[Shahrokh]

Hi,

In the first stage, I have to simulate the hull of planning boat in the water without wave for validate with experiment result. I need to calculate the drag force, pitch angle and heave.
At first time, I selected the Inlet and Opening boundary conditions, but the input mass flow rate was less that the output and so the water height decreased with time.
At second time, I selected the inlet (u=2.12 m/s) and Outlet with (massflow()@Inlet) boundary details for Outlet, because I thought that by doing this, the mass flow rate at the inlet and outlet would be same and the problem would be solved. Figure 1 shows that the mass flow rate of inlet and outlet are same.
But again, the height of the water decreases over time. I have to keep the water level constant. please help me.

Thanks

[ghorrocks]

I have deleted your duplicate posts which got moderated. I don't know why they got moderated, they look fine to me. No matter, this post got through OK so we will use this one. Don't worry about posts being sent to moderation in future, if they are legitimate posts I will approve them straight away.

Meanwhile, back on topic:
First of all, I would recommend you make the in flow side one single inlet, where you have defined the volume fraction to have the free surface level you want. I don't think splitting it into two parts is what you want. Likewise for the out flow side, I would just make that a single opening or outlet.

The free surface getting lower could be physical or it could be numerical.

A physical cause could be that the free surface needs to get lower as it travels through your domain to overcome friction on the outer walls and the body you put in there.

A numerical cause could be that you have a lot of dissipation in the volume fraction equation and volume fraction is slowly disappearing because of it. Make sure you are using the compressive advection scheme for volume fraction, try a tighter convergence tolerance, use double precision numerics, and use a high quality hex mesh with the hexes aligned to be orthogonal to the free surface. All these things will minimise dissipation and hopefully reduce spurious volume fraction loss.