[JJ122]

Hey all,
I am working on a harmonic response sim and I was hoping some one might have some advice for me on best practices. (I know this is a CFD forum but you all seem to be extremely well versed in all of the ansys software packages and if you know of somewhere else I should look please let me know)
To start I have a model with parts that are have a high surface area to volume ratio (I.E. they are thin) but a few of the parts are the opposite and have a low area to volume ratio. I would like to know what the resonant frequencies are and if it can survive a certain load at those frequencies. I have run a few sims and gotten some results, but of the 3 or 4 times I have gotten results they come out different, sometimes not giving frequencies even close to the previous sim.
Are there some rules of thumb that you use for a harmonic response? Things like should I keep the midside nodes or should I drop them?
I have looked at the help files but the mechanical help isn't as useful, to me, as the CFX help. I really wish there were tutorials for Mechanical (workbench) like there is for CFX and APDL.
Oh and I'm using Ansys 13.0 SP2
Thank you for any help,
JJ122