水-气界面下低速流体条带的喷射行为

STREAK-EJECTIONS UNDER SHEARED AIR-WATERINTERFACE

对剪切水－气界面下湍流低速条带的喷射现象进行了细致的观测．喷射过程中,条带的一部分迅速离开水面近区下侵,与深层高速流动发生激烈的相互作用．这意味着条带喷射对水面剪切层中的湍流输运有重要影响．根据流动显示的图像资料获得的定量结果揭示出喷射行为的基本特征,包括喷射频率、喷射流体的轨迹和速度分布等．文中将这些特征与固壁湍流边界层中的对应情形作了比较．实验观察和喷射、猝发事件的时间分布特点表明,低速条带喷射是猝发过程的组成部分,喷射的时间间隔一般显著小于猝发,两种现象的出现都具有很强的随机性．将喷射与湍动能产生率的空间分布作比较,它们的形状极为相似,反映出喷射过程可能是水面附近湍流的主要贡献者．

Described in this paper are careful visual and quantitative studies on ejection phenom-ena of low-speed streaks under sheared air-water interface. During an ejection process, a portion oflow-speed streaks rapidly migrates away from the water surface, and subsequeatly breaks down ina violent interaction with faster flow, demonstrating that such processes play an important role inthe turbulent transport between the interface and deeper layers. Quantitative data derived fromflow-visualization pictures reveals fundamental features in ejection events, including frequency,trajectories and velocities of ejected fluid. Comparisons of these findings with the correspondingfeatures of near-wall streak-ejections are also made. Temporal distributions of ejection and burstingevents, together with the visual observations, indicateo that a burst may involve either a single ejec-tion or multiple ejections closely grouped together, and the time interval between ejection events isgenerally much less than that between bursts. Furthermore, the occurrence of both ejections andbursts exhibits intense randomness. The comparison of a turbulence production profile with thecorresponding frequency distribution of ejections is presented. The remarkable agreement in theshape of two curves strongly suggests that ejection processes is the chief contributor of turbulentkinetic energy.