A new model for deflagration fronts in reactive fluids

We present a new way of modeling deflagration fronts in reactive fluids, the main emphasis being on turbulent thermonuclear deflagration fronts in white dwarfs undergoing a Type Ia supernova explosion. Our approach is based on a level set method which treats the front as a mathematical discontinuity and allows full coupling between the front geometry and the flow field. With only minor modifications, this method can also be applied to describe contact discontinuities. Two different implementations are described and their physically correct behaviour for simple testcases is shown. First results of the method applied to the concrete problems of Type Ia supernovae and chemical hydrogen combustion are briefly discussed; a more extensive analysis of our astrophysical simulations is given in (Reinecke et al. 1998, MPA Green Report 1122b).Comment: 11 pages, 13 figures, accepted by A&A, corrected and extended according to referee's comment

A computationally-efficient, semi-implicit, iterative method for the time-integration of reacting flows with stiff chemistry

A semi-implicit preconditioned iterative method is proposed for the time-integration of the stiff chemistry in simulations of unsteady reacting flows, such as turbulent flames, using detailed chemical kinetic mechanisms. Emphasis is placed on the simultaneous treatment of convection, diffusion, and chemistry, without using operator splitting techniques. The preconditioner corresponds to an approximation of the diagonal of the chemical Jacobian. Upon convergence of the sub-iterations, the fully-implicit, second-order time-accurate, Crank–Nicolson formulation is recovered. Performance of the proposed method is tested theoretically and numerically on one-dimensional laminar and three-dimensional high Karlovitz turbulent premixed n-heptane/air flames. The species lifetimes contained in the diagonal preconditioner are found to capture all critical small chemical timescales, such that the largest stable time step size for the simulation of the turbulent flame with the proposed method is limited by the convective CFL, rather than chemistry. The theoretical and numerical stability limits are in good agreement and are independent of the number of sub-iterations. The results indicate that the overall procedure is second-order accurate in time, free of lagging errors, and the cost per iteration is similar to that of an explicit time integration. The theoretical analysis is extended to a wide range of flames (premixed and non-premixed), unburnt conditions, fuels, and chemical mechanisms. In all cases, the proposed method is found (theoretically) to be stable and to provide good convergence rate for the sub-iterations up to a time step size larger than 1 μs. This makes the proposed method ideal for the simulation of turbulent flames

Center for Modeling of Turbulence and Transition (CMOTT). Research briefs: 1990

Brief progress reports of the Center for Modeling of Turbulence and Transition (CMOTT) research staff from May 1990 to May 1991 are given. The objectives of the CMOTT are to develop, validate, and implement the models for turbulence and boundary layer transition in the practical engineering flows. The flows of interest are three dimensional, incompressible, and compressible flows with chemistry. The schemes being studied include the two-equation and algebraic Reynolds stress models, the full Reynolds stress (or second moment closure) models, the probability density function models, the Renormalization Group Theory (RNG) and Interaction Approximation (DIA), the Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS)

Large Eddy Simulation of acoustic pulse propagation and turbulent flow interaction in expansion mufflers

A novel hybrid pressure-based compressible solver is developed and validated for low Mach number acoustic flow simulation. The solver is applied to the propagation of an acoustic pulse in a simple expansion muffler, a configuration frequently employed in HVAC and automotive exhaust systems. A set of benchmark results for experimental analysis of the simple expansion muffler both with and without flow are obtained to compare attenuation in forced pulsation for various mean-flow velocities. The experimental results are then used for validation of the proposed pressure-based compressible solver. Compressible, Unsteady Reynolds Averaged Navier-Stokes (URANS) simulation of a muffler with a mean through flow is conducted and results are presented to demonstrate inherent limitations associated with this approach. Consequently, a mixed synthetic inflow boundary condition is developed and validated for compressible Large Eddy Simulation (LES) of channel flow. The mixed synthetic boundary is then employed for LES of a simple expansion muffler to analyse the flow-acoustic and acoustic-pulse interactions inside the expansion muffler. The improvement in the prediction of vortex shedding inside the chamber is highlighted in comparison to the URANS method. Further, the effect of forced pulsation on flow-acoustic is observed in regard to the shift in Strouhal number inside the simple expansion muffler