【作者】 刘建华；

【导师】 姜楠；

【作者基本信息】 天津大学 ， 流体力学， 2009， 博士

【摘要】 本文工作的主要目的是,通过向湍流边界层中引入单一人造发卡涡,对在湍流边界层中对数律区的大尺度低速条纹结构（>20δ）的产生机理进行探讨。在层流边界中,流动显示和单丝热线探针测量的结果表明:4.8Hz、最大速度约1.1ms^-1（Rjet≈55%）的垂直于壁面的周期性射流,在自由来流约为2.0ms^-1、边界层厚度约为12mm的层流边界层中,形成单一发卡涡。在射流出口下游10mm处及其下游,相位平均的流向脉动速度和发卡涡标示函数在三维空间（运用Taylor冻结假设）中的分布表明,低速区形成于人造发卡涡的上游、内部区域,高速区形成于人造发卡涡的外侧。通过整合在流向六个横截面处的相位平均的流向脉动速度信号和发卡涡标示函数,得到在一个扰动周期时间内（209ms）,人造发卡涡在层流边界层中随时间的发展、演化过程,且与流动显示结果相吻合。同时,在水平截面内,流向瞬时脉动速度呈现类似Hutchins等2007年在湍流边界层对数律区发现的大尺度条纹结构（>20δ）。向来流约为2.0ms^-1、边界层厚度约为50mm的湍流边界层中,引入4.8Hz、最大速度约3.45ms^-1（Rjet≈172%）的垂直于壁面的周期性射流,同时,单丝热线探针在射流出口下游40mm处的（流向-展向）横截面内,对射流扰动的湍流边界层进行测量。相位平均的流向脉动速度与基于流向速度分量的发卡涡标示函数,呈现发卡形涡结构,而且在对数律区发现与Hutchins等2007年在湍流边界层对数律区发现的大尺度条纹结构相似的大尺度低速条纹结构。与较弱垂直于壁面的周期性射流（Rjet≈55%）在湍流边界层对数律区形成的低速条纹相比较,得知尾流区占主导地位的负展向涡是形成对数律区大尺度条纹结构的根本原因。由流向平均速度的二阶导数与（层流和湍流）边界层中条纹结构的对应关系,得知足够强的边界层剪切,对在边界层中产生条纹结构,是必要的。通过对比两个不同雷诺数的湍流边界平均速度的二阶导数,解释了对数律区的大尺度低速条纹结构在高雷诺数的湍流边界层中较强,而易于发现的原因。更多还原

【Abstract】 Mechanism of the large scale low-speed streaks （>20δ） found in logarithmic layer is experimentally investigated, by inducing a single synthetic hairpin vortex into turbulent boundary layers.It is proved by both flow visualizations and single hot-wire measurements that a single synthetic hairpin is generated by 4.8Hz pulsing jet normal to the wall with a maximum velocity of 1.1ms^-1 （Rjet≈55%） in laminar boundary layers with a thickness of around 12mm at a freestream velocity of about 2ms^-1. The distribution of phase-averaged u fluctuations and marker function in three dimensional area （by using Taylor’s hypothesis） at each x-location 10mm downstream the hole, demonstreates that low-speed region is formed inside and upstream of the synthetic hairpin, which is surrounded by high-speed region. The combination of the phase-averaged signals shows the dynamics process of synthetic hairpin within a cycle （209ms）, which is in consistent with the flow visualization in laminar boundary layers. Moreover, streaks similar to those found in logarithmic layer in turbulence flows by Hutchins et al. （2007） are observed in horizontal slices of the instantaneous u fluctuations.By introducing a normal jet at 4.8Hz with a maximum velocity of 3.45ms^-1 （Rjet≈172%）, phase-averaged u fluctuations and marker function indicate a single synthetic hairpin vortex is formed 40mm downstream the exit of the jet, in a turbulent boundary layer with 50mm thickness at around 2ms^-1. Moreover, low-speed streaks resemble those found in logarithmic layer of turbulence flows by Hutchins et al. （2007）. In comparion with the u fluctuations associated with a single hairpin vortex generated by a relatively weak normal jet （Rjet≈55%）, dominant passive spanwise vorticity in wake region is responsible for formation of the large scale streaks in logarithmic layer.Due to relationship between a second derivative of mean streamwise velocity and existence of streaks, a strong enough boundary-layer shear is necessary for formation of steaks. By comparion of second derivatives of mean velocity at two Reynolds number, it is figured out the reason why large scale low-speed streaks in logarithmic layer are stronger and easier to be observed.更多还原