分离式三箱梁空气动力学特性风洞试验

Wind Tunnel Study on the Aerodynamic Characteristics of Separated Triple-box Girders

分离式三箱梁因其优异的颤振稳定性,且对极端风环境的适应能力强,在超大跨度桥梁的设计和建设中逐渐被广泛应用。然而,因为双间隙的存在,分离式三箱梁极易发生涡激振动。桥梁涡激振动的起源与其所受的气动力及其绕流密不可分。为了揭示分离式三箱梁涡激振动的诱因,同时为工程设计和应用实践提供理论指导,以中国某座正在筹建的大跨度桥梁分离式三箱梁为研究对象,进行了精密化的风洞试验研究。通过试验对比了3种断面的表面压力特性,研究三者表面压力分布特征及频率特征。其频率分析结果表明:曲线腹板型式的箱梁具备“双频”现象,易导致涡振区的增大。其后,为揭示双频现象的诱因,进行了流场可视化烟线试验,试验结果表明:近尾流区没有发现剪切层的相互作用以及交替脱落的旋涡,单侧剪切层的分离和再附诱发了分离式三箱梁的气动不稳定性;线性腹板的断面间隙流场更为紊乱,尤其是下游间隙流场。还采用节段模型风洞试验对成桥状态下分离式三箱梁的压力分布特性及绕流场特征进行了初步研究。研究结果表明:与线性腹板的断面相比,曲线型腹板能够有效降低分离式三箱梁在成桥状态下的升阻力系数;剪切层的分离和再附依然是诱发分离式三箱梁气动不稳定性的主要原因。

Triple-box girders have been widely used in the design and construction of super-long-span bridges because of their excellent flutter stability and strong adaptability to extreme wind environments.However,owing to the existence of double gaps,triple-box girders are prone to vortex-induced vibrations(VIVs),whose origin in the bridge is closely related to the aerodynamic force and surrounding flow field.To determine the cause of VIVs in a separated triple-box girder and develop a theoretical guidance for engineering design and application practices,the separated triple-box girder of a long-span bridge under construction in China was investigated using a precise wind tunnel test.Based on the tests,the surface pressures of the three bridge types were compared considering their pressure distribution and frequency characteristics.The frequency analysis results show that a“dual-frequency”phenomenon occurs in the bridges with curved webs,and can be easily increased in the VIV region.Consequently,smoke-wire flow visualization was conducted to reveal the cause of the“dual-frequency”phenomenon,and the results show that the interactions between the shear layers and alternative vortex shedding in the near-wake region cannot be observed.Moreover,the separation-reattachment of a single shear layer induces aerodynamic instability in the triple-box girder and the gap flows of the bridges with linear webs are more turbulent,particularly the downstream gap flow.The pressure distribution and flow field characteristics of the completed bridge with a triple-box girder were then preliminarily studied using a segmental model wind tunnel test.The results show that compared to the bridges with linear webs,curved-web bridges can efficiently reduce the lift and drag coefficients of a completed bridge with a triple-box girder,and the separation reattachment of the shear layers is the main cause of aerodynamic instability in the triple-box girder.