PREVENTION OF AEROELASTIC INSTABILITIES
Aeroelastic oscillations cannot always be prevented. But it is desirable to limit the amplitude of oscillation within a safe bound for the entire range of flow speeds. For a cylindrical body, the resonance oscillation between the structure and the shedding vortices can be avoided by providing the structure with sufficient stiffness, so that the natural frequency of the structure is much higher than the frequency of the vortices. For an unstable aerodynamic section, the only way of preventing large amplitude oscillations is to provide sufficient damping in the system. For a suspension bridge, instability occurs when the reduced frequency of the bridge falls below a critical value, and can be stabilized by raising its natural vibration frequency. Stiffening and damping are the two most useful methods in controlling aeroelastic oscillations. Dampers, however, must be used with care, for they can be destabilizing. Cf. §6.11, p. 242.
One of the most important factors in aeroelasticity is the geometrical shape of the structure. In a civil engineering structure, aerodynamic forces being undesirable (unlike in airplane structures), an ideal section is one that produces no lift and small drag.
The application of this idea can be illustrated by the design of the Second Tacoma Narrows Bridge.2-11’2-12 The new design uses deep open trusses as the stiffening members (instead of plate girders), open trussed floor beams (instead of solid), and streamlined rail sections. Trusses, with small frontal area, are clearly aerodynamically ineffective. Tests in the laboratory for the new design showed complete stability even at higher angles of attack (up to 15°). These tests also revealed that the concrete deck, fitted with open steel grid slots of varying widths between each of the four traffic lanes and at the curb, has remarkable benefit.
On the other hand, one may try to design the structure streamlined so
that no separation occurs. Then, flutter, if any, will be of the “classical type” whose critical speed is higher than that of the stall flutter and can be predicted with good accuracy.
It is generally beneficial for aeroelastic stability to design the structure so as to have the least projected frontal area against wind. Decreasing the projected area decreases the magnitude of the aerodynamic forces. This follows from the fact that the aerodynamic forces are proportional to the vorticity strength, which in turn is proportional to the profile drag. A reduction in projected frontal area reduces the profile drag, and hence reduces the effective aerodynamic force.