arXiv subjects

Ming-Jiu Ni

Publications and source records attributed to Ming-Jiu Ni.

12 recordsLinked to original sources

arXiv2026

A Sharp and Conservative VOF Method for Multicomponent Liquid--Gas Mass Transfer: Bubble Dissolution and Droplet Evaporation

We present a sharp and conservative geometrical VOF--finite-volume method for multicomponent liquid--gas mass transfer across deformable interfaces. The method solves problems in which multiple species are coupled through interfacial mass balances, latent-heat exchange, and vapor--liquid equilibrium. The key novelty is a fully sharp two-field treatment of scalar transport: the species and temperature equations are solved separately in the liquid and gas phases, while the one-sided Robin conditions for species and the two-sided flux jump for temperature are imposed directly on the reconstructed interface through an embedded-boundary discretization. This avoids both volumetric regularization of interfacial source terms and explicit coupling based on previous-time-step interfacial data. A consistent geometrical advection scheme is used for volume, momentum, energy, and species transport, and a sequential coupling strategy is developed to determine the partial interfacial mass fluxes, close the temperature equation, and update the thermodynamic-equilibrium state. The method is validated through single- and multicomponent bubble dissolution, single-component droplet evaporation, non-ideal ethanol--isooctane droplet evaporation, and sessile water--glycerol droplet evaporation. The results demonstrate second-order accuracy, accurate interfacial flux prediction, good mass and energy conservation, and the ability to capture complex multicomponent effects such as gas replacement, azeotropic volatility reversal, and composition-driven Marangoni flow.

physics.flu-dyn↗

arXiv2026

Flow Reversal in Low-Prandtl-Number Convection via Lateral Confinement

A prevailing consensus holds that flow reversals of the large-scale circulation (LSC) are suppressed in low-Prandtl-number (Pr) fluids, as high thermal diffusivity rapidly dissipates the energy required to fuel the corner-vortex mechanisms. Here, we report Direct Numerical Simulations of liquid metal convection (Pr=0.029) revealing that strong lateral confinement defies this consensus, enabling sustained LSC reversals. We show that confinement triggers a ``plume condensation" transition, reorganizing chaotic thermal plumes into highly coherent, quasi-linear structures. A thermal dissipation analysis demonstrates that this coherence drastically reduces heat loss during transport, allowing plumes to deliver sufficient buoyancy to corner vortices to drive reversals. We map a distinct ``island of reversal" in the parameter space, establishing lateral confinement as a control parameter capable of overcoming the stabilizing effects of high thermal diffusivity.

physics.flu-dyn↗

arXiv2026

A 3D sharp and conservative VOF method for modeling the contact line dynamics with hysteresis on complex boundaries

We propose a sharp and conservative 3D numerical method for simulating moving contact lines on complex geometries, developed within a coupled geometric Volume-of-Fluid (VOF) and embedded boundary framework. The first major contribution is a modified VOF advection and reconstruction scheme specifically designed for mixed cells containing liquid, gas, and solid phases. This formulation ensures strict local mass conservation in the presence of arbitrarily shaped embedded boundaries. To overcome the severe time-step limitation caused by small cut cells, a redistribution advection strategy is introduced, which completely removes the CFL constraint while preserving both local and global volume conservation. The second key contribution is a novel 3D contact angle imposition technique built upon the height function framework. By incorporating a pre-fitting paraboloid procedure, the method achieves robust curvature estimation and accurate enforcement of contact angle conditions on irregular solid surfaces. In addition, contact angle hysteresis is modeled to capture more realistic wetting dynamics. A series of challenging benchmark tests have been conducted to demonstrate the accuracy, robustness, and superiority of the proposed method compared with existing sharp-interface approaches. This study, for the first time, establishes a fully geometric and conservative VOF-based scheme capable of accurately resolving contact line dynamics on arbitrarily complex 3D surfaces.

