Scramjet Geometries

To characterize the impact of thermal nonequilibrium in shock-induced combustion scramjets, several thermal equilibrium and nonequilibrium computational fluid dynamics simulations have been performed. Specifically, the effects that would be encountered in shock-tunnel testing are of interest. Therefore, thermal equilibrium and nonequilibrium simulations are run for a Mach 6 shock-tunnel nozzle and a radical farming scramjet model with total enthalpies between 3.3 and 4.9 MJ/kg. Simulating both the nozzle and scramjet with thermal nonequilibrium represents the shock-tunnel test, whereas equilibrium inflow into a scramjet simulated in thermal nonequilibrium provides the scenario encountered in actual flight. For all nonequilibrium simulations, the Landau-Teller model in combination with modified Millikan and White thermal relaxation coefficients is used. It is shown that modeling the entire flow as being in thermal equilibrium results in higher effective temperatures within the local high-temperature regions where ignition occurs, which causes the ignition length to decrease and thus the combustion process to be more efficient compared with the nonequilibrium cases. Additionally, the conditions for ignition in simulations representing shock-tunnel experiments are found to be more favorable than in the simulations representing a flight test, assuming the same enthalpy and boundary conditions. Furthermore, these differences are found to be insensitive to the total enthalpy.

Numerical Investigation of the Mixing Process in Inlet-fuelled Scramjets

For decades, researchers have pursued the development of supersonic combustion ramjet (scramjet) engines. The scramjet is an airbreathing hypersonic propulsion system that utilizes atmospheric oxygen for combustion. Hence, the oxidizer does not have to be carried on-board, which makes scramjets more efficient than rockets and thus very attractive for hypersonic transportation or access-to-space systems. However, hypersonic speeds cause, amongst other challenges, very short residence times within the scramjet combustor. Therefore, achieving a high combustion efficiency poses a major challenge. The combustion efficiency is closely coupled with the mixing efficiency, since most scramjets are mixing limited. Thus, extensive studies focusing on mixing enhancement have been conducted in the past. Different injector shapes and angles [1, 2], flame holding devices [3] and mixing enhancing hypermixers [4] have been investigated in scramjets.

Fluorescence studies of jet mixing in a hypersonic flow

Laser-induced fluorescence (LIF) imaging is a widely used experimental technique [1, 2] that allows for non-intrusive measurements in gases.

The Magnetohydrodynamic Richtmyer-Meshkov Instability: The Oblique Field Case

The Richtmyer-Meshkov instability (RMI) occurs when a perturbed interface separating fluids with different densities is impulsively accelerated, typically by a shock wave [1, 2]. Fig. 1(a) shows the canonical situation where the RMI occurs. The effect of the instability on the interface is shown in Fig. 1(b): it has become highly distorted, which can lead to significant mixing between the two fluids. When the fluids involved are in the plasma state, the RMI can be affected by a magnetic field [3]. This can clearly be seen by comparing Fig. 1(b), which shows simulated postshock- interaction density contours when no magnetic field is present, and Fig. 1(c), which shows the result of an identical simulation carried out in the presence of a normal magnetic field. The observed suppression of the instability in this case is caused by changes to the shock refraction process at the interface with the application of a magnetic field that leave the interface vorticity free [4].

Simulation of laser-induced-plasma ignition in a hypersonic crossflow

A numerical study of a laser-induced-plasma ignition system in a scramjet-like geometry is conducted. Simulations are based on experiments in which hydrogen is injected into hypersonic air-crossflow, compressed by a 9o ramp from a freestream Mach number of 8 to below hydrogen autoignition conditions. Ignition of the jet is conducted using a laser pulse, modelled in the simulations as a small region of high temperature and pressure corresponding to the energy input measured in the experiments. To capture the highly unsteady nature of the fuel-air interface and its coupling with the dynamics of the expanding laser spark, Wall-Modelled Large Eddy Simulations are used to resolve the majority of turbulent motion. Detailed non-equilibrium thermochemistry models are included in the calculations. Simulations have revealed that the cloud of hydroxyl radicals detected in the experiments is created by the blast wave which forms from the expanding plasma kernel merging with the jet’s bow shock. This event raises the temperature and pressure in the upstream mixing layer and spontaneous ignition of the fuel is observed. Shortly after this ignition, combustion ceases due to hostile conditions for ame anchoring. Downstream of the jet, the simulations suggest that the plasma kernel remains can initiate some radical formation in the wake region, but no stable ame forms due to nominally low pressures in the experimental condition studied.

Blast Wave-Induced Mixing in a Laser Ignited Hypersonic Flow

A laser ignition system suitable for a hypersonic scramjet engine is considered. Wall-modeled large eddy simulation (LES) is used to study a scramjet-like geometry with a single hydrogen injector on the inlet, at a Mach 8 flight condition with a total enthalpy of 2.5 MJ. Detailed chemical kinetics and high fidelity turbulence modeling are used. The laser forms a kernel of high temperature plasma inside the fuel plume that briefly ignites the flow and leads to massive disruption of the flow structures around the jet, due to the expanding plasma kernel driving a blast wave that collides with the surrounding flow. The blast wave produces vorticity as it passes through the fuel-air interface, but comparably less than that produced by the jetting of the hot gas affected by the laser as it expands outward into the crossflow. The remnant of the plasma rolls up into a powerful vortex ring and noticeably increases the fuel plume area and the volume of well mixed reactants present in the simulation. These results indicate that the laser ignition system does more than just supply the energy to ignite the flow; it also substantially alters the flow structure and the mixing process.

Combustion regimes in inlet-fueled, low compression scramjets

This paper investigates the combustion regimes that are present in inlet-fueled, low compression scramjets. Injection of fuel in the inlet allows mixing to take place prior to ignition, and permits the fuel plumes to interact with strong shocks and rarefactions at the combustor entrance, which accelerates the mixing process. Consequently, the combustion is partially premixed. Wall-modeled large-eddy simulations (WMLES) are used to accurately resolve or model the turbulent flow structures occurring during the supersonic combustion process. Based on the WMLES data, spatial distributions of turbulent and chemical time scales can be extracted and used to determine the representative turbulent Damkohler and Reynolds numbers. The collective WMLES data is visualized using the Williams diagram, which shows that a wide range of combustion regimes are present throughout the supersonic combustion process.