The variance and spectra of wall-normal velocities are investigated for direct numerical simulations of turbulent flow in a channel, pipe and zero-pressure-gradient boundary layer across a decade of friction Reynolds numbers. Spectra along the spanwise wavenumber have a pronounced peak well described by the turbulent dissipation rate and the local shear stress throughout the bottom half of the boundary layer. Deviations in the local stress from the surface shear velocity $U_\tau$ account for almost all of the differences in wall-normal velocity variance observed across different canonical flows, including for plane Couette flow. The dependence on the local stress is attributed to the fact that wall-normal motions are predominately ‘active’ per Townsend’s attached eddy hypothesis and directly contribute to the local shear stress, noting this hypothesis assumes simplified ideal conditions with constant turbulent shear stress. A semi-empirical fit applied to the Reynolds-number dependence of the variance matches the simulations across the lower half of the boundary layer and aligns with observed values in the literature. The fit extrapolates to a value between 1.45 and 1.65 times the local shear stress in the high-Reynolds-number limit, consistent with previous predictions relative to $U_\tau$ including for the vertical velocity in the near-neutral atmospheric boundary layer. However, universality in the exact proportional constant is precluded by small discrepancies in the variances corresponding to dissimilarity in the low-wavenumber contributions across different flow configurations and wall-normal positions. We speculate the dissimilarity is due to relatively weak ‘inactive’ wall-normal motions that are excluded from Townsend’s original hypothesis.

Due to volume fluctuations, bony prominences contacting prosthetic sockets are susceptible to skin breakdown due to constant pressure and shear stresses from lateral and vertical displacements throughout gait. A low-profile unloading cushion that can be controlled with a millisecond response to dynamically provide localized socket fit at the fibular head may offer stress relief during gait. The study presents a low-profile (<0.5 mm thick) dynamic fluidic cushion that can provide targeted cushioning during gait to mitigate high contact pressures at bony prominences while minimizing duration of pressure and shear. The proof-of-concept study comprised benchtop testing using a residual limb model. The dynamic cushion effectively unloaded high pressure caused by the body weights only during the stance phase at the fibular head for up to 80 kg by mitigating and redistributing contact pressures away from the fibular head to the immediately surrounding soft tissues. Results demonstrated that the fibular head peak contact pressure could be reduced by as much as 94% (98 to 6 kPa) with a response time of 250 milliseconds in benchtop tests, fast enough to turn on during stance and off during swing. The proposed dynamic fluidic cushion has the potential to offer unloading during gait to prevent skin damage at pressure-sensitive spots, notably benefiting individuals with complex limb geometries. We introduce a new method with the potential to be integrated into prosthetist workflows to locally adjust socket fit (e.g., at the fibular head) using elastomer sheets and a handheld heat press.

The effects of non-physiological shear rate gradient, frequency, and amplitude on platelet aggregation.

The high-temperature performance of asphalt mastic is a critical factor influencing the resistance of asphalt mixtures to permanent deformation. Despite the importance of this material phase, no standardized test exists for evaluating asphalt mastic behaviour at high temperatures. Therefore, researchers often use the Multiple Stress Creep Recovery Test (MSCRT), originally designed for asphalt binder, although its applicability to asphalt mastic is limited. This study proposes a novel rheological method referred to as Single Shear Creep Test (SSCT) as a more robust alternative for assessing the performance of asphalt mastic at high temperatures. The SSCT applies a constant shear stress over an extended period, allowing for the determination of the steady-state creep rate as a rheological performance indicator. A comprehensive experimental program involving 45 asphalt mastic variants, produced by using 11 asphalt binder types, 15 mineral fillers, and different filler-to-asphalt binder ratios. Each variant was tested using both MSCRT and SSCT in a Dynamic Shear Rheometer (DSR). The results demonstrated that SSCT provides more consistent and rheological meaningful differentiation between materials. The results show that asphalt binder type and the filler-to-bitumen (f/b) ratio strongly influence asphalt mastic behaviour at high temperature. Filler type has a limited influence, except for hydrated lime.

