Shear Stress Research Articles

Blood vessel wall shear stress determines regions of liposome accumulation in angiogenic vasculature.

Nanoparticles used for drug delivery often require intravenous administration exposing them to fluid forces within the vasculature, yet the impact of blood flow on nanoparticle delivery remains incompletely understood. Here, we utilized transgenic zebrafish embryos to investigate the relationship between the accumulation of fluorescently labeled PEGylated liposomes and various hemodynamic factors (such as flow velocity, wall shear stress (WSS), and flow pattern) across a wide range of angiogenic blood vessels. We reconstructed 3D models of vascular structures from confocal images and used computational fluid dynamics to calculate local WSS, velocities, and define flow patterns. The spatial distribution of fluorescently labeled liposomes was subsequently mapped within the same 3D space and correlated with local hemodynamic parameters. Through the integration of computational fluid dynamics and in vivo experimentation, we show that liposomes accumulated in vessel regions with WSS between 0.1-0.8 Pa, displaying an inverse linear correlation (R2 > 0.85) between time-averaged wall shear stress and liposome localization in vivo. Interestingly, flow pattern did not appear to impact liposome accumulation. Collectively, our findings suggest the potential of stealth liposomes for passive targeting of low-flow vasculature, including capillaries and intricate angiogenic vasculature resembling that of tumor vessel networks.

A machine learning-based method for predicting the shear behaviors of rock joints

In this study, machine learning prediction models (MLPMs), including artificial neural network (ANN), support vector regression (SVR), K-nearest neighbors (KNN), and random forest (RF) algorithms, were developed to predict the peak shear stress values and shear stress-displacement curves of rock joints. The database used contained 693 records of peak shear stress and 162 original shear stress-displacement curves derived from direct shear tests. The results demonstrated that the MLPMs provided reliable predictions for shear stress, with the mean squared errors (MSEs) between their predicted and measured shear stress varying from 0.003 to 0.069 and the coefficients of determination (R2 values) varying from 0.964 to 0.998. The feature importance values indicate that the joint surface roughness coefficient (JRC) is the most important influential factor in determining the peak shear stress, followed by the joint wall compressive strength (JCS), basic friction angle (φb), and shear surface area (As). Similarly, for the shear stress-displacement curve, the JRC is the dominant factor, followed by As, φb, and JCS. Additional direct shear tests were conducted for model validation. The validation shows that the MLPM predictions demonstrate improved consistency with the experimental results in relation to both the peak shear stress and peak shear displacement.

Analysis of normal and shear stresses and vertical displacements in cross-laminated timber panels through plate theories, numerical and simplified methods

Resumo Amadeira lamelada colada cruzada (MLCC) é um produto empregado na forma de painéis que apresenta como desafio a consideração conjunta da disposição ortogonal de lamelas com o comportamento ortotrópico da madeira. Para avaliar o comportamento mecânico de painéis de MLCC, em flexão simples, é possível recorrer a teorias de placas, métodos numéricos e simplificados. Dentro desse contexto, este trabalho se propôs a analisar as tensões, normais e de cisalhamento, bem como os deslocamentos verticais em um dado painel de MLCC por meio de três teorias de placas, três métodos simplificados, método dos elementos finitos e um método analítico proposto. Para o estudo, foi considerado um painel de MLCC com cinco camadas de 20 mm de espessura, inicialmente quadrado com lados de 3 m, sendo, posteriormente, realizadas análises paramétricas da razão entre as dimensões em planta. Com as análises estruturais, foi possível a obtenção das tensões, bem como os deslocamentos verticais do referido painel. As análises paramétricas realizadas permitiram averiguar a sensibilidade do painel de MLCC proposto, em termos de tensões e deslocamentos verticais, frente às variações da razão entre as dimensões em planta do painel.

Structure and characterization of an extracellular polysaccharide from Paenibacillus polymyxa 88A.

