# Calculate velocity inflow on a 2D surface in 3D

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May 6, 2009, 20:07
Calculate velocity inflow on a 2D surface in 3D
#1
Senior Member

TWB
Join Date: Mar 2009
Posts: 118
Rep Power: 9
Hi,
I need to calculate velocity inflow on a 2D surface in 3D. I can vaguely recall but I 'm not too sure. i have the coordinates of the 2D surface and the velocity vector

The steps are:
1. using 2 vector on the 2d surface (v1 n v2), calculate its unit cross product vector. this new vector will be perpendicular to the 2d surface

2. do a dot product between the new perpendicular vector and the velocity vector (vel vect). tt 'll give the velocity flowing into the 2D surface.

is this correct? tks!
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 May 7, 2009, 08:06 #2 Senior Member   Andrew Join Date: Mar 2009 Location: Washington, DC Posts: 195 Rep Power: 9 isn't this just the curl?

 May 7, 2009, 09:19 #3 New Member   Konstantin Metodiev Join Date: Apr 2009 Location: Bulgaria Posts: 3 Rep Power: 9 I think (actually I know with confidence) that you will take the right steps if you follow what you have suggested in the list. As regards the curl mettler asked about, he obviously doesn't know know that the "rot" operator is obtained through a CROSS product between the nabla operator and another vector.

 May 10, 2009, 05:07 #4 Senior Member   Rami Ben-Zvi Join Date: Mar 2009 Posts: 148 Rep Power: 9 There is one step missing in the suggested process: The v1 x v2 product will indeed create a normal, but you will then have to normalize it to make it a unit normal. Only then do the scalar (dot) product with the velocity vector to have the normal velocity. The direction of the latter (into or out of the domain) will depend on whether v1 x v2 is directed into the domain or outwards.

 May 10, 2009, 05:54 #5 Senior Member   TWB Join Date: Mar 2009 Posts: 118 Rep Power: 9 Thanks Rami and samolet4e, ya i missed out tt normalising step. tks for pointing it out.

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