Formation mechanism of cavitating spalls

Abstract

The dynamics of formation of cavitation zones in a liquid upon reflection of a shock pulse from the free surface is studied numerically in a one-dimensional formulation using the Iordanskii-Kogarkovan Wijngaarden two-phase model. It is shown that the formation of a system of cavitation zones (clusters) with a dynamically increasing volume concentration of the gas phase near the free surface is due to oscillations of the structure of the rarefaction wave profile. The fast relaxation of tensile stresses in the cavitation zone ends in the formation of a quasistationary mass-velocity field, which provides for almost unbounded growth of cavitation bubbles in subsurface clusters and explains the occurrence of the spall layers observed in experiments.Introduction. The problem of liquid breakdown in intense rarefaction waves is directly related to the notion of strength, which does not have a precise and clear definition in the mechanics of liquids under dynamic loading, unlike in solid mechanics. This is due primarily to the fact that real liquids, even in the unperturbed state, contain cavitation nuclei in the form of microheterogeneities of the type of free gas microbubbles, solid microparticles and their combinations [1][2][3]. The development of cavitation on these nuclei leads to significant changes in the parameters and structure of the external wave field, because of which the notion of critical tensile stresses for liquids is not defined.Nevertheless, in some experimental studies (see, for example, [4,5]), the occurrence of a mass-velocity jump on a free liquid surface upon reflection of the shock pulse from it is treated as a manifestation of spall type fracture. In this case, according to the data of [4], the value of the spall strength is determined from the formula p s = 0.5ρ 0 c 0 (W 0 − W min ) and depends on the velocity jump W 0 − W min (W 0 is the maximum velocity of the free surface at the moment of reflection and W min is its minimum value which is recorded before the spall pulse).In [5], based on experimental results on the behavior of the free surface of a water layer, it is concluded that the quantity p s is almost constant and varies in the range 38-42 MPa, depending on loading methods. However, this conclusion is not unambiguous because it does not follow from the results of [5] for the smallest values of the strain rateε = (2.0-2.7) · 10 4 sec −1 obtained at p 0 = 40 MPa and p 0 = 1050 MPa. In particular, in the case p 0 = 40 MPa, the spall strength p s > 38 MPa, i.e., close to the amplitude of the initiating shock pulse. Consequently, the energy of the shock pulse is not consumed in spall formation or the spall strength is much smaller than the value indicated above. We note that, in [4], the occurrence of spalling in a metal sample under shock-wave loading is explained by features of the free-surface dynamics recorded experimentally. The same approach was used in [4,5] to describe the behavior of liquids under similar loading conditions. As noted above, there is no direct analogy between the f...

Physicomathematical Modeling of the Explosive Dispersion of Liquid by a Centrifugal Atomizer

“…The element is partially filled with liquid water and water vapor, and its density 1 w is approx. 0.8 [1,2]. This is consistent with the cavitation index k = V w /V e % 0.8, wherein V e is the volume of the element and V w is the liquid volume in the cavitation element.…”

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On the question of the fine fragmentation of liquid-metal droplets during steam explosions

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