Abstract Flocculation, a critical process in coastal and estuarine systems, plays a significant role in sediment transport, nutrient cycling, and ecological health. This study develops a cohesive sediment transport modeling framework tailored to the wave bottom boundary layer under dilute and equilibrium conditions, explicitly incorporating flocculation effects via a Population Balance Equation (PBE). Using Direct Numerical Simulation, six baseline cases, each with a distinct sediment concentration profile resulting from a constant settling velocity and critical erosion shear stress, are generated to drive the PBE flocculation model for given floc yield strength and stickiness. Results reveal that flocculation significantly influences sediment concentration profiles promoting three distinct stages, well‐mixed, transition to lutocline, and well‐developed lutocline. At low concentrations with well‐mixed profiles, cohesive floc properties are less significant, and turbulence is a main flocculation driver. In contrast, as concentration increases, cohesive floc properties become crucial, facilitating lutocline formation. The analysis also highlights limitations of depth‐averaged settling velocity as a parameterization. It is suitable for well‐mixed and transitional profiles but fails in well‐developed lutoclines, where empirical formulations that explicitly incorporate turbulent shear rate and sediment concentration better capture variability. This study underscores the necessity of incorporating flocculation effects into sediment transport models to enhance predictions of sediment dynamics in wave bottom boundary layers.

Understanding the coupling between the dynamics of microstructuraldeformation and the bulk flow behavior of colloidal suspensions iscrucial for both fundamental studies and practical applications ofthis important class of soft matter systems. In this study, we investigatedthe flow behavior of cellulose nanofibril (CNF) suspensionsrenewable,sustainable materials with low environmental impactusing the“Rheo-Iris,” a rheo-polarized imaging system we developedto visualize two-dimensional microstructural changes in fluid underapplied stress. Creep tests under constant shear stress revealed aninitial elastic material response, a yield transition, followed byviscoplastic flow at long times. Simultaneous polarized imaging identifiedthree distinct retardation patterns, depending on the applied shearstress. At low stresses (≤10 Pa), or small strain immediatelyafter stress application, the phase retardation remained uniformlylow, and the orientation axis of the microstructure was randomly distributed,indicating that the homogeneously dispersed CNFs form an isotropic,entangled network structure. At a stress near the yield point (40Pa), a spiral-shaped region of high retardation appeared, and theorientation axis shifted to 60–85° away from the flowdirection. This corresponds to the formation of rosary-like structuresaligned in the vorticity direction, exhibiting spatially nonuniformbirefringence and an oriented microstructure. At higher stresses farabove the yield point (200 Pa), this “log-rolling” mesostructurecollapsed, and smaller CNF aggregates became aligned in the flow direction,leading to spatially uniform oriented birefringence across the entirefield. Both cases represent distinct fiber orientation phenomena,and our noninvasive rheo-polarization method clearly distinguisheshow the spatial orientation distribution in the field changes withapplied stress. The Rheo-Iris system enables real-time, quantitativeanalysis of internal microstructural evolution under imposed shearstrain or stress and offers a powerful tool for exploring the orientationdynamics in soft matter systems, opening a new eye on complex fluidrheology in colloidal dispersions.

Abstract Today, computational fluid dynamics (CFD) is widely used for atmospheric dispersion at building scale. This type of simulation requires boundary conditions for wind, turbulence, and temperature. A classical way of imposing representative flow at open boundaries is to derive (universal) functions corresponding to idealised situations (e.g. constant shear stress, constant heat flux, such as in the Monin–Obukhov theory). In this paper, we first propose an analysis of universal functions for the surface layer and compare them to a 5-year data base of measurement on the SIRTA observatory (France). The conclusion is that these functions are in good agreement with the experimental data for moderate ratios of $$\zeta $$ (distance to the ground divided by Obukhov length), but most of them do not fulfil asymptotic behaviours in the very stable or convective limit. We also compare the measurements with the extensions proposed by Gryning et al. (Bound Layer Meteorol 124(2):251–268, 2007), which take into account the height of the atmospheric boundary layer. Then, following the work of Nieuwstadt (Noct Bound Layer J Atmos Sci 41(14):2202–2216, 1984. https://doi.org/10.1175/1520-0469(1984)041$lt$2202:TTSOTS$gt$2.0.CO;2), we propose universal functions able to reproduce the Ekman spiral and consistent with moment-turbulence closures for stably stratified atmospheres.

