Wenming Yang

Ferrofluid sealing in liquid environments involves strongly coupled magnetic, interfacial, and hydrodynamic effects that are difficult to resolve using conventional two-phase models. In this study, a ternary phase-field ferrohydrodynamic framework is developed to simulate the interfacial evolution and failure of an air-ferrofluid-water sealing system. The framework couples magnetic-field-induced stresses, ternary phase-field dynamics, interfacial tension, wetting, shaft rotation, and pressure-driven flow within a unified computational model. Its numerical reliability is systematically examined through mesh independence, time step sensitivity, and phase-field interface-thickness sensitivity. The model is further validated against three benchmark problems involving magnetic deformation of a ferrofluid droplet, a three-phase liquid lens, and the critical sealing pressure of a conventional ferrofluid gas seal, showing good agreement with analytical, experimental, and published numerical results. The validated framework is then applied to investigate steady-state interface morphology, leakage-channel formation, and critical sealing pressure in liquid environments. The results demonstrate that liquid-environment sealing exhibits a distinct failure pathway from gas sealing, characterized by progressive interface deformation, liquid penetration, necking, and continuous leakage-channel formation. Increasing magnetic field intensity enhances sealing stability, whereas shaft rotation promotes interface instability. A dimensionless failure regime map based on the magnetic Bond number, Weber number, and critical pressure Bond number is established to distinguish inertia-dominated, transitional, and magnetically stabilized regimes. The proposed framework provides a computational basis for predicting and optimizing multiphase ferrofluid sealing systems.

This study investigates magnetization relaxation effects in ferrofluids using a coupled molecular dynamics and Lattice Boltzmann simulation framework. The model incorporates dipole-dipole and short-range (Lennard-Jones) interactions between particles, couples hydrodynamic interactions, and employs a bidisperse model for particle distribution. The magnetization component along the magnetic field direction, Mx*, increases and saturates with the Langevin parameter αL. However, larger shear rates γ* or pressure differences P* (beyond a threshold) slow this saturation due to the greater difficulty larger particles face in aligning with the magnetic field. The off-field magnetization component My*, arising from relaxation, exhibits a non-monotonic dependence on αL, first increasing and then decreasing under shear and pressure difference. This behavior is attributed to the evolving chain-like microstructures within the ferrofluid at different field intensities. Higher particle concentrations φm or dipole coupling parameters λ lead to larger maximum values of My*. The competition between the magnetic field and shear flow, along with shear-rate-dependent chain size distributions, leads to a larger γ* requires a stronger magnetic field for My* to peak, and the peak value itself increases with γ*. The pressure difference P* influences magnetization by modifying the local shear rate distribution, with its hydrodynamic effects becoming significant only after P* exceeds a critical value.

The maintenance of railway rails relies heavily on accurate profiling and wear assessment. In this research, a rail profile detection system using line-structured light machine vision technology is developed. Traditional image processing algorithms for rail profile measurement involve Zhang’s camera calibration, radial distortion correction, Gaussian filtering, and the iterative closest point (ICP) algorithm for point cloud registration. Building upon these conventional algorithms, an integrated error correction framework comprising projective transformation and system offset compensation is proposed. We introduce a method to dynamically determine the direction vector of the rail alignment and the angle between the laser plane and the rail cross-section for the projective transformation. Compared to the same hardware system without error correction, this method improves measurement accuracy from 0.0686 to ±0.015mm at the lateral wear measurement points and from 0.0678 to ±0.020mm at the vertical wear measurement points in profile detection.

The idea of ferrofluid pumping in pipes is extended to scenarios where a uniform magnetic field gradient is employed, yet without a pressure difference between the inlet and outlet of the pipe. The governing equations, including the phenomenological magnetization equation for ferrofluid pipe flow, are solved by a custom-developed OpenFOAM solver. After the validation of this solver, ferrofluid pipe flows under the application of a magnetic field gradient are numerically predicted. The findings reveal that both pumping volume and pressure distribution can be adjusted by varying the reference magnetic field intensity and field gradient. A stronger reference magnetic field and a steeper field gradient result in higher flow rates and accelerated pressure increases along the field gradient direction. In a circular tube with a radius of 1 mm, when the dimensionless magnetic field gradient is 0.1 and the magnetic Reynolds number is 1000, the maximum velocity can attain 10.2 μm/s, the flow rate can reach 0.016 μL/s, and the equivalent average pressure gradient achieves 0.15 Pa/s. Notably, in a gradient magnetic field, the effective viscosity of a ferrofluid flowing in a pipe can be significantly reduced, achieving approximately 70% of its intrinsic viscosity in this study. These promising results lay the groundwork for the design of ferrohydrodynamic pumps that harness the potential of constant magnetic fields.

Ferrofluids, a synthetic intelligent liquid material, exhibit fascinating flow phenomena when subjected to magnetic fields. This book systematically examines the fundamentals, applications, mathematical derivations, and properties of non-equilibrium ferrohydrodynamics and explores the magnetization and viscosity properties of ferrofluids. The book begins with an introduction to the basic concepts and applications of non-equilibrium ferrohydrodynamics and transport theory. It then goes on to discuss the equations of non-equilibrium and quasi-equilibrium ferrohydrodynamics and the magnetic levitation force in ferrofluids. The author discusses two crucial properties of ferrofluids: magnetization properties and viscosity properties, presenting the ferrofluid magnetization equations and the magnetoviscous effect, an important physical manifestation of non-equilibrium theory. Different types of flows of ferrofluids under the application of different magnetic field configurations are also elucidated, illustrating specific manifestations of non-equilibrium ferrohydrodynamics in different flow patterns. This book will be a valuable reference for researchers and students of mechanical engineering with an interest in fluid dynamics, computational mechanics, and electromagnetism. Professionals working in areas of ferrofluid applications such as mechanics, petrochemical engineering, environmental sciences, and biomedicine will also benefit.

This chapter delves into the magnetic properties of ferrofluids, encompassing the equation of equilibrium magnetization, the magnetization relaxation, and the equations of non-equilibrium magnetization. Expressions for the initial susceptibility under limiting conditions of weak and strong magnetic fields during equilibrium magnetization are presented. Two mechanisms of magnetization relaxation, Brownian and Néel relaxation mechanisms, and their corresponding representations of relaxation time are introduced. A comprehensive discussion is offered on various mathematical forms of phenomenological magnetization equation I, microscopic magnetization equation, and phenomenological magnetization equation II, along with their respective scopes of application.