Centrifugal And Numerical Modeling On Structures Impacted By Underwater Explosion

Underwater explosion is probably the most dangerous threat to hydraulic or marine structures in their whole life cycles.Failure of which may greatly endanger the public safety.However,if properly control blasting procedure,underwater explosion could be effectively used to dismantle structures,such as cofferdams or landslide dams.It is necessary to investigate the mechanism of underwater explosion.Generally,underwater explosion consists of shock wave and bubble oscillation,each individually taking up approximately 50% of the explosive total energy,may cause severe damage to vessels or hydrodynamic structures.Unfortunately,a paradox of UNDEX scaling is that it is impossible to satisfy both the similitudes of Mach and Froude numbers if gravity is constant.Despite bubble motion can be well comprehended by model tests in a vacuum or a pressurized tank,errors of bubble pulsating pressure can exceed 50%,which limit the advances of underwater theories.The results of dimensional analysis show that Mach and Froude similitudes are possible to be achieved simultaneously under hyper gravity,which would result in identical shock wave and bubble oscillation as prototypes.It has been widely acknowledged that centrifuge is effective to provide similar gravity field as that for the prototype in geotechnical engineering.Since stress field is similar,stress-strain relationship would be same as prototypes,which provides possibilities for investigation of injuries of concrete gravity dams or earth and rockfill dams.The potential application of centrifuge in modeling underwater explosion is worth attention.This paper improves the blast simulation equipment in China Institute of Water Resource and Hydropower Research,which would be more suitable to record the high frequency and wide range signals generated by underwater explosion.The technique for modeling underwater explosion is established for further engineer application.A series of underwater explosion experiments are undertaken based on this equipment.Signal processing methods for typical results and analysis of system errors are presented,results of which suggest system errors introduced by centrifuge is much lower compared with the errors of blast itself.This means it is reliable to use centrifuge to model underwater explosion.The underwater explosion loading is analyzed based on the observed results in centrifuge.According to dimensional analysis,the key parameters that could determine the characteristics of shock wave and bubble oscillation are obtained and validated by experiments.The observed results for shock wave and bubble oscillation in centrifuge are approximately the same as those in prototypes,which further prove that the capacity of centrifuge in modeling underwater explosion.Dimensional analysis significantly simplifies the investigation of shock wave and bubble oscillation,which could provide engineering formulae for underwater explosion loading.Moreover,the mechanism of structure vibration and deformation are investigated based on the observed results in centrifuge.It is found that the vibration is mainly induced by the peak pressure impacted on the structure,while the deformation is influenced by the impulse of loading.From the perspective of energy,the impact factor for spherical wave is derived to evaluate the shock environment of structure.Further,computation code is written based on potential flow theory and boundary element method to simulate underwater explosion procedure in centrifuge.The code could also be used to analyze effects,such as free surface and rigid boundaries,which would provide method for computation of underwater explosion.Since the boundary element method transform the integrations of whole fluid domain into boundary integrations,the element sum is remarkably reduced,which would overcome shortcomings of computational fluid dynamics,such as long computation duration and mesh distortion problem.