# Lagrangian Boundary Condition (interstitialInletVelocity

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March 14, 2014, 03:27
Lagrangian Boundary Condition (interstitialInletVelocity
#1
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Join Date: Jan 2010
Posts: 347
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Hello Foamers,

I have a question about interstitialInletVelocity inlet BC which is used in lagrangian/MPPIC/Goldschemidt testcase.
Code:
```    bottom
{
type            interstitialInletVelocity;
inletVelocity   uniform (0 0 1.875);
value           uniform (0 0 1.875);
phi             phi.air;
alpha           alpha.air;
}```
What is its difference with fixedvalue in application?
in its .h description it says:

HTML Code:
```Description
Inlet velocity in which the actual interstitial velocity is calculated
by dividing the specified inletVelocity field with the local phase-fraction.```
To find its difference with uniformFixedValue I compared average magnitude of inlet velocity (By using Integration over bottom patch) and found that in fixedvalue (0 0 2) the average of inlet velocity is 2, But in interstitialInletVelocity of ( 0 0 2 ) the average velocity is 4.8 m/s.
I calculated average alpha.air magnitude in this patch = 0.42 and understood that 4.8 m/s=2.0/0.42 as described in .h file.
Seeing the contour of inlet in this condition shows the magnitude of air varies between 4 to 6.51 m/s with the average of 4.89 (is attached) and I don't know why it increases the stated 2 m/s to higher magnitudes. If it considers the solid fractions in inlet patch so it should be some zero magnitude velocities on inlet but there is not seen.
Attached Images
 value2-bottomContour-interstitialInletVelocity.png (13.1 KB, 20 views)

March 23, 2014, 14:29
#2
Super Moderator

Bruno Santos
Join Date: Mar 2009
Location: Lisbon, Portugal
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Greetings Maysam,

If you take a look into the C file:
Quote:
 Originally Posted by src/finiteVolume/fields/fvPatchFields/derived/interstitialInletVelocity/interstitialInletVelocityFvPatchVectorField.C Code: ```void Foam::interstitialInletVelocityFvPatchVectorField::updateCoeffs() { if (updated()) { return; } const fvPatchField& alphap = patch().lookupPatchField(alphaName_); operator==(inletVelocity_/alphap); fixedValueFvPatchVectorField::updateCoeffs(); }```
It essentially defines that the fixed boundary value for U at the inlet should be defined as "inletVelocity_/alphap". Therefore, this apparently assumes that:
1. "inletVelocity_" is a reference velocity for a full "alphap" value, namely "1.0".
2. "alphap" can never be zero, otherwise it would result in a crash with a SIGFPE: http://en.wikipedia.org/wiki/SIGFPE#SIGFPE
3. The logic might be that the phase proportion is inversely proportional to the velocity, possibly due to a vacuum-like effect. I.e., when there is very little of this phase (smaller than 1.0), it acts as high-speed+low-pressure combination.

Best regards,
Bruno

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