Meaning of 3rd keypoint in LARC command

[MacEd]

Hello

I don't have access to ANSYS but I want to understand how an arc is defined with LARC.

The 3rd keypoint (PC), I assume lies in the plane of the arc, that makes sense. But it also has another function which seems to be to determine which one of the 4 possible arcs of the specified radius between the other two points to use. Can anyone explain how that decision is made? All I can find is this. Confusing part highlighted:

"Keypoint defining plane of arc and [highlight #FCE94F]center of curvature side (with positive radius)[/highlight]. Must not lie along the straight line from P1 to P2. PC need not be at the center of curvature."



FEA software feature chart

http://feacompare.com

 

[0rel]

This keypoint is kind of an information for Ansys for two things :
- You need 3 points to define a plane, thus Ansys can know in which plane to draw your arc
- Imagine a line between P1 and P2. Ansys needs to know on which side it has to put the center of your arc.

Ex :
Say you have P1 (1,0,0) and P2 (0,1,0) and you want to draw an arc from P1 to P2 with its center at the origin, ie C (0,0,0)
RAD = 1
You can define pretty much what you want as PC :
PC (0,0,0)
LARC,P1,P2,PC,RAD

or
PC (0.1,0.1,0)
LARC,P1,P2,PC,RAD

but also
PC (2,2,0)
LARC,P1,P2,PC,-RAD
(notice the - sign before RAD, telling Ansys "ok, PC helps you define the plane, but the center is on the other side of P1-P2")

I'd set PC to the center for clarity purpose though.

 

[0rel]

Oh, one more thing, you don't have 4 possible arcs but an infinity.

 

[MacEd]

Thanks Orel, that helps partly but I think it still leaves ambiguity

How does it know I don't want the 270 degree arc passing through the 2nd, 3rd and 4th quadrants? In 2D you can say the arc is counter-clockwise from P1 to P2, but it's a 3D arc so there doesn't seem to be any definition of which direction it should go.

So there are 4 arcs with radius=1, ending at P1 and P2 and in the XY plane where PC is.



FEA software feature chart

http://feacompare.com

 

[0rel]

Good point

But still, there are 4 arcs using P1 & P2 if you consider ansys knows the plane in which the arc is located and the center location or radius.

How does it know I don't want the 270 degree arc passing through the 2nd, 3rd and 4th quadrants?

It doesn't, and doesn't care either. Ansys just considers you're using LARC for its convenience.

In 2D you can say the arc is counter-clockwise from P1 to P2

Every arc is in 2D, that's the point of defining the plane.

but it's a 3D arc so there doesn't seem to be any definition of which direction it should go.

The direction you want is said implicitly by using the LARC command : the shorter path (thus colinear points are not allowed).



Ansys has a lot of commands that are meant to do the same thing but quite differently. Thus, to make a 270° arc in a "3D" plane, use circle and an angle <360°

Hope you'll find the info you wanted there.

 

[MacEd]

So LARC cannot make an arc greater than 180 degrees?

Just to make sure I got it right, the definition for PC with positive radius could be:
"The center of curvature of the arc is on the same side of the line through P1 and P2 as PC."



FEA software feature chart

http://feacompare.com

 

[0rel]

Exactly