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

对脉冲启动旋转圆柱诱导的密度分层圆流进行了实验研究。本研究的目的是对可视化流型进行定性分类，了解速度场和密度场变化的关系。为了实现这一目标，采用了多种实验技术，如电解沉淀和染料注射法、激光诱导荧光法（LIF）、粒子图像测速法（PIV）和阴影法。雷诺数(Re=r^<2>ω)/v=100-300，其中r为旋转圆柱体的半径和ω角速度)和弗劳德数（Fr=ω/N=4-12， N为浮力频率）是独立变化的。在旋转开始时，染色流型的不稳定性变得明显，然后卷起蘑菇型涡环阵列。染色涡型的演化导致了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。涡度曲线显示了相应的涡环阵列在圆柱周围的出现。此外，即使在维持大尺度层状地层的情况下，在柱体附近也观察到涡度阵对的出现。因此，远离圆柱体的振荡染色流运动是由于在圆柱体附近出现了涡量。雷诺数和弗劳德数影响层状结构的径向长度尺度，而波长几乎与雷诺数无关。在一定时间内，密度场与染色流型和涡量呈现相应的变化。然而，随着时间的推移，密度梯度的变化可以忽略不计，而染色图案仍然可以观察到显著的分层运动。这意味着当前的混合现象存在显著的时间尺度。

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.