Re: [ff3d-users] Problem with setting Boundary condition

[Stephane Del Pino]

Le vendredi 16 avril 2010, Juzar Thingna a écrit :
> 
> I want to set the derivative of w along the y-direction to be equal to wb
>  at ymin. I thought that setting dnu(w)=wb would do the trick, but
>  apparently I don't get the value of the slope at ymin = wb.
> Please could someone guide me as in where I'm going wrong?
> 
> The foll is my geometry file:
> 
> box {<0,7,0>,<20,37,20> pigment{color rgb <1,0,0>}}
> 
> Regards,
> Juzar Thingna
> Center for Computation Science and Engineering
> National University of Singapore
> 

Hello.

dnu(u) is not the normal derivative of "u" but the co-normal derivative as 
described in remark 14 of the user documentation (p50).
Your pde is 
        div(L*grad(w))-w=0
so the co-normal derivative is :
        dnu(w) = -L*dn(u)
where I noted here dn(u) to be the normal derivative. So, to get 
        dn(w)=wb
you need to specify
        dnu(w) = -wb/L

This is quite tricky ... To really understand how to set dnu(w) you need to 
understand variational formulas ...

Best regards,
Stéphane.