桥梁断面颤振导数的CFD全带宽识别法

EXTRACTION OF FLUTTER DERIVATIVES OF BRIDGE DECK AMONG FULL BANDWIDTH OF REDUCED WIND SPEEDS

由于需要在不同折算风速下重复进行大量试验或CFD(Computational Fluid Dynamics)模拟,现有风洞试验和CFD方法识别桥梁断面颤振导数耗时且效率低。提出一种基于CFD离散时间气动模型,快速识别感兴趣折算风速带宽内任意折算风速下桥梁断面颤振导数的全带宽识别法。该法基于任意拉格朗-欧拉描述的有限体积法和多层网格技术,首先计算作用在桥梁断面上的非定常气动力,CFD模拟时强迫桥梁断面以单自由度竖弯或扭转方式振动,位移模式为定义在感兴趣的频率范围内的指数脉冲时间序列。然后利用得到的气动力和该指数脉冲输入,通过系统识别建立起反映桥梁断面气动力系统特性的离散时间气动模型。随后利用该气动模型仿真桥梁断面在简谐位移激励下的气动力响应,并由该模型的输入和响应通过系统识别得到桥梁断面的颤振导数。该法在竖弯和扭转方向各仅需一次CFD模拟,就可构造离散时间气动模型,使得颤振导数识别的计算量显著降低。开展了三汊矶大桥加劲梁断面颤振导数识别和颤振临界风速计算,研究结果与风洞试验的一致性,证明了方法的可靠性和高效性。

Nowadays, either wind tunnel tests or CFD （Computational Fluid Dynamics） simulations are time-consuming and inefficient to identify bridge deck flutter derivatives, since a large number of tests or simulations must be performed repeatedly at various reduced wind speeds. In the work reported herein, a method, based on the CFD discrete-time model, is presented, which can effectively extract flutter derivatives of a bridge deck at arbitrary reduced wind speeds among full bandwidth of reduced wind speeds. First, unsteady aerodynamic forces acting on a bridge deck are obtained by using the Arbitrary-Lagrangian-Eulerian （ALE） descriptions in combination with f＇mite volume method and multigrid algorithm. In CFD simulations, forced vertical or torsional displancements in form of exponential pulse time series which defined on interesting range of frequencies are applied to the bridge deck. Then, based on the obtained aerodynamic forces and the displacement inputs, discrete time aerodynamic models can be developed which can represent the unsteady aerodynamic behaviors of the bridge deck. Finally, the aerodynamic models are simulated to obtain unsteady aerodynamic forces to simple-harmonic displacement inputs. With those input and obtained aerodynamic forces, bridge deck flutter derivatives can be identified through a system identification algorithm. The discrete-time aerodynamic models can be developed by the presented method which only employs one run of CFD simulation in heaving or pitching directions respectively, leading to a significant decrease in computing time. Flutter derivatives and flutter onset wind speeds of the Sanchaji Bridge are investigated. Reasonable agreements between results from the present method and those from wind tunnel tests demonstrate the reliability and efficiency of the method.