Preliminary aero-mechanical jitter analysis of hemispherical turrets

Abstract

This work presents preliminary results on aero-mechanical jitter of a hemispherical optical turret. A simplified geometry with a hemispherical shell and optics-holding canister was designed to reduce degrees of freedom and provide better insight into fundamental physics. Modal analysis of the turret and mounting plate to the wind tunnel, performed using finite e lement a nalysis ( FEA), r evealed s ignificant pl ate di splacements in th e lowest frequency modes. Three mounting plate thicknesses (1/4", 1/2", and 1") were tested. Wind tunnel tests at the University of Notre Dame's White Field Mach 0.6 wind tunnel assessed turret vibrations at speeds from Mach 0.2 to 0.5, using accelerometers and Shack-Hartmann tilt sensors. Two scenarios were tested: one with the turret inside the tunnel exposed to the flow, and another with the turret attached outside of the wind tunnel so that it is only excited by the base motion of the wind tunnel. The 1/4" plate showed tilt measurements ranging from 30 to 190 microradians when exposed to flow, compared to 10 to 50 microradians in the baseline c ase. The 1/2" and 1" plates exhibited lower tilts and less distinction between flow a nd b aseline c onditions. O verall, t he simplified turret only had about three vibration modes affecting t ilt, w ith s trong s patial a greement b etween experimental and FEA modal patterns.