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Unresolved and semi-resolved CFD-DEM coupling are frequently used for modeling particulate flows due to their high efficiency and satisfying accuracy. The grid size (in unresolved CFD-DEM) and smoothing distance (in semi-resolved CFD-DEM) are usually around three times of the particle diameter so as to properly reproduce the background flow field. An over-estimation of the grid size or smoothing distance can lead to low-fidelity flow fields, while under-estimations of these parameters can produce an inaccurate drag force. Therefore in practices, the size ratio is usually empirical, and how does it vary with cases remains unclear. In this paper, we provide an approach to identify the most suitable grid size or smoothing distance in coupled CFD-DEM. Firstly, resolved simulations based on immersed boundary method (IBM) are conducted to compute particles' drag forces. The obtained forces are then compared with those calculated by force models with certain smoothing lengths to identify an optimal smoothing length or effective grid size. During this process, integral scale of each case is also computed. Finally, linearity between integral length and the optimal smoothing distance or grid size is found. This correlation can offer guidance to CFD-DEM modeling in future and provide insights for novel drag force models. [Display omitted] • Optimal grid size in un-resolved CFD-DEM coupling is identified. • Optimal smoothing distance in semi-resolved CFD-DEM coupling is identified. • Optimal grid size and smoothing distance are determined by flow's integral scale. • Quantitative relation of the ratio of grid size to particle diameter is found. • Quantitative relation of ratio of smoothing distance to particle diameter is found. [ABSTRACT FROM AUTHOR]

Understanding the connection between physiology and kinematics of natural swimmers is of great importance to design efficient bio-inspired underwater vehicles. This study looks at high-fidelity three-dimensional numerical simulations for flows over an undulating American eel with prescribed anguilliform kinematics. Particularly, our work focuses on why natural anguilliform swimmers employ wavelengths shorter than their bodylengths while performing wavy kinematics. For this purpose, we vary the undulatory wavelength for a range of values generally observed in different aquatic animals at Strouhal numbers 0.30 and 0.40. We observe that our anguilliform swimmer is able to demonstrate more suitable hydrodynamic performance for wavelengths of 0.65 and 0.80. For longer wavelengths, the swimmer experiences large frictional drag, which deteriorates its performance. The wake topology was dominated by hairpin-like structures, which are closely linked with the underlying physics of anguilliform swimming found in nature. [ABSTRACT FROM AUTHOR]

• We extend a third-order weighted vartiational rDG(P 1 P 2) to solve the incompressible flows on unstructured grids. • High-order DoFs are reconstructed by minimizing a weighted interfatial jump integration function using variational method. • This new rDG(P 1 P 2) method is able to achieve the designed optimal third order of accuracy. • The computational costs of this new rDG(P 1 P 2) method are significantly reduced compared to the standard DG(P 2) method. In this paper, a third-order reconstructed discontinuous Galerkin (DG) method based on a weighted variational minimization principle, which is denoted as P 1 P 2 (WVr) method, is presented for solving the incompressible flows on unstructured grids. In this method, the first-order degrees of freedom (DoFs) are obtained directly from the underlying second-order DG method, while the second-order DoFs are reconstructed through the weighted variational reconstruction. Specifically, we first introduce a weighted interfacial jump integration (WIJI) function which represents a measure of the jump between the reconstructed polynomial solutions from two neighboring cells. Then, we build the constitutive relations by minimizing this WIJI function using the variational method. A number of incompressible flow problems in both steady and unsteady forms are presented to assess the performance of the proposed P 1 P 2 (WVr) method. The numerical results demonstrate that the P 1 P 2 (WVr) method is able to achieve the designed optimal third-order accuracy at a significantly reduced computational costs. Moreover, when a suitable value of the weight parameter is chosen to be used, the P 1 P 2 (WVr) method outperforms the reconstructed DG methods based on either least-squares or Green-Gauss reconstruction for the simulations of incompressible flows. [ABSTRACT FROM AUTHOR]

• The CFD-DEM method combined with micro-scale reconstruction is used to study proppant transport in hydraulic fracture. • Typical flow behaviors of proppant particles and fracturing fluid in different fracture models are analyzed. • The surface area ratio is utilized to characterize the cross-scale relation between proppant distribution and fracturing parameters. • The parameter effects of fluid, proppant, and fracture on proppant migration in fracture are discussed. A comprehensive investigation of the dynamical characteristics of proppant-laden flow systems in hydraulic fractures is important for the development of unconventional oil and gas resources. In this paper, we combine the micro-scale reconstruction technique and CFD-DEM method to numerically investigate the mobility and distribution rules of proppant-laden fluid in rough hydraulic fractures. To characterize realistic fracture surface morphology, six synthetic fracture models were assembled based on optical scanning data of real shale from three sets of hydraulic fracturing experiments. In order to accurately simulate the fluid-particle two-phase flow in rough fractures, the lift forces and particle rolling effect are considered in the CFD-DEM model. First, the effectiveness of the used CFD-DEM solver was validated by comparing it to the experimental data. We then examined mechanisms for the typical flow behaviors of proppant particles and fracturing fluid in smooth and rough fractures. Subsequently, the surface area ratio of the synthetic fracture model is adopted to characterize the cross-scale relationship between the micro-scale proppant distribution and fracture's geometric features induced by the macroscale fracturing parameters (i.e., confining stress, fluid viscosity, flow rate, and sand ratio). Lastly, the parameter effects of the fracturing fluid, proppant particle, and fracture aperture on the dynamic characteristics of particulate flow in the hydraulic fracture are further discussed. In summary, the presented numerical model and findings can help better understand particle transport and distribution in real rock fractures and bridge the connectivity of the micro-scale particle flow and macroscopic fracturing parameter, which is vital for hydraulic fracturing process optimization. [ABSTRACT FROM AUTHOR]

• Natural thermal convection induces fluid motion. • Particle shape-effects do play an important role in the particle and overall system dynamics. • Thermal heat exchange significantly affects the motion of the two-phase flow. • Multi-grid FEM-FBM is robust and potentially powerful tool for real particulate flow simulations. In particulate flows, particle-particle collisions play a very important role in determining the flow behavior of the fluid-particle two-phase systems. Thus, it is very important in the numerical simulations of particulate flows to treat the particle-particle interactions with a felicitous method. In this paper, a recently developed direct numerical simulation (DNS) technique, the Finite Element Fictitious Boundary Method (FEM-FBM), for thermal convective particulate flows is used for the simulations of dense particulate suspensions of elliptic shaped particles. The momentum and temperature flow fields are coupled with the aid of Boussinesq approximation. The thermal and momentum interactions between solid and fluid phases are handled by using the Fictitious boundary method (FBM). The continuity, momentum, and energy equations are solved on a fixed Eulerian mesh which is independent of flow features by using a multi-grid finite element scheme. A modified collision model is proposed that can handle not only the interactions between the circular particles but also effectively treat the collisions between the elliptic shaped particles. Firstly, we validate the newly developed collision model for isothermal circular particles together with a comparative study of "drafting, kissing and tumbling" (DKT) motion of both elliptic and circular shaped isothermal particles. Then we investigated the DKT motions of the hot and cold elliptic particles with energy exchange and studied the lateral behavior of the particles in numerous settings. Further, we studied the DKT motion of catalyst particles and investigate the particles behavior at different Grashof numbers. Numerical tests are performed to show that the present method is robust and provides an efficient approach for the simulations of particulate flows with a large number of elliptic particles. [ABSTRACT FROM AUTHOR]