Computers & Fluids最新文献

The Direct Simulation Monte Carlo (DSMC) method was widely used to simulate low density gas flows with large Knudsen numbers. However, DSMC encounters limitations in the regime of lower Knudsen numbers ($\mathrm{Kn}<0.05$). In such cases, approaches from classical computational fluid dynamics (CFD) relying on the continuum assumption are preferred, offering accurate solutions at acceptable computational costs. In experiments aimed at imaging aerosolized nanoparticles in vacuo a wide range of Knudsen numbers occur, which motivated the present study on the analysis of the advantages and drawbacks of DSMC and CFD simulations of rarefied flows in terms of accuracy and computational effort. Furthermore, the potential of hybrid methods is evaluated. For this purpose, DSMC and CFD simulations of the flow inside a convergent–divergent nozzle (internal expanding flow) and the flow around a conical body (external shock generating flow) were carried out. CFD simulations utilize the software OpenFOAM and the DSMC solution is obtained using the software SPARTA. The results of these simulation techniques are evaluated by comparing them with experimental data (1), evaluating the time-to-solution (2) and the energy consumption (3), and assessing the feasibility of hybrid CFD-DSMC approaches (4).

This work develops, for the first time, a face-centred finite volume (FCFV) solver for the simulation of laminar and turbulent viscous incompressible flows. The formulation relies on the Reynolds-averaged Navier–Stokes (RANS) equations coupled with the negative Spalart–Allmaras (SA) model and three novel convective stabilisations, inspired by Riemann solvers, are derived and compared numerically. The resulting method achieves first-order convergence of the velocity, the velocity-gradient tensor and the pressure. FCFV accurately predicts engineering quantities of interest, such as drag and lift, on unstructured meshes and, by avoiding gradient reconstruction, the method is less sensitive to mesh quality than other FV methods, even in the presence of highly distorted and stretched cells. A monolithic and a staggered solution strategies for the RANS-SA system are derived and compared numerically. Numerical benchmarks, involving laminar and turbulent, steady and transient cases are used to assess the performance, accuracy and robustness of the proposed FCFV method.

In this study, we utilize the simplified lattice Boltzmann method (SLBM) to investigate numerically the motion of buoyancy-driven deformable ferrofluid droplets through the orifice of varying widths and depths in two-dimensional (2D) space. Positioned directly beneath a plate with a central hole, the magnetic fluid droplets undergo acceleration to meet the plate under the influence of buoyancy and magnetic forces. We investigate the impact of magnetic field strength (Bo_m), pore ratio (PR), plate thickness ratio (WR), droplet viscosity (Re), and the plate's wettability (contact angle) on the dynamic behavior of ferrofluid droplets ascending through the orifice. Our results reveal significant effects on the efficiency and morphology of ferrofluid droplets passing through the hole. The introduction of a magnetic field facilitates a larger volume of liquid droplets passing through the hole at PR = 0.25. Moreover, increasing magnetic field intensity leads to the generation of secondary droplets during passage through the orifice. In practical applications, to prevent the generation of secondary droplets, we recommend Bo_m < 3 when the pore ratio falls within 0.35 < PR < 0.45 and plate thickness ratio WR = 1. Additionally, with increasing obstacle thickness, ferrofluid droplets on the hydrophobic wall can pass through the orifice more easily. Furthermore, when the magnetic field strength exceeds a certain threshold (Bo_m = 6.08), the droplets can pass through the orifice regardless of the wall's hydrophilicity or hydrophobicity. For practical applications with the pore ratio PR = 0.25 and plate thickness ratio WR > 1, we suggest Bo_m > 3.

This work presents a procedure to solve the Euler equations by explicitly updating, in a conservative manner, a generic thermodynamic variable such as temperature, pressure or entropy instead of the total energy. The presented procedure is valid for any equation of state and spatial discretization. When using complex equations of state such as Span–Wagner, choosing the temperature as the generic thermodynamic variable yields great reductions in the computational costs associated to thermodynamic evaluations. Results computed with a state of the art thermodynamic model are presented, and computational times are analyzed. Particular attention is dedicated to the conservation of total energy, the propagation speed of shock waves and jump conditions. The procedure is thoroughly tested using the Span–Wagner equation of state through the CoolProp thermodynamic library and the Van der Waals equation of state, both in the ideal and non-ideal compressible fluid-dynamics regimes, by comparing it to the standard total energy update and analytical solutions where available.

Undulated biomimetic propulsion has gained an extensive attention with upsurge of bionic applications. However, its performance in different flow environments is rarely discussed. In this paper, hydrodynamic behavior of an undulated beam in flow environments is studied, as well as its routing problem. The previously proposed loosely coupled partitioned algorithm is adopted. Motion of an undulated beam in still water is simulated to validate this algorithm. And then, hydrodynamic behavior of beam in flow environments with different directions and velocities is studied. It is found that velocity of beam is linearly affected by longitudinal flow and symmetric vortex structure still keeps. While transverse flow leads to the unequal amplitudes of velocity valley and crest, and symmetric vortex structure is lost. The influence of oblique flow could be regard as the combination of longitudinal and transverse flow components. Flow details are analyzed to reveal the mechanism of those hydrodynamic changes. Transverse flow component plays an important role. It significantly changes the pressure difference around beam and promotes the mixture of vortex. Besides, performance of beam in different flows and routing problem indicate that the straight path between the beginning and ending points is not always the best choice.

