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Old   August 6, 2020, 13:12
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Howdy Foamers:

I am using uniformFixedValue for one of my boundary conditions so that I can use a time varying profile to describe the evolution of one of my scalar variables.

My question is, while Function1 returns an interpolated value between two points, is there a function that will simply give me a step value until a new point of time is reached.

To illustrate, I have something like this for a generic scalar (phi)

t = 0, phi = 0
t = 1, phi = 0.1
t = 2, phi = 0.2

If this list of points are defined as a Function1 in my uniformFixedValue boundary, at t = 1.2, I would get 0.12. However, I want this value to stay at 0.1 until t = 2, in which case phi will stay 0.2 until a new point of time defined.

Thanks in advance, Gerry.
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Old   August 6, 2020, 13:27
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I'm not sure about prescribing a step function, but the first thing I thought of is using the tableFile option to read (from e.g. a csv) the values for a given time. You can also just provide the table directly if the number of points is sufficiently small. Here's an example that I've used for reading from a csv :

Code:
        type           uniformFixedValue;
        uniformValue   tableFile;
        uniformValueCoeffs
        {
            file                "$FOAM_CASE/data";
            format              csv;
            nHeaderLine         0;
            refColumn           0;
            componentColumns    (1);
            separator           ",";
            mergeSeparators     yes;
            outOfBounds         clamp;
            interpolationScheme linear;
        };
        value          $internalField;
And it looks like there's a tutorial for providing the table directly : https://github.com/OpenFOAM/OpenFOAM.../propeller/0/U

Caelan
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Old   August 21, 2020, 07:04
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Dear Caelan:

Thanks for the clarification, perhaps I wasn't too clear about my question. So this is kind of what I wanted to do. When I use a uniformFixedValue boundary, I define the following Function1 table:

Code:
// PATCH FIELD DEFINITION

uniformFixedValueBoundary  {
   type         uniformFixedValue;
   uniformValue  table  (
      ( 0   0.2 )
      ( 1   0.3 )
      ( 2   0.4 )
      ( 3   0.5 )
   )
}
When you run a simulation for, say, 4 seconds, OpenFOAM will determine the boundary values through interpolation, and the will remain at the closest value should the time be beyond the defined time range. That is, I will expect to get:

Code:
// WHAT I EXPECT TO GET FROM OPENFOAM

 TIME  VALUE
-------------
 0.0   0.20
 0.5   0.25
 1.0   0.30
 1.5   0.35
 2.0   0.40
 2.5   0.45
 3.0   0.50
 3.5   0.50
 4.0   0.50
HOWEVER, I would like to have the fixed values to be presented as a stepwise manner, i.e., not interpolate until the time has reached the next tabulated time value. In other words, this is what I want:

Code:
// WHAT I WANTED TO GET

 TIME  VALUE
-------------
 0.0   0.20
 0.5   0.20
 1.0   0.30
 1.5   0.30
 2.0   0.40
 2.5   0.40
 3.0   0.50
 3.5   0.50
 4.0   0.50
Is there a way to tell the Function1 object to do this? A workaround is to introduce additional points so that OpenFOAM will interpret this as piecewise constant, like this:

Code:
// WORKAROUND

uniformFixedValueBoundary  {
   type         uniformFixedValue;
   uniformValue  table  (
      ( 0      0.2 )
      ( 1      0.2 )
      ( 1.0001 0.3 )
      ( 1.0002 0.3 )
      ( 2      0.3 )
      ( 2.0001 0.4 )
      ( 2.0002 0.4 )
      ( 3      0.4 )
      ( 3.0001 0.5 )
      ( 3.0002 0.5 )
   )
}
The caveat is that OpenFOAM will only accept the first 6 significant figures. If you define extra points at 3.000001 and 3.000002, OpenFOAM will thorw in a fatal error because it cannot distinguish between the two.

Any ideas (wiuthout rewriting Function1) to do this would be most welcome.

Thanks again, Gerry.

Last edited by Gerry Kan; August 23, 2020 at 04:53.
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Old   August 21, 2020, 12:14
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I am not sure how to do it with Function1 -- perhaps someone else might. Another option if you want it to be instantaneous would be to run it in sections -- run a second, change the bc, and then run another second, etc.

Caelan
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