[swaldman]

I am new to CFD-Post, and am trying to use CFD-Post to compare two cases, in order to plot the differences in speed between them.

If I plot u-velocity or v-velocity and use the Compare Cases tool, this is straightforward. However, the flow is not parallel to either axis so these are not useful for this purpose.

If I plot simply "velocity", then all differences appear as positive, regardless of whether the "Use absolute value of difference" checkbox is checked. I speculate that this is because internally, the program is calculating the differences in u- and v-velocities and then deriving the magnitude from that - which of course is always positive.

How can I show the positive and negative differences in scalar speed between two cases? From some googling I get the feeling that I may be able to "do it myself" using CEL, but I'm not clear on how.

Many thanks for any advice.

[Lance]

I would do something like this

1) Create a CEL expression called velmag as sqrt(velocity u^2+velocity v^2+ velocity w^2)
2) assign expression velmag to a new variable called speed
3) perform case comparison.

[swaldman]

Originally Posted by Lance

I would do something like this

1) Create a CEL expression called velmag as sqrt(velocity u^2+velocity v^2+ velocity w^2)
2) assign expression velmag to a new variable called speed
3) perform case comparison.

Oh, that works perfectly, thank you. :-)
I think what I had missed was the need to first great an expression and then assign it to a variable - I had been trying to do this in one step and getting confused.