[paulathikalam]

Hii all,

Could you please help me with the formulations that is used to compute the effects of vegetation in B310?


Paul

[Felix_Sp]

Hello,

beware, that I am no expert from the REEF3D-team, but I am trying to understand the equations as well.

From my point of view, the Morison equation is basically used.

If you have a look in the files, vrans_veg_ini.cpp and vrans_veg_velsource.cpp from the source code, basically two things are done for the VRANS equations.

In the first file, the porosity is calculated based on:

a->porosity(i,j,k) =  1.0 - (p->B310_N[qn]*PI*pow(p->B310_D[qn],2.0)*0.25);

Which translates (approximately, since it is applied by a cell basis) into:



And is just the space not occupied by idealized vegetation.

In the second file, the velocity is adapted to the occurrence of vegetation:
The code reads:

 Fi = p->B309*0.25*PI*pow(D_val,2.0)*N_val*((a->u(i,j,k) - un(i,j,k))/p->dt);

and

 Fd = 0.5*Cd_val*N_val*D_val*a->u(i,j,k)*fabs(a->u(i,j,k));

Which is basically an incompressible form (no density) of the Morrison equation (https://en.wikipedia.org/wiki/Morison_equation), I guess:



This is then added to the RHS of the mathematical system for u and v only (do not know why not for w):

a->rhsvec.V[count] += (-Fi -Fd);

Since I am no expert I would be happy to be corrected, since I am also interested in this behavior and its applicability (maybe there is some literature on that?).



Cheers

[paulathikalam]

This is really helpful, thanks Felix

[valgrinda]

See here for the implementation: https://reef3d.files.wordpress.com/2...cej_porous.pdf

When using this VRANS model for vegetation, the alpha and beta values need to be adapted - depending on your vegetation.

I would not call the resulting momentum source terms Morison equations, but rather point to Darcy-Forchheimer. In VRANS, in addition to the source terms, the porosity is considered throughout all governing equations, see the Cm and n terms.