Search Results (139)

PVDF has expanded the application of ferroelectric materials in flexible and wearable electronics due to its flexibility, corrosion resistance, ease of processing, and low cost. However, the polarization of ferroelectric polymers is low, with a bottleneck value of 10 µC cm⁻². In this study, flexible ferroelectric composite films comprising Ca₂Nb₃O₁₀ (CNO) nanosheets and PVDF were fabricated via direct ink writing (DIW). By modulating the nozzle-to-substrate height in conjunction with flow-induced shear within the syringe and the application of additional shear force at the nozzle, effective alignment of low-content (2 wt.%) CNO nanosheets dispersed in a highly fluid ink was achieved. The enhanced orientation degree of the CNO nanosheets increased the breakdown strength of the PVDF–CNO composite films to 524 MV/m. Furthermore, the remanent polarization (P_r) was significantly increased by 207% compared to pure PVDF films, reaching a value of 11.6 µC cm⁻². This study provides a simple and efficient DIW-based strategy for improving filler orientation in composites and demonstrates the substantial enhancement in dielectric and ferroelectric properties achievable through such filler alignment. Full article

This study explores the feasibility and validity of using topology optimization (TO) to design the internal cooling of an airfoil-like geometry approximating a turbine guide vane. A conjugate heat transfer approach where the fluid flow physics are coupled with a convection–diffusion model for heat transfer is used in the TO. The objective is to minimize the maximum temperature on the outer surface of the vane with a constraint on the mass flow of the internal coolant. Two different flow models are investigated for the TO process: the Stokes model and the Reynolds-Averaged Navier–Stokes (RANS) equations with a simple zero-equation turbulence model. Velocity and temperature fields in topology-optimized designs are then compared to conventional conjugate heat transfer analyses performed on post-processed designs with body-fitted meshes and those using the shear stress transport (SST) RANS turbulence model. Designs obtained with the Stokes model exhibit different flow trajectories and mixing, while the use of RANS equations improves predictions but introduces uncertainties due to turbulence modeling limitations, particularly in the presence of flow separation. Thus, considering these limitations, the findings suggest that a simple flow model, such as Stokes in TO, with a comparatively low computational cost, can yield useful design concepts. However, the simplifications in the governing equations and their impact on physics should be considered carefully, and further aerothermal validation is required. Thus, the study findings, along with advances in robust meshing, enhance the practicality of topology optimization for industrial applications. Full article

The study systematizes the current state of knowledge on the morphological diversity of dispersed-phase particles in the most widely used lubricating greases, encompassing their shape, size, surface structure, and overall geometry. The extensive discussion of the diversity of grease thickener particles is supplemented with their microscopic images. Particular emphasis is placed on the influence of thickener particle morphology, the degree of their aggregation, and interparticle interactions on the rheological, mechanical, and tribological properties of grease formulations. The paper reviews recent advances in investigations of grease microstructure, with special emphasis on imaging techniques—ranging from dark-field imaging, through scanning electron microscopy, to atomic force microscopy—together with a discussion of their advantages and limitations in the assessment of particle morphology. A significant part of the work is devoted to rheological studies, which enable an indirect evaluation of the structural state of grease by analyzing its response to shear and deformation, thereby allowing inferences to be drawn about the micro- and mesostructure of lubricating greases. The historical development of rheological research on lubricating greases is also presented—from simple flow models, through the introduction of the concepts of viscoelasticity and structural rheology, to modern experimental and modeling approaches—highlighting the close relationships between rheological properties and thickener structure, manufacturing processes, composition, and in-service behavior of lubricating greases, particularly in tribological applications. It is indicated that contemporary studies confirm the feasibility of tailoring the microstructure of grease thickeners to specific lubrication conditions, as their characteristics fundamentally determine the rheological and tribological properties of the entire system. Full article

