# about large minus absolute pressure

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 December 5, 2003, 06:25 about large minus absolute pressure #1 richard Guest   Posts: n/a in my case, i simulate a tunnel with a 30 degrees downstream slope.there is a free surface in the tunnel with water flowing through it. the boundary conditions are: two inlets with velocity inlet for water and pressure inlet for air on the upstream, and the gauge total pressure is set to 0; and one outlet with pressure outlet, and the gauge pressure is set to 0.i initialized the porblem with water full of the tunnel and the x-orientation velocity is set to 23m/s,the water inlet velocity.then i start the simulation, and soon after the start, there is a very high pressure of about 5*10^6Pa existing near the inlet, and the solution is converged and the iterate goes on.the time step is 0.0001s and about 5s later, the absolute pressure becomes minus and decrease further more with iteration process. the absolute pressure limit of above 0 doesn't work! does someone meet such situation? how to explain such things i meet? thanks in advance.

 December 5, 2003, 09:12 Re: about large minus absolute pressure #2 Guest Guest   Posts: n/a the absolute pressure limit works only for compressible flows

 December 6, 2003, 08:46 Re: about large minus absolute pressure #3 richard Guest   Posts: n/a thanks, thanks, thanks so much, guest, you really help me.

 December 6, 2003, 09:39 Re: about large minus absolute pressure #4 richard Guest   Posts: n/a and some other questions, dear guest.i am now confused by them. in my simulation, there exists very pressure and velocity that shouldn't be there in real physics.and the calculation shows that there is minus pressure on the upstream and plus one downstream, which is also nonphysical.and the third strange phenomenon is that the pressure in the middle of the water domain(where there mixed in 1mm diameter air bubbles to some degree of the air volume fraction is about 0.2) is minus, where the pressure is the total of gauge pressure and pressure caused by gravity, which is above 0 at least. would you like to explain them for me?thanks a lot.