## Posts

Showing posts from September, 2017

### Mesh Convergence Study

In a previous post, I mentioned mesh convergence study is not done as often it should. I've been reminding myself to be careful in reporting stress results using sanity checks. A nagging question persists in my mind: "How much should the mesh be refined before calling it good? Is a factor of 2 good enough?"

A presentation here at Ruhr-Universität Bochum is quite interesting. It presented a linear Richardson Extrapolation being:
fex = fn+ k(fm - fn) where...
fexis the exact solution
fm is the solution at mesh size m
fn is the solution at mesh size n
k is what I'll call Richardson Extrapolation Factor:

k = (mα) / (mα - nα)
where α=2 for linear elements and α=3 for quadratic elements. To plot this factor k relative to the reduction factor (n/m) in Figure 1:

Figure 1: Richardson Extrapolation Factor
It is interesting that for quadratic elements, with a mesh size reduction by a factor of two, the factor is 114.3%. Hypothetically, wouldn't it be great if, for example, on…

### Do you double-click?

In Ansys Mechanical, there are a lot of drop-down selections. Instead of clicking on the down arrow, try double-clicking on the current selection. The selection would cycle to what's next on the list.

This is convenient when I want to create multiples of an object (e.g. contacts or bolt pretension) with the only difference being the named selection. After setting up the first object as I like it, I then Right-Mouse-Button "Duplicate" and double-click on the named selection to cycle through the named selection. This of course only works if the named selection was already created as part of the workflow that is named sequentially (e.g. bolt1, bolt2, bolt3... (or) fric1a, fric1b, fric2a, fric2b...)

Double-clicking also works for a slew of drop-downs in Analysis Settings.

p/s: Creating an ACT script would be more efficient but the conventional/lazy way above could be faster if future projects would rarely need it.

### Unintended Interference

If you're not careful, you'll end up like me with unintended interference. The problem can be illustrated with a pin and hole-part assembly. Three identical geometry assemblies are created. From left to right, firstly a Tet mesh, then two others of different Hex mesh controls (Figure 1). Though the pin has the same diameter as the hole diameter, there may be interference in the model due to geometry discretization.

Figure 1: Identical Geometry but Different Mesh
The difference in results are clear from the contact pressure shown in Figure 2:
Figure 2: Contact Pressure at Frictionless Contacts
The coarse Tet mesh does not 'curve' well (Figure 3) thus bumping into each other. The interference in the second assembly with hex mesh is less acute but still present (Figure 4). The third assembly with hex mesh with nodes of both the pin and hole aligned has no interference (Figure 5) hence almost no pressure.

Figure 3: End View Tet Mesh Shows Interference at Hole
Figure 4: End…

### Increasing Number of Iteration per Substep

Recently I came across a problem where the large deflection solution that has Body-to-Body beam connection that wouldn't converge. Ansys bisected at the maximum number of  iterations (26) and then halved the load step again... and again...

The trick was to add a command snippet in the solve section:

...with the following text containing NEQIT to increase the maximum number of iterations per substep :
neqit, 75

and VOILA! It Solves!