Computers & Fluids最新文献

Bubble nucleation, growth and separation from cavities on the bottom of a microchannel for subcooled flow boiling are investigated by pseudo-potential lattice Boltzmann method. The influence of subcooling temperature, wall superheat, wettability, cavity size, and cavity number on the flow boiling heat transfer is systematically studied. The results show that the bubble equivalent diameter is 1.9 times larger at subcooling temperature 0.05$T_c$ than that at 0.15$T_c$, and the heat flux is also 8 % higher at subcooling temperature 0.05$T_c$ than that at 0.15$T_c$. It is found that the flow boiling changes from nucleate boiling to film boiling with the increase of wall superheat. When the wall wettability changes from the hydrophobic wall (θ = 120°) to the hydrophilic wall (θ = 30°), the average Nusselt number (Nu_av) is reduced by 23 %. We also optimize cavity height and the uniformly distributed cavity number in the microchannel. It is found that the Nu_av is increased by 9.7 % when the cavity height changes from h = 20lu (lattice unit) to h = 60lu. However, there exists an optimal cavity height about h = 60lu, where the heat transfer performance cannot be improved with the cavity height over this value. In addition, the number of cavities in the microchannel can improve the boiling heat transfer. When the cavity number changes from 1 to 4, the Nu_av is increased by 10 %.

The mass-, energy-, and potential-enstrophy-conserving Poisson bracket numerical scheme introduced by Arakawa & Lamb (1981), and extended by Salmon (2004) and Stewart & Dellar (2016), is adapted to the problem of nonlinear shallow-water sloshing over a variable bottom surface in a rigid rectangular basin undergoing a prescribed coupled surge-sway motion. Adaptation to a finite domain requires a new approach to the boundary conditions at solid boundaries in the context of the Arakawa C grid. In this paper, the boundary condition at the vertical walls is taken to be vanishing of the normal derivative of the tangential velocity. This gives zero potential vorticity at the boundary, and is consistent with the material conservation of the potential vorticity. This condition, coupled to symmetric boundary conditions for the wave height arising from a reduced version of the evolution equations at boundaries, leads to an extension of the class of staggered C-grid Poisson-bracket schemes to interior flows with solid boundaries. The scheme is implemented, shown to preserve Casimirs, and applied to a range of problems in shallow water hydrodynamics including interaction of travelling hydraulic jumps at resonance, and X-type soliton interactions over a variable bottom surface. This structure-preserving scheme provides a robust building block for long-time simulation of floating ocean wave energy extractors.

The non-polynomial THINC (tangent of hyperbola for interface capturing) scheme has been reported to show both numerical simplicity and high fidelity for resolving contact interfaces. In this paper, two types of smooth sigmoid functions are employed to construct the non-polynomial reconstructions for capturing interfaces (similarly, called SFINC schemes, sigmoid functions for interface capturing). One type is that the exact jump location (a parameter introduced in the reconstruction) can be analytically calculated, and another type cannot. The algebraic function and the Gudermannian function belong to the first and the second types, respectively, and are investigated in this paper. An approximate method for calculating the jump location of the Gudermannian function is proposed. The method avoids the iteration process of determining the jump location, and hence is efficient and practical in applications. The numerical validations and comparisons of SFINC schemes are presented to show their performance for simulating complex compressible flow fields.

We introduce a novel positivity-preserving numerical stabilisation approach for high-order discontinuous spectral element approximations of compressible multi-species flows. The underlying stabilisation method is the adaptive entropy filtering approach (Dzanic and Witherden, J. Comput. Phys., 468, 2022), which is extended to the conservative formulation of the multi-species flow equations. We show that the straightforward enforcement of entropy constraints in the filter yields poor results around species interfaces and propose an adaptive switch for the entropy bounds based on the convergence properties of the pressure field which drastically improves its performance for multi-species flows. The proposed approach is shown in a variety of numerical experiments applied to the multi-species Euler and Navier–Stokes equations computed on unstructured grids, ranging from shock-fluid interaction problems to three-dimensional viscous flow instabilities. We demonstrate that the approach can retain the high-order accuracy of the underlying numerical scheme even at smooth extrema, ensure the positivity of the species density and pressure in the vicinity of shocks and contact discontinuities, and accurately predict small-scale flow features with minimal numerical dissipation.

There is a great influence of the melt filling process on the quality of castings. Smoothed particle hydrodynamics (SPH) that materials are approximated by free particles rather than by fixed grids is applied to accurately predict fluid flows involving complex free surfaces. In this paper we present a mathematical model of melt flow and heat transfer by using SPH method. A novel approach is used in the governing equations to ensure stable numerical schemes and homogeneous particle distributions. The SPH heat equation takes into account the thermal release during phase transition and is more suitable for alloy solidification. The solid wall boundary conditions are slightly modified to satisfy the filling simulations. Several case studies are carried out to predict significant details about the filling order and flow structures in the mold cavity. The velocity and temperature distributions during different stages of melt filling are also given. The results show that this proposed model allows us to understand the predisposition of defects such as gas porosity and weld lines in the castings. These predictions can be used as inputs for improving process parameters, venting and cooling systems design.

