Variety Of Geometric Conditions Research Articles

Effect of Variation of Johnson-Cook Parameters on Kinetic Energy and Simulation of 4340 Steel Projectile

Computer simulations have been used for decades in the modelling of high velocity impact and general large deformation problems. Several computer codes have been developed to simulate these types of problems under a variety of geometric conditions and for any variety of materials. The Johnson-Cook material model is widely used in the simulation of high velocity impacts and even penetration of target structures. This research work represents the study of effects of variation of Johnson-Cook material constants on ballistic impact and its effect on residual kinetic energy of projectile. In the Johnson-Cook damage model, there are five material constants viz. D1, D2, D3, D4 and D5, and it is very difficult to find out all of these material constants by experiments. The main objective of the present research is to determine the most sensitive parameter out of these constants. To accomplish this target, experimental testing was done and additionally the impact physics was likewise recreated numerically utilizing ABAQUS. For the impact phenomenon 4340 steel, 10 mm diameter projectile and a 1006 steel target plate were chosen. From this study, it is concluded that the most sensitive Johnson-Cook damage model constant is ‘D3’

Lunar surface roughness derived from LRO Diviner Radiometer observations

Sunlit and shaded slopes have a variety of temperatures based on their orientation with respect to the Sun. Generally, greater slope angles lead to higher anisothermality within the field of view. This anisothermality is detected by measuring changing emitted radiance as a function of viewing angle or by measuring the difference in brightness temperatures with respect to observation wavelength. Thermal infrared measurements from the Lunar Reconnaissance Orbiter Diviner Radiometer were used to derive lunar surface roughness via two observation types: (1) nadir multispectral observations with full diurnal coverage and (2) multiple emission angle targeted observations. Measurements were compared to simulated radiance from a radiative equilibrium thermal model and Gaussian slope distribution model. Nadir observations most closely match a 20° RMS slope distribution, and multiple emission angle observations can be modeled using 20–35° RMS slope distributions. Limited sampling of the lunar surface did not show any clear variation in roughness among surface units. Two-dimensional modeling shows that surfaces separated by distances greater than 0.5–5mm can remain thermally isolated in the lunar environment, indicating the length scale of the roughness features. Non-equilibrium conditions are prevalent at night and near sunrise and sunset, preventing the use of the equilibrium thermal model for roughness derivations using data acquired at these local times. Multiple emission angle observations also show a significant decrease in radiance at high emission angles in both daytime and nighttime observations, and hemispherical emissivity is lower than is apparent from nadir observations. These observations and models serve as a basis for comparison with similar measurements of other airless bodies and as an initial template for the interpretation of TIR measurements acquired under a variety of geometric conditions.

Flame-jet ignition of large fuel-air clouds

This paper reports on a series of large-scale flame-jet ignition tests conducted at the Defense Research Establishment Suffield in Canada. The experimental configuration consisted of a steel tube connected to a large plastic bag simulating an unconfined region. The test volumes were filled with fuel-air mixtures employing acetylene, ethylene, propane and vinyl chloride monomer fuels. The nature of the flame jet was controlled by an array of obstacles in the tube and, in many of the tests, by a central circular blockage installed in the exit plane of the tube. Transition to detonation was observed in the bag for acetylene-air mixtures under a variety of geometric conditions. In the case of a flame jet emerging from an open tube, high-speed cinematography and numerical calculations have shown that the entrainment of hot combustion products into the vortex established by the shock-induced flow ahead of the flame is responsible for a sequence of localized explosions which result in shock wave amplification sufficient to initiate detonation. Other modes of transition were observed with the acetylene-air system, including transition downstream of a central blockage, transition due to colliding flame tongues in an asymmetric jet, and transition due to flame/boundary interactions. When the data from all of these experiments are plotted in terms of flame-jet velocity versus the sensitivity of the mixture to detonation, they describe a well-defined relationship. Successful correlation of this diversity of data would suggest that the critical conditions are established by the flame jet and that nearby perturbing boundaries merely trigger the onset of detonation. Although transition to detonation was not observed with the other fuel-air systems, it was found that a significant explosion in the bag could nonetheless occur for the relatively sensitive ethylene-air system, generating pressures approaching 6 bar. Cinematographic coverage has revealed that this is likely the result of a series of “hot spots” or regions of high reactivity developing downstream of the tube exit. Although these mini-explosions do not persist long enough for the critical size of a detonation kernel to be exceeded, the resulting reactive blast wave takes considerable time to decay.

A Spherical Mask to Compensate for cos^3 and cos^4 Illuminance Variation

A dyed or pigmented gelatin mask cast in a shallow spherical mold cavity can provide excellent compensation for the cos(3)alpha or cos(4)alpha fall-off of illuminance on a flat surface. Computed data are shown for the imperfection of correction under a variety of geometric conditions, and some typical applications are described.