math.NA↗

arXiv2025

Magnetic modulation of flow reversals in liquid metal thermal convection

Flow reversals are rarely observed in low-Prandtl-number liquid metal convection due to the fluid's exceptionally high thermal diffusivity. Here, we demonstrate that an external transverse magnetic field can induce such reversals in a quasi-two-dimensional (Q2D) rectangular cell with an aspect ratio ($\itΓ$) of $0.2$. Our experimental observations reveal that the system initially exhibits periodic dynamics at the onset of reversals before transitioning to stochastic behavior as the ratio of Rayleigh number ($Ra$) to Hartmann number ($Ha$) increases. This transition is governed by the competition between buoyancy and Lorentz forces, with experimental data showing a linear scaling relationship between $Ra$ and $Ha$ at critical points. We develop a theoretical model that incorporates magnetic field effects in low-Prandtl-number convection to predict the reversal frequencies. These findings provide new insights into how magnetic fields can modulate flow regimes in low-Prandtl-number convection, establishing a controlled framework for investigating reversal dynamics in magnetohydrodynamic systems.

physics.flu-dyn↗

arXiv2025

Experimental investigation of a novel liquid metal plasma facing component with pre-filled microstructures

Regarding the plasma facing components (PFCs) in nuclear fusion, liquid metal PFCs with stable free surface flow on PFC surface are considered a promising alternative. However, due to the poor wettability of liquid metal on most solid substrates and the complex magnetohydrodynamic (MHD), the realization of stable free surface flow on PFCs surface is challenging. In the present study, using the 3D printed methods, we developed a novel liquid metal PFC surface with MIcrostructures pre-FIlled by Liquid Metal (MIFILM) to realize a stable free liquid metal surface flow. The experimental results demonstrated that due to the existence of MIFILM, the apparent contact angle (ACA) of liquid metal changes from 140$^{\circ}$ to approximately 20$^{\circ}$, indicating a transition from hydrophobic to hydrophilic. When the liquid metal flows on the MIFILM substrate, it is found that the liquid metal can completely spread on the surface with a stable and orderly free surface, even at a low flow rate. Moreover, the liquid metal could exhibit sustained spreading properties on the MIFILM substrate under a strong transverse magnetic field (up to 1.6 T). Results indicate that the magnetic field induces limited MHD drag but also accelerates the flow via two-dimensional effects. When the Stuart number $N<1$, the flow accelerates and the film thickness decreases. For $N>1$, both flow velocity and film thickness gradually stabilize. Therefore, the present novel MIFILM can offer a good choice for liquid metal PFC substrates in nuclear fusion.

physics.flu-dyn↗

arXiv2025

Theoretical analysis for non-linear effects of magnetic fields on unsteady boundary layer flows

This study investigates unsteady boundary layer phenomena in electrically conducting fluids subjected to static magnetic fields. Using a semi-explicit similarity transformation method, the momentum equation associated with the Stokes stream function is solved. The nonlinear closed analytical solutions for both stagnation flow and converging flow are derived. The results demonstrate that the boundary layer structure incorporates similar shock and solitary wave components which are promoted by Lorentz force. Under extreme magnetic fields, the flow exhibits sine and cosine wave patterns, which are motivated by the strong Lorentz force. An in-depth asymptotic analysis establishes the square root scaling laws that quantify the growth of friction and flux with increasing magnetic field strength. The boundary layer thickness scales inversely with the Hartmann number, a consequence of dominant Lorentz force, which differs from the conclusion of duct flow (Hunt 1965). These findings elucidate the physical mechanisms governing the nonlinear coupling between magnetic fields and the dynamics of the boundary layer.

physics.flu-dyn↗

arXiv2024

Three-dimensional simulation of film boiling on a horizontal surface with magnetic field

This study conducts a numerical investigation into the three-dimensional film boiling of liquid under the influence of external magnetic fields. The numerical method incorporates a sharp phase-change model based on the volume-of-fluid approach to track the liquid-vapor interface. Additionally, a consistent and conservative scheme is employed to calculate the induced current densities and electromagnetic forces. We investigate the magnetohydrodynamic effects on film boiling, particularly examining the pattern transition of the vapor bubble and the evolution of heat transfer characteristics, exposed to either a vertical or horizontal magnetic field. In single-mode scenarios, film boiling under a vertical magnetic field displays an isotropic flow structure, forming a columnar vapor jet at higher magnetic field intensities. In contrast, horizontal magnetic fields result in anisotropic flow, creating a two-dimensional vapor sheet as the magnetic strength increases. In multi-mode scenarios, the patterns observed in single-mode film boiling persist, with the interaction of vapor bubbles introducing additional complexity to the magnetohydrodynamic flow. More importantly, our comprehensive analysis reveals how and why distinct boiling effects are generated by various orientations of magnetic fields, which induce directional electromagnetic forces to suppress flow vortices within the cross-sectional plane.

