Transient simulation of intake manifold
 

I want to do transient simulation to intake manifold for a 2 cylinder 1000cc engine. 1 inlet and 2 outlet conditions.

My boundary conditions are, inlet as pressure inlet (atm pressure) and two outlets, one is pressure out and other is wall.
Venturi mixer is also there at the start of manifold, two fuel inlet coming into venturi. Here i have selected species transport model to see methane flow through the system.

It runs fine as steady state with some outlets being changed to a wall representing the valve closed. However now I wish to run it as a unsteady time dependant problem.

I have extracted (pressure Vs crank ) from GT power.
figured out total transient pressure drop.

From tutorial i didn't understand UDF but i have managed to write ".cas" file in notepad.

After doing all this, problem comes with reverse flows at outlet, solution is not converging in timestep ( max i gave to 250 iteration per time step size). model i select k-omega SST model, density based solver.

Some are suggesting in forum to change inlet condition to mass inlet vs time or velocity vs time but i want to see result in pressure boundary condition keeping inlet to atm. pressure only.

If some one familiar with these kind of manifold simulation plz do help me.