Experimental study for stratified flow around an impulsively started rotating cylinder using combined PIV and LIF

本文对脉冲起动旋转圆柱诱导的密度分层环流进行了实验研究。本研究的目的是对可视化流态进行定性分类，并了解速度场变化与密度场变化之间的关系。为了实现这些目标，采用了几种实验技术，如电解沉淀和染料注入法，激光诱导荧光（LIF），粒子图像测速（PIV）和阴影。雷诺数（Re=r^<2>ω）/v=100-300，其中r是旋转圆柱的半径和ω角速度）和弗劳德数（Fr=ω/N=4-12，N是浮力频率）独立地变化。旋转初期，染色流型的不稳定性变得显著，随后卷起蘑菇状涡环阵列。染色涡旋图案的演化导致随后的大规模层状形成。在分层形成和流型重排过程中，染色流表现出振荡运动的行为。涡量等值线显示了圆柱周围涡环阵列的相应出现。此外，即使在大尺度层状结构维持的情况下，也观察到圆柱附近涡量阵列对的出现。因此，远离圆柱的振荡染色流运动是由于圆柱附近涡量的发生。雷诺数和弗劳德数影响层状地层的径向长度尺度，而波长几乎与雷诺数无关。在一定的时间内，密度场的变化与染色流型和涡度的变化相对应。然而，密度梯度的变化变得可以忽略不计，但染色图案仍然观察到显着的分层运动。这意味着目前的混合现象存在着显著的时间尺度。

An experimental study has conducted for density-stratified circular flow induced by an impulsively started rotating cylinder. The objective of the present study is qualitative classification of visualized flow pattern and understanding of relationship between the variation of velocity field and that of density field. To achieve them, several experimental techniques, such as electrolytic precipitation and dye injection method, laser induced fluorescent (LIF), particle image velocimetry (PIV), and shadowgraph, are employed. The Reynolds number (Re=r^<2>ω)/v=100-300, where r is radius and ω angular velocity of rotating cylinder) and the Froude number (Fr=ω/N=4-12, N is buoyancy frequency) are varied independently. At the beginning of the rotation, instability of dyed flow pattern becomes significant, and then rolls up the mushroom-type vortex ring array. The evolutions of dyed vortex pattern lead to subsequent large-scale layered formation. During the layered formation and the rearrangement of the flow pattern, the dyed flow exhibits the behavior of oscillating motion. The vorticity contours show the corresponding occurrence of vortex ring array around the cylinder. In addition, occurrence of pairs of vorticity array nearby the cylinder is observed even in the situation where the large-scale layered formation is maintained. The oscillating dyed flow motion far from the cylinder, therefore, is due to the occurrence of vorticity nearby the cylinder. The Reynolds number and the Froude number affected the radial length scale of the layered formation, while the wavelength was almost independent of Reynolds number. The density field exhibits corresponding variations with those of dyed flow pattern and the vorticity within certain time period. However, variations of density gradient become negligible with time progress, while the remarkable layered motion of dyed pattern is still observed. It means that the significant time scale exists in the present mixing phenomenon.