Levan-type polysaccharides, produced by various organisms, are nontoxic, biocompatible, and biodegradable polymers with a wide range of biological activities. They have high potential for use in medicine, cosmetology, and industry. A large amount of levan (41.1 g L-1) was recovered by ethanol precipitation from a liquid nutrient medium of Paenibacillus polymyxa 88A that contained 15 % w/v sucrose as a carbon source. The levan was fractionated by gel-permeation and anion-exchange chromatography and was analyzed by DRIFT and NMR spectroscopy. It was found that levan was represented by slightly branched chains composed of β-(2→6)-Fruf residues. The average molecular mass of the levan was about 1.9 MDa. When shear stress was applied at different temperatures, aqueous levan solutions showed pseudoplastic behavior. As found by SEM, a freeze-dried powdered levan sample had a microporous structure. The levan had excellent emulsifying activity toward sunflower oil, forming an emulsion with long-term stability. Analysis of the antioxidant activity of the levan showed a higher, dose-dependent activity toward ABTS, as compared with that toward DPPH. Finally, the bioactivity of the levan was examined by MTT assay by using human cervical carcinoma (HeLa) cells.

Dynamic Fractal Ice Nano-Nucleators for Cancer Cryotherapy

We present cryo-responsive dynamic fractal ice nano-nucleators (DF-INNs) for cryo-cancer therapy applications. Our development of DF-INNs leverages their structural advantages, which inherently maximizes the number of active sites for heterogeneous ice nucleation. Owing to their radially attached nanocrystalline structure, DF-INNs expose an extensive array of grain boundaries. The rapid precipitation and subsequent radial attachments of nanocrystallites promote the exposure of facets with high Miller indices, intrinsically strained, five-fold twinned nanocrystals, and increasing point defects due to kinetically-limited precipitation. This unique fractal structure culminates in the elevating of freezing temperature compared to Euclidean-shaped ice nucleators. Additionally, the branched fractal structure of DF-INNs facilitates heterogenic ice formation within its nanoconfined region, leading to the production of numerous self-similar small fractal fragments. This fragmentation is primarily driven by nanoconfinement-induced delayed ice nucleation, similar to frost heaving. The shear stress can be easily relieved through grain boundary sliding within the radially stacked DF-INN, making itself prone to cryo-responsive fragmentation. Such dynamic attributes significantly enhance ice nucleating activity, presenting a powerful strategy to increase the efficacy of cryotherapy by enhancing cellular ice formation and in vivo tumor coverage.

Evolution of mechanical behavior in granular soil during fine particle loss simulated by salt dissolution: Insights from ring shear tests

Fine particle loss in soil is one of the main causes of slope instability and geotechnical structure failure. Loss of fines can cause instability in granular assembles by changing the fabric and microstructure of the sample. However, real-time monitoring of the evolution of mechanical behavior in granular soils during the particle loss process is still poorly explored. This study presents a novel approach by simulating fine particle loss through salt dissolution in ring-shear tests, offering real-time insights into the mechanical evolution of granular soils under realistic stress conditions. Meanwhile, the shear resistance, shear displacement, vertical displacement, salt content, and acoustic emissions were simultaneously recorded. The test results showed that the instability of the sample was triggered by the loss of fine particles. With a gradual loss of fine particles, both the vertical and shear deformations and the void ratio increased. The evolution of shear resistance in the sample can be divided into three stages: stress weakening, then strengthening, and finally recovery to the initial value. We infer that the evolution of shear resistance originated from the collapse and rearrangement of its granular fabric and microstructure. Additional evidence for this hypothesis was provided by high-frequency acoustic emissions (approximately 150 kHz), suggesting buckling of the force chains accompanying the particle loss process. Furthermore, the sample experienced greater shear deformations and stress weakening that developed under a larger initial fine content or a higher normal stress. This finding may provide valuable insights into the mechanical behavior of granular soil during the fine particle loss process.

