The Perfect Storm

    
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Wind, CFD, Structural Finite Element Analysis:
Subject a "geodesic," 16'-8" diameter, radome to straight line, hurricane, winds of 220 mph and evaluate the effect of flange and panel thicknesses on stress levels.

Special features:
3D Unsteady Flow without turbulence employing brick elements and a direction of flow, symmetry, plane. 3D model setup with elastic isotropic material model for composite radome panels. The composite panels are a sandwich of Fiberglas lay-ups with a foam interlayer of various thicknesses (reduce weight, increase strength and afford some damping) represented with 3D shell elements, whereas the flanges were only Fiberglas. The elastic material properties were experimentally determined for each design thickness. The panel side flanges were modeled with beam elements with specified moments of inertia.

Output:
Local wind velocity (air velocity over panels) and pressure as a function of initial velocity and position with respect to the wind; for this example, the wind is blowing horizontally at 220 mph. Use the pressure from the flow result to load the radome panels directly for stress and displacement.

Comments:
The first figure above (click on the thumbnail) shows one example of a "lay-up." The radome example shown has three different panel sizes arranged in a complex way to yield the shape shown. The panels are bolted together with microwave transparent fasteners through holes in the panel flanges (not shown). The model was setup with prescribed velocity boundaries. The upstream air flow is moving in the negative X-direction. Next, the air velocity result is shown animated (the storm has arrived). The third thumbnail shows the pressure profile in the flow (along the symmetry plane, XZ plane, refer to the model) and at the panel locations. The force on a given panel can be as much as 1,250 lb. The second to last thumbnail reveals a figure of the "worst case" tensile stresses in the panel flanges and the last slide shows the radome displacement magnitude (magnified 10 times) at 220 mph. The composite material is elastic/brittle so the figure of merit will be the maximum tensile stress compared with the Ultimate Strength of the lay-up. It is evident, that for this design, "The Perfect Storm" is just a breeze. Since this approach seemed to yield benign results for various designs it was not necessary to model the turbulent air flow (vortex shedding on the downstream side of the radome and the consequent lateral buffeting) and resulting cyclic stresses within the structure or model a small assembly of panels explicitly in 3D. Is it any wonder the "geodesic" dome is so strong and economically built?