有限长钝体尾流与气动力的双稳态特性与主动控制

The finite-length bluff body with one end mounted on a flat wall and the other free is frequently encountered in engineering applications. Its wake and aerodynamic forces differ significantly from those of a two-dimensional bluff body which attracts extensive investigations. Generally, bistable phenomenon occurs in the flow around a finite-length bluff body. The wake and aerodynamic forces present complete different behaviors for the two typical flow modes. This observation has significant effects on the aerodynamic forces and vibration response of the structure as well as the flow dynamics and momentum/heat transport in the wake. The present project plans to systematically investigate the effects of geometric properties (cross section, shape of free end, aspect ratio), Reynolds number, turbulence intensity and boundary layer conditions upon the near wake and aerodynamic forces. Based on the occurrence probability of bistable phenomenon, the present project aims to provide a description model from for the near wake and aerodynamic forces of finite-length bluff body. Active control using steady (pulsing) slot blowing or suction and synthetic jet near will be applied near cylinder free end and along cylinder span to manipulate the near wake and aerodynamic forces of finite-length cylinder. Control parameters will be optimized as well. The dynamics of near wake formation and evolution will be investigated for different control methods to reveal their mechanism. The present project also aims to develop a theory for controlling the near wake and aerodynamic forces based on the above mentioned investigations.

一端固定于壁面、另一端为自由端的有限长三维钝体在工程中广泛存在，其尾流与气动力特性相对于已得到广泛研究的二维钝体存在显著区别。大多数有限长钝体绕流存在双稳态现象，其尾流与气动力会在两种典型状态下表现出截然不同的特性。该现象对结构所受气动力、结构振动响应、尾流结构与传热传质特性等有重大的影响。本项目拟主要通过风洞实验并辅以数值模拟的方法，系统研究有限长钝体的几何特性（截面形式、端部形状、高宽比等）、雷诺数、湍流度、壁面边界条件等主要因素对其尾流与气动力的影响规律，从双稳态现象发生概率与非平稳特性的角度建立有限长钝体尾流与气动力的科学描述模型。分别在钝体自由端和展向施加定常（脉冲）狭缝吹/吸气与合成射流等控制方法对有限长钝体尾流与气动力进行主动控制，并优化控制参数。探明不同外部扰动下有限长钝体尾流的发展演化规律，揭示控制机理，建立针对有限长钝体尾流与气动力的控制理论。

一端固定于壁面，另一端为自由端的有限长三维柱体在工程中大量存在，如：高层建筑、潜艇附体等。由于其几何特征和来流边界层的影响，有限长三维柱体的绕流与气动力呈现强烈的三维效应，非常复杂。本项目通过风洞试验和大涡模拟等方法详细研究了有限长柱体绕流和气动力特性、影响因素和主动控制方法及其机理，获得主要成果如下：（1）发现了有限长圆柱气动力存在特殊雷诺数效应。尽管柱体处于均匀来流中，临界雷诺数沿高度仍存在显著差异。越接近自由端，临界雷诺数越小，即当自由端附近绕流处于临界或超临界状态时，底部仍可能处于亚临界状态。（2）阐明了对称和反对称涡脱落状态对有限长方形柱体气动力的影响规律。当展现涡同步脱落时，升力振幅很小，无明显周期性，展向相关性较弱；而当展向涡交错脱落时，升力呈现大幅周期性波动，展向相关性较强。（3）揭示了来流条件对有限长方柱气动力影响规律和机理。当柱体处于湍流度较高的湍流边界层中，气动力（尤其是脉动升力）远大于其处于均匀来流中的对应值。湍流边界层中柱体展向涡以反对称为主，而均匀流中则以对称为主，这是因为边界层内较高湍流度的扰动将不稳定的对称涡转化为稳定的反对称涡。（4）发现了有限长方柱绕流中除高卡门涡外，自由端附近存在一低频运动，其对应着自由端剪切流的上下拍动，频率约为卡门涡的十分之一。当自由端剪切流摆动至较高位置时，展向涡为对称脱落，而当其摆动至较低位置时，展向涡为反对称脱落，这一发现揭示了有限长柱体绕流中的两种典型状态相互转化的机理。（5）提出了自由端前边缘附近的定常狭缝吸气控制方法，并详细研究了吸气量对其气动力与涡激振动的影响规律，发现吸气速度等于来流速度时，控制效果最佳，脉动升力减小约45%，涡激振动基本完全抑制。并将顶部控制方法成功用于近地面类车体气动阻力的控制，实现12%的减阻。