In this work, two geometries are studied, the vane and the T-bar, which are best suited for assessing the start-up flow of thixotropic yield stress fluids because they minimize the sample disturbance. Based on step-shear measurements with the vane geometry at different angular velocities and on a wide range of products, mostly commercial toothpastes, we calculate the torque on the T-bar using computational fluid dynamics (CFD). The results are compared to the previously suggested approximate theory by Anderson and Meeten (AMT) and extensive original experiments. It turns out that the agreement between CFD, AMT, and the experimental data depends primarily on the shape of the flow curve which may be quantified by the fluid flow index, N, defined in the shear rate range which represents the flow around the rotating rod of the T-bar. While the CFD and AMT predictions agree well with each other (R² = 0.98), they both underestimate the experimental data although the experimental-to-predicted ratio also correlates to N (R² = 0.84) going up from 1 to around 2 as N increases from 0.1 to 0.5. This suggests that when using the T-bar for viscosity measurements, the user needs to take into account the flow index to which end a simple estimate of the effective shear rate is suggested also being a function of N.

在这项工作中，研究了两种几何形状，即叶片和 T 形杆，它们最适合评估触变屈服应力流体的启动流动，因为它们最大限度地减少了样品干扰。基于不同角速度下叶片几何形状的步进剪切测量和范围广泛的产品，主要是商业牙膏，我们使用计算流体动力学 (CFD) 计算 T 形杆上的扭矩。结果与之前由 Anderson 和 Meeten (AMT) 提出的近似理论和大量原始实验进行了比较。结果表明，CFD、AMT 和实验数据之间的一致性主要取决于可以通过流体流动指数N量化的流动曲线的形状，定义在剪切速率范围内，该范围代表围绕 T 型杆旋转杆的流动。虽然 CFD 和 AMT 预测彼此非常吻合 (R ²  = 0.98)，但它们都低估了实验数据，尽管实验与预测的比率也与N (R ²  = 0.84) 从 1 上升到 2 左右相关，如N从 0.1 增加到 0.5。这表明，当使用 T 形棒进行粘度测量时，用户需要考虑流动指数，因此建议对有效剪切速率的简单估计也是N的函数。