Session S1: Applied Physics

Development of a Laminar Flame Test Facility for Bio-Diesel Characterization

The relevance of applying testing standards established for diesel fuels to evaluate bio-diesel fuels motivates the design and fabrication of a vertical combustion chamber to be able to measure flame speeds of the varying strains of bio-diesel fuels and to attain more detailed kinetics information for biodiesel fuel. Extensive research is ongoing to understand the impact of fundamental combustion properties such as ignition characteristics, laminar flame speed, strain sensitivity and extinction strain rates on emission and stability characteristics of the combustor. It is envisioned that further flame studies will provide key kinetics validation data for biodiesel-like molecules -- the current test rig was developed with provisions for optical access and for future spectroscopic measurements. The current work focuses on laminar flame speeds since this important parameter contains fundamental information regarding reactivity, diffusivity, and exothermicity of the fuel mixture. It has a significant impact upon the propensity of a flame to flashback and blowoff and also serves as a key scaling parameter for other important combustion characteristics, such as the turbulent flame structure, turbulent flame speed and flame's spatial distribution etc. The flame experiments are challenging as the tested bio-fuel must be uniformly atomized and uniformly dispersed.   

Low Temperature Transient Performance of Polymer Organic Light-Emitting Diodes

Polymer Organic Light-Emitting Diodes (p-OLEDs) are conjugated polymers that conduct electric charges, enabling their use as semiconductors. Typical applications for p-OLEDs include high-resolution, high-efficiency displays, and when printed onto plastic substrates, thin and flexible patterned light sources such as vehicle dashboard displays and telephone keypads. We are investigating turn-on and turn-off transient effects in p-OLEDs that vary with temperature and the electrical driver. We have found that the turn-on transient is thermally activated, that light output is immediately proportional to current flow into the device, and that light emission continues from the device even after bias is removed. When these phenomena are fully characterized, they may explain transient effects seen in previous work, help describe the activation energies and rate kinetics in the device, and broaden the range of environments in which p-OLED devices may be used.   

Trap States in Organic Semiconductor Thin Films using Photogenerated Currents

Charge transport in semiconducting organic thin films for small molecules is strongly biased by the distribution of trap states present in the system. These trap states are mostly due to the inherently lower purity of small organic molecules and growth defects. Here, the trap states are explored and mapped using a time dependence study of photgenerated currents in thermally evaporated thin film copper phthalocyanine samples using the techniques described by Twarowski [1]. Although we use a different contact configuration and substantially thinner films than those used by Twarowski the method describes the system well up to first orders. The dependence of the recombination of photo-excited, dissociated charge pairs on the electric field also agrees with the Onsager mechanism [2] as predicted by Twarowski. We also explore the limitations of these models and discuss the potential of a more robust description for this type of system. This work is supported by NSF CAREER grant 0847552.   

Investigation on Novel Methods to Increase Specific Thrust in Pulse Detonation Engines via Imploding Detonations

Pulse Detonation Engines (PDE) is seen to be the next generation propulsion systems due to enhanced thermodynamic efficiencies. One of the limitations in fielding practical designs has been attributed to tube diameters not exceeding 5 inches, thus affecting specific thrust. Novel methods via imploding detonations were investigated to remove such limitations. During the study, a practical computational cell size was first determined so as to capture the required physics for detonation wave propagation using a Hydrogen-Air test case. Through a grid sensitivity analysis, one-quarter of the induction length was found sufficient to capture the experimentally observed detonation wave structure. Test case models utilizing axi-symmetric head-on implosions were studied in order to understand how the implosion process reinforces a detonation wave as it expands. This in effect creates localized overdriven regions, which maintains the transition process to full detonation. A parametric study was also performed to determine the extent of diameter increase for such that practical designs could be fielded. It was found in the study that diameters of up to 12 inches could be achieved with reasonable run length distances.   

Simulation, Construction, and Experimental Evaluation of a Twice-Augmented Railgun

Multi-rail augmentation is a simple method of improving railgun performance and achieving high projectile speeds from short railguns with moderate currents. Augmentation improves overall energy transfer efficiency by helping match the impedance of small railguns to laboratory power supplies. This paper presents the design and tests of a 50cm, easily assembled twice-augmented square-bore railgun. The design, consists of relatively thin, flat conductor and insulator plates held together with strong bolts. The plates to which the rear augmentation bolts are attached must be strong enough to withstand reaction forces that are transferred to the augmentation bolts when the projectile is launched. Prevention of surface electrical flash over across the thin insulator plates will be discussed. The 50-cm railgun reported here has accelerated 12-g projectiles to speeds greater than 1100 m/s with an overall efficiency of 8 percent; better performance is anticipated with further tests. The design is easily adapted to longer guns and larger bores. This design achieves higher speeds and better overall energy transfer efficiencies when compared to a 60-cm long gun with a single rail augmentation. Simulations of magnetic fields show that the field in the barrel is 39 percent higher for the 50-cm gun than for 60-cm gun at the same current. Simulations of the magnetic fields between the rails and the effect of rail configuration on performance will be discussed.   

NPS Gas Gun for Planar Impact Studies

The Naval Postgraduate School (NPS) commissioned a Gas Gun for shock wave studies on 9$^{th}$ October 2009, by performing the first experiment. The Gas Gun is the key element of NPS Shock Wave Research Program within the Physics Department, where well-characterized planar impacts are essential for obtaining high quality data, to characterize a solid material. This first experiment was very successful, and returned key data on the quality of the impact conditions created. The Gas Gun is designed by SANDIA NATIONAL LABORATORIES, and the NPS spent twelve months fabricating the components of the Gas Gun and six months assembling the Gas Gun. Three inch projectile are launched at velocities up to 0.5 km/s, creating high pressure and temperature states that can be used to characterize the fundamental response of relevant materials to dynamic loading. The projectile is launched from a `wrap around' gas breech where helium gas is pressurized to relatively low pressure. This gas is used to accelerate the projectile down a 3m barrel. Upon impact, the speed of the projectile and the flatness of the impact is measured, via a stepped circular pin array circuit. The next stage of development for the Gas Gun is to integrate a Velocity Interferometer System for Any Reflector (VISAR). The VISAR sees all the waves that flow through the target plate as a result of the impact. This is a key diagnostic for determining material properties under dynamic loading conditions.   

