Computers & Fluids最新文献

Knowledge of the bottom topography, also called bathymetry, of rivers, seas or the ocean is important for many areas of maritime science and civil engineering. While direct measurements are possible, they are time consuming, expensive and inaccurate. Therefore, many approaches have been proposed how to infer the bathymetry from measurements of surface waves. Mathematically, this is an inverse problem where an unknown system state needs to be reconstructed from observations with a suitable model for the flow as constraint. In many cases, the shallow water equations can be used to describe the flow. While theoretical studies of the efficacy of such a PDE-constrained optimisation approach for bathymetry reconstruction exist, there seem to be few publications that study its application to data obtained from real-world measurements. This paper shows that the approach can, at least qualitatively, reconstruct a Gaussian-shaped bathymetry in a wave flume from measurements of the free surface level at up to three points. Achieved normalised root mean square errors (NRMSE) are in line with other approaches.

We present an hybrid VOF/embedded boundary method allowing to model two-phase flows in presence of solids with arbitrary shapes. The method relies on the coupling of existing methods: a geometric Volume of fluid (VOF) method to tackle the two-phase flow and an embedded boundary method to sharply resolve arbitrary solid geometries. Coupling these approaches consistently is not trivial and we present in detail a quad/octree spatial discretization for solving the corresponding partial differential equations. Modelling contact angle dynamics is a complex physical and numerical problem. We present a Navier-slip boundary condition compatible with the present cut cell method, validated through a Taylor–Couette test case. To impose the boundary condition when the fluid–fluid interface intersects a solid surface, a geometrical contact angle approach is developed. Our method is validated for several test cases including the spreading of a droplet on a cylinder, and the equilibrium shape of a droplet on a flat or tilted plane in 2D and 3D. The temporal evolution and convergence of the droplet spreading on a flat plane is also discussed for the moving contact line given the boundary condition (Dirichlet or Navier) used. The ability of our numerical methodology to resolve contact line statics and dynamics for different solid geometries is thus demonstrated.

In this paper, an improved three-dimensional pseudo-potential multiphase flow model is proposed. A high-precision difference scheme is employed to improve the computational accuracy of the interaction forces to achieve super-large density ratio. The present model is verified to obtain two-phase density ratio up to hundreds of thousands for all selected equations of state, and the spurious currents can be suppressed to a relatively low level. A modification pressure tensor is introduced to implement a wide range of surface tension adjustments independently of the density ratio. The model is applied to simulate droplet impacts on a liquid film as well as droplet impacts on a dry surface. Numerical results show that the model still has good numerical stability in simulating complex fluid problems with very large density ratio, adjustable surface tension, high Reynolds number, or containing the wettable surface. In addition, to improve the computational efficiency, an efficient parallel algorithm based on graphics processing unit (GPU) is designed for the present model. A maximum speedup ratio of 687 times is obtained compared to the corresponding CPU-based serial algorithm, which can significantly accelerate the numerical simulation study.

The level-set method performs well when it captures relatively thick interface structures, but may not properly resolve thin interface structures when their sizes are comparable to or smaller than grid sizes. With coarse grids, these thin interface structures are lost through non-physical breakup during simulation. We propose a locally redistributed level-set method to sustain thin structures with reasonable grid resolution. The present method distributes additional level-set functions at the regions where two interfaces of a structure are about to contact. This method captures thin interface structures smaller than grid sizes. Moreover, it identifies adjacent interfaces separately, and enables an accurate calculation of surface tension. Numerical simulations are performed for binary droplet collisions at relatively high Weber numbers, and show that thin interface structures are well captured and their interface dynamics is accurately predicted.

The interaction of a moving shock wave and a microscale vortex is numerically studied by solving the BGK-type equation with the unified gas-kinetic scheme (UGKS) and the Navier-Stokes equations with the gas-kinetic scheme (GKS-NS). Different Knudsen numbers based on the core radius of the vortex are considered. The results indicate that GKS-NS tends to overestimate the dissipation rate of kinetic energy and the amplification of stress and enstrophy caused by the fully resolved shock wave, while underestimating the amplification of heat flux through the shock wave due to rarefied effects. It is also observed that as the core size of the vortex increases, the decay of the enstrophy over time slows down, while the amplification of enstrophy by the shock wave increases. Negligible rarefied effects can be assumed when the Knudsen number is below 0.01 where the overestimation of enstrophy amplification by GKS-NS is less than 5 %. However, when the Knudsen number exceeds 0.1, the difference of the enstrophy predicted by UGKS and GKS-NS is greater than 20 %, where rarefied effects need to be considered.

To study the hydrodynamics and structural dynamics of the semi-sealed cylindrical shells with different material properties during water entry, based on the STAR-CCM+and ABAQUS collaborative simulation method, the numerical simulation with different material properties is conducted in this paper. The results show that shells with the same material but different plasticity have similar velocity and displacement, but their deformation and stress distribution on the top position differ significantly. The cavity evolution of shells with different densities is evidently different. The volume of secondary cavity of the shell with higher densities is larger, and the concentration force and stress distribution on the inner upper wall are also greater. When shells of the same material shell penetrate into solutions with different densities, the depth of the shell gradually increases as the solution density decreases. Solutions with different densities can fill the inner space of the shell for the first time, but not all solutions with different densities can fill the inner space of the shell for the second time, the volume of solutions with lower densities which entering into the inner space is larger.

