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Raise pressure level in simulation to avoid negative pressures

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Old   June 12, 2012, 18:38
Default Raise pressure level in simulation to avoid negative pressures
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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?
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Old   June 12, 2012, 20:40
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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.
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Old   June 13, 2012, 00:22
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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!
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Old   June 13, 2012, 08:28
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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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