negative pressure

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 April 16, 2002, 06:27 negative pressure #1 Mark Render Guest   Posts: n/a Hi all, I simulate the flow around an hydrofoil under cavitating conditions. I use a simple VOF model, which converts water into vapour depending on the local pressure. Before the cavity starts at the leading edge there's a small area (0.02 chord) where the absolute pressure gets negative (like in calculations without considering cavitaion). Now my question: Is it physically possible or senseful to have negative absolute pressures in water ? Regards, Mark

 April 16, 2002, 07:53 Re: negative pressure #2 Philipp Beierer Guest   Posts: n/a In my opinion, in a given volume cannot be less than an absolute vacuum. Hence, you will never find a negative pressure in "real" life. Since the fluid is evaporating somwhat before that, your lowest absolute pressure should be the vapour pressure of the fluid. On the other hand I could imagine, too, that you can maybe find slightly lower pressures than that since cavitation must not necessarily start immediately as soon as the pressure is below p_vap (I'm thinking now e.g. at highly accelerated flow). Anyway, negative pressure is impossible!

 April 16, 2002, 08:25 Re: negative pressure #3 bosko Guest   Posts: n/a It depends what pressure you are specifying at the inlet. If you set an inlet pressure of 1Pa for example then the absolute pressure would be negative almost everywhere in the domain. Make sure you specify a reasonable value.

 April 16, 2002, 09:01 Re: negative pressure #4 bosko Guest   Posts: n/a It depends what pressure you are specifying at the inlet. If you set an inlet pressure of 1Pa for example then the absolute pressure would be negative almost everywhere in the domain. Make sure you specify a reasonable value.

 April 16, 2002, 11:30 Re: negative pressure #5 dav Guest   Posts: n/a it is possible in two-phase flow, i've a case. it is possible mathematically and physically. i.e. it depends in your formula, P = x + y + .... cfd

 April 19, 2002, 04:10 Re: negative pressure #6 Michael Guest   Posts: n/a This is a matter how you define a negative pressure. Pressure can be given a direction in the alignment with the acting normal force. A negative pressure is a tension acting on a fluid volume normal to the surface, so a positive stress leads to a negative pressure. The ability of a liquid to withstand a tension is expressd in (negative) pressure units. See the book by Trevena "Cavitation and tension in liquids". Even Reynolds has measured a negative pressure of water of about -5bar. This "negative" pressure energy is used to build up a phase interface, before cavitation starts. Once the interface between vapour and liquid has been created, the negative pressure vanishes because the energy now goes into vaporization. Current VOF cavitation models as e.g offered in Fluent and StarCD assume that cavitation starts at vapor pressure, so this "non-equilibrium" effect is not modelled ! In this case the negavite pressure is an artefact of the model due to an unphysical description of the pressure field and a wrong pressure - vaporization coupling. There a several ways to define a more realistic pressure value in a cavitating region, e.g. you can use a conditional ensemble averaging procedure.

 April 19, 2002, 06:30 Re: negative pressure #7 Philipp Beierer Guest   Posts: n/a Hi Michael, I agree with you that a slight positive normal stress on a fluid particle is possible and that in accordance to that the pressure is negative. On the other hand act impurities or dissolved gases in liquids as sites for cavitation. Hence, in practical situations, it is generally not possible to observe pressures falling below the vapourisation pressure of the liquid; not at least for longer than it takes for the cavitation process to occur. And, as you are mentioning that this "non-equilibrium" effect cannot be modeled by the commercial VOF cavitation models (as e.g. Fluent, etc), I believe, too, that in Mark's case the detected negative absolute pressure is a matter of the employed model rather than a copy of real phenomena.

 April 19, 2002, 12:28 Re: negative pressure #8 Mark Render Guest   Posts: n/a Hi Michael, thanks for answer ! So did I understand you correct, that in reality it is possible that the pressure could get negative if the water is very pure ? So why should a cavitation model that is based on bubble growth (Rayleigh-Plesset equation) not be able to predict that negative pressure just before the water vaporizes ? Regards, Mark