Granular Weissenberg Effect

拟议项目的目的是对“粒状魏森伯格效应”进行实验和数值表征，这是一种新发现的、尚未完全理解的现象，发生在非球形颗粒系统被剪切时。由非球形颗粒制成的颗粒材料在广泛的应用中很常见，例如制药和食品工业，各种技术领域和地质学。这种颗粒的剪切、浇注或震动会导致局部或全局取向秩序的形成，从而显著影响其宏观静态和动态特性。这会影响填料密度、休止角、流动特性和摩擦力等。在以前的研究中，我们发现了一个现象，现在还不明白：当剪切圆柱形几何容器中的非球形颗粒颗粒材料时，我们观察到旋转中心附近的表面明显升高，类似于非牛顿流体中众所周知的Weissenberg效应。虽然人们普遍认为这种材料中的剪切仅限于局部剪切带，但可以观察到材料的大规模重排，即二次流。详细情况见项目建议书。我们对这种流动的起源的假设是在剪切区或附近由正应力驱动的对流滚动的形成。所报道的效应仅在各向同性粒子明显的颗粒物质中观察到；对于球形或接近球形的颗粒，则不检测到它。尽管有明显的相似之处，但目前还不能确定这种现象是否与其他复杂流体中的Weissenberg效应有关（这一点也没有完全理解）。在这个建议中，我们使用术语“颗粒Weissenberg效应”作为工作假设，仅基于现象学相似性。拟议的研究项目旨在确定颗粒材料中观察到的效应的物理起源。为此，我们将实验研究与大规模数值模拟相结合。通过层析方法（x射线和核磁共振），我们可以对颗粒床进行三维表征。单粒子跟踪也是可能的。侵入性方法（如挖掘）可以作为一种成本效益高、节省时间的替代方法。计划的数值模拟允许有针对性地改变材料参数，并直接获得微观和宏观系综尺寸，以便与实验进行比较。我们计划描述长形和扁形粒子的系综，并将它们与球形粒子的行为进行比较。我们从假设效果很大程度上受宽高比和颗粒形状（如圆柱体、椭球体）的影响开始，但研究不应仅限于这些数量。

The aim of the proposed project is the experimental and numerical characterization of a "granular Weissenberg effect", a newly discovered and not yet fully understood phenomenon occurring when a system of non-spherical particles is sheared.Granular materials made of non-spherical particles are common in a wide range of applications, such as pharmaceutical and food industries, various technical fields and in geology. Shearing, pouring or shaking of such granulates can lead to the formation of local or global orientational orders, which significantly influence the macroscopic static and dynamic properties. This affects, for example, packing densities, repose angles, flow behaviour and friction forces.In previous investigations we found a phenomenon, which by now is not understood: when shearing granular material of non-spherical particles in a container of cylindrical geometry, we observe a significant elevation of the surface near the center of rotation, similar to the well known Weissenberg effect in non-Newtonian fluids. Albeit it is generally acknowledged that shear in such materials is restricted to localized shear bands, a large-scale rearrangement of the material, that is, a secondary flow, is observed. Details are described in the project proposal. Our hypothesis of the origin of this flow is the formation of a convection roll driven by normal stresses in or near the shear zone. The reported effect is only observed in granular matter of pronounced isotropic particles; for spherical particles or nearly spherical particles it is not detected. Despite obvious similarities, at present time it cannot be decided whether this phenomenon is related to the Weissenberg effect in other complex fluids (which is not fully understood as well). In this proposal, we use the term "granular Weissenberg effect" as a working hypothesis, based only on the phenomenological similarity. The proposed research project aims to identify the physical origin of the effect observed in granular materials.To this end, we combine experimental investigations with large scale numerical simulations. By means of tomographic methods (X-ray and NMR) we can perform three-dimensional characterization of the granulate bed. Single particle tracking is also possible. Invasive methods (such as excavation) can supplement as a cost-effective and time-saving alternative. The planned numerical simulations allow a targeted variation of material parameters as well as direct access to microscopic and macroscopic ensemble sizes for comparison with the experiment. We plan to characterize ensembles of prolate and oblate particles and compare them with the behaviour of spherical particles. We start from the hypothesis that effect is largely influenced by the aspect ratio and the shape of the particles (e.g. cylinder, ellipsoid), however, the research shall not be restricted to these quantities only.