Dennis E. Wilson

CAVITY DYNAMICS IN HIGH-SPEED WATER ENTRY

A method is presented for modeling the cavity formation and collapse induced by high-speed impact and penetration of a rigid projectile into water. The approach proposes that high-speed water-entry is characterized by a cavity that experiences a deep closure prior to closure at the surface. This sequence in the physical events of the induced cavity dynamics is suggested by the most recent high-speed water-entry experimental data, by results from numerical experiments using a hydrocode, and by an understanding of the fundamental physics of the processes that govern surface closure. The analytical model, which specifies the energy transfer for cavity production as equivalent to the energy dissipated by velocity-dependent drag on the projectile, provides accurate estimates for variables that are important in characterizing the cavity dynamics, and reveals useful knowledge regarding magnitudes and trends. In particular, it is found that the time of deep closure is essentially constant and independent of the i...

A mathematical model for removing volatile subsurface hydrocarbons by miscible displacement

A mathematical model is developed for analyzing the forced venting method of controlling hazardous vapors escaping from underground spills. Equations for predicting concentration profiles are derived from the fundamental laws governing the isothermal flow and dispersion of two-component, miscible, compressible fluids in a porous medium. The resulting equations are solved numerically by finite difference methods to predict concentration profiles for a two-dimensional venting process. Concentration profiles were found to be more sensitive to the dispersion coefficient than to porosity or permeability for the flow rates examined. A comparison of the model profiles with laboratory measurements indicated that realistic predictions are feasible.

Modeling of the Electrogun Metal Vapor Plasma Discharge

The electrogun metal vapor plasma discharge is a new high-power pulsed device for producing vapors of different metals. The process employs electrode erosion to vaporize one of the discharge electrodes. The eroded metal vapor is subsequently ionized to form a dense plasma in which a high current discharge is sustained. The vapor plasma then exits the discharge where it can be used in the synthesis of novel materials or as an ignitor for electrothermal - chemical guns. A model for the electrogun discharge bore plasma flow, bore wall erosion, and simplified models for the cathode and anode regions are presented in this paper. The bore plasma flows under local thermodynamic equilibrium conditions and is weakly nonideal. Two electrogun shots involving different cathode materials (aluminum and titanium) are simulated. Results reveal the physics underlying the operation of the electrogun discharge. High pressures of the bore plasma result in thermal choking at bore exit. Cathode material erosion dominates over the bore wall erosion, thus ensuring a relatively pure metal vapor yield. The exit parameters of the electrogun are predicted and can be used as input boundary conditions for analysis of the external plasma jet flow.

The significance of intermediate hydrocarbons during wall quench of propane flames

A one-dimensional model is used to study end-on wall quench using a detailed chemical kinetics mechanism for propane. Previous models using detailed chemical kinetics mechanisms for methane, methanol, and acetylene revealed that intermediate hydrocarbons exist at much lower levels than unreacted fuel molecules during quench, and thus led to the general conclusion that one-step global chemistry (which accounts only for the rate of disappearance of the fuel) is adequate for studying hydrocarbon evolution during wall quench. However, these fuels are extremely simple, low molecular weight molecules with very limited paths available for forming intermediate hydrocarbons. In the present study, wall quench is studied for propane-air mixtures at equivalence ratios of 0.9, 1.0, and 1.1; pressures of 1, 10, and 40 atmospheres; and wall temperatures of 400 and 500 K. It is shown that intermediate hydrocarbons exist at higher levels and at greater distances from the wall during quench than unreacted fuel. Furthermore, the intermediate hydrocarbons are oxidized less rapidly and persist at significant levels much longer after quench. The persistence of the intermediate hydrocarbons is aggravated by lower wall temperatures, lower pressures, and equivalence ratios both greater than and less than stoichiometric. At lower pressures, the rate of oxidation of the intermediate hydrocarbons is slowed to an even greater extent than is the fuel oxidation rate. The conclusion that intermediate hydrocarbons contribute significantly to wall quench hydrocarbon evolution indicates that one-step global chemistry is inadequate for studying turbulent wall quench, bulk quench, and crevice volume quench of higher hydrocarbon fuels and thus casts doubt on the use of the results of previous theoretical wall quench studies to obtain general conclusions regarding wall quench hydrocarbon evolution in practical engines using practical fuels.

