Theoretical,Experimental And Numerical Study On The Unsteady Aerodynamic Loads Of Bluff Bridge Sections

Aerodynamic loads developed on bluff bridge deck sections and on streamlined sections differ from each other significantly.Due to flow separation,flow fields around the bridge deck sections are no longer satisfied with the assumption of the potential flow,let alone assumptions that the trail of vortices are one dimensional and vortex strength is time-invariant.Nevertheless,some methods and conclusions from airfoil theories are still being widely used in bridge wind engineering.However,rationality of these applications to bluff sections has not been verified,and hence has resulted in several long-existing cruxes in bridge wind engineering.In this paper,the unsteady aerodynamic load theories of 2-dimensional thin airfoils are reviewed,including the mechanism and basic assumptions as well.As far as applications of these theories to bridge wind engineering are concerned,issues in the current buffeting-flutter theory are pointed out.Finally,in virtue of CFD simulations and wind tunnel tests,these isues are investigated and disscussed.Main contents of this work are given as follows:(1)The achievements of Von Karman and Sears regarding unsteady aerodynamic loads acting upon two-dimensional thin airfoils are reviewed,and mechanisms and basic assumptions involved are clarified.Based on these assumptions,the traditional flutter-buffeting theory in bridge wind engineering are re-examined and the following problems are pointed out:the aerodynamic admittances of the bluff bridge deck sections are no longer uniquely depend on the reduced frequency but also on properties of the oncoming turbulence;existing methods that focus on determining aerodynamic admittances according to known flutter derivatives are illogical;the self-excited and the buffeting loads,respectively due to the body motion and oncoming wind fluctuating,can not be superposed with each other.(2)Three numerical methods and computing strategies for identifying aerodynamic admittances are discussed.Among the three methods,the turbulent method has a moderate computational efficiency and poor smoothness;the harmonic fluctuating method,while of lowest efficiency due to the need of sweeping throughout the entire concerned range of frequency,leads to smooth results;the Kussner method,which is of the highest efficiency,has an obvious shortcoming of error accumulation and transmitting.Furthermore,it is only applicable to sections of which the aerodynamic admittances are completely independent on wind fields.(3)Based on CFD numerical simulations,aerodynamic admittances of a thin plate,a box-girder section,a double-girder section and a rectangular section are numerically identified.Two types of oncoming flow are considered,one is harmonic fluctuating flow and the other is broadband turbulent flow.Numerical results indicate that,for the thin plate and box girder sections,aerodynamic admittance functions are insensitivity to the oncoming wind field;as for the double-girder and rectangular sections,aerodynamic admittance functions exhibit strong dependence on the approaching wind field.Pressure measurement tests are carried out on the thin plate and rectangular sections in grid-turbulence wind fields in wind tunnel,and the results are in accord with the CFD numerical simulations.(4)Rationality of two types methods of identifying aerodynamic admittances based on known flutter derivatives is investigated.For the substitutive Kυssner function method,with the increase of the reduced frequency,the gap between this method and the measured results gradually increases due to ignoring of higher-order terms of velocity,and eventually it results in wrong limiting characteristics.In comparison,the equivalent Theodorsen method is in accord with the measured results in terms of general trends and limit characteristics as well;however,as the reduced frequency increases,results of this method exhibit a significant oscillation,especially for the rectangular section.(5)CFD method is adopted to investigate superposability of self-excited and buffeting loads,with thin plate and rectangular sections concerned.Emphasis is focused on the effects of turbulence characteristics,reduced velocity on the superposability.The results show that,for the plate section and rectangular section,the turbulence can have weak effects on flutter derivatives and self-excited loads at low reduced velocities.As the reduced velocity increases,however,the effect of turbulence on them becomes progressively significant.In general,as far as flutter derivatives and self-excited loads are concerned,the bluff rectangular section is more affected by the turbulence than the flat plate.Furthermore,without consideration of the vortex-induced loads,the structural motion itself has no influence on the buffeting loads for both plate and bluff rectangular sections.