Jae-Ryul Shin

DES Study of Base and Base-Bleed Flows with Dynamic Formulation of DES Constant

DES method is applied to an axisymmetric base and base-bleed flows at supersonic mainstream. The model is based on the Spalart-Allmaras (S-A) turbulence model in the RANS mode, and is based on the sub-grid scale model in the Large-eddy simulation (LES) mode. Accurate predictions of the base flow field and base pressure are successfully achieved by using the DES methodology which is less expensive than LES. Flow properties at the edge of base, such as boundary layer thickness, momentum thickness and skin fraction are compared with Dutton’s experimental data to proper prediction of base flow filed. From the present results, The DES accurately resolves the physics of unsteady turbulent motions, such as shear layer rollup, large-eddy motions in the downstream region and small eddy motions inside the recirculation region. Moreover, the DES analysis of strong compressibility flow, LES mode is intentionally performed in boundary layer with the conventional empirical constant CDES value of 0.65. In this study, an expression is suggested to determine the CDES value dynamically by using a distribution function of the ratio of turbulence length scale and wall distance which is used in S-A DDES model for RANS mode protection. The application of the dynamic CDES presents better prediction than previous results those used constant but different CDES values.

Speed of Sound and Real Gas Solution Algorithm for Supercritical Flows

The definition of the speed of sound is reexamined since it is crucial in the numerical analysis of compressible real gas flows. The thermodynamic speed of sound (TSS), ath, and the characteristic speed of sound (CSS), ach, are derived using generalized equation of state (EOS). It is found that the real gas EOS, for which pressure is not linearly dependent on density and temperature, results in slightly different TSS and CSS. in this formalism, Roes approximate Riemann solver was derived again with corrections for real gases. The results show a little difference when the speeds of sound are applied to the Roes scheme and Advection Upstream Splitting Method (AUSM) scheme, but a numerical instability is observed for a special case using AUSM scheme. It is considered reasonable to use of CSS for the mathematical consistency of the numerical schemes. The approach is applicable to multi-dimensional problems consistently.

Dynamic Correction of DES Model Constant for the Advanced Prediction of Supersonic Base Flow

The DES analysis of strong compressibility flow, LES mode is intentionally performed in boundary layer with the conventional empirical constant value of 0.65. In this study, an expression is suggested to determine the value dynamically by using a distribution function of the ratio of turbulence length scale and wall distance which is used in S-A DDES model for RANS mode protection. The application of the dynamic presents better prediction than previous results those used constant but different values.

Numerical Modeling of Hydrazine-Fueled Arcjet Thruster

The computational fluid dynamic analysis has been conducted for the thermo-chemical flow field in an arcjet thruster with mono-propellant Hydrazine (N2H4) as a working fluid. The Reynolds Averaged Navier-Stokes (RANS) equations are modified to analyze compressible flows with the thermal radiation and electric field. The Maxwell equation, which is loosely coupled with the fluid dynamic equations through the Ohm heating and Lorentz forces, is adopted to analyze the electric field induced by the electric arc. The chemical reactions of Hydrazine were assumed to be infinitely fast due to the high temperature field inside the arcjet thruster. The chemical and the thermal radiation models for the nitrogen-hydrogen mixture and optically thick media respectively, were incorporated with the fluid dynamic equations. The results show that performance indices of the arcjet thruster with 1kW arc heating are improved by amount of 180% in thrust and 200% in specific impulse more than frozen flow. In addition to thermo-physical process inside the arcjet thruster is understood from the flow field results.

Three-Dimensional Simulation of Ignition Transient in Hyshot Scramjet Configuration

HyShot scramjet combustor flowfield, where gaseous hydrogen is transversely injected into a supersonic cross flow, is simulated with DES turbulence and finite-rate chemistry model. The injection pressure is considered over a broad range which corresponds to equivalence ratio from 0.3 to 1.0. This work features detailed resolution of the flow and flame dynamics in the combustor, which was not typically available in most of the previous studies. Results indicate that the DES model is properly activated in the high equivalence ratio, where there exits the large scale eddies. The numerical pressure distribution shows that a high equivalence ratio causes a strong pressure wave generated by the explosive combustion. As a result of the strong pressure wave the combustor inner flowfield changes to the subsonic mode and converges to unstarting procedure.

Detonation Wave Simulation of Thermally Cracked JP-7 Fuel/Oxygen Mixture using Induction Parameter Modeling

The detonation wave characteristics of JP-7 and oxygen mixture is investigated by one-step induction parameter model (IPM) obtained from a detailed chemistry mechanism. A general procedure of obtaining reliable one-step kinetics IPM for hydrocarbon mixture from the fully detailed chemistry is described in this study. The IPM is obtained by the reconstruction of the induction time database obtained from a detailed kinetics library. The IPM was confirmed by the comparison of the induction time calculations with that from detailed kinetics. The IPM is later implemented to a fluid dynamics code and applied for the numerical simulation of detonation wave propagation. The numerical results show the detailed characteristics of the detonation wave propagation in JP-7 and oxygen mixture at affordable computing time, which is not be possible by the direct application of the detailed chemical kinetics mechanism of hydrocarbon fuel combustion.

Three-dimensional Detonation Wave Dynamics in a Circular Tube

The three-dimensional structure of detonation wave propagating in a circular tube was investigated using a parallel computational code developed previously. A series of parametric study for a circular tube of a fixed diameter gave the formation mechanism of the detonation cell structures depending on pre-exponential factor, k. The unsteady results in three-dimension showed the mechanisms of two, three and four cell mode of detonation wave front structures. The detonation cell number was increased but cell width and length were decreased with increased pre-exponential factor k. In the all multi-cell mode, the detonation wave structure and smoked-foil records on the wall are made by the moving of transverse waves. The detonation wave front structures have the regular polygon and windmill shapes periodically.