SOME AEROELASTIC PROBLEMS IN CIVIL. AND MECHANICAL ENGINEERING

Aeroelastic oscillations are generally sustained by aerodynamic forces induced by the structure itself. Many types of structures may develop excessively large elastic deformation, or suffer sustained or divergent oscillations in certain ranges of wind speeds. For a structure designed to maintain static equilibrium such aeroelastic trouble can be serious.

The failure of the original Tacoma Narrows Bridge is an example of aeroelastic oscillation. If the wind speed and the mode and frequency of the structural oscillation are such that energy can be absorbed from the wind by the structure, and if the energy absorbed is larger than that dissipated by the structural damping, the amplitude of oscillation will continue to increase and will finally lead to destruction. The Tacoma Narrows Bridge failed at a wind speed of 42 mph, whereas the structure as built should have been able to resist a steady wind of at least 100 mph if no oscillation had occurred. Since all structures exposed to wind oscillate under some disturbances, it becomes apparent that it is essential for the designer to predict the critical wind speed at which the structure may become aeroelastically unstable.

In this chapter we shall outline the general principles of aeroelasticity for some civil and mechanical engineering applications. The main character of the problems discussed in this chapter is that the structures concerned are mainly unstreamlined and hence unamenable to theoretical treatment. We must rely heavily on experimental results to understand the nature of the aerodynamic forces.

Several typical examples of aeroelastic oscillations are given in §2.1. Some aerodynamic considerations are given in § 2.2. The flow around a cylinder is then reviewed in § 2.3. The aeroelastic oscillation of a cylin­drical body is treated in § 2.4, and that of an H-shaped section is discussed in § 2.5. Finally, means of preventing aeroelastic instabilities are discussed in § 2.6.

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