physics.flu-dyn↗

arXiv2024

Development of the Multichannel Pulsed Ultrasonic Doppler Velocimeter for the measurement of liquid metal flow

In the present study, by adopting the advantage of ultrasonic techniques, we developed a Multichannel Pulsed Ultrasonic Doppler Velocimetry (MPUDV) to measure the 2D2C velocity fields of liquid metal flow. Due to the specially designed Ultrasonic host and post-processing scheme, the MPUDV system can reach a high spatiotemporal resolution of 50 Hz and 3 mm. The flow loop contains a cavity test section to ensure a classical recirculating flow was built to validate the accuracy of MPUDV in velocity field measurement. In the initial phase of the study, water with tracer particles was selected as the working liquid to ensure the velocity field measurements by the well-developed Particle Image Velocimetry (PIV). A comparison of the data obtained from the PIV and MPUDV methods revealed less than 3 differences in the 2D2C velocity field between the two techniques during simultaneous measurements of the same flow field. This finding strongly demonstrates the reliability of the MPUDV method developed in this paper. Moreover, the ternary alloy GaInSn was selected as the working liquid in the flow loop to validate the efficacy of the MPUDV in measuring 2D-2C velocity fields. A series of tests were conducted in the cavity at varying Reynolds numbers, ranging from 9103 to 24123. The measurements demonstrated that the MPUDV could accurately measure the flow structures characterized by a central primary circulation eddy and two secondary eddies in the opaque liquid metal. Furthermore, it was found that the vortex center of the primary circulating eddy and the size of the secondary eddies undergo significant alterations with varying Reynolds numbers, indicating the influence of inertial force on the flow characteristics in the recirculating flow. It is therefore demonstrated that the current MPUDV methodology is applicable for measuring a 2D2C velocity field in opaque liquid metal flows.

physics.flu-dyn↗

arXiv2023

Flow states and heat transport in liquid metal convection

We present an experimental study of Rayleigh-Bénard convection using liquid metal alloy gallium-indium-tin as the working fluid with a Prandtl number of $Pr=0.029$. The flow state and the heat transport were measured in a Rayleigh number range of $1.2\times10^{4} \le Ra \le 1.3\times10^{7}$. The temperature fluctuation at the cell centre is used as a proxy for the flow state. It is found that, as $Ra$ increases from the lower end of the parameter range, the flow evolves from a convection state to an oscillation state, a chaotic state, and finally a turbulent state for $Ra>10^5$. The study suggests that the large-scale circulation in the turbulent state is a residual of the cell structures near the onset of convection, which is in contrast with the case of $Pr\sim1$, where the cell structure is replaced by high-order flow modes transiently before the emergence of the large-scale circulation in the turbulent state. The evolution of the flow state is also reflected by the heat transport characterised by the Nusselt number $Nu$ and the probability density function (PDF) of the temperature fluctuation at the cell centre. It is found that the effective local heat transport scaling exponent $γ$, i.e., $Nu\sim Ra^γ$, changes continuously from $γ=0.49$ at $Ra\sim 10^4$ to $γ=0.25$ for $Ra>10^6$. Meanwhile, the PDF at the cell centre gradually evolves from a Gaussian-like shape before the transition to turbulence to an exponential-like shape in the turbulent state. For $Ra>10^6$, the flow shows self-similar behaviour, which is revealed by the universal shape of the PDF of the temperature fluctuation at the cell centre and a $Nu=0.19Ra^{0.25}$ scaling for the heat transport.

physics.flu-dyn↗

arXiv2022

Three-dimensional dynamics of a pair of deformable bubbles rising initially in line. Part 2: Highly inertial regimes

