Fluid Flow Research Articles

Migration and deformation of a droplet enclosing an active particle

The encapsulation of active particles, such as bacteria or active colloids, inside a droplet gives rise to a non-trivial shape dynamics and droplet displacement. To understand this behaviour, we derive an asymptotic solution for the fluid flow about a deformable droplet containing an active particle, modelled as a Stokes-flow singularity, in the case of small shape distortions. We develop a general solution for any Stokes singularity and apply it to compute the flows and resulting droplet velocity due to common singularity representations of active particles, such as Stokeslets, rotlets and stresslets. The results show that offsetting of the active particle from the centre of the drop breaks symmetry and excites a large number of generally non-axisymmetric shape modes as well as particle and droplet motion. In the case of a swimming stresslet singularity, a run-and-tumble locomotion results in superdiffusive droplet displacement. The effect of interfacial properties is also investigated. Surfactants adsorbed at the droplet interface counteract the internal flow and arrest the droplet motion for all Stokes singularities except the Stokeslet. Our results highlight strategies to steer the flows of active particles and create autonomously navigating containers.

Mean square flow of fluid in the fractures during rock failures

Flows are vivid and diversiform physical phenomena that reflect the fluid motion in different conditions. Fluid flow in the fractures is the typically motion mode of the fluid in rocks. This study investigated, using enhanced X-ray imaging digital radiography (XIDR), a special motion mode of fluid flow through the fractures during rock failures. Our experiments visually elucidated the process of mean square flow (MSF) of the fluid in rock fractures and its response to external loading. The X-ray imaging fluid filled micro-cracks and fractures in rocks, as observed using the XIDR scanner. These flow processes are strongly connected with rock failures. The results show that fluid flow in the fracture obeys a mean square function with respect to rock failure degree. The MSF reflects the fluid flow motion in the fractures in response to multilevel loading, which can provide guidance for predicting rock mass stability and numerical simulation of geofluid in hydro-rock engineering.

Simulation and analysis of heat transfer in counter-flow helical double-pipe heat exchangers using CFD

Heat exchangers play a vital role in industrial applications by facilitating efficient thermal energy exchange between two distinct fluid streams. These fluids are separated by a solid barrier, which prevents mixing, conserves energy and reduces operational costs. To enhance the efficiency of heat exchangers, wire inserts are often incorporated into the fluid paths. These inserts increase turbulence, improve fluid mixing, and boost heat transfer rates, making the system more effective. In this study, a counter-flow helical double-pipe heat exchanger was analyzed using Computational Fluid Dynamics (CFD) through the simulation software Ansys CFX. The simulations were conducted for cold fluid temperatures ranging from 12°C to 22°C and hot fluid temperatures between 32°C and 52°C, with Reynolds numbers varying from [Formula: see text] to [Formula: see text] to capture a broad spectrum of flow behaviors. The CFD model demonstrated excellent agreement with experimental results, achieving high correlation coefficients of 0.96 for the hot fluid and 0.95 for the cold fluid. This high level of accuracy validates the robustness of the model in predicting real-world heat transfer dynamics. The inclusion of wire inserts within the cold fluid flow path played a critical role in enhancing heat exchanger performance. By inducing turbulence, the wire inserts disrupted the thermal boundary layer, allowing for greater heat transfer between the hot and cold streams. This resulted in a substantial improvement in heat transfer rates, reaching a 25% increase under optimized conditions. This also led to an improved temperature difference between the fluid at the inlet and outlet, optimizing the thermal performance of the exchanger. Alternative materials such as aluminum or titanium could also be considered for wire inserts, potentially improving thermal efficiency at varying costs.

