Learning Forum

[Kamukyo]

Hello,

I tried to use the cylindrical coordinate system to see the axial velocity patterns in my model.

As you can see the figure below, the axial velocity sign is positive at the inlet but, negative at the outlet (actually, after the curve, the sign of the axial velocity was negative all the way down to the outlet..

It looks like z-direction velocity contour in the cartesian coordinates. As far as i know, the axial velocity sign should be positive at both the inlet and outlet in the cylindrical coordinate system.

I set a point on the inlet for rotation-axis origin and 0,0,1 for rotation-axis direction.

Could you give me any advice to fix this?

Thank you

 

 

[Rob]

The cylindrical coordinates are based on the rotation axis (as defined in the panel). As not all of your domain is aligned with this axis you'll get sensible results where it is, and nonsense elsewhere. 

[Kamukyo]

Thank you for the reply.

Is there anyway to get sensible results in all of my domain when it comes to the axial velocity?

[Rob]

It'll be possible with a UDF, but not something I'd recommend, and certainly not something we could help with much via the Community.  Breaking the domain into short lengths could also work, but may be more effort than it's worth. 

What are you trying to see on the plot as there may be an alternative approach. 

[Kamukyo]

What I am trying to see is whether it has the negative direction of the axial velocity component as the flow pass through the curve so that i think the direction of bulk flow should be consistent. I can plot the velocity profile on each plane that i am going to make. 

What you mean is that after curve, the rotation axis is no longer available and is turned back into the Z-direction ? 

Thank you

[Rob]

No, the axial velocity is defined from the axis you set in the panel. As the geometry deviates from that definition the plotted values become nonsense. 

Try plotting vectors on the plane that cuts the model roughly in half, and see if any vectors reverse. Plotting y+ and/or surface shear will also help as those values will approach zero at the point that the flow changes direction: this will only show the effect on the near wall cell.