# Negative Pressure in H2 gas flow and other physical interrogation

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 April 3, 2012, 20:30 Negative Pressure in H2 gas flow and other physical interrogation #1 New Member   Join Date: Apr 2012 Posts: 7 Rep Power: 6 Hi everybody, I am quite new in ANSYS and more generally in fluid mechanics simulation. For my lab, I want modelize a basic micrometric Laval nozzle (where the H2 gas become supersonic) surrounding by some tubes connected to an accurate pressure controller ( set to 50 Torr) and to the vacuum of a vac chamber. ( ~below 10 mTorr) No problem a priori in the geometry or the CFX meshing. In order to solve my CFX simulation, I have many choice of boundary condition, but which are relevant ? For an "outlet" surface open on vacuum, if I set "static pressure" = 0, at the end of the simulation (10e-5 RMS) , the pressure of this surface is way bigger than mTorr ( ??) and for a big part of my tube, negative ! How a simulator can accept a negative pressure in a gas ? If I change the boundary condition on "Mass flow" with a rough estimation of this mass flow, its not better. For an "inlet" surface what's the difference between Total pressure and Static pressure ? and also "opening pressure entrainement or direction" for an "opening" surface. It doesn't make lot of sens, even by reading the help. And what is the difference during the post processing between "pressure" and "total pressure" ? Thank you for your patient reading, and , i hope, for your answer Florian Uploaded with ImageShack.us P.S. I join a picture and I Apologize for my english grammar, it's not my language Last edited by FloMol; April 3, 2012 at 20:54. Reason: add image

 April 4, 2012, 08:55 No negative pressure #2 New Member   Erik Bjerklund Join Date: Jul 2009 Location: Sweden Posts: 3 Rep Power: 9 The pressure is not unphysically negative. Look at the variable Absolute pressure . The pressure is negative relative to your reference pressure which is specified in the domain model. Total pressure= static pressure + (rho v^2 )/2

 April 9, 2012, 19:57 #3 New Member   Join Date: Apr 2012 Posts: 7 Rep Power: 6 The Absolute pressure is negative as well, and i set the reference pressure at 0 Pa. Thanks for the difference between totale pressure and pressure anyway. F.M.

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