Moderate Aspect Ratio Research Articles

(Digital Presentation) The Influence of Pore Geometry on Water Transport in the Gas Diffusion Layer of PEM Fuel Cells

The water transport behavior in the Gas Diffusion Layer (GDL) depends on the pore characteristics, which include the pore size distribution, the wetting properties of the fibers, and the shape of the pore. In this study we investigate the impact of pore geometry on the breakthrough pressure of the GDL. The study shows the importance of the inclusion of different types of pore shapes in the analysis of the water transport behavior. The water invasion process through a pore inlet is simulated using a surface energy minimization model. In a GDL coated with a non-wetting coating such as PTFE, water makes contact with non-wetting fibers and with defects which pin the gas-liquid interface. The mixed-non-wetting (MNW) pore inlet is defined by fiber segments of varied wetting properties, in order to capture the water percolation process in the presence of coating defects. The shape of the pore is quantified according to its aspect ratio (AR). The high aspect ratio pores present the characteristics of micro-cracks. High to moderate aspect ratio pores up to a value of one, corresponding to the unskewed pore, are considered. For wetting pores with AR close to one, the changes in breakthrough pressure are minimal. The breakthrough pressure changes significantly in the case of mixed-non-wetting pores. MNW pores with few coating defects are further investigated by varying the AR and determining the corresponding Laplace pressure for each case. The breakthrough pressure and the droplet volume at breakthrough have direct implication on water management, through their influence on liquid finger formation. The changes in breakthrough pressure with the aspect ratio point to the necessity to include the pore shape distribution in porous media models, to accurately predict the water transport behavior in the GDL.

Bifurcation Instability Modulated by a Connecting Channel Leads to Periodic Water Partitioning in a Simple Channel Network

AbstractWater and mass transport in distributary channel networks play an important role in nourishing fluvial and coastal wetlands, and are largely determined by the morphological configurations of channel bifurcations. While the morphological equilibrium of a single channel bifurcation has been extensively studied, the equilibrium configurations of channel networks with connecting channels linking the bifurcating branches, that is, the “bifurcation‐connecting channel” units that are commonly found in rivers, deltas and estuaries, remain elusive. In this simple yet representative channel network of the “bifurcation‐connecting channel” unit, we observed through numerical simulations an oscillatory water partitioning under moderate Shields stress and channel aspect ratio, in addition to the steady‐state solutions reported in previous studies. The oscillatory water partitioning indicates a newly discovered periodic solution, which is an emergent behavior under constant boundary conditions. We found that the periodic solution is primarily due to the dynamic interactions between bifurcation instability and water surface slope advantage in the two branches modulated by the reversable discharges through the connecting channel, under moderate Shields stress and channel aspect ratio. In such cases, the developed slope advantage in the subordinate branch can suppress the deepening of the dominant branch and eventually lead to the shifting of the dominant branch. In contrast, the channel network attains a steady‐state solution when the slope advantage or the bifurcation instability is dominant with relatively low and high Shields stress (or channel aspect ratio). Our results improve the understanding on the evolution and restoration of channel networks under increasing human interventions in global deltas.

Wing Efficiency Enhancement at Low Reynolds Number

The aerodynamic performance of wings degrades severely at low Reynolds number; lift often becomes non-linear, while drag increases significantly, caused by large extents of separation. Consequently, a non-conventional wing design approach is implemented to assess its ability to enhance performance. The design methodology is that of wing segmentation, where the wing is divided into spanwise panels that can be separated, thereby yielding small gaps between the panels. A moderate aspect ratio wing comprised of four separate wing panels was manufactured and wind tunnel tested through a Re range from 40,000 to 80,000. Force balance data and surface flow visualization were used to characterize performance. The results indicate that segmentation is effective in significantly augmenting efficiency at Reynolds numbers at which the fused wing (i.e., no gaps) shows large extents of open separation. Drag is greatly reduced, while lift is increased, and stall is delayed. The benefit of segmentation was noted to diminish at higher Re where the fused wing’s performance improves markedly. Wing segmentation could find application in micro-unmanned-aerial-vehicle and drone design. Further study would entail the effects of AR and the number of spanwise panels on performance.

