The behaviour of elongated non-spherical particles in wall-bounded turbulent shear flows: Analysis, Modelling and Validation

非球形颗粒在流体中的传输存在于许多工业过程中，也与环境流动有关。通常，这些过程涉及限制，例如，在混合容器、管道和分离设备中。一般来说，这样的过程是在湍流区运行的，并且表现出强烈的剪切层。对这些工艺的设计和优化进行数值研究非常有吸引力，因为涉及的成本很低，而且对相关运输过程有详细的了解。然而，到目前为止，大多数粒子都被假设为完美的球形。为了能够使用基于点粒子的欧拉/拉格朗日方法对上述技术过程进行可靠的数值计算，本项目旨在彻底扩展细长非球形粒子的建模框架，特别是关于强剪切(非均匀)流动、湍流和壁面相互作用。重点将放在明显的细长非球形颗粒形状上，如纤维和板状颗粒，在点-颗粒近似下很难准确建模。为此，将采用多尺度方法，使用粒子分辨直接数值模拟(PR-DNS)来推导适当的阻力系数，用于力、力矩以及颗粒-流体相互作用模型。获得的模拟结果将被收集到一个公共数据库中，并通过小规模的三维粒子分辨成像实验进行验证。在这些研究的基础上，将发展拉格朗日点-质点模型，并在现有的数值框架中实现。在湍流模拟方面，将考虑RAN和LES互补的方法。具体地说，需要建立剪切和颗粒旋转引起的阻力、附加质量、Basset力和横向升力以及颗粒旋转引起的俯仰力矩和扭矩的模型。PR-DNS还将根据墙的存在对所有这些力进行分析，并得出适当的相关性。特别是在具有细长非球形颗粒的液体流动中，润滑作用是非常重要的，对近壁取向有重要影响。最后，需要通过详细的实验获得点颗粒Euler/Lagrange模拟的验证数据，使用阴影成像技术来确定湍流壁面边界剪切流动中非球形颗粒的完全分辨行为。为此，将开发一种同时捕捉流体流动和颗粒运动的三维成像技术。这项研究提案的成功结果将提供知识和模型，从而能够在各种相关条件下准确预测具有细长非球形颗粒的壁面边界湍流流动。

The transport of non-spherical particles in a fluid is found in many industrial processes and also relevant for environmental flows. Quite often these processes involve a confinement as, for example, in a mixing vessel, in pipelines and in separation devices. Generally, such processes are operated in the turbulent flow regime and exhibit strong shear layers. Numerical investigations for the design and optimisation of these processes are very attractive due to the involved low cost and the detailed insight into the relevant transport processes. However, so far mostly the particles are assumed perfect spheres.In order to allow a reliable numerical computation of these mentioned technical processes using a point-particle based Euler/Lagrange approach, the proposed project aims at a thorough extension of the modelling framework for elongated non-spherical particles, especially with respect to strong shear (inhomogeneous) flows, turbulence and wall interactions. The focus will be on pronounced elongated non-spherical particle shapes such as fibres and plates which are very challenging to accurately model in a point-particle approximation.For that purpose, a multi-scale approach will be adopted using particle-resolved direct numerical simulations (PR-DNS) for deriving appropriate resistance coefficients for the forces, torques, as well as particle-fluid interaction models. The obtained simulation results will be collected in a publically available database and validated by small-scale three-dimensional particle-resolved imaging experiments. Based on these studies the Lagrangian point-particle models will be developed and implemented in the available numerical frameworks. With respect to turbulence modelling, complementary RANS and LES approaches will be considered. Specifically, model developments are needed for the drag, added mass, Basset force and transverse lift forces due to shear and particle rotation as well as pitching torque and the torque caused by the rotation of the particles. All these forces will be also analysed by the PR-DNS with respect to their modification by the presence of walls and appropriate correlations will be derived. Especially in liquid flows with elongated non-spherical particles lubrication effects are expected to be quite important, having an essential effect on the near-wall orientation.Eventually, validation data for the point-particle Euler/Lagrange simulations need to be obtained through detailed experiments, using shadow imaging techniques to determine the fully-resolved behaviour of non-spherical particles in turbulent wall bounded shear flows. For that purpose, a three-dimensional imaging technique which simultaneously captures fluid flow and particle motion will be developed. The successful outcome of this research proposal will deliver knowledge and models which will enable accurate predictions of wall-bounded turbulent flows with elongated non-spherical particles for a wide range of relevant conditions.