The work is devoted to researching the effects of reflected gas molecules’ states on aerodynamic properties and surface characteristic quantities distributions. Within the framework of GUKA, the Maxwellian type gas surface interaction model is implemented. After the validation of the GKUA in representative cases, the simple geometry model is initially introduced to study the variations of aerodynamic properties with different gas molecules states. Then the simulations around simplified Tianzhou-5 cargo spacecraft and its symbolic components are conducted at typical reentry trajectory points. The results are useful to predict the disintegration trajectories and evaluate the distributions of survival spacecraft objects falling down the ground.

We present a finite element framework for the numerical prediction of cavitating turbulent flows interacting with flexible structures. The vapor–fluid phases are captured through a homogeneous mixture model, with a scalar transport equation governing the spatio-temporal evolution of cavitation dynamics. High-density gradients in the two-phase cavitating flow motivate the use of a positivity-preserving Petrov–Galerkin stabilization method in the variational framework. A mass transfer source term introduces local compressibility effects arising as a consequence of phase change. The turbulent fluid flow is modeled through a dynamic subgrid-scale method for large eddy simulations. The flexible structure is represented by a set of eigenmodes, obtained through a modal decomposition of the linear elasticity equations. While a partitioned iterative approach is adopted to couple the structural dynamics and cavitating fluid flow, the deforming flow domain is described by an arbitrary Lagrangian–Eulerian frame of reference. We establish the fidelity of the proposed framework by comparing it against experimental and numerical studies for both rigid and flexible hydrofoils in cavitating flows. Under unstable partial cavitating conditions, we identify specific vortical structures leading to cloud cavity collapse. We further explore features of cavitating flow past a rigid body such as re-entrant jet and turbulence-cavity interactions during cloud cavity collapse. Based on the validation study conducted over a flexible NACA66 rectangular hydrofoil, we elucidate the role of cavity and vortex shedding in governing the structural dynamics. Subsequently, we identify a broad spectrum frequency band whose central peak does not correlate to the frequency content of the cavitation dynamics or the natural frequencies of the structure, indicating the induction of unsteady flow patterns around the hydrofoil. Finally, we discuss the coupled fluid–structure dynamics during a cavitation cycle and the underlying mechanism associated with the promotion and mitigation of cavitation.

Computational Fluid Dynamics (CFD) traditionally relies on long-standing numerical simulation strategies for the Navier–Stokes equations. Recently, interest in data-driven hybrid CFD solvers has spiked, leveraging pre-computed datasets to enhance various weak links inside existing solvers, such as closure models, under-resolved physics, or even to guide numerical resolution strategies. Running these hybrid solvers, notably in High Performance Computing (HPC) environments, presents specific challenges. In particular, context-aware deep learning (e.g. Convolutional (CNN) and Graph (GNN) Neural Networks) is promising for this task, but requires passing data representations between the physics solver and the neural network. In relevant industrial configurations, CFD meshes can be Cartesian but highly irregular, or unstructured, both of which do not match the pixel/voxel structure needed to run CNNs. In addition, discrepancies in programming language and libraries are common between CFD and machine learning applications. This work explores the many challenges of running a parallel hybrid solver in an HPC context, through the coupling of the AVBP CFD solver with neural networks in turbulent combustion and wall friction modeling applications. The knowledge gained is showcased in this article, as well as assembled in an actionable open-source library.

Numerical simulation is a well-established way of analyzing compressible flows. Due to high computational demands of solvers for such flow problems, their verification is typically limited to two-dimensional (2D) cases. However, 2D simulations suppress fundamental three-dimensional (3D) aspects of the flow evolution in (compressible) turbulent flows or shock-bubble and shock-drop interactions. With increase of computational power, 3D simulations become more feasible for routine analyses. The verification of 3D simulation frameworks is often limited to transformations of lower dimensional test cases in the 3D space. There is a lack of strictly 3D reference test cases for gas dynamics. In this work, we present a set of genuine 3D Riemann problems in order to validate and verify numerical solvers for compressible flows. The problems are designed such that each octant’s constant initial data connects two neighboring states by an elementary wave only. The problem design is inspired by well-established 2D Riemann problems most prominently posted by Lax and Liu (1998). In contrast to the twenty published 2D cases, more than 300 distinct combinations can be found in 3D. We provide example solutions for the particularly interesting ones of these case combinations and show how the cases help to expose shortcomings of numerical solvers. We provide reference data from computations with an open-source compressible multiresolution flow solver. For the reference solutions, we employ the Harten-Lax-van Leer contact (HLLC) Riemann solver and a weighted essentially non-oscillatory (WENO) reconstruction stencil of fifth order. The reference solutions use an effective resolution of one billion cells. We additionally make the full compute pipeline of this work publicly available, so interested researchers may reproduce and extend the current work.