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276    FLUID COUPLING

            8.5 OTHER APPLICATIONS

            The same principles employed with coupled simulations involving explosive induced
            fracture and fragmentation apply to problems where most of the domain is filled by solid
            particles; the fluid fills most or at least some of the cracks and voids between the solid
            particles, and fluid flow occurs through the network of interconnected cracks and voids.
              All of these problems are characterised by relatively small domains associated with
            fluid flow and the CFD part of the coupled model. These domains (voids, cracks) are
            discontinuous, and are distributed in between solid particles. The mass of fluid is therefore
            small in comparison to the mass of the surrounding solid. For instance, in the explosive
            induced solid fragmentation the mass of solid is 1000 to 30,000 greater than the mass of
            the detonation gas. Consequently, the inertia of the fluid is small in comparison to the
            inertia of the solid.
              The moving solid takes with it the cracks and voids acting as a non-inertial frame of
            reference for the fluid locked inside these cracks and voids. It can be said that each crack
            or void has its own moving Eulerian grid. Inertia effects due to the grid moving, and
            therefore accelerating, are so small that their effects on, for instance, pressure distribution
            inside a crack or void are negligible. The CFD formulation can in most cases ignore inertia
            effects due to the acceleration of Eulerian grids. In some cases (heavy fluid, small pressure
            differentials, larger size of voids), these inertia effects have to be taken into account.
              The common characteristic of this type of coupled problem is that the Eulerian CFD
            grids are superimposed on Lagrangian finite element grids. With the increase in the size of
            voids and space between solid particles becoming larger, there comes a point when solid
            particles become completely submerged in fluid. These then become coupled problems of
            the first type, with a Lagrangian grid being superimposed on the Eulerian grid that is now
            fixed in space. These types of problems are in essence CFD problems, and are outside
            the scope of this book.
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