Anatomical Analysis of Vertebral Arteries in Vertebrobasilar Dolichoectasia: a Multi-center Study

BackgroundVertebrobasilar dolichoectasia (VBD) is a rare disease with significant morbidity. Its propensity for posterior circulation and relationship with aneurysms is poorly understood. Here, we aimed to describe the anatomical characteristics of the vertebral arteries (VA) in patients with VBD. MethodsWe conducted a multi-center retrospective cohort analysis on patients diagnosed with VBD between January 2009 and December 2022. Anatomical variables were measured after manual segmentation on CTA. Length of the VAs, basilar artery (BA), sectional diameters, and tortuosity index were measured and compared with a control group. Results124 patients were included: 38 patients with VBD and 86 controls. VBD patients had longer VAs (right VA 262.5 vs. 228.7mm, p<0.01; left VA 253.4 vs. 222.1mm, p<0.01), longer BAs (41.9 vs. 30.0mm, p<0.01), and larger cross-sectional diameters of right V3 (5.1 vs. 4.6mm, p<0.05) and V4 segments (4.5 vs.2.9mm, p<0.05) and left V1-V4 segments (V1 6.1 vs. 4.2, V2 7.2 vs.4.8mm, V3 6.4 vs.5.0mm, V4 6.5 vs. 3.0mm, p<0.01). Patients with VBD and fusiform aneurysms had longer VAs and BA than patients without aneurysms (p<0.01). The VAs tortuosity index was higher in VBD than controls (60.69 vs. 44.18, p<0.01). No cases of vertebral stenosis were detected. ConclusionsOur study offers an anatomical study of VBD. While the natural history is poorly understood, the higher tortuosity index with associated wall shear stress, provides a possible mechanism for disease progression. Further studies are needed to better understand the

Research on heat transfer characteristics and prediction methods of supercritical liquified natural gas flowing horizontally under the influence of buoyancy

An exhaustive elucidation of the heat transfer performances and mechanisms of supercritical LNG under water-bath heating conditions is paramount for optimizing heat transfer and designing related heat exchangers. This study employs experiments and numerical simulations to explore the heat transfer mechanisms and prediction methods of supercritical LNG flowing horizontally under buoyancy. The experimental investigation was executed within a small SCV using the N2-Ar mixture as a substitute medium for safety considerations. Concurrently, the accuracy of the numerical model employed is rigorously authenticated through a comparative analysis with the empirical findings obtained from the experimental endeavor. The investigation delves into the impacts of operating conditions, structural parameters, and medium composition on heat transfer through numerical simulations. The influence mechanism of buoyancy on heat transfer is meticulously explicated by examining the distribution of flow fields and thermophysical properties within the boundary layer. The introduction of a novel criterion, Bufull, derived from the analysis of wall shear stress variation caused by buoyancy, facilitates a quantitative assessment of the impact of buoyancy on heat transfer. Additionally, a simplified buoyancy criterion, FBu, is introduced, dependent solely on fundamental operating parameters and tailored for engineering applications, with a determined critical value of 1/ FBu = 60.3. Based on the dramatically variable thermophysical and transport properties, that occurs near the critical point, as well as buoyancy correction, a correlation is formulated for predicting mixed convection heat transfer, exhibiting an MAE of merely 3.04% and accurately predicting 81.28% of the data within a ± 5% margin of error, surpassing existing correlations. Finally, the proposed heat transfer calculation method is validated using five groups of operational data from two in-service SCVs. The maximum absolute discrepancy between the computed outlet temperature and the monitored value is 1.95K. The findings of this paper can support practical applications.

Investigating the Impact of Surface Textured Solar Photovoltaic Dummies for Soiling Mitigation: A Computational Fluid Dynamics Study

