Granular Weissenberg Effect

该计划的目的是对“颗粒魏森伯格效应”进行实验和数值表征，这是一种新发现的、尚未完全了解的现象，由非球形颗粒组成的颗粒材料在制药和食品工业、各种技术领域和地质学等广泛应用中很常见。这种颗粒的剪切、浇注或摇动会导致局部或全局取向有序的形成，从而显著影响宏观的静态和动态性质。例如，这会影响堆积密度、休止角、流动行为和摩擦力。在以前的研究中，我们发现了一个至今尚不清楚的现象：当在圆柱形容器中剪切非球形颗粒的颗粒材料时，我们观察到旋转中心附近的表面显著上升，类似于众所周知的非牛顿流体中的魏森伯格效应。尽管人们普遍认为这种材料中的剪切仅限于局部剪切带，但观察到材料的大规模重排，即二次流。具体内容见项目建议书。我们关于这种流动的起源的假设是，在剪切带内或附近形成了由法向应力驱动的对流辊。报道的效应只在明显各向同性颗粒的颗粒物中观察到；对于球形颗粒或近球形颗粒没有检测到。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魏森伯格效应有关(这一点也没有完全理解)。在这个建议中，我们使用术语“颗粒魏森伯格效应”作为一个工作假设，仅基于现象学的相似性。该研究项目旨在确定在颗粒材料中观察到的效应的物理来源。为此，我们将实验研究与大规模的数值模拟相结合。借助于层析成像方法(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)可以作为一种经济有效和节省时间的替代方法来补充。计划中的数值模拟允许有针对性地改变材料参数，以及直接获得微观和宏观总体尺寸，以便与实验进行比较。我们计划对长圆形和扁圆形粒子的集合进行表征，并将它们与球形粒子的行为进行比较。我们从假设粒子(如圆柱体、椭球体)的长径比和形状很大程度上影响效应开始，然而，研究不应仅限于这些量。

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.