Moving Body Treatment 
There are many CFD problems that can not be simulated using a fixed computational mesh. Such problems typically involve flows around moving or deforming bodies. Industrial examples of these flows include flows in pipes with moving valves, flows in combustion chambers with moving pistons, the classical store separation problem, and many others.


Theoretical Background 
When the computational mesh moves as a function of time, the mesh velocity enters the analysis and must be included in discretizing the governing differential equations. Basically, the mesh motion affects the convective fluxes of mass, momentum, energy, and other scalar dependent variables. In integral form, the continuity and the generalized transport equations can be written as follows: Continuity Equation
General Transport Equation
where V is an arbitrary moving volume, A is the surface of V, is any scalar quantity, and S are the diffusive flux and source terms for the corresponding variable. After the characteristics of the grid motion are specified, the mass and other convective fluxes across the cell faces are calculated according to the local fluid flow conditions and grid velocity. The cell volume, face area, and face direction cosines are recalculated at every time step. 