Superposability Of The Unsteady Buffeting Loads On Bridge Decks

The2-dimensional unsteady aerodynamic forces, in the context of both thin airfoilswhere theory of potential flow is always applicable and bluff bridge deck sections whereseparated flow is typically induced, are investigated from a point of view of whether or notthese forces, under various states of structural motions and wind gusts, conform to theprinciple of linear superposition. It is shown that the basic preconditions, which lead to thelinear superposability of the unsteady aerodynamic forces in cases of thin airfoil sections, areno longer valid for a bluff section. However, theoretical models in bridge aerodynamics, suchas the one related to flutter-buffeting analysis and those concerning aerodynamic admittance(AA) functions, necessitate implicitly this superposability. The contradiction revealed in thisthesis may throw light on the perplexing problem of AA functions pertaining to thedescription of buffeting loads of bridge decks. Some theoretical AA models employed ratherextensively in bridge aerodynamics, which have been derived from flutter derivativesaccording to interrelations resulted from thin airfoil theories, are demonstrated to a greatextent to be illogical. With full understanding of the linear superposability of the unsteadyaerodynamic forces, based on the suggestions in connection with experiment-based AAfunctions presented, wind tunnel test results of identification of AA functions of flat plate andbridge deck sections are investigated to estimate the superposability of the unsteadyaerodynamic loads. The major work and conclutions are as follows:(1) A comprehensive review centered on theoretical models in regard to flutter-buffetinganalysis and AA functions is provided.(2) Steady and unsteady aerodynamic forces on thin airfoil are derived from some basicconcepts and principles to emphasize some basic preconditions that lead to the linearsuperposability,and the applicability of these preconditions in cases of bluff body sections.(3) Different kinds of grid induced turbulence are induced in wind tunnel to identify sixAA functions of flat plate and two types of bridge deck sections based on the theoreticalexpression of the cross-power spectrum. The grid is specially designed so that the turbulenceinduced can be regarded approximately as two-dimiensional.(4) The fundamentals of the dentification of AA functions on the basis ofcross-power-spectrum are introduced. The procedure of this identification is justified bynumerical simulation, and the influences of some basic parameters about the spectrumestimation involved in the identification are discussed. Then, tests in wind tunnel areconducted, and the experimental results show that AA functions of bluff bridge decks identified vary more obviously than that of flat plate sections, which imply that thesuperposability of the unsteady buffeting loads on bluff bridge decks are no longer valid.