J.K. Clutter

NUMERICAL METHODS FOR TREATING DISPARATE SCALES IN HIGH-SPEED REACTING FLOWS

Multiple length and time scales exist in high-speed reacting flows and need to be treated with due care when such flows are modeled. In this study, two numerical aspects relevant to reacting flow computations are addressed: (I) alternatively a fully implicit and a prediction-correction splitting procedure for treating the stiff source terms arising in the chemically reacting flow field, and (2) a source term reseating procedure to accommodate the disparate scales in the flow field. A shock-induced combustion scenario resulting from a hypersonic projectile flow is used as the test problem to facilitate investigation of these issues. It is found that the splitting procedure can capture the coupling of the fluid dynamics and chemical reactions accurately, while reducing the computational requirements, and the reseating treatment can compensate for the error caused by inadequate numerical resolutions.

Computation of high-speed reacting flow for gun propulsion applications

The launch cycle of a conventional gun system is simulated using a computational fluid dynamics code that has been developed as a design code for gun muzzle devices. A primary phenomenon of interest is the blast overpressures generated by the gun. The governing equations, which include both fluid dynamics and chemical processes, are solved to predict the peak pressures in the field exterior to the gun. A finite rate chemistry model to simulate reactions is implemented to assess the effects of reactions on the blast overpressures. The physical models and the solution algorithm are discussed with emphasis on accuracy and efficiency. Simulations for both large- and small-caliber guns are carried out and compared with experimental data to ascertain the utility of the design code. Computations prove the reaction processes are key in determining the overpressure levels throughout the field.

CFD approach to firearms sound suppressor design

Suppression of muzzle blast is important in both large and small caliber gun designs. Key goals in the case of small caliber systems are the reduction in the incidence of hearing loss due to the acoustic signal and signature reduction for military applications. Various devices have been used to reduce the muzzle blast and the design of these devices have relied heavily on experimental investigation. The current study evaluates the utility of computational models in the design of suppressors for small caliber guns. Experimental measurements are made for a representative suppressor design and simulations are performed to determine the level of model sophistication needed to correctly predict the effects of the device. The current simulations correctly capture both the levels and characteristics of the acoustic signal generated by the bare muzzle and suppressor configurations. These findings support the use of computational models in the suppressor design process.

Evaluation of source term treatments for high-speed reacting flows

Numerical treatments for addressing the scales associated with the chemical aspects of high-speed reacting flows are evaluated in a shock-induced combustion flow field. The error associated with the source term is found to be correlated to the cell Damkohler number. Both a scaling and an interpolation method are evaluated and are found to improve the comparison between a computed flow field and experimental data. Performance of the methods are also found to be dependent on the particular reaction scheme used in the modeling.

Study of two-equation based modelling for compressible, turbulent flows

Effects of compressibility and chemical reaction on the turbulence structure are two important but difficult issues in turbulence modelling. Several proposed treatments dealing with the dilatation dissipation and the pressure dilatation correlation are discussed in the context of the two-equation model. Also, two new modifications are proposed to account for extra terms that appearfor compressible flows. An attempt is made to validate and calibrate these modifications against experimental data for supersonic flow over an axi-symmetric afterbody. These compressibility modifications are also tested for certain simple reacting flows in order to estimate their predictive capabilities. Additionally, proposed modifications to account for the imbalance between the production of turbulent kinetic energy and its rate of dissipation are contrasted against the standard modelling procedure.