Lighter-than-air parakites deployed at sea in the close proximity of wind turbines may offer the possibility of mitigating wake losses encountered in large offshore wind farms. Such devices, having an order of magnitude similar to wind turbine rotors, can divert the stronger winds available at high altitudes to the lower level within the atmospheric boundary layer to enhance the wind flow between turbines. Mooring the parakites directly to the offshore wind turbine support structures would avoid the need for additional offshore structures. This paper investigates a novel and simple approach for mooring a parakite to an offshore wind turbine. The proposed approach exploits the lift forces of the inflatable parakite to reduce the tower bending moment at the base of the turbine induced by the rotor thrust. An iterative numerical model coupling the parakite loads to a catenary cable piecewise model is developed in Python 3.12.7 to quantify the bending moment reduction and shear load variations at the wind turbine tower base induced by the different kite geometries, windspeeds, and mooring cable lengths. The numerical model revealed that the proposed approach for mooring parakites can substantially reduce the tower bending loads experienced during rotor operation without considerably increasing the shearing forces. It was estimated that the tower bending moment decreased by 7.7% at the rated wind speed, where the rotor thrust is at its maximum, while the corresponding shear force increased by 0.6%. At higher wind speeds, where the magnitude of the rotor thrust decreases, the percentage reduction in bending moment gradually increases to 51.7% at a wind speed of 24 m/s, with the corresponding shear force increasing by only around 4.6%. Furthermore, while upscaling the parakite augments the tower bending moment reduction, changes in cable length had little effect on bending moment reduction and shear increase. Full article

Organ-on-a-Chip (OOC) platforms are microfluidic systems that recreate key features of human organ physiology in vitro via controlled perfusion. Fluid mechanical stimuli strongly influence cell morphology and function, making this important for cardiovascular OOC applications exposed to pulsatile blood flow. However, many existing OOC devices employ relatively simple chamber geometries and steady inflow assumptions, which may cause non-uniform shear exposure to cells, create stagnant regions with prolonged residence time, and overlook the specific effects of pulsatile perfusion. Here, we used computational fluid dynamics (CFD) to investigate how chamber geometry and inflow conditions shape the near-wall flow environment on a cell culture surface at a matched cycle-averaged volumetric flow rate. Numerical results demonstrated that pillarized chambers markedly reduced relative residence time (RRT) versus the flat chamber, and the small pillar configuration produced the most uniform time-averaged wall shear stress (TAWSS) distribution among the tested designs. Phase-resolved analysis further showed that wall shear stress varies with waveform phase, indicating that steady inflow may not capture features of pulsatile perfusion. These findings provide practical guidance for pillar geometries and perfusion conditions to create more controlled and physiologically relevant microenvironments in OOC platforms, thus improving the reliability of cell experimental readouts. Full article

Pain and variable uptake remain practical barriers to needle-based delivery. Device-level vibration has emerged as a simple strategy for improving tolerability and dispersion, but its fluid-mechanical basis remains incomplete. Using a lattice Boltzmann model with a porous-media skin surrogate, we applied time-periodic inlet pressures at 0%, 16.6% ($\mathsf{\Delta }{P}_1$), and 35.1% ($\mathsf{\Delta }{P}_2$) amplitudes to Newtonian, model shear-thinning, and clinically measured protein formulations. We quantified the wall shear stress, wetted area, dispersion length, and pressure cost over one cycle. Vibration increased the normalized wetted area by 10.6% for Newtonian flow and by 15.9% and 21.3% for the non-Newtonian cases at $\mathsf{\Delta }{P}_1$ and $\mathsf{\Delta }{P}_2$, respectively, while advancing the penetration front and lateral dispersion. The one-cycle pressure cost per wetted area decreased by 3.9% for Newtonian flow and by 5.96% and 7.80% for non-Newtonian flows. For shear-thinning fluids, the wall-shear history was reshaped, with a brief early amplification and late-phase mean reductions of 10.3% and 13.3% at $\mathsf{\Delta }{P}_1$ and $\mathsf{\Delta }{P}_2$. These results establish a fluid-mechanical mechanism linking clinically relevant vibration amplitudes to reduced sustained shear exposure, deeper and broader depot formation, and improved conditions for drug uptake. Full article

