非零攻角下超低马赫数湍流边界层诱导空腔自激振荡机理研究

The self-sustained oscillation of cavity flow at very low Mach number is distributed widely in the ground transportation, and the fluctuated pressure induced by the self-sustained oscillation will affect the ride quality seriously. At present, the mechanism of self-sustained oscillation of cavity flows has been understood well at supersonic and subsonic Mach number relevant to configuration encountered in aeronautic transport or high speed trains. However, little data and theory are available for ground transportation where Mach numbers are typically around 0.1. Furthermore, to date, most of the studies have focused on the situation that the direction of the airflow is perpendicular to the cross-section of cavity (zero attack angles) and little attention has been given to the situation that there is certain angle between the direction of the airflow and the normal direction of the cavity's cross-section (non-zero attack angles). Moreover,the coherent structure in the turbulent boundary layer will affect the formation and convection of the vortical structure in the shear layer, and then affect the characteristic of the self-oscillation. Therefore, in current research proposal, the wind tunnel experiment combined with numerical simulation will be employed to investigate the flow field struture and flow characteristics around the cavity opening, and pressure fluctuation inside the cavity, to analyse the impact induced by the attack angle to the characteristics of turbulent boundary layer, to reveal the sufficient conditions for the self-sustained oscillation induced by turbulent boundary layer at low Mach number at non-zero attack angle. In order to improve the caculation accuracy of unsteady flow at very low Mach number, the governing equations of LES based on the weakly compressibility flow model will be constructed. The project aims to reveal the mechanism of self-sustained oscillations induced by turbulent cavity flow at very low Mach number at non-zero attack angles and make the theoretical system of cavity flow more perfect, and then lay a theoretical basis for the ground transportation design and control of self-sustained oscillation at very low Mach number at non-zero attack angle.

超低马赫数空腔流自激振荡广泛存在于地面交通工具中，其产生的非定常载荷会严重影响驾乘的舒适性。目前，对来流方向垂直于空腔横截面(零攻角)的超、次音速等高马赫数空腔流自激振荡的机理已能较好解释。而对地面交通工具涉及的超低马赫数且气流方向与横截面法线方向存在夹角（非零攻角）的情形则较少涉及。再者，湍流边界层中的拟序结构会影响剪切层中涡的形成与对流并进而影响自激振荡的特性。因此，本项目拟采用实验测量并结合数值计算分析空腔开口处的流场结构、流动特性以及腔内压力场变化；分析攻角对空腔前缘湍流边界层特性的影响；揭示非零攻角下湍流边界层诱导空腔自激振荡的充分条件；构建弱可压缩流模型的大涡模拟方程以提高超低马赫数空腔流自激振荡计算的精度。本项目旨在揭示非零攻角下超低马赫数湍流边界层诱导空腔自激振荡的机理，更加完善空腔流研究的理论体系，为地面交通工具设计和抑制非零攻角下超低马赫数空腔流自激振荡奠定理论基础。

超低马赫数空腔流自激振荡广泛存在于地面交通工具中,其产生的非定常载荷会严重影响驾乘的舒适性。目前, 对来流方向垂直于空腔横截面(零攻角)的超、次音速等高 马赫数空腔流自激振荡的机理已能较好解释。而对地面交通工具涉及的超低马赫数且气流方 向与横截面法线方向存在夹角(非零攻角)的情形则较少涉及。再者,湍流边界层中的拟序结构会影响剪切层中涡的形成与对流并进而影响自激振荡的特性。因此, 本项目采用实验 测量并结合数值计算分析空腔开口处的流场结构、流动特性以及腔内压力场变化;分析攻角对空腔前缘湍流边界层特性的影响;揭示非零攻角下湍流边界层诱导空腔自激振荡的充分条 件;构建弱可压缩流模型的大涡模拟方程以提高超低马赫数空腔流自激振荡计算的精度。本项目率先开展了非零攻角下空腔流自激振荡研究，揭示了自激振荡的机理及攻角、空腔几何参数对于自激振荡特性的影响规律、 非零攻角下空腔流诱发自激振荡的充分条件，项目的研究成果有助于进一步完善空腔流自激振荡研究理论体系以及提出抑制自激振荡的措施。首次提出了采用非光滑表面用于抑制空腔流自激振荡，研究结果表明凹坑非光滑表面能有效增加近壁面湍流边界层的厚度，改善近壁面平均流场的稳定性并能有效抑制空腔流自激振荡的强度。研究成果可用于抑制汽车天窗、侧窗风振噪声，具有较好的应用前景。利用非光滑表面能将近壁面层流边界层转化为湍流边界层，改善近壁面平均流场的稳定性的特性，将非光滑表面用于钝体模型减阻及汽车尾气温差发电系统换热器强化传热。研究结果表面，凹坑非光滑表面能有效减小钝体模型的压差阻力及强化换热器的传热能力。