Evaluating the influencing mechanism of solvents on the self-assembly synthesis of carbon nitride towards a high photocatalytic performance

The micro-structure of photocatalyst is an underlying factor affecting its activity. This work aims to clarify and get an applicable knowledge concerning the influences of different solvents on the formation of carbon nitride (CN) with different microstructures. In this bottom-up preparation of CN, solvent with smaller molecule size and lower viscosity benefits the insertion, avoiding a bulk structure. Furthermore, solvent with stronger polarity tends to cause rough and irregular surface. The CN sample prepared using ethanol as solvent (CN-E) shows the lowest PL emission and the highest photocatalytic activity compared to its counterparts, owing to its ideal hollow structure with moderate aspect ratio. The results synergistically address that: 1) Solvent with smaller molecule size，lower viscosity and moderate polarity results in an ideal hollow microstructure; 2) this microstructure is beneficial to the mass transfer and further photocatalytic process; 3) light-harvesting and conversion efficiencies are greatly improved thanking to the multi-reflection of light brought by the hollow structure. It can be concluded that the photocatalytic activity of carbon nitride is closely related with its microstructure which can be effectively tuned through the use of solvents, and we expect that by establishing this correlation, applicable knowledge could be acquired and so as to apply.

Collisions among elongated settling particles: The twofold role of turbulence

We study the collision rates of settling spheres and elongated spheroids in homogeneous, isotropic turbulence by means of direct numerical simulations aiming to understand microscale-particle encounters in oceans and lakes. We explore a range of aspect ratios and sizes relevant to the dynamics of plankton and microplastics in water environments. The results presented here confirm that collision rates between elongated particles in a quiescent fluid are more frequent than those among spherical particles in turbulence due to oblique settling. We also demonstrate that turbulence generally enhances collisions among elongated particles as compared to those expected for a random distribution of the same particles settling in a quiescent fluid, although we also find a decrease in collision rates in turbulence for particles of the highest density and moderate aspect ratios (A=5). The increase in the collision rate due to turbulence is found to quickly decrease with aspect ratio, reach a minimum for aspect ratios approximately equal to 5, and then slowly increase again, with an increase up to 50% for the largest aspect ratios investigated. This non-monotonic trend is explained as the result of two competing effects: the increase in the surface area with aspect ratio (beneficial to increase encounter rates) and the alignment of nearby prolate particles in turbulence (reducing the probability of collision). Turbulence mixing is, therefore, partially balanced by rod alignment at high particle aspect ratios.

A Conceptual Model for the Flutter Mechanism in Interlocked Turbine Blades

Abstract Many turbine rotor blades adopt the interlock configuration; among its advantages, there is a large increase in the natural frequencies with respect to the simpler cantilever configuration. Most widespread assessment methods estimate that this frequency increase should lead to a large improvement to the flutter stability of the interlocked rotor rows; some well-known approaches, such as the Panovsky–Kielb stability maps, suggest that flutter should be very unlikely for interlock configurations. Nevertheless, both the experience and detailed simulations indicate that it is possible to find flutter in interlocked blades, even with moderate aspect ratio. In this paper, the authors study the underlying physics behind flutter for interlocked blades, and present a very simplified model that is able to reproduce the main modal characteristics of interlocked blades, including the frequency increase and change in mode-shape as a function of the nodal diameter. This model also identifies a flutter mechanism, which is dependent on the aerodynamic interaction between the bending and torsion motion of the blade; even if both basic motions are individually stable, their interaction may lead to a coupled aeroelastic instability. The results from this simplified model are fully consistent with detailed three-dimensional linearized unsteady computational fluid dynamics simulations of representative turbine rotor blades, and should be of application in many realistic cases of interlock flutter. Finally, the authors present a study on the influence of the main parameters of the simplified model on the flutter stability, and derive some conclusions about the implications on flutter-free interlock design.

Pre-Smectic Ordering and the Unwinding Helix in Monte Carlo Simulations of Cholesteric Liquid-Crystals.