Soiling of solar collectors is a significant factor causing energy losses in solar photovoltaic power plants. The present study explores the concept of passive soiling mitigation by utilizing surface-textured solar photovoltaic dummies to minimize soiling on solar photovoltaic panels. Notably, this approach has not been extensively explored in the literature, despite its promising potential for enhancing photovoltaic system performance. Employing Computational Fluid Dynamics methodology, the dust-capturing capabilities of these surface textured photovoltaic dummies were explored. The investigation adopted the Discreet Phase Modelling approach coupled with the shear stress transport k-ω model for turbulence modelling. The study systematically analysed the optimal positioning of the dummy within distances between panel and dummy ranging from 2 m to 3 m and varied heights and tilts of installation. The wind analysis of the study area revealed an average wind speed of 3.31 m/s, with stronger winds primarily from the East of South-East to North-East. Experimental designs, incorporating dummy distance, height, and tilt angle, demonstrated that the dummy accumulated 58 % more dust compared to the photovoltaic panel due to micro-turbulences induced by its textured surface. The dummy tilt angle was identified as a critical factor influencing dust deposition, with higher angles increasing turbulent kinetic energy and enhancing dust capture. Additionally, the installation height and distance between the dummy and panel significantly impacted soiling, with closer distances offering better protection. Surface impressions on the dummy further increased dust deposition by 23 % through enhanced friction and reduced airflow. This study highlights the promising potential of surface-textured photovoltaic dummies in mitigating soiling, a concept not widely explored in existing literature. The study advocates for the widespread adoption of these photovoltaic dummies, particularly in environments prone to frequent soiling and where the installation configuration does not allow for easy application of other methods of soiling mitigation.

Effects of cavitation number and pitching frequency on hysteresis characteristics of a pitching hydrofoil

In this study, the effect of cavitation number σ and pitching frequency f* on hysteresis characteristics of a pitching hydrofoil, has been investigated. The angle of attack α changed from 5° to 15°, then from 15° to 5° at each pitching period. A pitching 2D(Two-dimensional) Clark-y hydrofoil is analyzed with high-speed video recording for the cavitation pattern. The hysteresis phenomenon is observed in increasing and decreasing of α, and the hysteresis effects changed with σ and f*. The cavitating flow around the pitching hydrofoil during different conditions are simulated by revised k-ω SST(Shear Stress Transfer) turbulence model and Merkle cavitation model. The calculation results of hysteresis characteristic parameters showed that the decrease of σ leads to the increase of hysteresis effect. As σ decreased, the evolution of cavity vortex is more complex. Especially in the downstroke, there are complex phenomena such as vortex decomposition and merging, which leads to the intensification of hydrodynamic hysteresis effect. There is a nonlinear relationship between the hysteresis characteristic parameters and f*. As f* increases, its influence on the cavitation flow in the downstroke becomes more and more serious. Besides, the evolution form of cavitation vortices also changes, which leads to the change of hysteresis characteristics.

Effect of Asphaltenes on Thermally- and Shear-driven Regimes of Wax Deposition

Waxes and asphaltenes co-exist in crude oils, yet few studies consider the effects of one component on another. The influence of asphaltenes on wax deposition and its properties is studied using a bespoke cold rotating finger which allows measurements to be made in both static and dynamic (sheared) conditions. For static deposition, adding asphaltenes reduces the rate at which the wax deposit is formed, as well as the overall thickness of the deposit. This change is attributed to changes in the fluid properties, with asphaltenes lowering the wax appearance temperature, meaning a greater degree of subcooling is needed to achieve the same amount of precipitated wax. For dynamic measurements, both the rate of deposition and deposit thickness are unaffected by asphaltenes, demonstrating that hydrodynamics strongly govern behavior. However, the average molecular weight and the wax content of the deposit increases significantly with asphaltenes present. With the deposit thickness being similar with and without asphaltenes, it suggests that the yield strengths of the two deposits are also similar, since thickness in the hydrodynamic regime is affected by the fluid shear stress. The changes in composition and content of the wax deposit will increase yield strength, yet this is balanced by a reduced wax gel yield strength in the presence of asphaltenes. These two factors effectively cancel one another to result in the same deposit thickness when sheared. The study provides new insights into the mechanisms by which asphaltenes modify wax deposition and deposit properties.