Solute dispersion in an electroosmotic flow of Carreau and Newtonian fluids through a tube: analytical study

Solute dispersion in an electroosmotic flow of Carreau and Newtonian fluids through a tube: analytical study

Numerical study of particle suspensions in duct flow of elastoviscoplastic fluids

Numerical study of particle suspensions in duct flow of elastoviscoplastic fluids

Transient non-soluble noble metal transport in hydrothermal ore systems

The transport of noble metals (Au, Ag) by metal-rich melts in hydrothermal ore systems is now acknowledged as a complementary mechanism to complexing ligands in solution. However, it is unclear where/when both mechanisms coexist and whether metal-rich melts can be physically transported by hydrothermal fluids. Here we show evidence for a suspension-like transport of nano-to-micron-sized metal-rich sulfide-sulfosalt melts within epithermal fluids at <400 °C, forming irregular and bleb-like polymineral inclusions of Ag-Au-Cu-Pb(-Fe-Zn)-As-Sb-S-Se upon cooling. These polymineral inclusions, 5 nm to 40 µm in size, are cogenetic with fluid inclusions in quartz. Numerical modeling based on particle fluidization and settling theory shows hydrothermal fluids can mechanically transport metal-rich sulfide-sulfosalt nano-micromelts at fluid flow rates <10–1 m/s. The chemical similarity between nano- and micron-scale polymineral inclusions suggests the coalescence of nanomelt precursors during transient transport from their source(s) to deposition sites, playing a key role in noble metal mineralization.

Molecular dynamics analysis of fluid transport in nanochannels with non-uniform pore arrangements

ABSTRACT This study employs molecular dynamic simulations to investigate how non-uniform pore sizes affect fluid flow within nanochannels. In these simulations, the nanochannel walls are composed of copper atoms while the fluid consists of argon atoms. Three different non-uniform pore arrangements were examined: 30-40-50, 50-40-30 and 40-40-40. We evaluated parameters including fluid flow rate, pressure, density and velocity to understand the influence of pore distribution. Despite having the same average pore radius, different distribution patterns alter fluid flow properties by modifying velocity and density distributions at both the channel surface and centre. As the fluid passes through consecutive pores, atomic adsorption and compression reduce the flow rate. The simulations reveal that the 30-40-50 and 40-40-40 configurations incur a flow loss of approximately 26%, whereas the 50-40-30 arrangement exhibits a 39% loss. The relatively uniform 40-40-40 arrangement yields a more even pressure distribution and the highest flow rate. In contrast, a transition from wide to narrow pores (50-40-30) restricts the flow. Furthermore, calculations indicate that pore arrangement affects the number of atoms and their interaction energies near the channel walls, with the 50-40-30 configuration showing higher values than the other two setups.

Real-Time Photocatalytic Measurement of Dental Materials in an Open System.

It is common to encounter discrepancies between in vitro and in vivo studies, particularly when assessing the antibiofilm efficacy of dental materials. Typically, dental materials are tested in a closed system where fresh nutrients are not replenished, the test conditions are static, and the same planktonic bacteria persist. However, real environments are characterized by the continuous supply of fresh nutrients, dynamic saliva flow, and the periodic removal of planktonic bacteria through swallowing. To address these differences, we used an open system approach using microfluidic chips that simulate the nutrient and fluid flow conditions of the mouth. This setup enables the spatiotemporal development of biofilms, facilitates real-time observation, and provides deeper insights into the biofilm formation and removal processes. Photocatalytic dental materials are particularly suitable for use with microfluidic chips, as these devices allow real-time tracking of biofilm dynamics, both with and without light exposure. Nitrogen-doped titanium dioxide effectively produces reactive oxygen species (ROS) under visible light conditions, even when embedded in a resin matrix. These ROS have been shown to inhibit Enterococcus faecalis biofilms. The evaluation of the photocatalytic effects of dental materials using microfluidic chips showed that both new and established biofilms were disrupted by ROS production. ROS weakens the interface between the biofilm and dental material, allowing the biofilm mass to be removed by fluid flow. Furthermore, the open system provided by microfluidic chips demonstrated higher accuracy in evaluating antibiofilm efficiency than the conventional system did. Thus, the developed microfluidic chip is a novel and promising tool for assessing antibiofilm properties, with potential applications in various fields.

