hi all

I am running transient analysis with pressure -pressure boundary condition. i have one pressure in and 4 outlets (i.e pressure- pressure boundary condition). At any time one outlet is pressure and the rest all are wall. the pressure outlet interchanges with respect to time. This can be compared with air Induction in IC engine i.e when intake valve is opened its pressure and when closed it changes to wall.The problem i am facing is that when the boundary changing from wall to pressure or vice versa i am getting Negative density at the boundary. Is the solver is unstable during the change of boundries. Is there any way to get rid of it. I am running in MARS mode for momentum and Temp. Could any one help in this regared....

I am using Starcd 3.26

try underrelaxation of piso:

also piso node 20 0.15 0.8

- check .info file if the problem is temperature convergence: set residual tolerance for temperature and scalars to 1e-3

- save .pst every time step

- post process density to see which cells are -ve

- reduce the time step: if it still give -ve density: your mesh is no good.

and try UD at least initially....

Had you considered driving the model with velocity/massflow boundaries? The level of pressure does not set the mass in/out of the domain it is the gradient, which you are not setting.

Also instaneously changing the boundary type to wall will almost certainly cause problems. You are trying to stop any massflow from entering and leaving essentially instaneously, which will naturally cause a very large negative or positivepressure pulse.

If you are driving this with the output of one of the 1D engine flow solvers then you will have a lot better luck driving it with a massflow condition (or possibly coupling it in).