open TELEMAC-MASCARET The mathematically superior suite of solvers

Hello everybody !

I'm starting working with dragforce and triing to understand it, so I wonder in that part of the program what is the use of 'PARAMETER :: N=1'

INTEGER IELEM,I,I4,NSOM,DISCLIN
DOUBLE PRECISION UNORM,AIRE,SOM,XSOM(4),YSOM(4),X4,Y4
! DOUBLE PRECISION, PARAMETER :: CD=1.56D0,DIAM=2.D0
DOUBLE PRECISION, PARAMETER :: CD=1.34D0,DIAM=1.D0
INTEGER, PARAMETER :: N=1
!
DOUBLE PRECISION P_DSUM
EXTERNAL P_DSUM
Thank you !

Hi Albin

N is intended to be the number of cylindrical drag elements within the quadrilateral area defined by XSOM(4), YSOM(4). You can see that the drag force components are multiplied by this value within the subroutine - i.e. a larger number of cylinders per unit area results in higher overall drag.

However, I would suggest to take care when choosing the drag coefficients you apply with this approach. For example, using the drag coefficient for an isolated cylinder is only appropriate when the cylinder spacing is large enough that there is limited interaction (in which case, the area-averaged approach may not be appropriate anyway, and you may want to define separate drag elements). When cylinders in an array become more densely packed, sheltering effects can reduce the drag on downstream elements, so the value for an isolated cylinder may not be appropriate. In this case, you may want to omit N (or set N = 1) and apply a "bulk" drag coefficient for the cylinder array that is a function of the packing density. For further explanation, see for example: Nepf (1999) "Drag, turbulence, and diffusion in flow through emergent vegetation", in Water Resour. Res.

Good luck.

Enda

Ah nice ! Thank you for your answer !