is it possible to calculate div (vector,vector) ?

SHUBHAM9595 · January 28, 2020, 12:30

Dear Foamers,

I'm trying to compute a source term for the velocity field which is shown in the attached image.

So I tried the following way (Here M and H are both vector Fields) but I'm getting an error which is not surprising as If I'm not wrong then OF only calculates the divergence of a vector field with a scalar prefix.

But is it possible to calculate something like fvc::div (vector, vector) ?

// KELVIN FORCE DENSITY INITIALIZATION

Code:

volVectorField KFD = munot*(fvc::div(M, H)); // M dot del H

Is there any workaround to do so? It'll be really helpful if you guide me to proceed in the right direction.

geth03 · June 20, 2022, 11:54

Openfoam can calculate divergence from a volVectorField or surfaceScalarFields.
If M*H results a volScalarField, it will probably return an error.

überschwupper · June 21, 2022, 03:58

I would forward you to this page:
https://cpp.openfoam.org/v9/classFoa...ts_1_1div.html

But I'm not sure if the math is correct here in your approach. The expression inside the brackets results in

$(\vec{M} \cdot \nabla) = M_x \frac{\partial}{\partial x} + M_y \frac{\partial}{\partial y} + M_z \frac{\partial}{\partial z}$

which defines a new "Scalar operator" which has to be applied to each component of your vector H. Thats why you dont need the divergence here. Those are the components here of the resulting vector

$M_x \frac{\partial H_x}{\partial x} + M_y \frac{\partial H_x}{\partial y} + M_z \frac{\partial H_x}{\partial z}$
$M_x \frac{\partial H_y}{\partial x} + M_y \frac{\partial H_y}{\partial y} + M_z \frac{\partial H_y}{\partial z}$
$M_x \frac{\partial H_z}{\partial x} + M_y \frac{\partial H_z}{\partial y} + M_z \frac{\partial H_z}{\partial z}$

This is not that trivial as someone would assume. I cannot provide a solution right now. But you need to define a function that is calculating the differentials of H and multiply them with M.x(), M.y(), M.z()

June 20, 2022, 11:54		#2
geth03 Senior Member Join Date: Dec 2019 Location: Cologne, Germany Posts: 355 Rep Power: 8	Openfoam can calculate divergence from a volVectorField or surfaceScalarFields. If MH results a volScalarField, it will probably return an error. Last edited by geth03; June 21, 2022 at 04:14.*

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June 21, 2022, 03:58		#3
überschwupper Member Join Date: Jan 2022 Location: Germany Posts: 72 Rep Power: 4	I would forward you to this page: https://cpp.openfoam.org/v9/classFoa...ts_1_1div.html But I'm not sure if the math is correct here in your approach. The expression inside the brackets results in $(\vec{M} \cdot \nabla) = M_x \frac{\partial}{\partial x} + M_y \frac{\partial}{\partial y} + M_z \frac{\partial}{\partial z}$ which defines a new "Scalar operator" which has to be applied to each component of your vector H. Thats why you dont need the divergence here. Those are the components here of the resulting vector $M_x \frac{\partial H_x}{\partial x} + M_y \frac{\partial H_x}{\partial y} + M_z \frac{\partial H_x}{\partial z}$ $M_x \frac{\partial H_y}{\partial x} + M_y \frac{\partial H_y}{\partial y} + M_z \frac{\partial H_y}{\partial z}$ $M_x \frac{\partial H_z}{\partial x} + M_y \frac{\partial H_z}{\partial y} + M_z \frac{\partial H_z}{\partial z}$ This is not that trivial as someone would assume. I cannot provide a solution right now. But you need to define a function that is calculating the differentials of H and multiply them with M.x(), M.y(), M.z()