 For low Mach numbers and incompressible media formula_8 is assumed to be infinite, which is the reason for the above continuity equation to reduce to

The way of obtaining a velocity field satisfying the above, is to compute a pressure which when substituted into the momentum equation leads to the desired correction of a preliminary computed intermediate velocity.Applying the divergence operator to the compressible momentum equation yields

formula_11 then provides the governing equation for pressure computation.The idea of pressure-correction also exists in the case of variable density and high Mach numbers, although in this case there is a real physical meaning behind the coupling of dynamic pressure and velocity as arising from the "continuity equation"

formula_3 is with compressibility, still an additional variable that can be eliminated with algebraic operations, but its variability is not a pure artifice as in the compressible case, and the methods for its computation differ significantly from those with formula_14