Experimental investigation of stratorotational instability using a thermally stratified system: instability, waves and associated momentum flux
Experimental investigation of stratorotational instability using a thermally stratified system: instability, waves and associated momentum flux
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DOI：
10.1080/03091929.2018.1488971
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发表时间：
2018-07
影响因子：
1.3
通讯作者：
T. Seelig;U. Harlander;M. Gellert
中科院分区：
文献类型：
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作者：
T. Seelig;U. Harlander;M. Gellert
层旋不稳定性（SRI）被认为是开普勒吸积盘中向外角动量输运的一种机制。特别设计的具有轴向分层的Taylor-Couette实验室实验适合于研究不稳定性。底部端板被冷却，顶部端板被加热以实现轴向分层。由于结构的限制，终板在视觉上是不可靠的，并且在同向旋转框架中的定量测量技术只能通过观察外圆柱体来完成。为此，我们建立了一个同向旋转的小型PIV（粒子图像测速）系统与相机具有倾斜的视角，关于水平激光片。本研究的目的是（i）量化的不确定性的迷你PIV连同所使用的校准技术和（ii）比较SRI的实验结果与理论预测。我们进行测量的方位角和径向分量的速度在轴向稳定分层泰勒-库埃特流，考虑速度分布和频率滤波和流动分解。的mini-PIV系统的绝对误差为2%，我们意识到，分层泰勒-库埃特流有较小的Ekman端壁效应比均匀的。尽管如此，Ekman泵送对流动有影响，并且可能是忽略端壁的数据和理论模型之间的差异的原因。在这里，我们专注于在过渡到SRI，SRI模式的漂移率和径向动量通量作为雷诺数的函数的流动结构。而被困的边界开尔文模式和漂移率的形式的结构对应以及与早期的预测，动量通量显示出非线性的依赖性相对于雷诺数。远离过渡区，理论模型显示线性关系。实验和理论模型之间的失配的几个可能的原因进行了讨论。最重要的是，我们的实验表明，在瑞利稳定的流动制度的SRI可以提供大量的向外的动量通量，这使得这种不稳定性有趣的吸积盘的背景下，也旋转和分层的大气涡旋也发挥了重要作用。
ABSTRACT Stratorotational instability (SRI) has been proposed as a mechanism for outward angular momentum transport in Keplerian accretion disks. A particular designed Taylor–Couette laboratory experiment with axial stratification is suitable for studying the instability. Bottom endplate is cooled and top endplate is heated to achieve axial stratification. Due to constructive constraints, endplates are visually unamenable and quantitative measurement techniques in the co-rotating frame can only be done by looking through the outer cylinder. For this purpose, we built a co-rotating mini-PIV (Particle Image Velocimetry) system with a camera having a tilted viewing angle regarding the horizontal laser sheet. The aim of this study is (i) to quantify the uncertainty of the mini-PIV together with the used calibration technique and (ii) to compare experimental findings on SRI with theoretical predictions. We perform measurements of the azimuthal and radial component of the velocity in axial stably stratified Taylor–Couette flows, consider velocity profiles and do frequency-filtering and flow decomposition. The absolute error of the mini-PIV system is 2% and we realised that stratified Taylor–Couette flows have smaller Ekman endwall effects than homogeneous ones. Still, Ekman pumping has an impact of the flow and might be responsible for differences between the data and theoretical models ignoring the endwalls. Here we focus on the flow structure during transition to SRI, the drift rate of SRI modes and the radial momentum flux as a function of the Reynolds number. Whereas the structure in form of trapped boundary Kelvin modes and the drift rate corresponds well with earlier predictions, the momentum flux shows a nonlinear dependency with respect to the Reynolds number. Away from the region of transition, theoretical models show a linear relationship. Several possible reasons for the mismatch between the experimental and theoretical models are discussed. Most important, we experimentally demonstrated that in the Rayleigh stable flow regime the SRI can provide a significant amount of outward momentum flux which makes this instability interesting in the context of accretion disks and also of atmospheric vortices where rotation and stratification also play a significant role.