Particle-laden flows in non-Newtonian fluids are encountered in a variety of industrial applications, such as concrete pumping and battery electrode slurry processing, where accurate prediction of particle migration is essential for performance and product quality. This work investigates fully developed suspension flows in circular tubes, combining the shear-induced diffusion framework of Phillips et al. with the Krieger–Dougherty relative viscosity and the Carreau–Yasuda constitutive model. Unlike previous studies that generally rely on Newtonian or simple non-Newtonian rheology models, we employ the Carreau–Yasuda model, a more sophisticated constitutive relation that captures both shear-thinning behavior and Newtonian plateau regimes. By applying nondimensionalization and variable transformations, we reduce the governing coupled differential equations to a system of nonlinear algebraic equations, which allows for efficient computation of both particle concentration and velocity profiles. A systematic parametric study was conducted to evaluate the influence of several factors, including the pressure gradient, average particle concentration, and the five parameters of the Carreau–Yasuda model. Additionally, the migration parameter $\alpha =K_c/K_{\eta }$ was considered. The results reveal how the increased rheological complexity of the Carreau–Yasuda model significantly alters the migration dynamics when compared to simpler models. These novel findings have direct implications for optimizing industrial processes involving highly loaded suspensions, offering more accurate predictions of particle behavior under varying flow conditions. For the validation of our findings, experimental data in the literature was used. Full article

This study investigates the impact of eddy viscosity on equatorially trapped waves and the instability of the background shear in a simple barotropic–baroclinic model. It is the first study to include eddy viscosity in the study of tropical wave dynamics. This study also unifies the study of baroclinic and barotropic instabilities by using a coupled barotopic and baroclinic model of the tropical atmosphere. Linear wave theory is combined with a systematic Galerkin projection of the baroclinic dynamical fields onto parabolic cylinder functions. This study investigates varying shear strengths, eddy viscosities, and their combined effects. In the absence of shear, baroclinic and barotropic waves decouple. The baroclinic waves themselves separate into triads, forming the equatorially trapped wave modes known as Matsuno waves. However, when a strong eddy viscosity is included, the structure and propagation characteristics of these equatorial waves are significantly altered. Different wave types interact, leading to strong mixing in the meridional direction and coupling between meridional modes. This coupling destroys the Matsuno mode separation and offers pathways for these waves to couple and interact with one another. These results suggest that viscosity does not simply suppress growth; it may also reshape the propagation characteristics of unstable modes. In the presence of a background shear, some wave modes become unstable, and barotropic and baroclinic waves are coupled. Without eddy viscosity, instability begins with small scale and slowly propagating modes, at arbitrary small shear strengths. This instability manifests as an ultra-violet catastrophe. As the shear strength increases, the catastrophic instability at small scales expands to high-frequency waves. Meanwhile, instability peaks emerge at synoptic and planetary scales along several Rossby mode branches. When a small eddy viscosity is reintroduced, the catastrophic small-scale instabilities disappear, while the large-scale Rossby wave instabilities persist. These westward-moving modes exhibit a mixed barotropic–baroclinic structure with signature vortices straddling the equator. Some vortices are centered close to the equator, while others are far away. Some waves resemble synoptic-scale monsoon depressions and tropical easterly waves, while others operate on the planetary scale and present elongated shapes reminiscent of atmospheric-river flow patterns. Full article