Development of hybrid robust model based on computational modeling and machine learning for analysis of drug sorption onto porous adsorbents

This study investigates the utilization of three regression models, i.e., Kernel Ridge Regression (KRR), nu-Support Vector Regression (-SVR), and Polynomial Regression (PR) for the purpose of forecasting the concentration (C) of a drug within a specified environment, relying on the coordinates (x and y). The analyses were carried out for separation of drug from a solution by adsorption process where the concentration of drug was obtained in the solution and the adsorbent via computational fluid dynamics (CFD), and the results of concentration distribution were used or machine learning modeling. The model considered mass transfer and fluid flow equations to determine concentration distribution of solute in the system. The hyperparameter optimization was carried out using the Fruit-Fly Optimization Algorithm (FFOA), a nature-inspired optimization technique. Our results demonstrate the performance of each model in terms of key regression metrics. KRR achieved an R2 score of 0.84851, with a Root Mean Square Error (RMSE) of 1.0384E-01 and a Mean Absolute Error (MAE) of 7.27762E-02. -SVR exhibited exceptional accuracy with an R2 of 0.98593, accompanied by an RMSE of 3.5616E-02 and an MAE of 1.36749E-02. PR, a traditional regression method, attained an R2 score of 0.94077, an RMSE of 7.2042E-02, and an MAE of 4.81533E-02.

Robust Slippery Liquid-Infused Porous Surfaces with Fast Self-Replenishment Properties for Anticontaminant, Anti-Icing, and Anticorrosion Applications on Magnesium Alloys.

Slippery liquid-infused porous surfaces (SLIPSs), a class of functional bioinspired surface, have earned a position at the forefront of many areas such as anticorrosion, anti-icing, antifogging, antibacterial, anticontaminant, microflow control, and drag reduction. However, the fast self-replenishing ability of SLIPS remains extremely challenging due to limited lubricant storage capacity or microstructure impeding fluid flow. Herein, a superhydrophobic surface was prepared by spraying the sepiolite/zeolitic imidazolate framework-8 hybrid material with a layered three-dimensional (3D) fibrous porous network structure on the magnesium (Mg) alloy surface. Subsequently, SLIPS with fast self-replenishing properties was obtained by injecting silicone oil on the superhydrophobic surface. The unique internal structure of SLIPS not only served as a container for storing the lubricant but also provided a channel for the flow of lubricants. In addition, the SLIPS possessed favorable self-replenishment, anticontaminant, and anti-icing properties. Moreover, compared with the superhydrophobic surface, SLIPS exhibited better corrosion resistance, which was attributed to the stable continuous lubricating oil replacing the air trapped in the rough structure and effectively blocking the invasion of corrosive ions. Significantly, the as-prepared SLIPS still maintained excellent corrosion stability on the Mg alloy after soaking in 3.5 wt % NaCl solution for 30 days. The research provides an avenue for designing green environmental protection, sustainable anticorrosion, and rapidly self-replenishing SLIPSs with great application potential.

Investigating the Effect of Relaxation Time on Richtmyer-Meshkov Instability under Reshock Impact: A Two-Component Discrete Boltzmann Method Study

Abstract The Richtmyer-Meshkov (RM) instability plays an important role in various natural and engineering fields, such as inertial confinement fusion. In this work, the effect of relaxation time on the RM instability under reshock impact is investigated by using a two-component discrete Boltzmann method. The hydrodynamic and thermodynamic characteristics of the fluid system are comprehensively analyzed from the perspectives of the density gradient, vorticity, kinetic energy, mixing degree, mixing width, and non-equilibrium intensity. Simulation results indicate that for larger relaxation time, the diffusion and dissipation are enhanced, the physical gradients decrease, and the growth of the interface is suppressed. Furthermore, the non-equilibrium manifestations show complex patterns, driven by the competitive physical mechanisms of the diffusion, dissipation, shock wave, rarefaction wave, transverse wave, and fluid instabilities. These findings provide valuable insights into the fundamental mechanism of compressible fluid flows.

