Identification Of Aerodynamic Rational Function And Multivariate Analysis Of Flutter Stability

Direct identification of aerodynamic rational function can meet the preliminary information of aerodynamic force for time domain and frequency domain of flutter analysis. So the establishment of corresponding theory and test technique has important practical engineering value. In order to investigate the influence factors to flutter stability, it is necessary to investigate the effect of cables caused by aerodynamic force and the vibration of cables, and the aerodynamic nonlinear effect induced by aerostatic additional Angle of attack, and the aerodynamic control measures. Therefore, this thesis carried out the following aspects of work:(1) The aerodynamic rational function identification theory which is based on three degrees of freedom sectional model both for free vibration and forced vibration was established, and aerodynamic rational functions identification program was developed. The analysis results show that proper selection of two test wind speed can identify rational functions and the corresponding aerodynamic derivatives. It is evident that this method reduces drastically wind tunnel experiment work, and can be applied in actual projects(2) The fast technique for fitting rational function coefficients by particle swarm optimization algorithm for global optimum is presented, and it provides global optimal values. Based on the rational functions-based unsteady forces identified above, a complete time-domain method for analyzing bridge is developed and implemented in ANSYS. The aerodynamic stiffness and damping portions of aerodynamic forces is modeled by Matrix27in ANSYS while the remaining recursive portions are treated as external loadings during time-domain simulation.(3) The explicit expressions for unsteady aerodynamic forces were derived for stay cables and a FORTRAN program based on this theory was implemented in special flutter analysis program NACS to consider the effect of unsteady aerodynamic forces of cables on flutter. A new finite element model of cable-stayed bridges is developed that condenses the mass of multiple elements of a cable to cable end node to effectively eliminate the local cable modes during modal analysis. By attaching MATRIX27element at cable elements’ nodes, the aerodynamic and cable vibration effects on flutter stability of cable-stayed bridge were taken into account. It is shown that the combined effect of these two factors promote flutter critical wind velocity. The forced vibration device for flutter derivatives identification can circumvents the additional attack angles which exists inherently for free vibration device. The flutter analysis accounting for the additional attack angle effect can be realized in ANSYS automatically for considering aerodynamic nonlinearity effectively.(4) The mechanism of central stabilizer for improving flutter performance is investigated based on modal energy participation in flutter state and3D flutter analysis. It is the increase of the energy participation of vertical bending degrees of freedom that leads to energy transfer of unstable torsion modes to the stable bending modes, and therefore promoting flutter velocity. Through the CFD numerical simulation and PIV wind tunnel test, micro mechanism of aerodynamic mechanism of improvement of flutter stability of long-span truss-girder suspension bridge using central stabilizer was studied.