In this paper, molecular chirality is studied for liquid-crystal fluids represented by hard rods with the addition of an attractive chiral dispersion term. Chiral forces between molecular pairs are assumed to be long-ranged and are described in terms of the pseudotensor of Goossens [W. J. A. Goossens, Mol. Cryst. Liq. Cryst. 1971, 12, 237-244]. Following Varga and Jackson [S. Varga and G. Jackson, Chem. Phys. Lett. 2003, 377, 6-12], this is combined with a hard-spherocylinder core. We investigate the relationship between molecular chirality and the helical pitch of the system, which occurs in the absence of full three-dimensional periodic boundary conditions. The dependence of the wavenumber of this pitch on the thermodynamic variables, temperature, and density is measured. We also explore the use of a novel surface boundary interaction model. As a result of this approach, we are able to lower the temperature of the system without the occurrence of nematic droplets, which would interfere with the formation of a uniaxial pitch. Regarding the theoretical predictions of Wensink and Jackson [H. H. Wensink and G. Jackson, J. Chem. Phys. 2009, 130, 234911], on the one hand, we have qualitative agreement with the observed non-monotonic density dependence of the wavenumber. Initially increasing with density, the wavenumber reaches a maximum, before falling as the density moves toward the point of phase transition from cholesteric to smectic. However, further analysis for shorter rods, in the presence of novel boundary conditions, reveals some disagreement with the theory, at least in this case; the unwinding of the cholesteric helix in the cholesteric phase occurs simultaneously with subtle increases in smectic ordering. These pre-smectic fluctuations have not been accounted for so far in theories on cholesterics but turn out to play a key role in controlling the pitch of cholesteric phases of rod-shaped mesogens with a small to moderate aspect ratio.

Heat transfer measurements in an inclined low aspect ratio Rayleigh–Bénard convection cell under feedback control

We report experimental results on the inclined Rayleigh–Bénard convection of a moderate aspect ratio convection cell using a high Prandtl aqueous solution of glycerol. The main goal of the present work is to measure continuously the transition from a convective condition into a purely conduction regime by a smooth quasi-static variation of the tilt angle of the convection cell with respect to the gravity vector in 180° at Rayleigh numbers just above of convection onset. Flow circulation inside the convection cell is indirectly detected by temperature sensors located at the bounding active plates. Changes in the temporal evolution of the heat transfer curve appear directly related to flow transitions between characteristic flow patterns from transversal rolls evolving into an unicellular single roll convective pattern and then a conductive state. For small tilt angles 6°≤γ≤20° time signal fluctuations on both the Nusselt number and temperature readings were observed. Flow visualizations and PIV measurements provided a complementary view of the convective flow pattern being associated to transitions in the Nusselt number evolution with the tilt angle of the cell. For small tilt angle transitions, we observed a loss of rolls process followed by a reorganization of the flow which might be the responsible of the Nusselt unsteadiness.

Gyrokinetic simulations of neoclassical electron transport and bootstrap current generation in tokamak plasmas in the TRIMEG code

For magnetic confinement fusion in tokamak plasmas, some of the limitations to the particle and energy confinement times are caused by turbulence and collisions between particles in toroidal geometry, which determine the “anomalous” and the neoclassical transport, respectively. Neoclassical effects are also responsible for the intrinsically generated bootstrap current, and only the self-consistent modeling of neoclassical and turbulent processes can ultimately give accurate predictive results. In this work, we focus on the implementation of neoclassical physics in the gyrokinetic code TRIMEG, which is a TRIangular MEsh-based Gyrokinetic code that can handle both the closed and open field line geometries of a divertor tokamak. We report on the implementation of a simplified Lorentz collision operator in TRIMEG. For comparison with neoclassical theory, the calculation of flux surface averages is necessary. Since the code uses an unstructured mesh, a procedure for calculating the flux surface averages of particle and energy fluxes and the bootstrap current is derived without relying on the poloidal coordinate, which is useful also for other simulations in unstructured meshes. With the newly implemented collision operator, we study electron transport and bootstrap current generation in a plasma with a finite density gradient but uniform temperature for various simplified and realistic geometries. Compared with neoclassical theory, good agreement is obtained for the large aspect ratio case regarding the particle and energy fluxes as well as the bootstrap current. However, some discrepancies are observed at moderate aspect ratio and for a case with the realistic geometry of the ASDEX Upgrade tokamak. These deviations can be explained by different treatments and approximations in theory and simulation. In this paper, we demonstrate the capability to calculate the electron transport and bootstrap current generation in TRIMEG, which will allow for the self-consistent inclusion of neoclassical effects in gyrokinetic simulations in the future.