Constant and cyclic shear stress regulate collagen fibers self-assembly

The stress state changes in shallow gassy sand strata under anthropogenic disturbances can be simplified as failure problems of gassy sand under constant shear stress drained (or undrained) stress paths. However, related research remains relatively limited. To investigate the mechanical properties of gassy sand, this study utilized a high-pressure gas dissolution saturator and employed a CO2-saturated aqueous solution degassing method to prepare high-saturation gassy sand specimens. A series of triaxial tests under constant shear stress paths were conducted under both drained and undrained conditions, focusing on the effects of relative density, saturation, and deviatoric stress levels in constant shear stress paths. The effective stress paths and instability time-response patterns of gassy sand under constant external loads with internal stress variations were obtained. Experimental results revealed that in the p'-q plane, for loose or dense gassy sand with varying saturations, the stress path entry points into an unstable state consistently lie within the potential instability zone bounded by the critical state line (CSL) and instability line (IL). In constant shear stress drained (CSD) tests, lower sand saturation corresponds to longer times to reach instability, whereas the opposite trend was observed in constant shear stress undrained (CSU) tests. These findings enhance the understanding of the mechanical behavior of gassy sand and provide theoretical support for safety assessments in engineering projects involving shallow gassy strata.

Abstract At 23:59 (UTC + 8) on 18 December 2023, an earthquake of Ms 6.2 struck Jishishan County in Gansu Province, China, and triggered a large‐scale, flow‐like loess landslide in Zhongchuan Town, resulting in some 20 deaths. Originated from relatively gentle terrain, the loess flow displayed high mobility with a run‐out distance of 3,200 m, suggesting that pore‐water may play a critical role in the mobility of Zhongchuan flowslide. Following onsite investigations and soil sampling, we replicated the initiation process of the flowslide through dynamic back pressure direct shear tests under a constant shear stress condition. Two types of tests were conducted on saturated loess samples: elevated back pressure tests to simulate instability induced by high pore‐water pressure, and dynamic loading tests to examine the evolution of pore‐water pressure under seismic loading conditions. The experimental results, supported by microscopic analysis, indicate that elevated pore‐water pressure is the key factor driving the progressive transformation of shear displacement from accelerated motion to instantaneous runaway. Meanwhile, dynamic loading substantially amplifies the generation of excess pore‐water pressure. Moreover, the initial pore‐water pressure was found to be a critical factor in both the initiation and high mobility of the Zhongchuan flowslide. These experiments quantitatively capture the in situ evolution of pore‐water pressure throughout the liquefaction process, providing a physically based framework for understanding the mechanisms of loess landslides.

This study uses the diffusion analogy (Miyake, Sci. Rep., 5R-6, 1965, Univ. of Washington, Seattle, USA) to predict the full growth behaviour of internal boundary layers (IBLs) induced by a roughness change for neutrally – and especially stably – stratified boundary layers with finite thickness. The physics of the diffusion analogy shows that the streamwise variation of the IBL thickness is dictated by $\sigma _w/U$ at the interface, where $\sigma _w$ and $U$ represent wall-normal Reynolds stress and mean streamwise velocity, respectively. The existing variants of the model, summarised by Savelyev &amp; Taylor (2005, Boundary-Layer Meteorol., vol. 115, pp. 1–25), are tailored to IBLs confined within the constant shear stress layer. To extend the applicability of the model to the outer region, we investigate the relation between $\sigma _w/U$ and $U/U_\infty$ in the outer region across varying stratification, where $U_\infty$ is the free-stream velocity. Our analysis reveals that wind tunnel data from a number of facilities collapse onto a master curve when $\sigma _w/U$ is premultiplied by a height-independent parameter, which is a function of the ratio of Monin–Obukhov length to the boundary layer thickness. The scaled $\sigma _w/U$ decreases inversely with $U/U_\infty$ in the surface layer, transitioning to a linear decrease as $U/U_\infty$ increases. The new model, which integrates these findings, along with the effects of streamline displacement and acceleration, captures the complete characteristics of IBLs as they develop within turbulent boundary layers of finite thickness.

