Normal to the boundary

Dear all,

I want to specify a blowing on airfoil wall as v_j=v.n_w in which n_w is the normal to the airfoil. How this normal can be defined?. Is N.y for normal to the boundary or it is just a unit vector in y direction?

best regards,


Hi Zeinab,

in FreeFEM, N.x and N.y on the boundaries are x and y components of the surface normal vector. Note that as per standard convention, the normal vector points outside of the defined domain.

I hope this helps.


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Dear Ugis

Thank you for your response. for example for my case as the figure below, I have the following expression:
V_j=uN.x+vN.y ? because I had V_j=|V_j|.n
so now I have two components which should be specified. do I need to find the tangent to the surface and find a relation between u and v to diminish the unknowns?


Indeed, that could be needed in some cases. But only if you need to also impose tangential velocity.

But this is up to your specification of the problem and not that much about the solver code … In general, for interface normal velocity only with magnitude U, in vector form you would have vec{u} = U*(N.x+N.y). Consequently, x component is UN.x and y component is UN.y.


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