[KateBradbrook]

The code for VariableHeightFlowRateInletFlowVelocity is:

scalar avgU = -flowRate_/gSum(patch().magSf()*alphap);
vectorField n(patch().nf());
operator==(n*avgU*alphap);

I cannot understand why avgU is multiplied by alphap.....

I have a function object to track the inflow (and also one for outflow to see when they have equalised):
volFlowIn
{
type swakExpression;
valueType patch;
patchName inlet;

verbose true;
aliases {
alphaWater alpha.water;
}
expression "alphaWater*U&normal()";
accumulations (integrate);
outputControl timeStep;
outputInterval 20;
}

And, predictably, this is always slighlty less than the flow rate I specify in boundary as the mass flow I calculate in any cell is effectively avgU*alphaWater^2 which is wrong in any cells where alphaWater<1.

I therefore made a GroovyBC to implement it as:
inlet
{
type groovyBC;
refValue uniform (0 0 0);
refGradient uniform (0 0 0);
valueFraction uniform 1;
value uniform (0 0 0);
valueExpression "-vel1*normal()";
gradientExpression "vector(0,0,0)";
fractionExpression "1";
evaluateDuringConstruction 0;
variables
3
(
"area1=sum(alphaWater*mag(Sf()));"
"flowRate1=650;"
"vel1= (alphaWater>1e-3) ? (flowRate1/area1) : 0;" //the threshold is arbitrary - need to check implications
);
timelines (
);
lookuptables (
);
aliases {
alphaWater alpha.water;
}
}

I would be very grateful for confirmation of my interpretation from experienced users to reassure me in what I have done, or an explanation as to why the code in VariableHeightFlowRateInletFlowVelocity is correct and my function object flow calc wrong!

[KateBradbrook]

OK. So I wrote: "I cannot understand why avgU is multiplied by alphap..... " I think it is for stability - my GroovyBC above introduces instabilities. So instead, I just adjust the velocities to account for the alphap^2 effect, so my variables become:

"area1=sum(alphaWater*alphaWater*mag(Sf()));"
"flowRate1=$inletFlowRate;"
"vel1= (flowRate1/area1)*alphaWater;"

So far, so good, but I'll post again if I need to revise further!