Decoupling of global signal and cerebrospinal fluid inflow is associated with cognitive decline in patients with obstructive sleep apnoea.

Decoupling of global signal and cerebrospinal fluid inflow is associated with cognitive decline in patients with obstructive sleep apnoea.

Understanding a frequency shifting phenomenon interfering with in-line acoustic monitoring of an extrusion compounding process for polymer composites

This study investigated an anomalous frequency shift observed in collected spectra from an inline monitoring system based on guided ultrasonic waves, with the changing flow rate of an extrusion compounding process for fiber-reinforced thermoplastics. Three possible process parameters to explain the ultrasonic peak shifting, namely melt temperature, velocity, and fiber length were evaluated. The unlikely potential of a doppler moment due to melt velocity, was readily dismissed in the analysis since fluid flow through the die was too slow and while resonance frequency variation may be possible from the related fiber damage associated with increasing flow rates, there was insufficient physical evidence of this anticipated effect in this study. Melt temperature variation associated with viscous dissipation was concluded to be the dominant cause for the frequency shifting noted in the acoustic spectra. The changes in material temperature through which the sound travelled were varying the extent and frequency of the dispersion modes in the polymer melt. These findings are new guidance to processors on setting up a system using active ultrasonics for in-line monitoring in the polymer composites industry.

Initiation of transthyretin aggregation at neutral pH by fluid agitation

The transthyretin (TTR) tetramer, assembled as a dimer of dimers, transports thyroxine and retinol binding protein in blood plasma and cerebrospinal fluid. Aggregation of wild type (WT) or pathogenic variant TTR leads to transthyretin amyloidosis, which is associated with neurodegenerative and cardiac disease. The trigger for TTR aggregation under physiological conditions is unknown. The tetramer is extremely stable at neutral pH, but aggregation via tetramer dissociation and monomer misfolding can be induced in vitro by lowering the pH. To elucidate factors that may cause TTR aggregation at neutral pH, we examined the effect of shear forces such as those that arise from fluid flow in the vascular system. Fluid shear forces were generated by rapidly stirring TTR solutions in conical microcentrifuge tubes. Under agitation, TTR formed β-rich aggregates and fibrils at a rate that was dependent upon protein concentration. The lag time before the onset of agitation-induced aggregation increases as the total TTR concentration is increased, consistent with a mechanism in which the tetramer first dissociates to form monomer that either partially unfolds to enter the aggregation pathway or reassociates to form tetramer. NMR spectra recorded at various time points during the lag phase revealed growth of an aggregation-prone intermediate trapped as a dynamically perturbed tetramer. Enhanced conformational fluctuations in the weak dimer-dimer interface suggest loosening of critical intersubunit contacts which likely destabilizes the agitated tetramer and predisposes it toward dissociation. These studies provide insights into the mechanism of aggregation of WT human TTR under near-physiological conditions.

Vortex Dynamics in the Sinus of Valsalva

Patients undergoing aortic valve repair or replacement with associated alterations in stiffness characteristics often develop abnormalities in the aortic sinus vortex, which may impact aortic valve function. The correlation between altered aortic sinus vortex and aortic valve function remains poorly understood due to the complex fluid dynamics in the aortic valve and the challenges in simulating these conditions. The opening and closure mechanism of the aortic valve is studied using fluid–structure interaction (FSI) simulations, incorporating an idealized aortic valve model. The FSI approach models both the interaction between the fluid flow and the valve’s leaflets and the dynamic response of the leaflets during pulsatile flow conditions. Differences in the hemodynamic and vortex dynamic behaviors of aortic valve leaflets with varying stiffness are analyzed. The results reveal that, during the systolic phase, the formation of the sinus vortex is closely coupled with the jet emanating from the aortic valve and the fluttering motion of the leaflets. As leaflet stiffness increases, the peak vorticity of the sinus vortex increases, and the phase space of the vortex core develops a pronounced spiral trajectory. During the diffusion phase, the vortex strength decays exponentially, and the diffusion time is longer for stiffer leaflets, indicating a longer residence time of the sinus vortex that reduces the pressure difference on the leaflet during valve closure. Changes in leaflet stiffness play a critical role in the formation and development of sinus vortices. Furthermore, the dynamic characteristics of vortices directly affect the pressure balance on both sides of the valve leaflets. This pressure difference not only determines the opening and closing processes of the valve but also significantly influences the stability and efficiency of these actions.

