[Nek5000-users] Tangential velocity / gradients

[nek5000-users at lists.mcs.anl.gov]

Hi Paul,

This post has its relevance to an earlier post with regards to calculating
shear stress [
http://lists.mcs.anl.gov/pipermail/nek5000-users/2010-April/000261.html] .
You had earlier mentioned an approach to get the wall shear stress by
obtaining the tangential velocity and then differentiating it. I found the
tangential velocity at the first grid point close to a wall by this eqn :
->       ->    ->
U_t  = U - ( U . nhat ) nhat ; % nhat is the unit normal vector pointing
away from the wall.

The gradient of this quantity (which is defined only near the wall) should
then give the shear stress. But calling gradm1 with the tangential
velocities
gives zero gradients near the wall. Is there any other routine / way to
calculate the gradient ?

Regards
Shriram
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