CFD Online Discussion Forums

CFD Online Discussion Forums (
-   CFX (
-   -   channel flow-velocity in the wall (

antonio July 1, 2010 13:20

channel flow-velocity in the wall
Hi all.

I am modelling a channel where the bottom consists of many triangles equally spaced. I have already a solution. When I plot a velocity vector the profile seems correct because there`s no velocity in the bottom (using hybrid values). However, when I export a .dat profile (using hybrid or conservative) the velocity in the wall (y=0) is not zero. In the documentation it says that for quantitative calculations one should use conservative values. My objective is to do a spatial average of several velocity profiles along the channel.

What is the best option/way for this objective?Using conservative values and consider the first point where y=0 (with u different from zero) garbage? A light would be very helpful.

Many thanks

ghorrocks July 1, 2010 18:39

Read the documentation about hybrid and conservative values. In short the conservative value of the wall node velocity is not zero as the centroid of the control volume is off the wall, and hence it has a non-zero value. The centroid of the control volumes away from the wall is close enough to the nodal values so no correction is needed.

antonio July 2, 2010 06:29

Hi all. Thanks Glenn for your suggestion. I read the documentation (sometimes it`s good to read something more than 1 time to really understand how things are:)...) .

But still I have a doubt. In my case points inside the contour of the triangles have velocity!? This is not possible because inside the triangles there is no fluid...I have drawn a line in a triangle zone and then export the velocity values on that line.Why do I have velocity "inside" the triangles?!


ghorrocks July 2, 2010 06:34

Can you show an image of what you are modelling. I have no idea what the triangles you refer to are.

But my guess as to what is happening is that the triangles are holes in the domain and should show no velocity. If you have limited sampling points along the line you might not have enough points and you jump over a triangle. Rather than specify a number of sampling points use the section option (I think that's its name) and it will give a value at each control volume the line intersects.

antonio July 2, 2010 06:56

Yes, I could but I am not being able to put here an image.

But basically the ideia is what you have mentioned. The bottom consists of triangles that have a hole. I don`t think the problem is the way I am taking the samples. I used the maximum samples allowed and the first y height is inside the hole...

antonio July 7, 2010 11:29


All times are GMT -4. The time now is 05:38.