Theoretical analysis of an external pulsed plasma jet

A theoretical description for a highly underexpanded, supersonic, pulsed plasma jet is presented, using an approximate analytical model. The model assumes an inviscid formulation and introduces an ad hoc asymptotic matching procedure with a perfect gas approximation. However, the specific heat ratio is allowed to be pressure and temperature dependent and the gas constant is a function of the degree of ionization. The model can resolve the shock structure and transient gas dynamic flow field for propellant discharges characteristic of the muzzle blast from conventional thermochemical guns and plasma discharges. Quantitative results are presented for the analytical model and compared to experimental data.

A similarity solution for the axisymmetric viscous‐gravity jet

A local similarity solution to the viscous‐gravity jet valid for large Reynolds number flows is given. The jet is divided into an inner core where axial gradients are small relative to the outer annular region. A similarity transformation is found for the outer region, reducing the resulting differential equations to a two‐point boundary value problem. This solution is matched to the inner solution through the boundary conditions. This approach effectively eliminates the mathematical difficulties associated with the unknown free surface and the stress singularity at the exit.

Modeling of the electrothermal ignitor metal vapor plasma for electrothermal-chemical guns

The solid propellant electrothermal-chemical (SPETC) gun is a hybrid propulsion concept which uses an ignitor whose electrical energy enhances the performance of standard solid propellant guns. This ignitor is an electrogun which rapidly vaporizes metals such as aluminum. The metal vapor is then ionized to form a dense plasma in which a high current discharge is sustained. Understanding and characterizing the plasma inside the electrogun is one step in understanding the overall performance of the SPETC gun. This paper describes a model to study the electrogun (ignitor) plasma flow field. The plasma flow field equations are solved for parameters that simulate the operational conditions of a small prototype electrogun. Realistic modeling of the plasma flow in the bore is obtained by a consideration of the energy release within the bore and solution of the conservation equations for continuity, momentum, and energy. This obviates the need to model the complicated phenomena at the cathode surface itself. Total potential drops across the electrogun discharge are matched with experimental data to determine values for the cathode surface parameters that yield realistic solutions.

Numerical simulation of the blast-wave accelerator

A concept for propelling a projectile to hypervelociti es is described, and the feasibility is demonstrated by numerical simulations. In theory, the concept employs imploding blast waves to accelerate a projectile as it travels down a launch tube. The launch tube can be open to the atmosphere or sealed and maintained at some low pressure to minimize internal drag. The launch tube has a liner that contains a suitable explosive or energetic material. The explosive is configured with inert annular rings to prevent upstream detonation. A suitable trigger detonates each explosive ring sequentially as the projectile passes. The resulting blast wave causes an elevated pressure on the afterbody of the projectile. The acceleration continues until the projectile exits the launch tube.

Added mass and damping coefficients fora hexagonal cylinder

An analytical investigation of the fluid coupling effects from a single hexagonal cylinder ina hexagonal cavity undergoing harmonic oscillations is presented. A closed form solution for the velocity and pressure is obtained under a thin gap approximation for the case of moderate frequencies. From this solution, the usual viscous and inertial fluid coupling coefficients are easily obtained. These analytically derived coefficients indicate a strong dependence upon gap spacing and oscillating Reynolds number.

The Effect of a Turbulent Wake on the Stagnation Point: Part II—Heat Transfer Results

A phenomenological model is proposed that relates the effect of free-stream turbulence to the increase in stagnation point heat transfer. The model requires both turbulence intensity and energy spectra as inputs to the unsteady velocity at the edge of the boundary layer. The form of the edge velocity contains both a pulsation of the incoming flow and an oscillation of the streamlines. The incompressible unsteady and time-averaged boundary layer response is determined by solving the momentum and energy equations. The model allows for arbitrary two-dimensional geometry; however, results are given only for a circular cylinder. The time-averaged Nusselt number is determined theoretically and compared to existing experimental data