I am modeling, steady state supersonic reacting jet flow using density based implicit method with RNG k-epsilon turbulence model. What should be CFL no?

with regards kumar

CFL Number will always be less than one by definition it is the ratio of fluid speed to grid speed grid speed is the ratio of minimum grid size to time step. and only used in transient analysis not for steady state if u are solving for seeking steady state solution u will get this, other wise u dont have to use cfl number. any way try using cfl number less that one. in certain methods as far as my knowledge is concerned only in one method it is less than 2 and in few cases it is less that 1/2

for further information u can refer any cfd book.

cfd books by chung, lomax pullliam, hirsh vol 1 and vol2 etc......

Dear Ramesh, Thanx for the information. But in the FLUENT documentation file, it is mentioned that the CFL no for Implicit density based solver should be more than 1. Even it should be more that 5 also.

So what shall i do?

I have the same question!!! If anyone knows what fluent means, please help!

Hi

It seems that when we use the characteristics to find the boundary conditions they are in the form of u+c, u-c and u so when u split these characteristics we get two velocities one pertaining to fluid speed (u) and the other one pertaining to acoustic or disturbance speed (c) so the CFL number used when the interest is on disturbance propagation is more than one and they have used above 5 a value 7 but when our interest is only the fluid it is of the order less than 1

this is according to my understanding so please check it and correct me if wrong