Comparative assessment of macrophage responses and antileishmanial efficacy in dynamic vs. Static culture systems utilizing chitosan-based formulations.

The discovery of novel anti-leishmanial compounds is essential due to the limitations of current treatments and the lack of new drugs in development. In this study, we employed the Quasi Vivo 900 medium perfusion system (QV900, Kirkstall Ltd, UK) to simulate physiological fluid flow, allowing us to compare macrophage responses and therapeutic outcomes under dynamic versus static conditions. After 24 hours, phagocytosis and macropinocytosis decreased in all cell types under flow conditions compared to static cultures. Under slow (1.45 x 10-9 m/s) and faster (1.23 x 10-7 m/s) flow conditions ((simulating in vivo lymphatic flow), phagocytosis decreased by around 42.55% and 56.98% in peritoneal macrophages (PEMs), 42.21% and 56.11% in bone marrow-derived macrophages (BMMs), and 49.75% and 63.32% in THP-1 cells, respectively. Similarly, macropinocytosis decreased by approximately 40.7% and 62.2% in PEMs, 34.8% and 60.9% in BMMs, and 33.3% and 59.3% in THP-1 cell line under this same conditions. In this study, we further assessed the impact of medium perfusion on drug efficacy and macrophage functions using a Leishmania major amastigote-macrophage assay. We evaluated the performance of both standard and nanoparticle-based drug formulations within dynamic and static culture systems. After 72 hours of medium perfusion, chitosan solution, blank chitosan-sodium tripolyphosphate (TPP) nanoparticles, and amphotericin B (AmB)-loaded chitosan-TPP nanoparticles exhibited a statistically significant reduction in antileishmanial activity by approximately 30-50% under slow flow conditions and 60-80% under faster flow conditions. In comparison, pure AmB showed a 40% decrease in efficacy at slow flow and a 67% decrease at faster flow, both statistically significant. These results highlighted the importance of considering fluid flow dynamics in in vitro studies for a more accurate simulation of in vivo conditions, potentially leading to better therapeutic strategies for cutaneous leishmaniasis (CL).

Modeling Lithium Adsorption in Column Chromatography Applications with Granulated Li/Al-LDH: A Lattice Boltzmann Modeling Approach

Abstract Given a crucial role of lithium (Li) on clean energy transition through effective decarbonization of various energy sectors, enhancing and diversifying the source of Li is regarded as an urgent priority. Producing Li from formation brines is a promising solution due to their abundant resources and environmental friendlessness to extract. In this study, we focus on Li extraction with ion sieve method utilizing Li/Aluminum-Layered Double Hydroxide Chlorides (Li/Al-LDH), by its significant stability, great scalability, and favorable techno-economic feasibility. In this regard, we set our goal to numerically quantify the adsorption performance of granulated Li/Al-LDH adsorbent for Li+ by quantitatively analyzing the impacts of controlling factors. To achieve the goal, we develop our numerical capability of addressing brine injection, fluid flow, component transport, and adsorption in column chromatography application, based on Lattice Boltzmann Method (LBM) modeling. To quantify the impact of operational conditions of Li+ adsorption performance with granulated Li/Al-LDH adsorbent, various values of porosity and radius of granule, Li+ concentration in injected brine, and brine injection velocity are considered. From the numerical simulations and coupled local sensitivity analysis, radius of adsorbent granule is found to be most influential on the adsorption performance, followed by granule porosity, concentration of Li+ in injected brine, and injection velocity. This study provides the conceptual and essential information of quantified impact of various operational conditions on Li+ adsorption performance that can be used to optimize the design of Li/Al-LDH adsorbent granule and column chromatography strategy, as achieving the techno-economically feasible Li+ extraction from formation brines.

