Flow Simulation of the Self-compacting Concrete by the Unresolved CFD-DEM Approach: Bingham Fluid and Multiphase Flow
Authors:   , You Sizeng 
Published: April 2022

The self-compacting concrete (SCC) is a material that has great flowability in fresh but sustains a high bearing capacity in the hardened state. It is widely used in projects where manual compaction is inconvenient such as high-rise buildings, deep foundations, and highly reinforced structures. However, contrary to its rapid growth in popularity, the study on the flow behaviour of fresh SCC is limited. Consequently, controlling the common defects in practices such as segregation, bleeding, and exclusion of rebars remains a great challenge. To provide a numerical technique for this problem, this thesis aims to establish a framework for the SCC flow simulation.Preliminarily, the fresh concrete is regarded as a homogeneous Bingham fluid and the software Fluent is used to investigate the flow behaviour of fresh SCC. Firstly, a parametric study is designed to correlate the performance of robust workability tests and the two-parameter Bingham model. It provides a convenient indirect method to obtain concrete rheology for numerical modeling. At the same time, it fills up the gap caused by the lack of reliable and consistent direct solutions. To demonstrate its the performance, a case of diaphragm wall is chosen as an example to show its application in real practice. According to the on-site test record, commercial concrete transported by different trucks varies greatly in the rheology. The simulation of construction process is configured referring to the on-site placement arrangement. Meanwhile, to show a comparison, the same construction process is repeated but uses only one type of concrete. This case study not only shows the distribution of concretes in the diaphragm wall but also indicates the influence of material homogeneity on concrete distribution.In practice, the size of coarse aggregate in concrete is usually close to the rebar. To study the interaction between fresh SCC and the reinforcement, the existence of aggregate cannot be ignored. The unresolved CFD-DEM approach is a powerful two-way coupled technique to study the particulate flow, which solves the fluid flow by the CFD, tracks the particle trajectory by the DEM, and couples the two by exchanging momentum. In the further study, the goal is to develop our in-house code based on this approach to realize the SCC flow simulation at the scale of aggregate size. On the initial step, an in-house code is developed for the basic CFD-DEM scenario: the coupling of a single Newtonian fluid and a discrete phase. Every part of our in-house including CFD solver, DEM solver and their coupling is validated by comparing with sophisticated software like OpenFOAM, LIGGGHTS and CFDEM. In the next step, the difficulties to accomplish the simulation of SCC flow are to be discussed: a general VOF-DEM method and acceptable computation cost.The complexity of SCC flow simulation lies in 3 aspects: 3 phases are involved including air, mortar, and aggregate; the geometry of flow domain could be very complicated; the comparable size of aggregate, rebar, and CFD cell. To overcome the first difficulty, the VOF method is implemented to solve the multiphase flow in the CFD domain which gives the so-called VOF-DEM method. While the latter two facts bring the challenge to the mapping between CFD and DEM domains. What’s more, the discontinuity nature of the interface between two fluids worsens the situation. Improper mapping methods will generate unsmooth CFD fields in the coupling stage and lead to numerical instability. To address this issue, the mapping of involved variables is grouped and different schemes are proposed for them. For example, the volume fraction is a variable in the space domain which is uniform in all directions. An improved statistic kernel method is developed to obtain a smooth volume fraction field because it could handle a wide range of cell/particle size ratio. Besides, the mapping of fluid velocity, drag force, torque, and so on are discussed. The performance of the VOF-DEM method with the general coupling scheme is demonstrated by 3 cases: a group of small particles fall into water from the air, a big particle falls into the water from the air and a big particle rotates on the interface.Due to the exchange of drag force between CFD and DEM, one extra time step criterion is introduced to each of the CFD and DEM solvers. In the case of concrete material, it becomes the bottleneck of computation efficiency. For Bingham fluid, when the shear rate is small, the fluid will show an extremely high apparent viscosity which results in a very small critical time step. In the case of fresh mortar, it can be as small as 1×10−7 s for both CFD and DEM, which makes the computation very expensive. To solve this problem, a relaxing scheme is proposed to release the drag-induced step criteria. It gives explicit time-step-independent solutions for drag force computing in both CFD and DEM solvers. The performance of each solution and their combination are validated step by step. The result proves that the relaxing scheme could release the drag-induced time step without sacrificing accuracy. At last, a case of SCC flow shows that it increases the CFD time step by 100 times which is dominant in simulation.Once the above two major challenges are settled, our in-house code was used to simulate the workability tests. According to the previously obtained relation chart between the indices of workability tests and Bingham parameters, the apparent properties of the mortar and aggregate mixture were obtained from the simulation results of the slump flow test and V funnel test. What’s more, using the L box test, the influence of aggregate gradation, the existence of rebars and aggregate were discussed one by one.As a conclusion, we have proposed a feasible numerical technique to simulate the fresh SCC flow: to study the feature of bulk flow, the concrete can be modelled by pure CFD; to study it at the scale of aggregate size, the proposed VOF-DEM method and the relaxing scheme should be used to enable the desired simulation with acceptable efficiency.