# Raise pressure level in simulation to avoid negative pressures

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 June 12, 2012, 17:38 Raise pressure level in simulation to avoid negative pressures #1 New Member   Join Date: Jul 2010 Posts: 20 Rep Power: 9 Hi! I am doing some simulations of hydraulic spool valves to improve pressure drop and flow force characteristics. At small openings, I get negative pressure values, in some cases in excess of -1000 bar. To avoid this problem I increased the pressure of my outlet BC (actually an opening) to a much higher value, thereby ensuring that no negative pressures occur. My question is whether this is a valid method, by which I can compare the performance (pressure drop and flow forces) of various valve geometries. Or, is this incorrect?

 June 12, 2012, 19:40 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,205 Rep Power: 110 I did some spool valve design work many years ago.... It is well known that spool valves cavitate when they crack open. That limits the pressures to approximately the vapour pressure of the oil. So your negative pressures is the sign that cavitation is present in an incompressible simulation.

 June 12, 2012, 23:22 #3 Senior Member   Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 715 Rep Power: 13 Geez, I swear Glenn, you seem to know something about just about everything! But I bet you don't know the square root of a million! Advection Boy likes this.

 June 13, 2012, 07:28 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,205 Rep Power: 110 Well, for the bonus prize: A big problem with spool valves is that when they crack open at high pressure the very high velocity just as it opens causes the pressure to collapse (due to bernoulli) and therefore it closes again, then the pressure builds up and it opens again. A poorly designed spool valve will then chatter itself to bits as it oscillates. You have to be very careful to not design a spool valve which will chatter. sqrt(1e6)=1000

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