Neutron reflectometry is a technique for measuring structure near planar interfaces that has been previously used to non-destructively characterize the polymer density of hydrated, dilute, and soft materials. Previous investigations have conducted neutron reflectometry measurements of liquids, gels, emulsion, and polymer solutions at rest, in compression, and subject to shear stress. However, correlating structure with tribological properties of soft materials presents significant experimental challenges for prior instruments due to wall slip, sample thickness, and structural heterogeneity (e.g., depth-wise gradients). A linear reciprocating tribometer offers several advantages for in situ neutron reflectometry studies, including uniform velocity profiles, constant shear stress over large regions of interest, and independent control of normal force and sliding velocity during measurements. This work outlines basic considerations for the design of a custom linear reciprocating tribometer that operates in a neutron beamline and includes commissioning measurements. The tribometer is designed to compress soft and hydrated materials against linearly reciprocating silicon disks. The three key design considerations for this tribometer are (1) safety, (2) neutron transmission, and (3) sample positioning. This instrument design will enable in situ studies of soft matter and illuminate the role of interfacial structure on tribological phenomena.

This paper presents a high-resolution characterization of tool–chip interfacial stress distribution in cutting of a ductile metal using full-field photoelasticity. Our findings reveal the complex nature of stress distribution and friction along the contact, influenced by tool geometry. Notably, for negative rake-angle tools, the measurements reveal a distinct zone near the tool tip where the shear stress decreases as one travels toward the tool tip. Stress measurements are complemented with in situ velocimetry of chip flow at the interface to enable correlation between stresses and velocity distribution at the interface. Based on the data, the tool–chip interface is categorized into three distinct zones: (1) a retardation zone near the tool tip, characterized by a high normal stress but small shear stresses, followed by (2) a uniform sliding zone with a relatively constant shear stress and (3) an elastic zone near the contact edge that obeys Coulomb friction. The paper challenges the customary practice of dividing the tool–chip contact into sticking and sliding zones and instead argues for a contact description, in terms of plastic and elastic zones that is more consistent with the experimental observations. The study also highlights the importance of high-resolution in situ characterization techniques for resolving the complex nature of friction in cutting and other similar sliding plastic contacts.Graphical abstract

Abstract This paper introduces a mechanical model for steel‐reinforced concrete prestressed with bonded CFRP rods and presents the findings of an experimental campaign conducted to validate the model. The study is part of a project aiming at developing a railway bridge system in Switzerland that utilizes stainless steel reinforcing bars combined with pretensioned sand‐coated CFRP rods to (i) maximize durability and (ii) achieve reasonable plastic deformation capacity despite the inherent brittleness of the prestressing material. The proposed Tension Chord Model for CFRP‐prestressed Structural Concrete (TCM‐cfrp) extends the established Tension Chord Model for Structural Concrete by introducing a constant, rigid‐perfectly plastic bond shear stress‐slip relationship between concrete and CFRP, allowing for computationally efficient closed‐form solutions to determine the load‐deformation behavior, crack widths, and reinforcement stresses. The validation against the experimental results reveals (i) satisfactory model predictions of the mean strains, the stresses at the crack and the crack widths and (ii) insensitivity of the structural behavior on local bond shear stress distributions, thus supporting the simplification of assuming constant bond shear stresses. The TCM‐cfrp is capable of capturing all stress states in tension chords of railway bridge girders and other structural elements relevant for serviceability, ultimate, and to some extent, fatigue limit state verifications. The limitations of the model lie in (i) the simplified treatment of the crack formation process, overestimating deformations but underpredicting crack widths prior to stabilized cracking and (ii) its inability to realistically model local strain and stress distributions between the cracks, which is however of limited relevance for most structural engineering purposes.

