The 2-dimensional unsteady aerodynamic forces,in the context of both a thin airfoil where theory of potential flow is always applicable and a bluff bridge-deck section where separated flow is typically induced,are investigated from a point of view of whether or not they conform to the principle of linear superposition in situations of various structural motions and wind gusts.It is shown that some basic preconditions that lead to the linear superposability of the unsteady aerodynamic forces in cases of thin airfoil sections are no longer valid for a bluff section.Theoretical models of bridge aerodynamics such as the one related to flutter-buffeting analysis and those concerning aerodynamic admittance(AA)functions,however,necessitate implicitly this superposability.The contradiction revealed in this work may throw light on the perplexing problem of AA functions pertaining to the description of buffeting loads of bridge decks.Some existing theoretical AA models derived from flutter derivatives according to interrelations valid only for thin airfoil theories,which have been employed rather extensively in bridge aerodynamics,are demonstrated to be illogical.Finally,with full understanding of the preconditions of the applicability of linear superposability of the unsteady aerodynamic forces,suggestions in regard to experiment-based AA functions are presented.
Mean wind response induced incompatibility and nonlinearity in bridge aerodynamics is discussed,where the mean wind and aeroelastic loads are applied simultaneously in time domain.A kind of incompatibility is found during the simultaneous simulation of the mean wind and aeroelastic loads,which leads to incorrect mean wind structural responses.It is found that the mathematic expectations (or Iimiting characteristics) of the aeroelastic models are fundamental to this kind of incompatibility.In this paper,two aeroelastic models are presented and discussed,one of indicial-function-denoted (IF-denoted) and another of rational-function-denoted (RF-denoted).It is shown that,in cases of low wind speeds,the IF-denoted model reflects correctly the mean wind load properties,and results in correct mean structural responses;in contrast,the RF-denoted model leads to incorrect mean responses due to its nonphysical mean properties.At very high wind speeds,however,even the IF-denoted model can lead to significant deviation from the correct response due to steady aerodynamic nonlinearity.To solve the incompatibility at high wind speeds,a methodology of subtraction of pseudo-steady effects from the aeroelastic model is put forward in this work.Finally,with the method presented,aeroelastic nonlinearity resulted from the mean wind response is investigated at both moderate and high wind speeds.
