Romain Fiévet

Numerical simulation of a scramjet isolator with thermodynamic nonequilibrium

Flow inside the isolator of a scramjet engine is likely to be in thermal non-equilibrium due to successive shock-based compressions and expansions. Given the short flow-through timescales in such engines, the flow might not reach equilibrium even in the fuel injection region. Since the distribution of energy in the internal modes affects chemical reactions, non-equilibrium has been shown to have significant impact on ignition and flame stabilization. In this study, detailed numerical simulation of a supersonic turbulent channel flow coupled with a multi-temperature model is used to quantify the impact of thermodynamic non-equilibrium on the shock train structure inside the isolator and the flow characteristics at the outlet.

Numerical Investigation of Shock-Train Response to Inflow Boundary-Layer Variations

A dataset of normal shock trains in a rectangular cross-section channel has been created from direct numerical simulations in an effort to quantify the impact of inflow confinement ratio on the sho...

Effect of equivalence ratio and turbulence fluctuations on the propagation of detonations

The effect of turbulence and inhomogeneities on the propagation of a 3-dimensional detonation is studied in a practical configuration. An in-house Navier-Stokes solver using detailed H2−O2 chemistry is first validated on known test-cases, then used on the target calculation. The geometry chosen is that of a linearized RDE with non premixed injection, operating at low initial pressure and temperature. The results are analyzed with an emphasis on two important metrics describing the nature of the detonation, shock front velocity and induction length.

Spontaneous Raman Scattering Temperature Measurements and Large Eddy Simulations of Vibrational Non-equilibrium in High-Speed Jet Flames

High-speed turbulent diffusion flames were investigated using time-averaged spontaneous Raman scattering to determine the vibrational and rotational temperature of the major diatomic species, N2 and O2. Mixing-induced thermal non-equilibrium is detected in the shear layer upstream of the turbulent hydrogen flame in N2 molecules but not O2. Rotational temperatures of the two species agree to within the measurement precision. The non-equilibrium is relaxed immediately beyond the average flame-base location due to the presence of combustion products. The non-equilibrium measured in a lower speed methanehydrogen flame is significantly weaker, as expected. The presence of non-equilibrium is confirmed using Rayleigh thermometry images to quantify the effect of translational temperature variation in the Raman measurement volume. The effect of interspecies vibrational energy transfer is investigated using large-eddy simulations of the experimental flow. Good agreement is found between the measurements and average simulated temperature fields when the interspecies vibrational coupling is very weak.

Numerical investigation of vibrational relaxation coupling with turbulent mixing

In flows where the relaxation rate of vibrational motion of the molecules to equilibrium is comparable to the flow through time scales, the presence of turbulence can alter the mixing and equilibration process. To understand the coupling between mixing and vibrational relaxation, a novel state-specific species model is solved in a background turbulent flow. The method is applied to mixing of two nitrogen streams at different static temperatures. The relaxation rates for each state are computed using quasi-classical trajectory analysis. For the flow conditions considered, the first ten vibrational levels are computed in the flow solver.The direct numerical simulation shows that population in different vibrational levels are significantly affected by turbulence and that the local distribution becomes nonBoltzmann. In certain locations in the jet, the population from the direct calculation can be several orders of magnitude different than the local-temperature based Boltzmann level. Last, while the bulk vibrational energy is inferior to its local equilibrium value throughout the mixing layer, the high energy level populations (levels 3 to 8) are on the opposite always over-populated. As chemical reactions are affected by these high vibrational energy populations, a simple temperature model would under-estimate the impact of nonequilibrium on combustion.

Numerical simulation of shock trains in a 3D channel

A dataset of normal shock trains in a rectangular cross-section channel has been created from Direct Numerical Simulations (DNS) in an effort to quantify the impact of inflow confinement ratio on the shock train structure. To this end, only the inlet boundary layer momentum thickness was varied while the bulk inflow and outflow conditions remained constant. The fully-resolved 3D turbulent boundary layer inflows correspond to atmospheric air isentropically expanded to Mach 2 and were obtained from auxiliary DNS. The simulations show that a change of inflow confinement ratio has a nonlinear impact on the shock train location, with a reduction in boundary layer momentum thickness leading to a displacement of the shock train downstream inside the isolator. As expected, an increase in boundary layer momentum thickness results in a reduction of the normal-like portion of the lambda-shock structures in the tunnel core. This leads to more numerous but weaker bifurcating shocks as well as an increase of the shock train length. It is also found that the growth rate of the boundary layer past the first bifurcating shock is dependent on both the inflow momentum thickness and the relative speed of the shock train compared to the bulk flow. When the inflow boundary layer thickness is varied temporally, the complex shock train response depends strongly on the excitation frequency. Its location along the tunnel is as expected more sensitive to lower frequencies while the shock train length exhibits a band-pass filter behavior.