International Journal for Numerical Methods in Fluids最新文献

We present a methodology for simulating melting and solidification within a lattice Boltzmann framework utilizing filter-matrix collision operators. This approach integrates a source-based enthalpy method for phase change and an immersed boundary scheme to enforce the no-slip condition at the evolving phase interface. The proposed methodology demonstrates excellent agreement with benchmark cases, including the Stefan problem and the analytical model for transient freezing in channel flow between two isothermally cooled parallel plates. Further validation is performed on two more complex scenarios: Ice layer formation in a cavity driven by natural convection and channel flow with a constant flux of heat removal, both of which show strong agreement with reference data. Given the enhanced stability of filter-matrix collision operators, future work could extend this approach to turbulent melting and solidification simulations.

The pressure-correction method is a well-established approach for simulating unsteady, incompressible fluids. It is well-known that implicit discretization of the time derivative in the momentum equation, for example, using a backward differentiation formula with explicit handling of the nonlinear term, results in a conditionally stable method. In certain scenarios, employing explicit time integration in the momentum equation can be advantageous, as it avoids the need to solve for a system matrix involving each differential operator. Additionally, we demonstrate that the fully discrete method can be written in the form of simple matrix-vector multiplications, allowing for efficient implementation on modern and highly parallel acceleration hardware. Despite being a common practice in various commercial codes, there is currently no literature available on error analysis for this scenario. In this work, we conduct a theoretical analysis of both implicit and two explicit variants of the pressure-correction method in a fully discrete setting. We demonstrate the extent to which the presented implicit and explicit methods exhibit conditional stability. Furthermore, we establish a Courant–Friedrichs–Lewy (CFL) type condition for the explicit scheme and show that the explicit variant demonstrates the same asymptotic behavior as the implicit variant when the CFL condition is satisfied.

Researchers have proposed multifarious passive methods for heat transfer augmentation over geometries with separation and reattachment. In this study, the turbulent forced convection flow of water-based CNT-TiO₂ hybrid nanofluid, ND-Ni hybrid nanofluid, and mono Ni nanofluid (with temperature-dependent properties) in a double forward-facing step channel with a converging/diverging bottom adiabatic wall is evaluated. The single-phase shear stress transport k-ω model is applied to solve the governing equations. Results indicate that the highest thermo-hydraulic performance (the value of figure-of-merit is equal to 1.1) can be achieved using TiO₂-CNT/water HyNf with ϕ = 0.002. Generally, as the velocity of the incoming stream is reduced, the thermal efficacy of HyNf improves. When the water-based NFs are not an effective heat transfer fluid (inducing a performance evaluation criterion lower than unity), the diverging channel (weakening of the contact between surfaces with constant heat flux and working fluid) can be employed instead of the DFFS channel while the Reynolds number of the incoming flow reduces as well. It is found that the thermal efficacy of water-based NFs in complex separated flows depends strongly not only on the deflection angle of the bottom adiabatic wall but also on the velocity of the incoming flow.

In injection molding processes, shrinkage and warpage cause deviations in the size and shape of produced parts compared to the cavity shape. While shrinkage is due to the change of material density during solidification, warpage is caused by uneven cooling and internal stresses within the part. One approach to mitigate these effects is by adjusting the cavity shape to anticipate the deformation. While finding the optimal cavity shape is often experience-based in practice, numerical design optimization can greatly assist in this process. In this study, we evaluate various numerical algorithms from existing literature to identify the optimal cavity shape. Each method is briefly outlined regarding how it adapts the geometry, and we discuss their respective strengths and weaknesses for different scenarios. We conduct comparisons using 3D geometries of varying complexity. Our findings demonstrate that, for geometric warpage compensation, the node-based reverse geometry method yields the least warpage and is computationally cost-effective. Furthermore, it is straightforward to implement and consistently performs well across different geometries.

This paper presents a new robust treatment for smoothed particle hydrodynamics (SPH) open (inflow/outflow) and solid boundary conditions, avoiding the unphysical fluctuations and numerical noise present in existing techniques. By novel use of concepts from finite volume methods, the fluid properties from sequential dynamic particles with different normal distances to the boundaries are extrapolated to ghost particles. No so-called mirror points are required, making the method computationally efficient and easy to implement. The new methodology is validated through a series of progressively challenging test cases. The effectiveness of the wall and inflow-outflow boundaries is evaluated for 2-D Poiseuille laminar flow. The performance of the wall boundary for complex geometries is demonstrated using a hydrostatic tank with a triangular wedge, followed by a conventional 2-D dam-break problem to capture impact pressures. A range of challenging vertical inflows rarely explored using SPH, with varying efflux velocities, demonstrate highly accurate performance of the boundary treatment, with results compared to STAR-CCM+. Finally, the robust performance is demonstrated for flow past circular and square cylinders over a range of Reynolds numbers, showing excellent results compared to reference results.

This study applies a reconstruction scheme, “hybrid MUSCL–THINC” for finite volume methods developed by Chiu et al., to magnetohydrodynamics (MHD) simulations. The scheme is a hybrid of monotone upstream-centered schemes for conservation law (MUSCL) and a tangent of hyperbola interface capturing (THINC) scheme. THINC sharply captures discontinuous distributions of physical quantities by using a hyperbolic tangent function. Our investigation reveals that hybrid MUSCL–THINC is more oscillatory in MHD simulations than in gas dynamics simulations, owing to the greater number of physical variables and associated complex waves in MHD. Analytical results demonstrate that artificial compression by THINC is excessive for MHD shock waves, whereas it is effective for linear discontinuities, such as contact discontinuities. Therefore, we propose a modification in which the artificial compression by THINC is weakened in the vicinity of nonlinear discontinuities and applied only to linear regions. The new scheme is tested using one- and two-dimensional MHD problems, and the results demonstrate that the scheme sharply captures linear discontinuities while avoiding numerical oscillations due to excessive artificial compression.