Multiscale formation mechanism and instability characteristic of the submerged vortices caused by a closed pump intake

The intake system serves as a crucial inlet structure for tidal pumping stations along the coastal regions. However, the various types of submerged vortices caused by the closed intake disrupt the energy-conversion stability, limiting the efficient and safe operation of the units. Especially when the multiple vortices are mixed enter the pump, the pump's energy consumption increases, and the hydraulic stability of the intake further decreases. This study used the high-precision simulation method to elucidate that the anisotropic turbulent structure around the roof-attached vortex (RAV) and the Reynolds shear stress are the critical factors for initiating small-scale RAV. The vertical and its enstrophy transport equation were used to explain that the vortex stretching term is the primary source of inducing the vertical vorticity evolution of RAV and floor-attached vortex (FAV). The tilting term provides a power source for vortex wandering. Based on the high-speed visualization experiment, the instability characteristics analysis of the vortices showed that the root of RAV dissipation was the existence of a critical stagnation point, SRAV, and the dissipation form exhibited “bubble-like fragmentation.” The unstable dissipation of the FAV does not have a vital stagnation point, and the dissipation form manifests as “vortex filament dissipation.” The results have important academic value and theoretical guidance significance for ensuring the safe and efficient operation of the tidal units.

A high-resolution 3D sediment-dynamics model of a morphologically complex mesotidal estuarine river system (New Zealand): insights from model calibration and validation

Land use change and the associated change in sediment runoff to the receiving environment is a topic of concern for estuarine and coastal governing agencies around the world. Fine sediments can enter the receiving environment and have a plethora of potential negative impacts (e.g., ecological, and recreational) and present no real opportunities or positive impacts. In the present study, a Delft3D numerical model is presented. The model has been calibrated considering water levels, currents, temperature, salinity, and Suspended Sediment Concentrations (SSC). Delft3D-Wave was also coupled to the hydrodynamic model to investigate the role of wave related fine sediment resuspension due to wave bed shear stresses. The extent of this model covered the Wairoa River and estuary, just south of Auckland, New Zealand. The motivation for this study was to produce a method that could link the upstream SSC contribution to the subsequent distribution in the receiving environment. From there it can readily be used as input to a regional scale model to investigate and understand the fate of sediments in the Hauraki Strait. As part of an ongoing research programme, the present study formed part of an extensive measurement campaign. Measurement instruments and results are presented alongside numerical model sensitivity analyses. The model performance is discussed, and the physical dynamics are described via extensive tables and summarising plots.

Investigation of the sintering behavior of nanoparticulate SiC by molecular dynamics simulation

Sintering is a valuable processing technique utilized to create bulk materials from powder compacts. In recent years, sintering has a garnered significant attention in the research community due to its versatility and applicability in various fields, including additive manufacturing. Herein, we investigate the sintering kinetics and mechanism of SiC nanoparticulate using molecular dynamics simulations. The surface morphology, microstructure, radial distribution function, mean square displacement, atomic displacement distribution, structural transformation, and diffusion coefficient are analyzed in detail. The free surface surrounding the neck undergoes a reconstruction to eventually reach an equilibrium dihedral angle. The neck rapidly grows and aligns with the surface, causing the SiC particle to transform from an original spherical shape into a twists shape. The concentration of shear stress results in a change of the local lattice structure from the face-centered cubic phase to amorphous phase in the surface. With an increasing sintering time, the dominant diffusion mechanism gradually shifts from surface diffusion to interface diffusion. This trend leads to a rise in the atomic migration, the consumption of interface energy, and the degree of particle fusion. Ultimately, the sintered SiC particles have the spherical shape, low surface energy, and stable structure. Additionally, the high mobility of melted atom causes a gradual dissolution of SiC particle into the large particle from the surface towards the interior. This process leads to high-altitude concentration structures that facilitate atomic migration. These results shed light on an atomic sintering mechanism in SiC nanoparticulate for applications in the preparation of nanostructured materials, and additive manufacturing.

