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 samiam1000 February 29, 2012 10:58

flowRateInletVelocity

Dear All,

I am trying to use the flowRateInletVelocity BC. There is something that I can not understand: what does the `value' field mean?

The code looks like this:
Code:

```inlet { type        flowRateInletVelocity; flowRate    0.2;        // Volumetric/mass flow rate [m3/s or kg/s] value      uniform (0 0 0); // placeholder }```
And what about the value (0 0 0)? What does placeholder stand for?

Thanks,

Samuele

 romant March 1, 2012 03:27

Quote:
 Originally Posted by samiam1000 (Post 346914) Dear All, I am trying to use the flowRateInletVelocity BC. There is something that I can not understand: what does the `value' field mean? The code looks like this: Code: ```inlet { type        flowRateInletVelocity; flowRate    0.2;        // Volumetric/mass flow rate [m3/s or kg/s] value      uniform (0 0 0); // placeholder }``` And what about the value (0 0 0)? What does placeholder stand for? Thanks, Samuele
The value field is a part of every boundary condition, since they derive from the same basic boundary condition. placeholder means just that. you need it in order for OF to use the boundary condition, but it doesn't really matter what you set it to.

 samiam1000 March 1, 2012 03:42

Thanks for ansering.

I have a doubt: isn't that right that those values are the component of the u vector that is used if we have a revers-flux through that surface?

Thanks a lot,

Samuele

 romant March 1, 2012 03:46

Quote:
 Originally Posted by samiam1000 (Post 347018) Thanks for ansering. I have a doubt: isn't that right that those values are the component of the u vector that is used if we have a revers-flux through that surface? Thanks a lot, Samuele

In the source code it doesn't say anything about that and it gives that the flux is always normal and inwards from the patch.

Code:

```Description     Describes a volumetric/mass flow normal vector boundary condition by its     magnitude as an integral over its area.     The basis of the patch (volumetric or mass) is determined by the     dimensions of the flux, phi.     The current density is used to correct the velocity when applying the     mass basis.     Example of the boundary condition specification:     \verbatim     inlet     {         type        flowRateInletVelocity;         flowRate    0.2;        // Volumetric/mass flow rate [m3/s or kg/s]         value      uniform (0 0 0); // placeholder     }     \endverbatim Note     - The value is positive inwards     - May not work correctly for transonic inlets     - Strange behaviour with potentialFoam since the U equation is not solved```

 samiam1000 March 1, 2012 04:03

Dear Roman,

pardon the huge number of messages I am writing, but I would like to better und this point.

I many read the code many times. There's a point that I can not understand: why should we specify something that does not have any effects on our simulation?

Could you try to explain this?

Sorry if I am teasing you. I am just to try to und.

Have a good day,

Samuele

 romant March 1, 2012 04:08

Quote:
 Originally Posted by samiam1000 (Post 347021) Dear Roman, pardon the huge number of messages I am writing, but I would like to better und this point. I many read the code many times. There's a point that I can not understand: why should we specify something that does not have any effects on our simulation? Could you try to explain this? Sorry if I am teasing you. I am just to try to und. Have a good day, Samuele

Hej,

OpenFOAM is based on C++ where new classes and libraries, such as boundary conditions can be derived from previous ones, which also means that they take input parameters and internal values with them. In this case this means that the new class flowRateInletVelocity, which is based on fixedValueFvPatchVectorField, also inherited the value component.

This in turn means that the value variable is part of flowRateInletVelocity and therefore must be specified, otherwise the class is missing an input, even though the variable is never used. Just try it without the variable and you will notice that it won't run.

 samiam1000 March 1, 2012 04:12

Thanks Roman,

thank you very much.

I perfectly understand what you mean. And I completely agree with you. I haven't thought about that.

By the way - though I am afraid I am going off topic - is there `a point' in which I am asked to insert the value of velocity that are used just in case of a reverse flow somewhere?

Thanks again,

Samuele

 anon_a March 1, 2012 04:53

I think that the "value" field is also needed because it's read by paraview.
At least I think that paraview crashes if I delete that field.
In the beginning it does not make a lot of sense I guess.

 Ahmed Khattab March 4, 2012 14:58

where could i find such this file for all cases?