Characterization of Nonequilibrium Interactions of Catalytic Microswimmers Using Phoretically Responsive Nanotracers.

Catalytic microswimmers convert the chemical energy from fuel into motion. They sustain chemical gradients and fluid flows that propel them by phoresis. This leads to unconventional behavior and collective dynamics, such as self-organization into complex structures. Characterizing the nonequilibrium interactions of microswimmers is crucial for advancing our understanding of active systems. However, this remains a challenge owing to the importance of fluctuations at the microscale and the difficulty in disentangling the different contributions to the interactions. Here, we show a massive dependence of the nonequilibrium interactions on the shape of catalytic microswimmers. We perform tracking experiments at high throughput to map interactions between nanocolloidal tracers and dimeric microswimmers of various aspect ratios. Our method leverages dual tracers with differing phoretic mobilities to quantitatively disentangle phoretic motion from hydrodynamic advection. This approach is validated through experiments on single chemically active sites and on immobilized catalytic microswimmers. We further investigate the activity-driven interactions of free microswimmers and directly measure their phoretic interactions. When compared to standard models, our findings highlight the important role of osmotic flows for microswimmers near surfaces and reveal an enhanced contribution of hydrodynamic advection relative to phoretic motion as the size of the microswimmer increases. Our study provides robust measurements of the nonequilibrium interactions from catalytic microswimmers and lays the groundwork for a realistic description of active systems.

Fluid-structure interaction aspects of the shape-morphing structures and robots

Abstract Advancement in intelligent materials for the past few decades enabled the development of functional morphing structures and robots operating in fluid environments. Fluid-structure interaction problems for functional morphing structures and robots were naturally accentuated. In this paper, the recent advancements in shape-morphing robots and structures in fluid flow across different Reynolds number scales are reviewed and summarized, from microrobots with Reynolds number of much lower than 1 to deformable aircrafts in turbulent flows. With the quest to improve the design and functionality of the morphing structures and robots, we discuss modeling methods, experiments, and materials for the morphing structures over a vast range of Reynolds number. The importance of understanding fluid-structure interaction to design morphing structures and robots are emphasized. Following up with several critical future questions to address, the potential applications of artificial intelligence and machine learning (AI/ML) techniques in improving the design of shape-morphing structures and robots are discussed. These shape-morphing structures are expected to have a significant impact in enhancing sustainable solutions for today's challenges, and exploring unknown of deeper oceans and outer space.

Revealing the sound, flow excitation, and collision dynamics of human handclaps

Hand clapping, a ubiquitous human behavior, serves diverse daily-life purposes. Despite prior research, a comprehensive understanding of its physical mechanisms remains elusive. To bridge this gap, we integrate human data, parametric experiments, finite-element simulations, and theoretical frameworks to investigate the acoustic properties of clapping sound and their connections with the fluid flow and soft matter collision. Motion-audio synchronization reveals the flow-excitation nature of the hand cavity resonance. The classical Helmholtz resonator model, incorporating occasional pipe standing wave contributions for finger grooves, reliably predicts clapping sound frequencies across various real and engineered hand configurations. Material elasticity, coupled with the dynamic collision process, has minor effects on the sound frequency but a major impact on the temporal evolution of the sound signals, as reflected by the quality factors of resonance. Both spatial and dynamic factors for sound intensity are examined. We establish a quadratic scaling relationship between hand cavity gauge pressure and clapping speed, elucidating the positive correlation between faster claps and louder sounds. Our work advances the knowledge of hand-clapping acoustics and offers insights into sound signal synthesis, processing, and recognition. Furthermore, these findings may facilitate low-cost acoustical diagnostics in architecture and enhance rhythmic sound patterns in music and language education. Published by the American Physical Society 2025