Nonlinear Aerodynamic Characteristics Of Limit Cycle Flutter Of Typical Bridge Deck Sections

The wind-induced vibration responses of long-span bridges under wind force perform nonlinear aerodynamic characteristics.In this paper,the research progress of divergent flutter and limit cycle flutter are reviewed.The shortcomings of using conventional free vibration device for large amplitude tests are analyzed.This article describes the invention of the research team for large free vibration tests.The nonlinear aerodynamic characteristics of limit cycle flutter are studied based on the Sutong Bridge and H-shape sectional models.The main contents and conclusions are summarized as follows:(1)For the traditional free vibration test device of the bridge sectional model,the torsional stiffness reduction rate expression is derived under some appropriate assumptions.The results show that the torsional stiffness reduction rate of the traditional test device is 3%~10% when the torsional displacement reaches 10°~20°.The tilted springs inevitably perform lateral vibration and local vibration during the model vibration process,which introduces additional damping.It adversely affects the tests.(2)As the traditional free vibration device is not suitable for large amplitude coupled vibration,the research team invented a new free vibration device with the advantages of large amplitude,small damping,low cost,wide use and easy operation.(3)The wind-induced behaviors of Sutong Bridge and H-shape sectional models under different test conditions(different wind speeds,angles of attack,excitations)are studied by using the new device.The results show that the flutter can be divided into three cases:(i)Flutter hysteresis zone,that is,the presence or absence of external excitation has an impact on the form of flutter.If there is external excitation,the vibration amplitude increases gradually from the zero to a certain value until the limit cycle flutter happens.If there is no external excitation,the section models stay still.(ii)Limit cycle flutter stability zone,that is,the amplitude of limit cycle flutter basically shows an linear relationship with the wind speed.(iii)Divergent flutter zone,that is,the flutter amplitude increases exponentially when the wind speed is greater than the critical wind speed.The critical flutter wind speed calculated by linear flutter theory is the starting point of limit cycle flutter stability zone or divergent flutter zone.Large vibration may occur before the critical wind speed with the external excitation.(4)The amplitude ratio of vertical and torsional vibration between divergent flutter and limit cycle flutter is discussed.The results show that for limit cycle flutter,the bending-torsion amplitude ratio decreases with the increase of amplitude until it is stable at a certain value,which is related to the wind speed.For divergent flutter,the bending-torsiol coupled amplitude ratio increases with increase of amplitude.The ratio of bending-torsion amplitude increases linearly with the increase of wind speed in the stable vibration process of limit cycle flutter.In the bending-torsion coupled flutter process,the position of the aerodynamic torsion center move between upstream and downstream,which is related to the wind speed,angle of attack and motion period.(5)The energy characteristics of two sectional models under different working conditions are studied.The results show that the structural mechanical energy of the system presents a wavy upward trend in the process of limit cycle flutter.For the stable vibration process of limit cycle flutter,the structural mechanical energy is closed to a sine curve changing with time but not a constant.The mechanical energy of the system increases and decreases alternately in a complete vibration cycle,but the aerodynamic force work is zero.