This study investigates the heat transfer coefficient (HTC) during flow condensation inside smooth inclined tubes, analyzing the combined effects of flow orientation, fluid properties and flow characteristics on the thermal performance. The literature review indicates that the channel inclination effect on the HTC remains insufficiently understood, highlighting the need for further investigation. Thus, a comprehensive experimental database comprising 4944 data points was compiled from 24 studies, including all flow directions, from upward, to horizontal, downward, and intermediate orientations. The study reveals that the influence of flow inclination on the HTC can be ruled by a criterion based on the liquid film thickness Froude number, ${\mathrm{Fr}}_{\delta }$. At ${\mathrm{Fr}}_{\delta }$ > 4.75, the effect of flow inclination becomes negligible, while under ${\mathrm{Fr}}_{\delta }$ < 4.75, the inclination can have a considerable effect on the HTC. The experimental data show that at low Froude numbers, upward flow typically exhibits higher HTC compared to downward flow, attributed to enhanced interfacial turbulence caused by opposing gravitational and shear forces. In contrast, under vertical downward flow, the annular pattern is more prominent, with reduced interfacial disturbances, limiting HTC performance. The compiled experimental database for inclined channels was compared against an update list of prediction methods, including seven correlations incorporating the inclination angle as an input parameter. Additionally, a new simple correction factor including the effect of inclined tubes was proposed based on the flow inclination angle and on the liquid film thickness Froude number. The proposed correction factor improved the prediction of well-ranked correlations in the literature by over 20% for stratified flow pattern conditions and by more than 5% for low Froude number values. These findings present new insights into how tube inclination can affect heat transfer in a two-phase flow. Full article

Background/Objectives: Coronary artery disease (CAD) remains the leading global cause of mortality, underscoring the need for functional assessments that extend beyond anatomical evaluation. The Hyperemic Stenosis Resistance (HSR) index combines invasive pressure and flow parameters to assess stenosis severity but faces limitations due to methodological and standardization challenges. This study aimed to introduce and validate a novel non-invasive computational equivalent of HSR (cHSR), derived from coronary computed tomography angiography (CCTA), and to compare its diagnostic performance with fractional flow reserve derived from computational fluid dynamics (FFR_CFD) and quantitative flow ratio (QFR). Methods: A retrospective analysis was conducted on 64 patients (106 coronary lesions) with suspected chronic coronary syndrome (CCS) who underwent both CCTA and invasive coronary angiography (ICA). Computational simulations incorporated patient-specific boundary conditions based on CCTA-derived left ventricular and aortic flow data. Diagnostic accuracy for predicting revascularization was compared among cHSR, FFR_CFD, and QFR. Results: FFR_CFD showed a strong correlation with invasive FFR (r = 0.87, p < 0.0001). The cHSR index achieved the highest diagnostic accuracy (96.2%) at an optimal cut-off of 0.75 mmHg/cm·s⁻¹, outperforming both FFR_CFD and QFR. No significant correlation was found between cHSR and shear stress parameters, including the Oscillatory Shear Index (OSI) and Time-Averaged Wall Shear Stress (TAWSS), indicating complex hemodynamic interactions beyond simple flow–pressure relationships. Conclusions: The computational hyperemic stenosis resistance (cHSR) index represents a promising non-invasive tool for the functional assessment of CAD, demonstrating superior diagnostic performance compared with existing imaging-based indices. Prospective multicenter studies with larger populations are warranted to confirm its clinical applicability and prognostic value in chronic coronary syndrome management. Full article

The Giesekus model has proven to be one of the most successful constitutive rheological models. Although Giesekus introduced the anisotropicity parameter as a constant, recent evidence suggests that it should not be. We elaborate herein on the implications of having a variable anisotropicity coefficient; to our knowledge, this is only the second such model. We find that the modification leads to important differences in the predictions of the second normal stress coefficient in simple shear flow, of which the most significant is the shift of the linear viscoelastic envelope of the second normal stress coefficient to higher values in the case of start-up simple shear flow, which is more in line with experimental data in the literature. Full article

