Hello! I'm trying to develop a code using fractional step metod but I have little practise. I use Runge-Kutta (for convection) and Crank-Nicolson(for diffusion) time stepping. In a substep I have to update velocities with eq. such as

U(n+1)=U(n)+C(U)*dt+D(U)*dt+G(P)*dt+F*dt

C ..conv,D ..diff.,G .. pressure gradient,F ext.force(I use immersed boundary method)

Can I devide it in separate steps like this?

U*=U(n)+C(U)*dt U**=U*+D(U*)*dt U***=U**+G(P)*dt U(no1)=U***+F*dt

And what order should I use?

Thanks for reply. Lada

U*=U(n)+C(U)*dt

U**=U*+D(U*)*dt

U***=U**+G(P)*dt

U(no1)=U***+F*dt

Sorry, I didn't expect it will write it that way.