The impact of a storm on the microtidal flat in the Yellow River Delta

Strong hydrodynamic forces generated by storms are key in shaping coastal tidal flats. Most tidal flats achieve equilibrium by adapting to hydrodynamic conditions and sediment inputs. However, high-energy wave activity during storms disrupts this equilibrium, causing rapid and significant changes, particularly in tidal flats, especially in microtidal flats, which are characterized by low tidal ranges. In this study, we conducted an 11-d field campaign on a microtidal flat in the Yellow River Delta (YRD), capturing data during both stormy and calm weather conditions. We measured tidal currents, wave activity, suspended sediment concentrations and sediment grain sizes. The results demonstrated that the tidal flat maintained equilibrium under calm conditions, with minimal fluctuations in bed level (within ±2 mm). Contrastingly, severe erosion and sediment removal during the storm significantly altered the equilibrium of the area. The storm-induced high shear stresses, ranging from 1.02 to 1.48 , along with alongshore sediment transport, resulted in an elevation change of -10 mm. Furthermore, the subsequent bed level recovery was minimal and insufficient to offset the erosion. Compared to that of the mesotidal and macrotidal flats, post-storm recovery on microtidal flats was limited due to shorter inundation periods and weaker hydrodynamic forces. Therefore, frequent storms may lead to continuous shoreline retreat on microtidal coasts. Conclusively, the present findings underscore the significant impact of storm-induced erosion on the evolutionary processes of microtidal flats and suggest that greater attention should be given to protecting these areas during storms in the Yellow River Delta. The insights can guide the development of more effective coastal protection strategies, highlighting the need for enhanced measures to mitigate erosion and promote resilience in microtidal regions.

The effect of stress history on the critical shear stress of bedload transport in gravel-bed streams

Critical shear stress is a pivotal parameter that describes the bedload transport and riverbed stability. Recent studies indicate that different interevent periods in gravel-bed streams can lead to a realignment of riverbed structures, thereby disrupting the bedload transport. In this research, a series of flume experiments were conducted to study the effect of stress history on the critical shear stress of bedload transport in a gravel-bed channel. The results reveal the temporal-spatial variation patterns of critical shear stress. The critical shear stress has an increased trend after interevent with low flow and long duration. Conversely, a decreasing trend in critical shear stress observed after low flow and short duration conditions. After interevent durations with medium to high flow condition, no matter the duration, critical shear stress shows a decreasing trend. A significant positive correlation between the flow magnitude of the interevent period and the critical shear stress of the subsequent flood period is shown. Besides, critical shear stress has a significant spatial variation pattern on the gravel bed due to the presence of microtopography as revealed by the correlation analysis results among the spatial density of microtopography with different protrusion heights, riverbed statistical parameters, and critical shear stress. Moreover, the stress history, by adjusting the position of fine particles on the riverbed surface through different magnitudes and durations of the interevent period, affects the spatial density of the microtopography, thereby influencing the critical shear stress. The conventional bedload transport equations are deficit by only considering the riverbed slope or standard deviation of bed surface elevation. An optimization of the critical shear stress calculation method, based on parameter correlation analysis, is proposed, potentially enhancing the calculation accuracy of bedload transport in mountainous rivers.

Heat treatment effects on the shear performance and microstructure evolution of W-core SiC fibers

SiC fiber reinforced titanium alloy matrix composites employed in elevated temperature aeroengine components are subjected to complex loading conditions. Investigating the shear performance of SiC fibers after heat treatment can help ensure the load-carrying capability of composites at service temperatures. This study presents a novel double shear testing method for W-core SiC fibers. The shear strength and stiffness of the SiC fibers in as-received and heat-treated conditions were measured. The morphologies of SiC fibers’ shear fracture surfaces and C coatings were thoroughly analyzed. The evolution of the internal microstructure and the composition of reaction layers after heat treatments were investigated. The shear strength of W-core SiC fibers at 95% survival probability reached 588.64MPa after 200 h of heat treatment at 600°Celsius, representing a 55% improvement over both the as-received and 1100°Celsius heat-treated conditions. After heat treatment at 1100°Celsius, the reaction layer between the W core and the SiC sheath thickened exponentially due to extensive elemental diffusion, resulting in the formation of numerous Kirkendall voids. The voids facilitated crack initiation and propagation, leading to a deterioration in fiber shear performance. The insights gained in this study regarding the shear properties of SiC fibers can provide support for predicting the mechanical performance of composites under complex loading conditions.