The present study examines flow through Bi-Leaflet Mechanical Heart Valves (BMHV) at physiological conditions considering both Newtonian and non-Newtonian fluid models for blood rheology. It is well known that the non-Newtonian effects of blood are pronounced in small diameter arteries. Most of the earlier works on Mechanical Heart Valves (MHV) have considered blood as a Newtonian fluid as the flow involves large-diameter artery such as the aorta. In this work, we have reported the predicted parameters, such as leaflet kinematics, vortex structures, wall shear stress, and blood damage index for both blood models. It is found that the leaflet attributes smaller asynchronous motion in the case of non-Newtonian Carreau fluid model with slightly reduced angular velocity compared to the Newtonian assumption. Predictions on the blood damage index suggest a 21% higher damage while using non-Newtonian model than Newtonian model, which may be attributed to higher levels of mechanical stress within the fluid. However, vortex structures, time-averaged wall shear stress (TAWSS), and oscillatory shear index (OSI) are found to be similar in predictions using both the fluid models. We have used an in-house sharp interface immersed boundary method with fluid–structure interaction to simulate the coupled action of moving valves and pulsatile blood flow. Our findings suggest that the general consensus of using Newtonian model in large arteries may not be appropriate for prediction of leaflet kinematics and blood damage index in Mechanical heart valves.

In this article, we propose an approach for interface-resolved simulations of immiscible incompressible n-phase flows ($n\ge 3$). The standard level set (LS) method can accurately simulate the motion of two phases. However, when more than two phases are considered, the interfaces cannot intersect and numerical overlaps occur, leading to unphysical spurious velocities and unrealistic distortions of the interfaces. The proposed approach organizes a group of $n-1$ LS functions into a hierarchy to resolve the issue of numerical overlaps. The n-phase flow field is discretized using the extended finite element method (XFEM), which adequately addresses the discontinuities occurring in the pressure and velocity fields due to distinct fluid properties and surface tension effects at the interface between the fluids. This approach is applied to three-phase and four-phase fluid dynamics, involving gas and liquids under various conditions of surface tension. The $2D$ numerical tests prove that the proposed numerical methods can effectively model interactions between multi-fluid interfaces, avoiding numerical overlaps.

Vector form of intrinsic finite element (VFIFE) is a numerical method widely used in solid mechanics. However, it's hard to extend the VFIFE method to fluid mechanics since the traditional VFIFE method fails to reflect the analytical equilibrium of multiple variables in the continuum. Therefore, under the framework of analytical mechanics, this paper proposes Lagrange's equation of the second kind in fluid mechanics with the extremum condition of Lagrange power functional. And a vectorized motion equation of incompressible viscous fluids is deduced from Lagrange's equation. By using several efficient algorithms in the finite difference method (FDM) and the finite element method (FEM), the NS equation is decomposed into four governing equations of vector form for fluid mechanics. In addition, with the application of the classic Smagorinsky sub-grid scale model in large eddy simulation (LES), this paper puts forward turbulence modelling with VFIFE procedure, and a corresponding MATLAB program is developed. Two typical examples are given to demonstrate the applicability and efficiency of the proposed large eddy simulation with VFIFE method. The proposed algorithm can effectively eliminate the non-physical oscillation of the pressure, and obtain much accurate results with a small number of grids.

Artificial Compressibility Methods (ACM) rely on an artificial equation that links the pressure and velocity fields to model incompressible flows. These hyperbolic/parabolic equations can rapidly converge to a ‘nearly’ divergence-free flow field in contrast to the methods based on the elliptic pressure Poisson equation. We compare the computational efficacy of two ACMs, namely, the Bulk Viscosity ACM (BVACM) and Entropically Damped Artificial Compressibility (EDAC) recently proposed in the literature. The methods implemented in the in-house high-order finite difference solver, COMPSQUARE, are validated for the test cases of a 2D doubly periodic shear layer (DPSL), a 3D Taylor Green Vortex (TGV), and 2D/3D NACA0012 airfoil pitching about the quarter chord. The efficacy of these methods was also tested on static and dynamic grids using conservative metrics. Although both ACMs yield competitive results, the divergence of the velocity field is found to be more prominent in the highly unsteady regions. BVACM resulted in (a) a superior divergence-free velocity field and (b) $20-38\text{\%}$ higher maximum stable time than the EDAC, thereby increasing the computational speed. A higher value of the bulk viscosity coefficient, $A$, although ensures a stringent divergence-free velocity field, is shown to have minimal effect on the flow statistics and reduce the maximum stable time step. The parabolic–hyperbolic nature of the governing equations and the lack of dual time-stepping in BVACM and EDAC ensures that both these methods are highly scalable on massively parallel architectures. Since the energy equation is no longer required to compute the velocity field, both EDAC and BVACM approaches are found to be $8-10\text{\%}$ faster than the weakly compressible Navier–Stokes simulations under the low-Mach number limit.