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Drag coefficient implementation

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Old   November 30, 2020, 03:44
Default Drag coefficient implementation
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Sumit Peh
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Hi Foamers

I would like to apply equations of other Drag coefficient in 4 conditions of Renold's number.
[Re<1 // 1<Re<100 // 100<Re<1000 // Re>1000]

SchillerNaumann has 2 conditions of Re which are Re<1000 and Re>1000
Here is the SchillerNaumann code on ~dragModels
I supposed neg(Re - 1000) is this Re<1000 condition and pos0(Re - 1000) is the Re>1000.

Code:
{
volScalarField Re(max(Ur*phase1_.d()/phase2_.nu(), scalar(1.0e-3)));
volScalarField Cds
(
neg(Re - 1000)*(24.0*(1.0 + 0.15*pow(Re, 0.687))/Re)
+ pos0(Re - 1000)*0.44
);

return 0.75*Cds*phase2_.rho()*Ur/phase1_.d();
}
Is there a way to write code for more than 2 conditions ?

As my imagination, I guess maybe my case would be
Code:
{
volScalarField Re(max(Ur*phase1_.d()/phase2_.nu(), scalar(1.0e-3)));
volScalarField Cds
(
neg(Re - 1)*(Equation1)
+ neg1(Re - 100)*(Equation2)
+ neg100(Re - 1000)*(Equation3)
+ pos0(Re - 1000)*(Equation4)
);

return 0.75*Cds*phase2_.rho()*Ur/phase1_.d();
}
Please confirm if i'm right and please correct if i'm wrong.

Thanks in advanced

James
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Old   November 30, 2020, 05:04
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if the compiler does not complain about your code, you still need to verify it with easy to check simulation, maybe a quadratic pipe with one cell depth and height and 30 cells long. set different inlet velocities for both phases and use the calculator in paraview to calculate your drag coeff., also you can output the drag coeff. with openfoam, so you can cross check.
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Old   December 7, 2020, 23:02
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Sumit Peh
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Quote:
Originally Posted by geth03 View Post
if the compiler does not complain about your code, you still need to verify it with easy to check simulation, maybe a quadratic pipe with one cell depth and height and 30 cells long. set different inlet velocities for both phases and use the calculator in paraview to calculate your drag coeff., also you can output the drag coeff. with openfoam, so you can cross check.
Bad news, the compiler did complain about neg1, neg100.
So i tried this instead. Everything go well when compile but not sure its correct or not.

+ (1 - Re -100)
+ (100 - Re - 1000)
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Old   December 9, 2020, 04:52
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ok, let us first clarify what neg() and pos() mean:

neg(scalar s) checks if the value of s i negative, if yes, it will return
the value 1. so if neg(Re-1000) is negativ, than you will get a 1 back.

the same goes for pos(s) or pos0(s). pos(s): is s larger than zero,
pos0(s): is s larger than zero or 0. if yes, return 1.

which means, if you have neg(s) and pos0(s) in your code, it will only return
for one case 1, otherwise 0.

if you have many of that, as you did, it can and will be the case, that you will return more 1 than expected.

let me give an example:
if Re = 10 and you have

+ neg(1 - Re -100)
+ neg(100 - Re - 1000)

like you did, you will have

+1
+1

as return values, because in both cases, the if-check will give "true".
so, in that case, you will overpredict the drag coefficient.

as a workaround, i suggest you loop over all your cells and depending
on the Re, you can check in which value area Re is for each cell and save
Cds accordingly. afterwards you can insert that one in the place where this short formula would normally be.

i hope things are clear now.
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Old   December 10, 2020, 04:50
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Quote:
Originally Posted by geth03 View Post
ok, let us first clarify what neg() and pos() mean:

neg(scalar s) checks if the value of s i negative, if yes, it will return
the value 1. so if neg(Re-1000) is negativ, than you will get a 1 back.

the same goes for pos(s) or pos0(s). pos(s): is s larger than zero,
pos0(s): is s larger than zero or 0. if yes, return 1.

which means, if you have neg(s) and pos0(s) in your code, it will only return
for one case 1, otherwise 0.

if you have many of that, as you did, it can and will be the case, that you will return more 1 than expected.

let me give an example:
if Re = 10 and you have

+ neg(1 - Re -100)
+ neg(100 - Re - 1000)

like you did, you will have

+1
+1

as return values, because in both cases, the if-check will give "true".
so, in that case, you will overpredict the drag coefficient.

as a workaround, i suggest you loop over all your cells and depending
on the Re, you can check in which value area Re is for each cell and save
Cds accordingly. afterwards you can insert that one in the place where this short formula would normally be.

i hope things are clear now.
Hi geth03,

Thanks for your reply, you made me clear about selecting case for different Reynold's number.

I also found a Lain.C in /drags, it use the code like this so I changed it.

Code:
#include "Lain.H"
#include "phasePair.H"
#include "addToRunTimeSelectionTable.H"

// * * * * * * * * * * * * * * Static Data Members * * * * * * * * * * * * * //

namespace Foam
{
namespace dragModels
{
    defineTypeNameAndDebug(Lain, 0);
    addToRunTimeSelectionTable(dragModel, Lain, dictionary);
}
}


// * * * * * * * * * * * * * * * * Constructors  * * * * * * * * * * * * * * //

Foam::dragModels::Lain::Lain
(
    const dictionary& dict,
    const phasePair& pair,
    const bool registerObject
)
:
    dragModel(dict, pair, registerObject)
{}


// * * * * * * * * * * * * * * * * Destructor  * * * * * * * * * * * * * * * //

Foam::dragModels::Lain::~Lain()
{}


// * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //

Foam::tmp<Foam::volScalarField> Foam::dragModels::Lain::CdRe() const
{
    volScalarField Re(pair_.Re());

    return
        neg(Re - 1.5)*16.0
      + pos0(Re - 1.5)*neg(Re - 80)*14.9*pow(Re, 0.22)
      + pos0(Re - 80)*neg(Re - 1500)*48*(1 - 2.21/sqrt(max(Re, SMALL)))
      + pos0(Re - 1500)*2.61*Re;
}
As your example, why Re = 10 get +1 result ? It isn't in the range of neg(100 - Re - 1000).

According to my understanding, we can write code by this way :
pos0(Re - 1)*neg(Re - 100)
for a condition of 1 < Re < 100
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Old   December 10, 2020, 06:28
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if Re = 10, then neg(100 - Re - 1000) = neg(-910).
neg(-910) will check if -910 is a negative value, and return +1
if true, which it is, in that case.

as in the Lain.C - case, you can use pos0(s) neg(s1) in combination
so if your Re falls between two transition values, both return
+1 and +1, otherwise +1 and 0, which makes +1 x 0 = 0, and
so other Re-ranges are neglected.

i didn't think about combining those pos() and neg(), that is way smarter
then looping over all cells.
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