Multi-mode instability interactions in twin jet configurations

Twin jet configurations are prone to a wide range of interactions due to flow instabilities driven by jet self-excitation and cross-excitation of one jet on another. These instability modes and associated phases are sensitive to many parameters chief among which is the exit profile of the nozzle. Extensive studies on Twin Circular nozzles and Large Aspect Ratio (AR) Twin rectangular nozzles have shown various mode coupling but, not much is known about twin jets with moderate aspect ratios (AR < 3). This study aims to understand the effect of the exit nozzle geometry on the generation of flow instabilities by comparing a Twin Rectangular nozzle (AR 2) to a Twin Square nozzle (AR 1) with identical design parameters through experimental analysis. High speed Schlieren imaging is coupled with Particle Image Velocimetry (PIV) to explore both qualitative (internal shock cell structure) and quantitative aspects (flow vorticity and turbulence) of these jets. Near field and far field acoustics are also studied to provide a wholistic understanding of the effect of mode coupling exhibited by these jets. Generation and propagation mechanisms of multi-mode instabilities which effect both nozzle orientations are also studied through Spatial and Temporal Fourier analysis.

A Circular Approach for the Valorization of Tomato By-Product in Biodegradable Injected Materials for Horticulture Sector.

This study focuses on the use of tomato (Solanum lycopersicum L.) by-product biomass from industrial plants as reinforcement for designing a range of new degradable and biobased thermoplastic materials. As a novel technique, this fully circular approach enables a promising up-cycling of tomato wastes. After an in-depth morphological study of the degree of reinforcement through SEM and dynamic analysis, mechanical characterization was carried out. Our mechanical results demonstrate that this circular approach is of interest for composite applications. Despite their moderate aspect ratio values (between 1.5 and 2), the tomato by-product-reinforced materials can mechanically compete with existing formulations; PBS-Tomato fiber, for example, exhibits mechanical performance very close to that of PP-flax, especially regarding strength (+11%) and elongation at break (+6%). According to the matrix and particle morphology, a large range of products-biobased and/or degradable, depending on the targeted application-can be designed from tomato cultivation by-products.

Comparative Study of Beam and Plate Theories for Moderate Aspect Ratio Wings

The main objective of the present study is to identify the best structural model for both aeroelastic and structural analysis of unswept rectangular wings with aspect ratios between 2 to 5. The paper presents a detailed study of the similarities and differences between the classical beam and plate theories, which provides new insight into this classical problem. To perform the aeroelastic analysis, the three-dimensional Peters aerodynamic model is coupled with classical plate theory. This combination leads to a new simpler three-dimensional aeroelastic model that can reduce the computational time. The results have been evaluated using both the unpublished and highly cited experimental data. Interestingly, by performing both structural and aeroelastic analyses, a comment can be stated that, even for relatively low aspect ratios () of thin rectangular cantilever plates, the model based on the beam theory is closer to the experimental model compared to the plate theory. This finding may be unexpected by many researchers. Also, using a novel approach, it has been shown that the most significant difference between the beam and plate theories results arises from a subtle difference in the displacement field, which has not been given attention so far.

Shape matters: Competing mechanisms of particle shape segregation.

It is well known that granular mixtures that differ in size or shape segregate when sheared. In the past, two mechanisms have been proposed to describe this effect, and it is unclear if both exist. To settle this question, we consider a bidisperse mixture of spheroids of equal volume in a rotating drum, where the two mechanisms are predicted to act in opposite directions. We present evidence that there are two distinct segregation mechanisms driven by relative overstress. Additionally, we showed that, for nonspherical particles, these two mechanisms (kinetic and gravity) can act in different directions leading to a competition between the effects of the two. As a result, the segregation intensity varies nonmonotonically as a function of aspect ratio (AR), and, at specific points, the segregation direction changes for both prolate and oblate spheroids, explaining the surprising segregation reversal previously reported. Consistent with previous results, we found that the kinetic mechanism is dominant for (almost) spherical particles. Furthermore, for moderate aspect ratios, the kinetic mechanism is responsible for the spherical particles' segregation to the periphery of the drum, and the gravity mechanism plays only a minor role. Whereas, at the extreme values of AR, the gravity mechanism notably increases and overtakes its kinetic counterpart.