The buoyancy-driven dynamics of a pair of gas bubbles released in line is investigated numerically, focusing on highly inertial conditions under which isolated bubbles follow non-straight paths. In an early stage, the second bubble always drifts out of the wake of the leading one. Then, depending on the ratios of the buoyancy, viscous and capillary forces which define the Galilei ($Ga$) and Bond ($Bo$) numbers of the system, five distinct regimes specific to such conditions are identified, in which the two bubbles may rise independently or continue to interact and possibly collide in the end. In the former case, they usually perform large-amplitude planar zigzags within the same plane or within two distinct planes, depending on the oblateness of the leading bubble. However, for large enough $Ga$ and low enough $Bo$, they follow nearly vertical paths with small-amplitude erratic horizontal deviations. Increasing $Bo$ makes the wake-induced attraction toward the leading bubble stronger, forcing the two bubbles to realign vertically one or more times along their ascent. During such sequences, wake vortices may hit the trailing bubble, deflecting its path and, depending on the case, promoting or hindering further possibilities of interaction. In some regimes, varying the initial distance separating the two bubbles modifies their lateral separation beyond the initial stage. Similarly, minute initial angular deviations favour the selection of a single vertical plane of rise common to both bubbles. These changes may dramatically affect the fate of the tandem as, depending on $Bo$, they promote or prevent future vertical realignments.

physics.flu-dyn↗

arXiv2021

Three-dimensional dynamics of a pair of deformable bubbles rising initially in line. Part 1: Moderately inertial regimes

The buoyancy-driven motion of two identical gas bubbles released in line in a liquid at rest is examined with the help of highly resolved simulations, focusing on moderately inertial regimes in which the path of an isolated bubble is vertical. Assuming first an axisymmetric evolution, equilibrium configurations of the bubble pair are determined as a function of the buoyancy-to-viscous and buoyancy-to-capillary force ratios which define the Galilei ($Ga$) and Bond ($Bo$) numbers of the system, respectively. The three-dimensional solutions reveal that this axisymmetric equilibrium is actually never reached. Instead, provided $Bo$ stands below a critical $Ga$-dependent threshold determining the onset of coalescence, two markedly different evolutions are observed. At the lower end of the explored $(Ga,\,Bo)$-range, the tandem follows a Drafting-Kissing-Tumbling scenario which eventually yields a planar side-by-side motion. For larger $Ga$, the trailing bubble drifts laterally and gets out of the wake of the leading bubble, barely altering the path of the latter. In this second scenario, the late configuration is characterized by a significant inclination of the tandem ranging from $30^\circ$ to $40^\circ$ with respect to the vertical. Bubble deformation has a major influence on the evolution of the system. It controls the magnitude of vortical effects in the wake of the leading bubble, hence the strength of the attractive force acting on the trailing bubble. It also governs the strength and even the sign of the lateral force acting on this bubble, a mechanism of particular importance when the tandem is released with a small angular deviation.

physics.flu-dyn↗

arXiv2021

Direct numerical simulation of quasi-two-dimensional MHD turbulent shear flows

Direct numerical simulations (DNS) are performed to study the turbulent shear flow of an electrically conducting fluid in a cylindrical container. The flow is driven by the interaction between the radial electric currents ($I$) injected through a large number of small electrodes at the bottom wall and an axial magnetic field. All the numerical parameters, including the geometry of the container, the total injcected currents and the magnetic field, are in line with the experiment performed in J. Fluid Mech. 456, 137-159. First, witth laminar Hartmann layers, three dimensional simulations recover experimentally measured quantities (global angular momentum, velocity profiles). The variation laws for the wall shear stresses, the energy spectra and visualizations of flow structures near the side wall highlight separation and turbulence within the side wall layers. Furthermore, a parametric analysis of the flow reveals that Ekman recirculations have significant influence on the vortex size, the free shear layer, and the global dissipation. Second, we recover the scaling laws of the cutoff scale that separate the large quasi-two-dimensional scales from the small three-dimensional ones (J. Fluid Mech. 118, 507-518), and thus establish their validity in sheared MHD turbulence. Furthermore, we find that three-componentality are and the three-dimensionality appear concurrently and that both the two-dimensional cutoff frequency and the mean energy associated to the axial component of velocity scale with $N_t$, respectively as $0.063N_t^{0.37}$ and $0.126 N_t^{-0.92}$.

physics.flu-dyn↗