  The sensitizing effect of cavities in the form of microbubbles on the shock
initiation of a homogeneous liquid explosive is studied computationally. While
the presence of voids in an explosive has long been known to induce so-called
hot spots that greatly accelerate the global reaction rate, the ability to
computationally resolve the details of the interaction of the shock front with
heterogeneities existing on the scale of the detonation reaction zone has only
recently become feasible. In this study, the influence of the spatial
distribution of air-filled cavities has been examined, enabled by the use of
graphic processing unit (GPU) accelerated computations that can resolve shock
initiation and detonation propagation through an explosive while fully
resolving features at the mesoscale. Different spatial distributions of
cavities are examined in two-dimensional simulations, including regular arrays
of cavities, slightly perturbed arrays, random arrays (with varying minimum
spacing being imposed on the cavities), and randomly distributed clusters of
cavities. The presence of the cavities is able to reduce the time required to
initiate detonation---for a given input shock strength---by greater than 50%,
in agreement with previous experimental results. Randomly distributing the
cavities results in a 15-20% decrease in detonation initiation time in
comparison to a regular array of cavities. Clustering the cavities---as would
occur in the case of agglomeration---results in an additional 10% decrease in
detonation initiation time in comparison to random arrays. The effect of
clustering is shown not to be a result of the clusters forming an effectively
larger cavity, but rather due to interactions between clusters upon shock
loading occurring on the microscale.
