In order to accelerate the calculation, the pressure correction equation is modified as
where  is the overrelaxation factor. A value of  is commonly used. The value of  giving most rapid convergence, should be determined by numerical experimentation. After calculating  , the pressure in the cell  is adjusted as
Now the pressure and velocity components for each cell are corrected through an iterative procedure in such a way that for the final pressure field, the velocity divergence in each cell vanishes. The process is continued till a divergence-free velocity is reached with a prescribed upper bound; here a value of 0.0001 is recommended.
Finally, we discuss another important observation. If the velocity boundary conditions are correct and a divergence-free converged velocity field has been obtained, eventually correct pressure will be determined in all the cells at the boundary. Thus, this method avoids the application of pressure boundary conditions. This typical feature of modified MAC method has been discussed in more detail by Peyret and Taylor (1983). However it was also shown by Brandt, Dendy and Ruppel (1980) that the aforesaid pressure-velocity iteration procedure of correcting pressure is equivalent to the solution of Poisson equation for pressure. As such from Eqn. (33.11) we can directly write as
The Eqn. (33.16) can be solved implicitly using appropriate boundary condition for p' at the confining boundaries.
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