Effects of laminar burning velocity to friction velocity ratio on turbulent premixed flame-wall interaction within turbulent boundary layers

Direct numerical simulation of flame-wall interaction (FWI) for two flame configurations under different laminar burning velocity to nonreacting wall friction velocity ratio, SL/uτNR, have been performed. The first configuration considered is a V-flame in a turbulent channel flow with isothermal inert walls and represents oblique wall interaction. The second configuration is representative of head-on interaction (HOI) of a planar flame interacting with a wall in a fully developed turbulent boundary layer. In the case of the V-flame with SL/uτNR=0.4, a smaller flame angle is obtained when compared with that of the V-flame with SL/uτNR=0.7. This is a consequence of the differences in the turbulent burning velocities between the two V-flame cases, where the case with SL/uτNR=0.4 has a lower turbulent burning velocity when compared with that of the case with SL/uτNR=0.7. By contrast, the ratio of turbulent to laminar burning velocity is higher for the SL/uτNR=0.4 HOI case when compared to the corresponding HOI case with SL/uτNR=0.7. The flame orientations with respect to the streamwise component of velocity and wall-normal direction in addition to the SL/uτNR values have been found to have a significant impact on the variations of wall heat flux, wall shear stress, and wall friction velocity in premixed FWI within turbulent boundary layers. The behaviors of nondimensional streamwise velocity, u+, and nondimensional temperature using wall units, T+, have also been investigated for different SL/uτNR ratios and it is found that the standard log-law profiles in the case of u+ and T+ are not valid for the two configurations and the two SL/uτNR ratios considered. Published by the American Physical Society 2024

Thermoresponsive Polymers as Viscosity Modifiers: Innovative Nanoarchitectures as Lubricant Additives

The world of lubricants is driven by the constant pursuit of improved performance in response of the requests of new engine generations. Engine oils play a critical role as lubricants in mitigating wear, reducing friction and ensuring optimal engine operation under diverse conditions. Modern commercial engine oils are complex formulations, comprising of a base oil, generally coming from petroleum sources, formulated with specific, important additives able to optimize the viscosity, thickening and shear stress in the operating temperature range. Such additives are produced in the thousand tons per year scale range. The most important class of additives for modern lubrication is made of organic polymers with variable architectures and topologies, generally referred as “viscosity modifiers” (VMs): they act as “moderators” of viscosity at different working temperatures. The tremendous advances in polymer science have been reflected in the realm of VMs, allowing the commercialization of products obtained by controlled polymerization techniques, and the experimentation of a broad variety of different macromolecular architectures and topologies as VMs. In this review we introduce the reader, together with the basic principles of viscosity modification and thermal‐dependent rheological response, to the fascinating chemistry towards the improvement of VMs, through optimization of macromolecular design and architecture.

Analysis of Water Film Distribution and Aerodynamic Performances of High-Speed Train Under Rainfall Environment

AbstractThe current research on the aerodynamic performance of the train running in rainy weather is primarily concerned with the trajectory of the raindrops and the aerodynamic variation of trains caused by raindrops. In fact, water film will generate on the train body when raindrops hit the train, which interacts with the flow field around the train, and would probably affect the aerodynamic performance of the train. In this paper, based on shear stress transport (SST) k-w turbulence model and Euler-Lagrange discrete phase model, the aerodynamic calculation model of a high-speed train under rainfall environment is established. The LWF (Lagrangian wall film) is used to simulate the water film distribution of the high-speed train under different rainfall intensities, and the aerodynamic performance of the train are studied. The calculation results show that raindrops will gather on the train surface and form water film under rainfall environment. With the extension of rainfall time, the thickness and coverage range of water film expand, and the maximum thickness of water film can reach 4.95 mm under the working conditions in this paper. The average thickness of water film on the train body increases with the rainfall intensity. When the rainfall intensity increases from 100 mm/h to 500 mm/h, the average water film thickness will increase by 3.26 times. The velocity of water film in the streamlined area of head car is larger than that in other areas, and the maximum velocity is 22.14 m/s. Compared with the rainless environment condition, the skin friction coefficient of the high-speed train increases and the average value will increase by 10.74% for a rainfall intensity of 500 mm/h. The positive pressure and resistance coefficient of the head car increase with the rainfall intensity. This research proposes a methodology to systematically analyze the generation of water film on the train surface and its influence on the train aerodynamic performance; the analysis can provide more practical results and can serve as a reference basis for the design and development of high-speed trains.