Local Damage And Protection Mechanism Of Warship Underwater Contact Explosion

Underwater explosion releases enormous heat in a very short period of time, andproduces high-pressure shock wave and bubble oscillation which can cause devastatingdamage of warships and other structures in water. Because of the important militarysignificance，Western naval power poured a lot of manpower, material and financial resourcesinto studying underwater explosion theories and related numerical techniques, and haveformed relatively mature numerical analysis methods of underwater non-contact explosion.The methods have integrated into ABAQUS and other commercial software. However,underwater contact explosion has characteristics of large deformation, moving materialinterface, free surface, high degree of non-uniformity and other features, which is easy tocause wrong results due to high distortion of the grid while simulating with grid methodintegrated in the above software. To overcome the inherent grid distortion problems of gridmethod, many scholars begin to simulate underwater explosion with smooth particlehydrodynamics (SPH) method which has the nature of Lagrangian and particle, and havemade certain progress. However, because of the complexity of the underwater explosionphysical process, the simulation is not yet mature, and there are still many mechanisticproblems to be solved.Therefore, the present research situation of warship underwater explosion and SPHmethod are reviewed firstly. Through the review we find: the grid method is still the mainaccess to simulations of underwater contact explosion, but simulations with SPH method arerare; Researches on torpedo defense cabin in China are restricted to theoretical andexperimental studies, and it is difficult to quantify the protection role of each layer, so theattenuation effects and protection mechanisms of torpedo defense cabin are not clear; Shockwave reflection effects of free surface and seabed are also confined to qualitative analysis, andthe differences of seabed reflection coefficient selection during engineering calculation stillexist; At the same time, simulation of underwater explosion with SPH is also limited to twodimensions, and three-dimensional underwater explosion simulation with SPH method orsymmetric SPH method is very rare; Simulations of underwater explosion mainly focus on thestage of shockwave, and are lack of the whole process of shockwave, bubble oscillation andbubble jet. So, responsing to the above questions, this paper mainly studies on the damagemechanism of warship underwater contact explosion, protection mechanism of torpedodefense cabin, shockwave reflection effects of different boundaries and the whole simulationprocess of three-dimensional underwater explosion.As for the present situation that it is unable to simulate multiphase flow problems oflarge density ratio with the standard SPH method, the modified SPH method based on the volumn approximation is raised, and the numerical models of modified SPH method areestablished. As for the shortcomings of traditional variable smoothing length, excessiveiterations to ensure the accuracy, the fully variable smoothing length algorithm, whosecomputing efficient is high, is put forward. The example that tapered charge explosion drivethe metal jet shows that the modified SPH method can solve multiphase flow problems ofhigh-speed, high compression and high-density ratio compared to the standard SPH method.Besides, the modified SPH method has the characteristics of simple and small amount ofcalculation compared with the grid method. The efficiency of fully variable smoothing lengthcalculation method is much higher than the traditional method, so the computing time is savedand the accuracy is guaranteed at the same time. The above model can be applied to theunderwater contact explosion damage, torpedo defense cabin protection, explosion nearboundary and other numerical simulations with the characteristic of high-density ratio, strongshock, and density fluctuation fiercely.At the engineering background of warships and submarines attacked by the torpedocontact explosion, the simplified models of single shell and double shell contact explosion areestablished. The models are simulated with the modified SPH method. The shock wavepropagation and the damage process of steel plate under the impact of underwater contactexplosion are reproduced. Researches found: the destruction of steel plate is due to the strongshock wave from the TNT detonation; The failure mode has gone through from the "cloud"debris damage to the plugging destruction and then to cracking with the increasing of the steelthickness, so it can be achieved that the damage mode of warship underwater contactexplosion is "cloud" debris damage. Besides, when the shock wave spreads to the steel platewhose back is air, the plate of high impedance will reflect shock wave, and thereby directivewave is strengthened or the second peak is formed; However, the air of low impedance willreflect rarefaction wave, decay shock wave pressure rapidly, and form the phenomenon of“cut off”. The role of strengthen and weaken exists in the vicinity of waters near the plateswhose back is air, which should be taken seriously in the design of submarine, hull structureand other engineering applications.According to the characteristics of torpedo defense cabin, the calculation model aboutprotection mechanism of expansion tank and absorption tank are abstracted. The shock waveattenuation process of expansion tank and the high-speed shrapnel decay process ofabsorption tank are simulated with modified SPH method. Through the quantitative study onthe attenuation law of different parameters of expansion tank and absorption tank, thefollowing conclusions and suggestions can be illustrated as: the expansion tank caneffectively attenuate shock wave taking advantage of the low impedance of air; In the contactexplosion simulations, the shock wave peak pressure can reduce up to55%when the thickness of expansion tank is equal to that of explosion, shock wave attenuation effectdecreases as the thickness continues to increase, which all can give a reference to theexpansion tank design. The absorption tank can effectively attenuate high-speed shrapneltaking advantage of the low compressibility of water; As the increase of absorption tank width,the shock wave pressure, which is generated by high speed shock, attenuates; As a result, inthe design of broadside torpedo defense cabin, the absorption tank should be designed as wideas possible; Also, the attenuation effect of outside board of liquid tank is not apparent, so theoutside board thickness of liquid tank should be designed normally.According to the engineering requirements of blast load produced by the seabed torpedoexplosion and the explosion near free surface, the process of which are reproduced with theSPH method, and the load features are also studied. As for the explosion near free surface,rarefaction wave reflected from the free surface reduces the shock wave peak pressure only inthe non-regular reflection region. The majority area of the explosion near free surface isregular reflection area, and the “cut off” phenomenon can not affect the peak pressure. But theshock wave impulse can attenuate significantly. As for the seabed torpedo explosion, thereflection of seabed sediment can increase shock wave. Therefore, when the blast loadingcalculated in engineering, the reflection coefficient of seabed can be selected as1.2.The three-dimensional SPH numerical models are established, and the simulation ofthree-dimensional underwater explosion with SPH method is achieved. The simulation resultsof shock wave propagation are consistent with the basic theory of underwater explosion.Besides, shock wave peak pressure and the attenuation trends are in good agreement withZamyshlyayev empirical formula. Compared with the grid method of CEL, SPH method has ahigh accuracy when solving underwater explosion problems, so the program can provide thetechnical basis of engineering applications for three-dimensional underwater explosion.Spherically symmetric, axisymmetric, plane symmetric SPH smooth functionexpressions and the discrete equations are derived based on spherical coordinates, cylindricalcoordinates, Cartesian coordinates, and the corresponding numerical models are established.Mirror particle algorithm to solve the problem of particle penetrating symmetric axis isproposed. Through verification of underwater explosion examples, the model have overcomethe particle penetrating symmetric axis, and has a higher accuracy near the symmetric axis;The pressure-time curves are consistent with the Zamyshlyayev empirical formula, and theSPH model has obvious advantages compared with others, which can be further applied tounderwater weapon designs, warships underwater explosion and other engineering problems.As for different stages of underwater explosion have different physical characteristics,the whole underwater explosion process of shock wave propagation, bubble oscillation,bubble jet is simulated taking the full advantages of BEM method and SPH method, the combination of which is achieved. The numerical simulation of bubble jet phase is achievedbased on axisymmetric SPH method. The results are consistent with the basic law and theexperimental data, which shows the feasibility of bubble jet simulation with axisymmetricSPH method. The simulation has overcome the drawbacks of manual intervention andnumerical smoothing technologies while simulating bubble jet with BEM method.