地面效应对桥梁气动性能的影响及其作用机理研究

"Ground effect" comes from the aeronautical field, and refers to the physical phenomenon of the aircraft lift force suddenly increase and the drag reduction, when the aircraft flights near the ground or the water. Similarly, when the bridge is closer to the ground or the water, the effect is also present under the action of the wind. With the construction of a large number of the urban bridges and the cross-sea bridges, there are more bridges close the ground or the sea. Affected by the rough surface of the ground or the impact of waves, the aerodynamic performance of the bridge may be more obvious changes. Therefore, it is necessary to study the extent of its impact and mechanism of action. Firstly, based on large eddy simulation, dynamic mesh generation and VOF two-phase flow simulation technique, this research builds a numerical platform for establishing ground effect of bridge. And then, with the numerical simulation technique and the wind tunnel test, the research obtains the wind boundary layer of different roughness and the wave shape under the combined action of wind and waves. And also invents the device which can control the movement speed of the wave boundary. Based on the streamlined box girder and blunt body side girders, the static pneumatic parameters, vortex vibration and flutter performance of bridge are studied by numerical simulation and wind tunnel test. These studies consider the wind field type, the bridge height from the ground, the wave form, the wind angle and other state parameters. At last, the ground effect of the bridges’ aerodynamic characteristics, intrinsic influencing factors and its action mechanism are obtained. The research results can provide suggestions and guarantee for the wind resistance design of the bridges which close to the ground or the sea.

“地面效应”一词源于航空领域，指飞行器靠近地面或水面飞行时会出现升力陡然增加阻力减小的物理现象；类似当桥梁距地面或水面较近时，在来流风作用下该效应也同样存在。随着我国城市及跨海桥梁的大量建设，出现了较多靠近地面或海面的桥梁，受近地表粗糙元或波浪的影响，桥梁的气动性能可能会变化明显，因此有必要研究其影响程度及作用机理。本课题首先基于大涡模拟、动网格生成及VOF两相流模拟技术建立桥梁地面效应数值平台；然后利用数值模拟及风洞试验，获取不同粗糙类别的近地边界层风场及风浪联合作用下的波浪形态，并研发装置实现波浪边界速度可控的移动。研究流线型箱梁及钝体型边主梁在不同 的离地高度、地表类别、波浪形态、风场类别及风攻角等状态下桥梁断面的静气动参数、涡振及颤振性能，获取地面效应对桥梁气动性能的影响范围、内在影响因素及其气动作用机理，为近地或近海桥梁的抗风设计提供建议与保障。

当桥梁距地面或水面较近时，在来流风作用下主梁的气动性能可能发生较为明显的变化，即“地面效应”。随着我国城市及跨海桥梁的大量建设，受近地表粗糙物或波浪等影响，该效应广泛存在，因此有必要研究其影响程度及作用机理。. 主要研究内容：（a）建立了桥梁地面效应数值模拟平台关键技术，基于大涡模拟技术考虑气流湍流效应，结合动网格生成技术来保证流场网格与动边界的同步匹配，并利用两相流技术实现液面边界形态变化的模拟。（b）分别选取流线型箱梁断面和钝体型边主梁断面，完成了不同的离地高度、风场类别以及来流风攻角等状态参数下桥梁断面的测压和测力研究，分析了地面效应的作用距离范围、静气动参数影响程度、速度压力场变化规律等。（c）通过风洞试验及数值模拟，研究地面效应的作用距离范围、涡激振动的振幅及风速区间、速度压力场变化分布等特点。（d）研究地面效应对颤振的影响，重点分析发生颤振前后，桥梁断面受地面效应影响下系统振动能量变化规律。. 主要研究结论：（a）静力三分力系数在大攻角和小攻角下的变化趋势存在差异，三分力系数与离地高度的变化趋势有明显不同，地面效应使结构风荷载增大，产生不利影响。（b）随着桥梁断面离地高度的降低，断面St不断升高，地面效应将导致断面的涡激振动区间提前。（c）正攻角情况下升力和升力矩系数随着离地高度的降低而增大，负攻角下结果则相反。（d）钝体型主梁涡振的起振风速在离地高度降低时会略有提前，涡振的锁定区间也同时略微发生了前移，导致了涡振发生的可能性增加。涡振振幅对离地高度的变化较为敏感，会随着离地高度的降低而增大。（e）地面效应使得涡激气动力的卓越频率会向主梁自振频率逐渐靠近，同时又会促进涡激气动力的能量在结构频率附近聚集。（f）地面粗糙度的存在对断面静风稳定性和颤振稳定性均会产生不利的影响。地面粗糙度的存在会加快断面的旋涡脱落，导致涡激振动锁定区间提前，涡激振动现象提前发生。