i am using coupled solution in FLUEENT. I found that if set CFL number high as 200, the residual of the continuity equation decreases fast but when it decrease to a certain value around 0.14, it will not drop down again. when CFL number is lower as 1, the residula of continuity equation decreases very slow and also will not drop down when it decreased to a certain value around 0.12. So can anybody know how to set CFL number (usually how much is more approporiate) to make the residual of continuity equation decrease continously? the convergence problem can also be due to the high skewness mesh?

Thank you in advance.

Hi,

CFL number is nothing but u*delt/delx which signifies that the wave moves a unit cell in unit time (hyperbolic equation). The CFL number can be calculated once u know ur velocity, delt and delx. For coupled formulation - explicit case always keep the CFL condition as low as possible take for instance 0.05 and then start the iteration. Once the solution starts converging u may increase it till 1. In the case of implicit formulation u may start with a CFL number of 1 and keep on increasing till 20 (its advisable not to go beyond that).

Regards, Rameche Candane. S