Magnetostatic interaction in oriented assembly of elongated nanoparticles

The elements of the magneto-dipole (MD) interaction matrix are calculated for a pair of oriented spheroidal magnetic nanoparticles with a semiaxis ratio a/b = 1.25, 1.5, 2.0 and 3.0 as functions of the distance between the particle centers. It is shown that the spherical approximation for the MD interaction matrix is already incorrect at aspect ratios a/b > 1.25, if the distance R between the particle centers is of the order of particle sizes. However, for moderate particle aspect ratios, a/b < 1.5, the first order correction to spherical approximation with respect to a small parameter (a2 – b2)/R2 is shown to be in a good agreement with numerical data. Using exact MD interaction matrix, the quasi-static hysteresis loops of dilute assemblies of oriented clusters of spheroidal nanoparticles with different filling densities are calculated, depending on the direction of the external magnetic field with respect to the particle orientation axis.

Topological transition and helicity conversion of vortex knots and links

Topological transition and helicity conversion of vortex torus knots and links are studied using direct numerical simulations of the incompressible Navier–Stokes equations. We find three topological transitional routes (viz. merging, reconnection and transition to turbulence) in the evolution of vortex knots and links over a range of torus aspect ratios and winding numbers. The topological transition depends not only on the initial topology but also on the initial geometry of knots/links. For small torus aspect ratios, the initially knotted or linked vortex tube rapidly merges into a vortex ring with a complete helicity conversion from the writhe and link components to the twist. For large torus aspect ratios, the vortex knot or link is untied into upper and lower coiled loops via the first vortex reconnection, with a helicity fluctuation including loss of writhe and link, and generation of twist. Then, the relatively unstable lower loop can undergo a secondary reconnection to split into multiple small vortices with a similar helicity fluctuation. Surprisingly, for moderate torus aspect ratios, the incomplete reconnection of tangled vortex loops together with strong vortex interactions triggers transition to turbulence, in which the topological helicity decomposition fails due to the breakdown of vortex core lines.

Pitch/Plunge Equivalence for Dynamic Stall of Unswept Finite Wings

Pitch/plunge equivalence for deep dynamic stall of a straight finite wing is considered by employing large-eddy simulations. The flowfields are computed with a well-established time-implicit high-fidelity large-eddy simulation approach. The wing has a moderate aspect ratio of and a NACA 0012 section incorporating a rounded tip. The flow parameters are a freestream Mach number of and chord Reynolds numbers of . Pure pitching and equivalent plunging maneuvers are considered with a reduced frequency of and minimum and maximum effective angles of attack of 4 and 22 deg, respectively. The pitch and plunge dynamic stall cases are compared in detail in terms of the unsteady three-dimensional flow structure, surface pressure, and loading. Differences in the lift and moment coefficient are shown to be correctable, employing a steady thin-airfoil theory for a cambered airfoil that accounts for the rotation-induced apparent camber present in the pitching case. This pitch/plunge equivalence is extendable to the dynamic stall of a swept wing, as demonstrated in another paper by the authors [“Pitch–Plunge Equivalence for Dynamic Stall of Swept Finite Wings,” AIAA Journal (submitted for publication)].

