It has been demonstrated through experimental investigation that shock wave passing through water results in phase transition from liquid water to ice VII. However, there exists no literature which demonstrates the thermodynamic pathway of phase transition at a molecular level which this manuscript addresses. The manuscript demonstrates that without presence of external nucleators shock waves of speed 4-4.5 Km/sec is able to induce phase transition of bulk pure liquid water to ice VII which has been verified through radial distribution functions and spectral investigations. The molecular dynamics (MD) study is able to determine for the first time in literature the time in nano-seconds after the passage of shock at which phase transition occurs, percentage of crystallinity of the sample and interpenetration distance of the simple cubic lattices to form the bcc crystal structure of ice VII vs. time.
However, given the computational resource limitations it is not possible to initiate an MD study to study shock wave reflections and impulse transmission to a structure submerged in water. A continuum mechanics approach (based on discontinous form of the Euler equation or Rankine-Hugoniot jump conditions) considering nonlinear compressibility to model water medium is then presented in this manuscript to demonstrate the effect of reflections from and impulse transmission to a underwater rigid submerged structure which is able to move perpendicular to the direction of the impulse and thereby create significant changes in pressure behind the structure. Phase transition phenomenon of water has also been considered as part of the developed theory. The continuum theory presented in the manuscript demonstrates nonlinear compressibility of the water medium further enhances the beneficial effects of fluid-structure-interaction in reducing impulse transmitted to the structure. Fluid-structure interaction effect was observed to be more predominant for cases where the rigid structure moves on shock wave impact resulting in significant increase in pressure of the medium behind the structure. For low plate masses (as is prevalent in sandwich composite construction for marine structure hulls) it was observed that this new theory (considering nonlinear compressibility of the medium) gives significant different results from theories presented earlier by various researchers.
When a sandwich composite structure is being considered as underwater submerged structure, one of the primary mechanisms of energy dispersion is through compression of the soft core. Thereby the continuum theory presented above has been extended for porous soft core sandwich panels in which the above presented theory has been combined with the rigid-perfectly-plastic model for core to provide estimates for compression of the core. Energy conservation carried out as part of this study demonstrate increase in plastic dissipation energy, increase in rate of decay in kinetic energy, and decrease in contribution of work done by pressure rise on back of the plate along with increase in the ratio of core to face-sheet mass.
The comprehensive study considering both molecular dynamics and continuum mechanics approach demonstrates the physics of intense shock transmission in water and also explains impulse transmission in underwater submerged structure, especially for structures with low mass.
NOTE: Contents in this manuscript have either been published or being reviewed in journals (as referred below).
References:
- Neogi and N. Mitra, Shock induced phase transition in water, submitted to Physical Review Letters, 2014.
- Ghoshal and N. Mitra, 2014. On core compressibility of sandwich composite panels subjected to intense underwater shock loads, Journal of Applied Physics, 115(2), 024905.
- Ghoshal and N. Mitra, 2012. Non-contact near-field underwater explosion induced shock-wave loading of submerged rigid structures: nonlinear compressibility effects in fluid structure interaction. Journal of Applied Physics. 112(2), 024911.
