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Modal Analysis in Ansys to get natural frequencies and compare them with theory results
Hi, I'm Pietro, a short term Italian incoming student at VT (virginia tech).
My advisor, in order to start my thesis there about skin friction coefficients in a scramjet combustion chamber, gave me a Modal analysis to do on a gage described as follow:
a cantilever beam (1mm diameter and 3cm long cylindrical shape) with 1mm depth and 1cm diameter cylindrical flat "hat" attached upon.
Base constraints: zero displacements, zero rotation
On the top is applied a costant, mono direction wall shear stress of 200 Pa, so a 0.0157 N of wall shear force (but i don't think it needs for modal analysis).
Material: Nickel bellows: density=8880 kg/m^3, Poisson ratio:0.25, E=195 GPa (I ask more:is it correct?)
My problem is:THERE'S NO AGREEMENT BETWEEN THEORY AND SOFT. ANSYS ANALYSIS ABOUT NATURAL FREQUENCIES (e.g. ANSYS gave me a first mode frequency of 300Hz roughly,on the other hand, theory by f=(Kn/2*PI)*sqrt(EI/w*L^4); where w=width=1mm,L=3cm,Kn=3.52 for first mode roughly, gave me E+4 m order of magnitude)
WHAT'S WRONG?...WHAT HAVE I TO DO TO HAVE AGREEMENT?IS THERE SOMETHIN' WRONG IN THEORY FORMULA OR IN SOFT. ANALYSIS?
Thanks a lot, Best Regards.Pietro