Knots are ubiquitous in polymers and biological macromolecules such as DNA and proteins, yet their behavior and functionality are still not sufficiently explored. Here we investigate the impact of Poiseuille flow on simple knots in flexible polymers placed in a quasi-rectangular micro-channel by systematically varying the flow strength for different chain lengths. Hydrodynamic interactions are accounted for by means of Multi-Particle Collision Dynamics (MPCD). We find that initially loosely localized knots in polymer coils typically tighten under shear to several segments beyond a certain body force threshold. At higher shear rates, intermittent transition from chain stretching to tumbling is observed which correlates with strong fluctuations in the knot size. Somewhat unexpectedly, our results indicate that the influence of channel width on tightening steadily increases with growing width even at equal mean shear rate $\overline{\dot{\gamma }}$. Full article

An analytical study is conducted on unsteady, one-directional magnetohydrodynamic (MHD) flows of electrically conducting, incompressible, and viscous fluids, where the viscosity varies with pressure following a power-law relationship. The flow takes place within a planar channel and is driven by the lower plate, which moves along its own plane with an arbitrary, time-dependent speed. The effects of gravitational acceleration are also considered. General exact formulas are derived for both the dimensionless velocity of the fluid and the resulting non-zero shear stress. Moreover, these are the only general solutions for the MHD motions of the fluids considered, and they can produce precise solutions for any motion of this type for respective fluids. The proposed analytical method leads to simple forms of analytical solutions and can be useful in the study of other cases of fluids with viscosity depending on pressure. As an example, solutions related to the modified Stokes’ second problem are presented and confirmed through graphical validation. These solutions also help highlight the impact of the magnetic field on fluid dynamics and determine the time needed for the system to achieve a steady state. Graphical representations indicate that a steady state is reached more quickly and the fluid moves more slowly when a magnetic field is applied. Full article

This study presents an innovative data-driven methodology to model the hardening behavior of sheet metals across a broad strain range, crucial for understanding sheet metal mechanics. Conventionally, true stress–strain data from such tests are used to analyze plastic flow within the pre-necking regime, often requiring additional experiments to inverse finite element methods, which demand extensive field data for improved accuracy. Although digital image correlation offers precise data, its implementation is costly. To address this, we integrate experimental data from standard tensile tests with a machine-learning approach to estimate the flow curve. Subsequently, we conduct finite element simulations on uniaxial tensile tests, using materials characterized by the Swift constitutive equation to build a comprehensive database. Loading force-gripper displacement curves from these simulations are then transformed into input features for model training. We propose and compare three models—Models A, B, and C—each employing different input feature selections to estimate the flow curve. Experimental validation including uniaxial tensile, plane strain, and simple shear tests on the DP590 and DP780 sheets are then carefully considered. Results demonstrate the effectiveness of our proposed method, with Model C showing the highest efficacy. Full article

Bluff body flows, while commonly assumed to be isolated, are often subject to confinement effects due to interactions with nearby objects. In this study, a simple approximation of such a flow configuration is considered, where a cylinder is placed symmetrically within an infinite channel. The presence of walls implies the wake is physically confined and introduces interactions between the wake and the boundary layer along the wall. To isolate the effect of confinement, simulations are conducted with slip channel walls, removing the boundary layers. Comparisons of flow statistics between simulations of slip and no-slip channel walls show minor differences at a low blockage ratio, $\beta$ (defined as the ratio of cylinder diameter to channel height), while for larger blockage ratios, the differences are significant. Spectral analysis is also performed on the wake and shear layers. At the lowest blockage, $\beta =0.3$, little modification is made to the wake, and we find that Kármán vortices are one-way coupled to the boundary layers along the walls. For $\beta =0.5$, wall–wake interactions are determined to significantly contribute to wake dynamics, thus to two-way coupling Kármán vortices and the wall boundary layers. Finally, for $\beta =0.7$, the absence of Kármán shedding couples Kelvin–Helmoltz vortices in the shear layer, separating off the cylinder to the wall boundary layer. Full article