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nico765 October 22, 2007 14:04

Hello, Is there a way to se

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).


hjasak October 22, 2007 14:13

Fluid density is constant: tha
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).



mkraposhin October 23, 2007 04:52

Dear Hrvoje Jasak! Please, ans
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!

hjasak October 23, 2007 05:02

Requires a Napoleonic answer h
Requires a Napoleonic answer

Multiply by the density


mkraposhin October 23, 2007 05:11

But it is negative!!! for exam
But it is negative!!! for example, pressure in simpleFoam ranges from -1 to 1 - what does it means?

mkraposhin October 23, 2007 05:24

And, another question (i hope,
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?

hjasak October 23, 2007 05:24

As you know, in incompressible
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.


mkraposhin October 23, 2007 05:39

Many Thanks, Hrvoje Jasak, for
Many Thanks, Hrvoje Jasak, for Your advice!
Many Thanks!

I'll give it a try.

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