Laminar flow over a flat plate

Nick R · November 21, 2010, 04:22

Hi ALL

I'm trying to simulate flow over a flat plate (Laminar Steady State). I have a box as my domain whose bottom is defined as Wall and its left side is the inlet.
I have set the inlet to a certain normal velocity (0.001 m/s). However when the computation has converged and I plot the velocity vectors at the inlet I notice that next to the region where the inlet and the wall meet I get velocity vectors pointing upwards so the velocity at inlet is no longer the value I set before. I know this is supposed to be true away from the inlet because of the presence of boundary layer but I get this AT THE INLET!
Why does CFX change my Boundary Condition? What can I do to fix this?

Thanks in advance
Nick R

ghorrocks · November 21, 2010, 06:23

You apply boundary conditions on element faces but the velocity vectors are from control volume centroids. You specified the inlet velocity at the element face but the first control volume centroid has a normal component as it tries to sort the boundary layer out.

joey2007 · November 21, 2010, 06:48

For some post-processing operations you have the choice between "conservative" and "hybrid" (which is also conservative). Don`t have in mind if you have this also for vector plots. If yes you should change to hybrid.

Nick R · November 21, 2010, 20:43

Thank you guys. Very helpful.

November 21, 2010, 04:22	Laminar flow over a flat plate	#1
Nick R Senior Member Nick Join Date: Nov 2010 Posts: 126 Rep Power: 16	Hi ALL I'm trying to simulate flow over a flat plate (Laminar Steady State). I have a box as my domain whose bottom is defined as Wall and its left side is the inlet. I have set the inlet to a certain normal velocity (0.001 m/s). However when the computation has converged and I plot the velocity vectors at the inlet I notice that next to the region where the inlet and the wall meet I get velocity vectors pointing upwards so the velocity at inlet is no longer the value I set before. I know this is supposed to be true away from the inlet because of the presence of boundary layer but I get this AT THE INLET! Why does CFX change my Boundary Condition? What can I do to fix this? Thanks in advance Nick R

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November 21, 2010, 06:23		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	You apply boundary conditions on element faces but the velocity vectors are from control volume centroids. You specified the inlet velocity at the element face but the first control volume centroid has a normal component as it tries to sort the boundary layer out.

November 21, 2010, 06:48		#3
joey2007 Senior Member Join Date: Mar 2009 Location: Europe Posts: 169 Rep Power: 17	For some post-processing operations you have the choice between "conservative" and "hybrid" (which is also conservative). Don`t have in mind if you have this also for vector plots. If yes you should change to hybrid.

November 21, 2010, 20:43		#4
Nick R Senior Member Nick Join Date: Nov 2010 Posts: 126 Rep Power: 16	Thank you guys. Very helpful.