Results comparison
Results comparison
(OP)
Thought i'd throw a question out there:
If you have a coarse grid model of say 9 Quad4 elements in a 3 rows of 3 columns (Nastran) and apply a load to the model to generate shears and endloads within the quads. And then use hand calcs to size the panels, all well and good.
If a finer grid model of say 81 Quad4 elements in a 9x9 similar group (so 9 elements take the place of the original element) and apply the same loading. How is the best way to compare the results of the new model against the old.
So if the original quad had 10N/mm shear flow, how do you compare (combine?) the new more detailed elements back to the original.
Hope that makes sense.
If you have a coarse grid model of say 9 Quad4 elements in a 3 rows of 3 columns (Nastran) and apply a load to the model to generate shears and endloads within the quads. And then use hand calcs to size the panels, all well and good.
If a finer grid model of say 81 Quad4 elements in a 9x9 similar group (so 9 elements take the place of the original element) and apply the same loading. How is the best way to compare the results of the new model against the old.
So if the original quad had 10N/mm shear flow, how do you compare (combine?) the new more detailed elements back to the original.
Hope that makes sense.





RE: Results comparison
clear as mud, no!?
RE: Results comparison
I would plot the results (whatever you are interested in displ, and or stress components) for each mesh vs. position ( an x-y plot) and see how the plotted data overlays...You know that (assuming both models are correctly put together and solved) the finer mesh will have better results and you should be able to get a reasonable estimate of the correlation between the meshs......I also think this would give a better picture of what is going on than rb's method....
Ed.R.
RE: Results comparison
I ran 2 quick models today, and was going to upload the details, but typically i forgot, will do it tomorrow.
However, basically i had the 3x3 grid and applied a transverse load at teh top of the sqaure, and contrained at the bottom of the sqaure. As would be expected the centre quad conatined only shear (around 23N/mm) and no endloads. The same loading etc applied to a 9x9 square grid and once again the (smaller) centre grid contains only shear, but the magnitude is just over double (around 58N/mm).
Plotting the shear across the centre line of the square grid it can be seen that the more detailed plot (9 elements across) is trying to form a parabola, where as the coarse grid (3 elements across) is only able to form a pointy 2 line shape.
The mangitude of difference however was a bit of a surprise
RE: Results comparison
Sounds like you are seeing a behaviour similar to beam shear across a beam cross section....The 3x3 is trying to show the average shear across the section while the 9x9 is showing a much more realistic set of shear values....If you think of a beam shear situation the average shear values times 1.5 are the max values of the actual parabolic shear values (for rectangular x-sections).....given that your section is probably more like a deep beam I suspect that double is not too bad a value (you could probably check a deep beam solution in a handbook for a better estimate of the values).....
Ed.R.
RE: Results comparison
You should also be able to do a hand calculation of this situation.
RE: Results comparison
I have uploaded a wordpad file which contains BOTH input decks, so if your interested then you would have to just copy out the separate parts before using.
RE: Results comparison