This study examines the behavior of anisotropically consolidated granular assemblies under undrained cyclic true triaxial loading paths. To achieve this, the Discrete Element Method (DEM) is conjugated with the Coupled Fluid Method (CFM) to account for fluid-solid interaction in undrained conditions. The examined loading paths include two phases: anisotropic consolidation and undrained cyclic true triaxial loading. During consolidation, samples are sheared at various Lode angles to reach a spectrum of initial static shear stress levels. In the second stage, undrained cyclic loading is applied with constant shear stress amplitudes at various Lode angle values. The results indicated that the monotonic and cyclic Lode angle, initial static shear stress, and amplitude of deviatoric stress have pronounced effects on the secant shear modulus degradation and the rate of excess pore water pressure generation of granular assemblies. In tandem with macro-scale observations, the evolution of the microstructure within assemblies is analyzed using the coordination number, redundancy index, inter-particle contact fabric tensor, and particle orientation fabric tensor. The micro-scale findings confirm that the anisotropy induced by changes in the loading direction significantly impacts the shear strength of the assemblies. Additionally, the fabric of assemblies aligns along the preferential direction corresponding to the major principal stress, influencing the dilative response.

This study investigates interface sliding behavior in composite I-steel–concrete beams reinforced with a composite material plate by analyzing various connection configurations combining shear stud connectors and adhesive bonding. The degree of composite action, governed by the shear stiffness at the steel–concrete interface, plays a critical role in structural performance. An analytical model was developed based on the elasticity theory and the strain compatibility approach, assuming constant shear and normal stress across the interface. Five connection modes were considered, ranging from fully mechanical (100% shear studs) to fully adhesive (100% bonding), as well as mixed configurations. The model was validated against finite element simulations, demonstrating strong agreement with relative differences between 0.3% and 10.7% across all cases. A parametric study explored the influence of key factors such as interface layer stiffness and composite plate reinforcement material on the overall interface behavior. The results showed that adhesive bonding significantly reduces slippage at the steel–concrete interface, enhancing bond integrity, while purely mechanical connections tend to increase interface slippage. The findings provide valuable guidance for designing hybrid connection systems in composite structures to optimize performance, durability, and construction efficiency.

The glycocalyx (GCX), a multicomponent coating on endothelial cells (ECs), plays a critical role in various cellular behaviors, including barrier formation, vasodilation, and mechanotransduction. Mechanical perturbations in the vascular environment, such as blood vessel stiffness, are sensed and transduced by ECs via the GCX. Hypertension-induced stiffness disrupts GCX-mediated mechanotransduction, leading to EC dysfunction and atherosclerotic cardiovascular diseases. Understanding GCX-regulated mechanotransduction necessitates an in vitro model that closely mimics in vivo conditions. Existing models are insufficient, prompting the development of the system described in this manuscript. Here, we report on a new system to model varying EC substrate stiffness under sustained physiological fluid shear stress, providing a realistic environment for comprehensive examination of EC function. Gelatin methacrylate (GelMA) substrates with stiffnesses of 5 kPa (physiological) and 10 kPa (pathological) were seeded with human umbilical vein ECs (HUVECs) and subjected to constant physiological shear stress (12 dyn/cm2) for 6 h. Analysis focused on heparan sulfate (HS), sialic acid (SA), hyaluronic acid (HA), syndecan-1 (SDC1), cluster of differentiation 44 (CD44), and Yes-associated protein (YAP). Compared to the 5 kPa conditions, HS coverage and thickness decreased at 10 kPa, indicating impaired barrier function and increased susceptibility to inflammatory agents. SA density increased despite decreased coverage, suggesting enhanced binding site availability for inflammatory recruitment. HA expression remained unchanged, but the amount of the HA core receptor, CD44, was found to be increased at 10 kPa. Consistent with previously published interactions between CD44 and YAP, we observed increased YAP activation at 10 kPa, as evidenced by increased nuclear translocation and decreased phosphorylation. These findings, bridging biomaterials and mechanobiology approaches, deepen our understanding of how mechanical stimuli influence the EC GCX function. The results underscore the potential of mechanotherapeutic strategies aimed at preserving vascular health by modulating the endothelial function.