In this paper, we are concerned about the lattice Boltzmann methods (LBMs) based on vector-kinetic models for hyperbolic partial differential equations. In addition to usual lattice Boltzmann equation (LBE) derived by explicit discretisation of vector-kinetic equation (VKE), we also consider LBE derived by semi-implicit discretisation of VKE and compare the relaxation factors of both. We study the properties such as H-inequality, total variation boundedness and positivity of both the LBEs, and infer that the LBE due to semi-implicit discretisation naturally satisfies all the properties while the LBE due to explicit discretisation requires more restrictive condition on relaxation factor compared to the usual condition obtained from Chapman-Enskog expansion. We also derive the macroscopic finite difference form of the LBEs, and utilise it to establish the consistency of LBEs with the hyperbolic system. Further, we extend this LBM framework to hyperbolic conservation laws with source terms, such that there is no spurious numerical convection due to imbalance between convection and source terms. We also present a D2Q9 model that allows upwinding even along diagonal directions in addition to the usual upwinding along coordinate directions. The different aspects of the results are validated numerically on standard benchmark problems.

The coalescence-induced jumping of nanodroplets on superhydrophobic surfaces has recently gained research attention due to its application in energy harvesting, self-cleaning, and cooling of nanoscale electronic devices. This study aims to investigate the jumping behavior of water nanodroplets in a high Ohnesorge number regime, where 0.45 $<$ Oh $<$ 1 and identify the critical size of droplets where jumping terminates. The study utilized molecular dynamics simulations to analyze the jumping characteristics of droplets ranging from 1.5 nm to 7 nm in radius. The findings of this research developed a universal jumping mechanism for droplets of all sizes, identified the lower limit of droplet size, below which coalescence-induced jumping does not occur, and explained a special phenomenon of jumping velocity becoming maximum before it approaches zero. The study also investigated how jumping terminates due to the size difference between droplets. These findings align well with prior micro-scale studies and experimental predictions. Surface energy, viscous dissipation, kinetic energy, and varying surface tension have been identified as the dominating factors influencing nanoscale droplet jumping at such a high Oh regime. The findings will provide insights for developing various applications at this scale.

We investigate the generation of free-stream perturbations at a relatively low characteristic Reynolds number of 1000 by means of direct numerical simulations using a synthetic turbulence generation method. This approach consists of generating turbulent fluctuations by means of digital filtering and a source term formulation in the Navier–Stokes equations. To assess its validity in the framework of decaying turbulence, we compare the results with those obtained with a physically-based, grid-induced turbulent flow in terms of spatial decay, evolution of characteristic length-scales and energy spectra. Also, we highlight relevant differences such as those in the streamwise development length and the anisotropy of the largest scales. Then, we characterize the generated perturbations when systematically varying the input parameters, namely the initial integral length-scale and turbulence intensity. Here, we notice differences in the streamwise decay of the turbulence intensity and the development length as we vary these parameters. By inspecting the evolution of the characteristic length-scales and the micro-scale Reynolds number, we also identify that the effective scale separation is highly sensitive to these variations.

Direct simulations of external flow and the associated noise radiation are studied by an improved lattice Boltzmann method, i.e., the generalized interpolation-supplemented cascaded lattice Boltzmann method (GICLBM). In this method, the cascaded collision scheme is used to improve the numerical stability of the conventional collision schemes, and the generalized interpolation approach is used in the particle streaming process so as to allow a non-uniform and body-fitted mesh partition. With that, both near- and far-field flow dynamics and noise radiation are resolved simultaneously. In order to capture sound waves, the perfectly matched layer is also implemented so as to avoid waves reflecting to and polluting the inner acoustic field. Moreover, a novel index technique is developed for the GICLBM to enable implicit streaming, which brings an efficient memory reduction. Three cases are then performed to showcase the feasibility, accuracy, extensibility, and efficiency of the present framework, including flow past a square cylinder, flow past an elliptic cylinder, and flow past a NACA 0012 airfoil, each implemented with a type of body-fitted mesh. Both the fluid dynamic and noise radiation are found to be in good agreement with results using the Navier–Stokes solvers. This study demonstrates the potential of the GICLBM for accurately and efficiently simulating external problems as well as sound generation and propagation.

The stability of fluid–structure interaction (FSI) problems using immersed boundary (IB) method is an active area of research. In this regime, strong coupling is generally used to ensure stability and robustness. Strong coupling, however, is computationally expensive owing to its iterative nature. In the present work, we showcase the application of loose coupling algorithm for FSI problems using the sharp interface IB method specifically for low to moderate mass ratios (defined as the ratio of the mass of the structure to the mass of the fluid at the same volume). We demonstrate several test cases: vortex-induced vibration (VIV) of a cylinder, the effect of hinged leaflets attached to the exit of a piston in a channel, sedimentation of a circular disk, and bi-leaflet mechanical heart valves (BMHV) made of lightweight materials in physiological flow. We found our loose coupling method to be stable in all the test cases and obtained a linear relationship between the grid resolution employed and the lowest mass ratio for stable computations in the case of VIV of cylinder. Thus, a significant finding of our work is that with a reduction in grid spacing, one can achieve stable FSI simulation involving lower mass ratios using loosly-coupled schemes. We have deployed the present technique to investigate the dynamics of very low-density cylinders and hinged leaflets due to the fluid forces on them. The current method is extended to handle flexible bodies, such as vortex-induced vibrations of an elastic plate attached to a rigid cylinder and stable simulations are obtained when the Young’s modulus of the elastic plate is varied.