Quote:
 Originally Posted by romant (Post 347020) In the source code it doesn't say anything about that and it gives that the flux is always normal and inwards from the patch. Code: ```Description     Describes a volumetric/mass flow normal vector boundary condition by its     magnitude as an integral over its area.     The basis of the patch (volumetric or mass) is determined by the     dimensions of the flux, phi.     The current density is used to correct the velocity when applying the     mass basis.     Example of the boundary condition specification:     \verbatim     inlet     {         type        flowRateInletVelocity;         flowRate    0.2;        // Volumetric/mass flow rate [m3/s or kg/s]         value      uniform (0 0 0); // placeholder     }     \endverbatim Note     - The value is positive inwards     - May not work correctly for transonic inlets     - Strange behaviour with potentialFoam since the U equation is not solved```
please roman could you tell me the directory where this file exists?

 romant March 5, 2012 02:41

Quote:
 Originally Posted by rebel ahmed (Post 347580) please roman could you tell me the directory where this file exists?
You can find it under \$FOAM_SRC/finiteVolume/fields/fvPatchFields/derived/flowRateInletVelocity/flowRateInletVelocityFvPatchVectorField.H

 Ahmed Khattab March 5, 2012 11:11

 Ahmed Khattab March 5, 2012 11:11

solvers

roman,
i searched for discription for another type, slip type but this is the only description available. is there is moer detailed description.

Description
Foam::slipFvPatchField

is this available discription for solvers also?, if yes where?

 ntchuyen August 13, 2013 04:42

Dear all guys,

I used the type flowRateInletVelocity in my case for the velocity field at the inlet. The solver ran well. However this error occured when I open the paraView.

"--> FOAM FATAL IO ERROR:
Please supply either 'volumetricFlowRate' or 'massFlowRate' and 'rho'"

I've used OpenFOAM version 2.1.1.

Can you instruct me to repair it?

Best regard.

"The basis of the patch (volumetric or mass) is determined by the dimensions of the flux, phi."

Do you know where I can find the dimension of phi?

 vineet_bhola October 7, 2015 08:50

Hello James,
"phi" Files are created at run time. So let your solver run for some iterations and then you can check in the timestep folders. There you will either see m3/s or kg/s. You can change the number of solving iterations and write iterations in system/controlDict
Thanks
Vineet

Thanks for helping me knowing the unite of my flux.

I found that I am using kg/s. Anyway to change that? I'd like to use m3/s as my temperature is varying and I cannot use a fixed rho value.

 vineet_bhola October 19, 2015 09:55

I guess it is the opposite way. If you are using a compressible fluid then phi values should be in kg/s and if you are using a incompressible one, it should be in m3/s. Though I think it depends on your solver also, you can try to change it by changing the thermoType in Thermophysicalproperties file in constant Folder of your fluid Region.
The reason for m3/s and kg/s is that for a constant rho in Navier Stokes equation it can be removed from both sides. Also check the dimensions of Pressure. It is sometime P and sometimes P/rho.

Hope that helps.
Vineet.

I am using compressible flow with the solver buoyantPimpleFoam but I know the volumetric flow rate of my extracting system so I'd like to use it.

I'd like to work with constant volumetric flowrate, varying rho and varying mass flowrate (rather than constant mass flowrate, varying rho and varying volumetric flowrate).

I checked P is P (not P/rho)

My thermoType is hRhoThermo<pureMixture<constTransport<specieThermo <hConstThermo<perfectGaz

I start looking for documentation about how to change it but if you have any hint, starting point, it would be appreciated.

 vineet_bhola October 19, 2015 10:40

If you can use a constant volume flow rate and varying mass flow rate, you would be either getting a nobel prize or defying the laws of physics. Chances of second Option are more though :P
Just kidding. Fluids too have to follow conservation of mass, momentum and energy. Infact These 3 are the Basis of Navier Stokes Equation.
So ask the guy who gave you volumetric flow rate to either give the values of density at that Point OR better to give mass flow rate values.
Thanks
Vineet

I may have poorly explained what I meant:

Let's say my outlet BC is mass flow rate = 1

At t=0 I may have
mass flow rate = 1
volume flow rate = 1
rho = 1

At t=1, rho changes because temperature goes up, I may have
mass flow rate = 1 (never changes, it's the BC)
volume flow rate = 2
rho = 0.5

Now let's say my outlet BC is now volume flow rate = 1

At t=0 I may have
volume flow rate = 1
mass flow rate = 1
rho = 1

At t=1, rho changes, I may have
volume flow rate = 1 (never changes, it's the BC)
mass flow rate = 0.5
rho = 0.5

In this case, due to mass conservation, my inlet mass flow rate will have to change too, ofc.

My temperature is not varying that much and I probably could use mass flow rate estimating my outlet temperature since I already have a few simulations. But still, I'm interested in finding a way (if there is one) to use volume flow rate, "for knowledge".

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