Optimal transport of surface-actuated microswimmers

We analyze the transport behavior of surface-actuated spheroidal microswimmers that locomote steadily with or without a spatiotemporally uniform external forcing. The surface actuation is in the form of either a tangential surface motion or a zero-net-mass-flux wall-normal transpiration. Starting from a general modal expansion in terms of an appropriate basis set, we link the surface actuation, the force exerted on the spheroid, and its forward speed through a Stokesian representation of the microhydrodynamics. Our analysis is generic and enables a systematic investigation over the complete range of aspect ratios from zero (streamlined needlelike spheroid) to infinity (disc-shaped spheroid). We identify a critical aspect ratio of 1.82 below and above which tangential and wall-normal surface actuations enable transport at minimal energetic cost, irrespective of whether the spheroidal microswimmer is free or forced. Crucially, we find the propulsive performance of a forced spheroidal swimmer to be appreciably higher than the one of an analogous self-propelled swimmer. Most importantly, the optimal energy expenditure minimizing tangential or wall-normal surface actuation for forced transport is passive overall so that the power requirement arises solely from the rate at which work is done by the external forcing. We highlight the complementing roles of external forcing and surface actuation over moderate and extreme aspect ratios and also exemplify the crucial disparities between optimal transport in free and forced environments. Our results indicate that a combination of external forcing and an optimal surface actuation could substantially enhance the transport of generic streamlined and bluff microswimmers.

Deconvolution of ferromagnetic resonance spectrum of magnetic nanoparticle assembly using genetic algorithm

The ferromagnetic resonance (FMR) spectra of dilute random assemblies of magnetite nanoparticles with cubic magnetic anisotropy and various aspect ratios are calculated using the stochastic Landau–Lifshitz equation at a finite temperature, T = 300 K, taking into account the thermal fluctuations of the particle magnetic moments. Particles of non-spherical shape in the first approximation are described as elongated spheroids with a given semiaxes ratio a/b, where a and b are the long and transverse semiaxes of a spheroid, respectively. A representative database of FMR spectra is created for assemblies of randomly oriented spheroidal magnetite nanoparticles with various transverse diameters D = 5–25 nm, moderate aspect ratios a/b = 1.0–1.8, and magnetic damping constants κ = 0.1, 0.2. The basic FMR spectra of assemblies with D = 25 nm at different aspect ratios can be considered as representatives of assemblies of single-domain magnetite nanoparticles with transverse diameters D > 25 nm. The database is calculated at exciting frequency f = 4.9 GHz (S-band) to clarify the details of the FMR spectrum that depend on the particle magnetic anisotropy nature. The data obtained make it possible to analyze arbitrary combined FMR spectra constructed as weighted linear combinations of FMR spectra of the base assemblies. In addition, using a genetic algorithm, the corresponding inverse problem is solved. The latter consists in determining the volume fractions of the base assemblies in some arbitrary nanoparticle assembly, which is represented by its FMR spectrum.

Periodically driven spheroid in a viscous fluid at low Reynolds numbers

In this paper, we study the motion of a spheroid of a moderate aspect ratio in a viscous fluid under the action of an external harmonic force. We first derive the dynamics equation of the particle oscillating along one of its axes and subject to damping, Basset memory, and second history integral forces at small Reynolds numbers, and then, we proceed to obtain an analytical solution of this equation at resonance. With graphical representation, we observe that for a prolate spheroid, the conventional Q-curves show a greater variation with respect to the particle aspect ratio, particle–fluid density ratio, and natural frequency; the variation is significantly larger for the curve corresponding to the second history force. Furthermore, we find that all three forces affect the amplitude of motion: the amplitude increases with the strength of damping as well as the second history integral forces, whereas the presence of Basset memory decreases it. Remarkably, Basset memory causes a phase-shift in the oscillations, while the other two forces have no effect on the phase. Since our solutions are analytical, they may have valuable application in experiments involving more complex systems, in particular, to understand the effect of external force on the transport of micro-particles.

Magnetostatic Interaction in Oriented Assembly of Elongated Nanoparticles

The elements of the magneto-dipole (MD) interaction matrix are calculated for a pair of oriented spheroidal magnetic nanoparticles with a semiaxes ratio a/b = 1.25, 1.5, 2.0 and 3.0 as a function of the distance between the particle centers. It is shown that the spherical approximation for MD interaction matrix is incorrect already at aspect ratios a/b > 1.25, if the distance R between the particle centers is of the order of particle sizes. However, for moderate particle aspect ratios, a/b < 1.5, the first order correction to spherical approximation with respect to a corresponding small parameter is shown to be in a good agreement with numerical data. Using exact MD interaction matrix, the quasi-static hysteresis loops of dilute assemblies of oriented clusters of spheroidal nanoparticles with different filling densities are calculated, depending on the direction of the external magnetic field with respect to the particle orientation axis.