Problem with pressure distribution

Andrea

Pressure distribution (traction) applied to model with all tetra respect the total load (sum). Same pressure distribution applied to a model with C3D8 (also C3D20 tried) doesn't reach the theoretical load. I tried to reduce mesh size but not negligible different remains
I reproduce 3 different simple example

Victor

Non-uniform loads are interpolated at a few points over each element face so there will be some discretization error when it's a nonlinear function like this.

But it's odd that hex20 is worse than tet. Would you mind sharing the files, especially the coarse hex mesh?

Andrea

Problem is related to apply parabolic pressure distribution to a partial hole inner face. In this case the face forms an angle of 90°. Hole center, for pressure equation, should be 0,0,0. On attached image the theory behind

Victor

I get the same result (-9.41 kN) for both hex8 and hex20 with flat faces but it's much more accurate (-9.9994614 kN) with hex20 and fitting the midside nodes to the circular shape.



I think the main source of error is that due to the faceted shape, the x and z values in the formulas will be consistently inside the cylinder. I changed the formula to replace 5 with sqrt(x^2+z^2) to represent the actual radius, and that improved the reaction force to -9.97 kN on the original hex8 mesh. The remaining error is probably because the total area is smaller than the ideal cylinder's.



Curiously, this seems to be the same problem with a solution described by @kennethfugate recently https://mecway.com/forum/discussion/comment/7514

Andrea

Ok...but changing elements shape from linear to quadratic, did you changed manually the coordinate of mid side nodes or there is a trick?

Victor

I used Node coordinates -> Coordinate system = Cylindrical (Y axis) and set the radius.

JohnM

My favorite thing about this forum is some of the small discoveries I make with the gems that get offered up in conversation - like Node coordinates cylindrical, brilliant, and right in front of me!