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Old   October 22, 2007, 14:04
Default Hello, Is there a way to se
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nicolas
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Hello,

Is there a way to setup the fluid density in simpleFoam?

I would like to use water, so far i have set up my cases to run at similar reynolds number using high velocities (~100m/s) in my cases. Not sure if this is the best way, since the cases are more unstable when running at higher velocities (same y+ values).

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Old   October 22, 2007, 14:13
Default Fluid density is constant: tha
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Fluid density is constant: that is why we use the kinematic viscosity. Therefore, divide your dynamic viscosity (in Pascal seconds) bu fluid density and specify that in the constant/transportProperties. Be careful to put back the density if you need wall forces or similar from the pressure (for consistency, p is kinematic pressure).

Enjoy,

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Old   October 23, 2007, 04:52
Default Dear Hrvoje Jasak! Please, ans
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Matvej Kraposhin
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Dear Hrvoje Jasak! Please, answer a stupid qustion: how can i convert relative pressure from simpleFoam to normal (total Pressure (kg*m/s^2)??

Many thanks for advice!
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Old   October 23, 2007, 05:02
Default Requires a Napoleonic answer h
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Requires a Napoleonic answer

Multiply by the density

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Old   October 23, 2007, 05:11
Default But it is negative!!! for exam
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But it is negative!!! for example, pressure in simpleFoam ranges from -1 to 1 - what does it means?
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Old   October 23, 2007, 05:24
Default And, another question (i hope,
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And, another question (i hope, i'm not too importunate, and sorry for bad English!)

This question arises when i increase mesh density in cavity tutorial by 2 (both in x and y dimensions) - pressure range increases by 4...

i think, it arises from Bernoulli eqn
i mean, p/rho + w^2/2 + g*z = const...

specific volume (= 1/rho) decreases by 4, so pressure range increasses 4 to satisfy equation. isn't it?
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Old   October 23, 2007, 05:24
Default As you know, in incompressible
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As you know, in incompressible flows, the pressure level does not matter: the flow is driven by the pressure gradient. Therefore, you can add any offset to the pressure field that you like - remember the pressure on the boundary being e.g. zero or the pressure in the reference point usually set to zero as well.

Therefore, if you know the absolute pressure in any point of the domain, just shift the complete pressure field for this number, maybe adding 101325 Pascal if it makes you feel better.

None of this actually matters when you are calculating the forces unless you've got vacuum outside.

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Old   October 23, 2007, 05:39
Default Many Thanks, Hrvoje Jasak, for
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Many Thanks, Hrvoje Jasak, for Your advice!
Many Thanks!

I'll give it a try.
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