Development of a Laboratory Scale Test Facility (LSTF) to investigate Armor solutions against buried explosive threats

This LSTF will address the effects of High Velocity Sand Blast Impact; massive overpressures, impulsive effects, kinetic energy and momentum, from developing the type of flat sand- loading profile required for code validation purposes. The background of this study is to generate a planar shock-wave profile and a flat-loading profile from high velocity sand and air blast onto intended flat-plate targets, to properly characterize the codes under development; to do this we propose to use a flyer plate, which is explosively driven, so, we end with a design in which a slanted flyer plate, with an explosive layer underneath it, is set-up and detonated from one end, as the detonation wave runs through the explosive layer, it pushes the flyer plate. If all the geometry is carefully designed and the flyer plate/explosive layers are precisely positioned, in theory we should be able to produce a flat flyer plate that travels on the order of 1 to 2 km/s towards a layer of sand, therefore generating a shock wave within the sand that will eventually accelerate the sand with a flat top-hat profile towards the intended target, thus achieving a flat sand loading profile onto the target. Success in this domain will allow ease of testing of advanced armor concepts against simulate buried explosive threats, thus providing validation for numerical codes that will be used to perform optimization of novel armor designs at low costs.   

Numerical Calculation of Anelastic Seismic Pulse Propagation in a Hysteretic Elastic Material Along a Horizontal Surface Boundary of the Earth

The stress-strain relation for materials such as soil and sand exhibit hysteretic elastic behavior and are modeled using the Preisach-Mayergoyz method for a numerical calculation of a propagating seismic pulse. The source pulse is taken to be the result of pressure applied to the inner surface of a cylindrical cavity in order to simulate a two dimensional dynamite source. The anelastic differential equation of motion that is solved does not include traditional nonlinear elasticity terms appropriate to materials with atomic elasticity, but contains the dominant anelastic terms appropriate to consolidated materials that exhibit hysteretic elastic behavior. For parameters characteristic of sand at the Earth's surface, a comparison of anelastic to linear seismic pulse propagation gives an anelastic pulse with much slower propagation speed than a corresponding linear pulse with evidence of dispersion in the pulse. The simulated ground roll that results shows dramatic differences between the anelastic and linear cases. These results have important implications for the detailed behavior of strong seismic waves moving in soft sediments. Their dominant frequencies, amplitudes, and methods by which they may be attenuated will depend on getting the detailed pulse structure and its propagation correct.   

Optimal Control of Shock Tube Flow via Water Addition with Application to Ignition Overpressure Mitigation in Launch Vehicles

Ignition Overpressure (IOP) in launch vehicles occurs at the start of ignition when a steep rise in pressure propagates outward from the rocket nozzle. It is crucial to minimize the overpressure so as to decrease risk of damage to the rocket body. Currently, CFD studies exist on this situation but there are no optimization studies of the water addition as a means to suppress the IOP. The proposed dissertation will use a numerical method to compute an approximate solution for an optimal control problem constrained by the one-dimensional Euler PDEs of fluid dynamics as well as volume fraction conservation. A model for inter-phase transport of mass momentum and energy and fluid interface quantities will be given. The control will be water addition from external nozzles. The adjoint system of equations will be derived and discretized. Necessary optimal conditions will be derived. An SQP method will solve an optimal situation. Predictions will be validated against shock tube experiments at the NPS rocket lab.   

Anomalous Velocity Dependence of the Friction Coefficient of an Air Supported Pulley

A standard undergraduate lab exercise to verify Newton's law, F = ma, is to measure the acceleration a of a glider of mass m suspended on an air track. In our experiment the glider is accelerated by a thin tape attached to the glider at one end, and to a weight of mass M at the other end. The weight hangs vertically via a pulley over which the tape is suspended by air pressure. In the absence of friction, the force pulling the glider is F = (M m/(M + m)g, where g is the acceleration of gravity. To the accuracy provided by the fast electronic timers (accurate to 1/10000 second) used in our experiment to measure the velocity and the acceleration of the glider, we verified that the friction due to the air track can be neglected. But we found that this is not the case for the friction due to the air pulley which adds a component -v/T to the force F on the glider, where T is the friction coefficient. We have measured the dependence of this coefficient on v, and found an excellent analytic fit to our data. This fit deviates considerable from the conventional assumption that 1/T is a constant and/or depends linearly on v.   

Instrumentation for Calorimetric Measurements of Strongly Correlated Electron Materials

A calorimeter is used to make measurements of the internal energy of a material in order to probe its thermodynamic properties such as crystalline lattice stiffness, electronic effective mass, phase transitions' and entropy. Rare-earth metallic compounds are of interest in our lab because they are known to exhibit strongly correlated electron behavior, which gives rise to interesting phenomenon such as conventional and unconventional superconductivity, metal-insulator transitions, magnetism and the magnetocaloric effect. Therefore, the temperature dependence of specific heat is an important quantity to investigate these materials. With limited space of our cryogenic system we are unable to use a traditional semi-adiabatic method, instead; we use a thermal relaxation method for our calorimetric measurements. A discussion on the construction of the calorimeter will be presented.