Heeseok Koo

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...

Vibrational non-equilibrium effects in supersonic jet mixing

A joint experimental and computational study is being conducted to investigate the effects of vibrational non-equilibrium on supersonic combustion, although the focus of this paper is on mixing between a supersonic jet and a subsonic coflow. A new facility has been constructed that consists of a Mach 1.5 turbulent jet issuing into an electrically heated coflow. In the preliminary experiments reported here, air is used in both the jet and the coflow. The degree of non-equilibrium in the jet shear layers is quantified by using high-spectral resolution timeaverage spontaneous Raman scattering. The Raman scattering is complemented with planar temperature imaging using Rayleigh scattering. Much of the current work is focused on the extent to which vibrational non-equilibrium can be assessed by using time-averaged Raman scattering in a turbulent flow with large-scale temperature fluctuations. The experimental work is supported by direct numerical simulation of related jet flows. Preliminary DNS of turbulent jets in coflow with imposed vibrational non-equilibrium shows that vibrational relaxation effects have a first-order effect on the jet temperature field and mixing physics.

Interaction of vortices due to diaphragm on the wall blowing surface

LES analysis was conducted to investigate the role of diaphragm on the downstream flow dynamics. Special attentions are put on the evolution of turbulent coherent vortices, the interaction and merging phenomenon of vortex on the fluctuating flow dynamics in the vinicity of injection wall. Even though the chemical reaction was not considered, the calculation results can be useful in determining the turbulenct characteristics generated by the diaphragm. The calculation results show that the installation of diaphragm can introduce the decreasing or increasing trend of fluctuating motion on the downstream flow field depending on the axial location. Calculation results also revealed that the streamlines analysis and vorticity distribution could represent the phenomena of the decrease or increase local heat flux due to the vortices and oscillatory flow motion as in the actual hybrid rocket motor. This downstream flow phenomenon can be influenced by the interaction of the active and strong vorticity production and merging, and the axial length to the downstream after the recirculation zone.

LES of a sooting flame in a pressurized swirl combustor

Large eddy simulations (LES) of a model aircraft combustor at different pressure and operating conditions are conducted. Detailed models for soot formation and evolution is used along with minimally-dissipative numerical schemes in a fully unstructured mesh simulation of this complex geometry flow. Two slightly different swirl combustors, one operated at atmospheric pressure and the other at higher pressures (3-5 bars) are used. Both combustors are stabilized by strong swirl generated by inlet swirlers. In both cases, a set of secondary injection ports are present that mimic the rich-quench-lean combustor design. The objective of this work is to explore the role of soot trajectories on the intermittent nature of particulate generation. It is found that soot intermittency comes from the trajectories traveled by the soot particles. Only a small portion of the combustor exhibit conditions suitable for soot particle growth. Due to the chaotic nature of the turbulent flow, only a small fraction of the fluid elements pass through this region, which leads to spatial and temporal intermittency. Simulations at various pressures show that with increasing pressure, jet breakdown and mixing is more efficient, which somewhat curtails the generation of fuel-rich pockets needed for particle growth. It is also observed that intensity of soot-turbulence interaction becomes stronger as the operating pressure increases.

Improved Large-Eddy Simulation Validation Methodology: Application to Supersonic Inlet/Isolator Flow

A process was demonstrated that enables more meaningful comparisons of experimental and large-eddy simulation results of the same flowfield. This approach was motivated by recent studies, which show the importance of modeling the measurement device or process when making comparisons of this nature. In the current case, a large-eddy simulation of a Mach 5 inlet/isolator flow is the subject of the validation process, a flow that was previously validated using particle image velocimetry measurements. In the present study, the particle image velocimetry measurement process is emulated by adding particles with inertia to the large-eddy simulation and using synthetic particle image velocimetry to mimic the process by which velocity data are extracted. Analysis of the particle response reveals modifications to the underlying flow consistent with the original particle image velocimetry measurements, including a drastic decrease in rms velocities in the vicinity of shockwaves and weaker velocity gradients througho...

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.

LES for Turbulent Duct Flow with Surface Mass Injection and Vortex Shedding

Hybrid rocket shows interesting characteristics of complicated mixing layers developed by interactions between turbulent oxidizer flow and mass flow from surface due to fuel vaporization. In this study, compressible LES with a ring structure attached at the entrance of the combustor are performed. According to one recent report, adding a ring structure in the middle of the combustor helps increasing regression rate. From the numerical results, it is seen that vortex structures near the wall becomes stronger due to the interaction with surface mass injection, and the local heat flux increases due to the vortices. This phenomenon is obviously related to the generation of dimple structures which are seen in the number of experiments. Also, the ring structure at the entrance induces strong vortex flow which enhances heat transfer to the wall surface and mixing between fuel and oxidizer as well as reaction efficiency.

The Effect of a Block on Flow Oscillations near the Fuel Surface with Wall Blowing

The stability and combustion behavior in hybrid rockets are significantly affected by the formation of vortex and their interactions. Experimental results also show that the change in the flow conditions prior to entering the fuel grain can also alter behaviors of regression rate and pressure fluctuations. This study focuses on effect of vorticity interactions on the combustion behaviors by controlling the vortex formation in the pre-chamber. In this regard, an annular-shape obstacle located in front of mass-injection part was used in combustion tests. A numerical calculation with LES was done to simulate the evolution of flow interactions including vortex dynamics and pressure oscillations in the cylindrical configuration. It is found that flow structure is changed in that momentum and fluctuation are increased near wall, and an additional layer of vorticity is created due to the obstacle which interacts with near-wall vortices generated by mass blowing. Also due to mass injection through the wall, a number of positive azimuthal vorticity spots are created that effectively squeeze mainstream flow toward the wall.