微吹气对湍流平板边界层流动特性的影响及其减阻机理

微吹气技术能够改变平板湍流流场结构，减小平板表面的摩擦阻力。采用直接数值模拟方法，计算了来流马赫数0.7条件下，流场流过光滑平板和NASA-PN2多孔平板表面两种情况，通过对比这两个算例的相关流场特征，验证了微吹气控制减阻的有效性，局部最大减阻率达到了45%，并且由于微吹气控制的"记忆"功能，减阻效果在微吹气流域下游仍会持续一段距离，增加了减阻区域的流向面积。壁湍流摩擦减阻的原因在于近壁区域出现了一个低速的"湍流斑"，黏性底层厚度增加，速度型曲线被抬升。但与此同时，边界层内湍流速度脉动也得到了增强。进一步对流向脉动涡演化规律分析，发现微吹气对流向脉动涡发挥着多重作用。在增加流向脉动涡强度的同时，还使得流向涡团向远离壁面抬升，这样减小了流向涡与壁面之间直接作用。此外，微吹射流产生的冲击作用会在流向涡表面留下凹痕，使得流向涡分散成相对小的涡团结构。

Micro-blowing:Effect on flow characteristics in turbulent flat plate boundary layer and drag reduction mechanism

Micro-blowing technology can change the turbulent structure in a flat plate flow and reduce the wall friction drag. In this paper, two cases of inflow through a smooth plate and a NASA-PN2 porous plate at Mach number 0.7 are respectively resolved by direct numerical simulation. Comparison of the flow characteristics in the two cases proves the effectiveness of micro-blowing technology on drag reduction, with the maximum rate reaching 45%. Furthermore, because of the "memory" function controlled by micro-blowing, the effect will last for a certain distance in the downstream, thus expanding the area of drag reduction. The explanation for the drag reduction in a wall turbulent boundary layer is the production of a low-speed "turbulence spot" in the near-wall region, which increases the thickness of viscous sub-layer and uplifts the average velocity profile. However, the turbulent velocity fluctuations in the boundary layer are strengthened simultaneously. Further analysis of the evolution of stream-wise vortex fluctuations reveals that micro-blowing plays multiple roles. It not only enhances the intensity of stream-wise vortex fluctuations, but also uplifts the stream-wise vortex clusters away from the wall, hence directly reducing the interaction between the stream-wise vortex and the wall surface. In addition, the impact caused by micro-blowing will leave dents on the vortex surface, leading to more dispersed and finely broken vortex clusters.