均匀流场作用下ETFE气枕的流固耦合分析

Study on the Fluid-Structure Interaction of ETFE Cushions Under Uniform Flow Field

ETFE(Ethylene-Tetra-Fluoro-Ethylene)气枕由于自重轻、造型丰富等优点,广泛应用于土木工程领域的大跨空间钢结构如水立方等以及航空航天领域的空间可展结构。ETFE气枕属于风敏感结构,在风荷载作用下,易产生较大的变形和振动,进而使得周围风场发生变化,引起显著的流固耦合效应。这种耦合效应可以通过结构形状改变、附加质量、气动阻尼和气承刚度的变化来表征。目前相关研究主要对张拉膜结构在均匀流情况下的流固耦合作用进行了研究,尚未发现有关ETFE气枕等充气膜结构方面的研究报道。以方形ETFE气枕为研究对象,借助商用有限元软件ADINA,通过将CFD(Computational Fluid Dynamics)/CSD(Computational Structure Dynamics)分析结果与FSI(Fluid Structure Interaction)分析结果进行对比,探究其在均匀流场情况下的流固耦合作用的特点以及对动力响应的影响规律。数值计算中忽略能量的耗散,因此没有考虑气动阻尼的影响,通过有势流体考虑气承刚度的影响,着重研究了流固耦合作用中气枕形状改变和附加质量的影响作用。主要研究内容与结论如下:假定外部流场为均匀层流场,采用8结点层流单元进行离散;ETFE气枕外围薄膜为线弹性材料且其变形满足大转动、小应变条件,采用3D-4结点膜单元进行离散;ETFE气枕的内充气体为小扰动、无旋的理想流体,采用8结点线性势流体单元进行离散。在CFD模块,通过瞬态分析求得ETFE气枕刚性模型表面的风压分布和风荷载时程,然后将风压时程施加在CSD模块中ETFE气枕的膜面上,借助隐式时间积分进行时程分析,求得在不考虑流固耦合作用时气枕的风致响应。通过分别在CFD和CSD模块中设置流固耦合界面,并在CFD模块中引入ALE(Arbitary Lagrangian-Euler)网格离散计算域以考虑流固耦合边界的运动问题,对均匀流场中的ETFE气枕进行双向耦合分析,计算其在考虑流固耦合作用情况下的表面风压和风致响应。通过将CFD/CSD分析结果与FSI分析结果进行对比,研究流固耦合作用对ETFE气枕动力响应的影响规律。结果表明:两种情形下得到的风压分布和数值大致相同,说明在外部均匀流场情况下,气枕形状改变对流场的影响很小;而两种情形下的内压、位移、速度、加速度和应力响应时程结果差异较大,说明在外部均匀流场情况下,附加质量对ETFE气枕动力响应具有显著影响。可见,均匀流场与ETFE气枕的流固耦合作用,在不考虑气动阻尼的情况下,主要体现为附加质量的影响。

ETFE(Ethylene-Tetra-Fluoro-Ethylene)cushions are popularly used in large-span steel structures such as National Swimming Center,etc.Due to flexibility and wind-sensitivity,ETFE cushions under wind often suffer large deformation and vibration,which,however,further changes the surrounding wind field and therefore its action on the enveloping membrane,causing a significant fluid-structure interaction.The fluid-structure interaction can be characterized by the effects of change in the structural shape,added mass,aerodynamic damping and pneumatic stiffness.So far,the literature only focused on the fluid-structure interaction of tensioned membrane structures under uniform flow and the research on inflated membranes such as ETFE cushions still remains untouched.In view of this,this paper aims to numerically explore the fluid-structure interaction of ETFE cushions under uniform flow and its effect on the dynamic responses by comparing the results from the CFD/CSD(computational fluid dynamics/computational structure dynamics)analysis and the FSI(fluid-structure interaction)analysis.Since aerodynamic damping is not included in the analysis and pneumatic stiffness can be considered with the potential flow element in the CSD module of ADINA,attention is only paid in the paper to the influences of the change in the structural shape and added mass.The results indicate that the effect of FSI has a slight effect on the wind pressure but plays a significant role in the dynamic responses of ETFE cushions.