用叶片工具测量流动和非均匀性：磁共振成像测量
Flows and heterogeneities with a vane tool: Magnetic resonance imaging measurements
摘要：
我们研究了各种材料的局部流动特性在一个杯形叶片的几何形状。我们使用磁共振成像技术来测量流动的牛顿流体，屈服应力流体中的速度和颗粒浓度，并在屈服应力流体中的非胶体颗粒的浓缩悬浮液。在牛顿流体中，我们观察到$θ$平均应变速率分量$d_{r,θ}$随着间隙半径的平方反比而减小，这与Couette类比一致。这允许直接比较（没有端部效应校正）叶片和库埃特几何形状中的剪切阻力。在这里，在杯形叶片几何形状的平均剪切应力是略低于在相同尺寸的库埃特细胞，并略高于当叶片嵌入在一个无限的介质。我们还观察到，流动深入叶片之间的区域，导致显著的拉伸流动。在屈服应力流体中，与基于文献模拟结果的通常接受的图片相比，我们发现在低速下在叶片附近被剪切的材料层不是圆柱形的。因此，在分析中应考虑剪切的显著延伸分量。最后，令人惊讶的是，在悬浮液中，我们观察到，一个薄的非圆柱形的滑移层的纯间隙屈服应力流体迅速出现在剪切材料和材料之间的界面，作为一个刚体之间的叶片移动。该特征可归因于叶片边缘周围的非胶体颗粒的非对称轨迹。这一新的重要观察结果与普遍认为的叶片工具防止滑动并可能排除使用叶片工具研究具有大颗粒的糊状材料的流动的观点尖锐矛盾。
英文摘要：
We study the local flow properties of various materials in a vane-in-cup geometry. We use magnetic resonance imaging techniques to measure velocities and particle concentrations in flowing Newtonian fluid, yield stress fluid, and in a concentrated suspension of noncolloidal particles in a yield stress fluid. In the Newtonian fluid, we observe that the $θ$-averaged strain rate component $d_{r,θ}$ decreases as the inverse squared radius in the gap, in agreement with a Couette analogy. This allows direct comparison (without end-effect corrections) of the resistances to shear in vane and Couette geometries. Here, the mean shear stress in the vane-in-cup geometry is slightly lower than in a Couette cell of same dimensions, and a little higher than when the vane is embedded in an infinite medium. We also observe that the flow enters deeply the region between the blades, leading to significant extensional flow. In the yield stress fluid, in contrast with the usually accepted picture based on simulation results from the literature, we find that the layer of material that is sheared near the blades at low velocity is not cylindrical. There is thus a significant extensional component of shear that should be taken into account in the analysis. Finally and surprisingly, in the suspension, we observe that a thin non-cylindrical slip layer made of the pure interstitial yield stress fluid appears quickly at the interface between the sheared material and the material that moves as a rigid body between the blades. This feature can be attributed to the non-symmetric trajectories of the noncolloidal particles around the edges of the blades. This new important observation is in sharp contradiction with the common belief that the vane tool prevents slippage and may preclude the use of the vane tool for studying the flows of pasty materials with large particles.
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