We report numerical simulations of creep flow and yielding of particulate depletion gels under constant shear stress, combining data on different length and time scales. We characterize the statistics of mesoscale strand-breaking events in the gel, which are distributed homogeneously in space, corresponding to macroscopically ductile flow. At the microscale, a spatiotemporal analysis of structural and mechanical metrics connects properties of strands before and after they fail, indicating that strand breakage is statistically predictable. Using results from different scales, we discuss the interplay between creeping and aging dynamics, and we demonstrate a viscosity bifurcation.

The mechanical behavior of rocks under loading conditions depends on stress path and magnitude. With increasing load, rocks have an elasto-plastic behavior. Within the loading yield surface, constitutive models assume that rocks behave elastically and are independent of the stress path and magnitude (e.g., Mohr–Coulomb models). We performed tests on unconsolidated sands (no cementation), and under both loading and unloading conditions. We mapped the loading yield surface using a multi-stage triaxial test with the yield criterion as the point of positive dilatancy. We studied the yield behavior of the two different unloading stress paths: a constant axial stress unloading test (reducing mean effective stress and increasing shear stress) and a constant shear stress unloading test (reducing mean effective stress and keeping shear stress constant). The results show that unloading-based tests reach yield point at a lower shear stress than expected from the loading-based yield surface. The unloading-based yield surface is also dependent on the stress path. The application of this research includes a prediction of the geomechanical behavior of unconsolidated sands under injection conditions. Often, a constitutive model derived from loading stress paths is used for injection with the ad hoc assumption that the loading and unloading models are identical. These constitutive models provide results for injector design parameters, injection performance prediction, and safe injection envelopes. Therefore, it is essential to have accurate constitutive models that are representative of unloading stress paths. In calibrating these models, we demonstrated that the yield criterion (point of positive dilatancy) is reached before the loading-based yield surface during injection (decrease in mean effective stress) is reached. We also developed a minimum yield surface model. With a calibration using three tests, this model can predict the yield point for any stress path and at any initial stress state (within the bounds of the experiments).

Cryptosporidium hominis is the dominant Cryptosporidium species infecting humans, but most advances in developing robust in vitro culturing platforms for Cryptosporidium have utilised C. parvum. Consequently, there is relatively little available information specific to the biology and life cycle of C. hominis. The present study utilised a pumpless and tubeless gut-on-chip to generate a physiologically relevant in vitro environment by applying a constant fluid shear stress of 0.02 dyn cm-2 to HCT-8 cells. Gut-on-chips were fabricated using standard soft lithography. C. hominis oocysts isolated from human pathology samples were used to infect the human ileocecal colorectal adenocarcinoma (HCT-8) cell line under a constant fluid shear stress of 0.02 dyn cm-2. Parasite growth was assessed using a C. hominis-specific quantitative PCR, a Cryptosporidium genus-specific immunofluorescence assay, and scanning electron microscopy. Differences in the HCT-8 transcriptome with and without fluid shear stress, and the host-parasite interaction, were both assessed using bulk transcriptomics. Transcriptomic analysis of the HCT-8 cell line cultured within the gut-on-chip demonstrated a metabolic shift towards oxidative phosphorylation when compared to the same cell line cultured under static conditions. Extended C. hominis (subtype IdA15G1) cultures were sustained for up to 10 days within the gut-on-chip as shown by a C. hominis-specific qPCR and a Cryptosporidium genus-specific immunofluorescence assay, which demonstrated ~30-fold amplification in the gut-on-chip over the duration of the experiment. Scanning electron microscopy of infected monolayers identified trophozoites, meronts, merozoites, macrogamonts, microgamonts, and possible gamont-like stages at 48 h post-infection. The potential role of gamonts in the Cryptosporidium life cycle remains unclear and warrants further investigation. Transcriptomes of HCT-8 cells infected with C hominis revealed upregulation of biological processes associated with cell cycle regulation and cell signalling in C. hominis-infected cells under fluid shear stress compared to static culture. These data demonstrate that bioengineered gut-on-chip models support extended C. hominis growth and can be used to interrogate responses of host cells to infection. Owing to its relative simplicity, the pumpless and tubeless gut-on-chip can be accessible to most laboratories with established HCT-8 infection models for Cryptosporidium culture.