Low Aspect Ratio Research Articles

Investigation of fatigue behavior of laser powder bed fusion Ti-6Al-4V: Roles of heat treatment and microstructure

High cycle fatigue (HCF) performance of additive manufactured (AM) titanium products is of great significance for its structural and functional applications. However, tying fatigue performance to the complex microstructures in AM titanium alloys is challenging. The work here carried out a thorough investigation into the influence of microstructures (particularly the grain boundary α-phase (GB-α) with varied morphologies) on the fatigue performance of laser powder bed fusion (LPBF) Ti–6Al–4V (Ti-64). Results showed that the improvement in the high-cycle fatigue life of LPBF Ti-64 could be achieved by the formation of low aspect ratio α lath and discontinuous GB-α via optimized post-fabrication heat treatment. Discontinuous GB-α could fully accommodate the deformation, improving the fatigue crack propagation resistance. Moreover, α lath with a low aspect ratio could lead to less strain accumulation on the interface between adjacent α lath, and thereby inhibit the crack initiation at these interfaces. This study enhances the understanding of how LPBF-induced complex microstructures influence fatigue behavior, and provides a pathway for the improvement of fatigue performance of additive manufactured titanium alloy.

Transient thrust behavior of low‐aspect‐ratio rocket propulsion systems**

AbstractIn this work, the transient thrust force of the solid propellant rocket motor is numerically computed to illustrate the shortcomings of the common thrust formula in transient periods of operation. The internal ballistics of the solid rocket motor is simulated using the quasi one‐dimensional governing equations in which the finite volume method and the Roe′s scheme are utilized in calculation procedure. This is demonstrated that in transient operation of the propulsion systems, the common thrust formula may ignore some parts of physical phenomena because it has been derived based on steady state assumption, but there are some moving waves in transient operation in the combustion chamber which induce the fluctuations on the thrust force. So, the thrust‐time history must be computed by pressure force integration over the whole solid boundaries of the motor at any instant. The results show that such transient thrust behavior is more intense in low aspect ratio propulsion systems like small space thrusters.

Experimental Study: The Effect of Pore Shape, Geometrical Heterogeneity, and Flow Rate on the Repetitive Two-Phase Fluid Transport in Microfluidic Porous Media.

Geologic subsurface energy storage, such as porous-media compressed-air energy storage (PM-CAES) and underground hydrogen storage (UHS), involves the multi-phase fluid transport in structurally disordered or heterogeneous porous media (e.g., soils and rocks). Furthermore, such multi-phase fluid transport is likely to repeatedly occur due to successive fluid injections and extractions, thus, resulting in cyclic drainage-imbibition processes. To complement our preceding study, we conducted a follow-up study with microfluidic pore-network devices with a square solid shape (Type II) to further advance our understanding on the effect of the pore shape (aspect ratio, Type I: 5-6 > Type II: ~1), pore-space heterogeneity (coefficient of variation, COV = 0, 0.25, and 0.5), and flow rates (Q = 0.01 and 0.1 mL/min) on the repetitive two-phase fluid flow in general porous media. The influence of pore shape and pore-space heterogeneity were observed to be more prominent when the flow rate was low (e.g., Q = 0.01 mL/min in this study) on the examined outcomes, including the drainage and imbibition patterns, the similarity of those patterns between repeated steps, the sweep efficiency and residual saturation of the nonwetting fluid, and fluid pressure. On the other hand, a higher flow rate (e.g., Q = 0.1 mL/min in this study) appeared to outweigh those factors for the Type II structure, owing to the low aspect ratio (~1). It was also suggested that the flow morphology, sweep efficiency, residual saturation, and required pressure gradient may not severely fluctuate during the repeated drainage--imbibition processes; instead, becoming stabilized after 4-5 cycles, regardless of the aspect ratio, COV, and Q. Implications of the study results for PM-CAES and UHS are discussed as a complementary analysis at the end of this manuscript.

Thermal performance of rectangular serpentine mini-channel cooling system on lithium battery

The energy crisis and environmental damage urgently require clean and efficient energy conversion to replace the traditional petroleum fuel combustion. Lithium battery is an efficient energy storage and conversion system, and its use can effectively reduce the use of fossil fuels. However, its extremely sensitive temperature characteristics seriously limit its performance. Therefore, it is of great significance to design an efficient battery cooling system to protect the performance of lithium batteries. In this paper, a novel serpentine mini-channel cooling management system for lithium battery is studied with experiment research and numerical simulation. The simulation results and experimental data are in good agreement. The difference between the two results is roughly within 1 K. The maximum battery temperature increases with the increase of the discharge current. The cooling system has high efficiency and made battery temperature drop rapidly. When the Reynolds number is 200, the maximum battery temperature decreases from 327.4 K to 306.9 K at 30 A discharge current, from 313.5 K to 300.4 K at 15 A discharge current, and from 306.1 K to 298.7 K at 7.5 A discharge current. The maximum battery temperature drops rapidly when the Reynolds number ranges from 0 to 40 and decreases slowly from 40 to 200. Whereas, the Reynolds number has little effect on the battery temperature for Re is over 200. The low inlet temperature and high aspect ratio are beneficial for heat dissipation. The cooling effect of ethylene glycol is stronger than that of water and ammonia. When the Reynolds number is 200, ethylene glycol, water and ammonia reduce the temperature by 21.6 K, 20.6 K and 14.4 K respectively. The results provide a good guidance for the application and further study of serpentine mini-channel in lithium battery.

Effect of platy gangue minerals in sulfide flotation: Part 2: Mechanisms

The presence of the usually unwanted, hydrophilic ore particles (gangue) in a mineral flotation froth plays a critical role. While they reduce the grade of the concentrate, they are also an important influence on the dynamical structure of the froth and its ability to transport value particles into the launder for ultimate recovery. This paper discusses how the shape and size of these gangue particles affect their transport through the froth zone under conditions typical of commercial sulfide flotation, but without hydrophobic ore particles present.Advective transport of gangue particles via entrainment depends on slurry drag due to the upward motion of air bubbles and the drainage effects of gravity on the liquid and solid phases confined in the network of fluid channels in the froth. In a companion paper (Bhambhani et al., 2023a) a significant increase in gangue transport rates through the froth zone was observed when a copper sulfide ore was mixed with platy mica (higher aspect ratio) compared to when the same ore was mixed with globular (lower aspect ratio) silica. It was hypothesized that this increased upwards transport rate was related to the greater upward drag experienced by the platy mica particles as compared to the lower aspect-ratio silica particles of comparable size. This hypothesis was tested by conducting froth sampling experiments, reported here, under increased upward drag conditions for the suspension and thus increased upward advective forces on the particles. It was observed that the entrainment vs particle size curves shifted upwards for the coarser sizes for both mica and silica. This was unexpected. It was additionally hypothesized that as the particle size approached the size of the channels (Plateau borders), through which settling particles must pass if they are to move downward, the particles become more confined and on average experience less downward motion. Platy mica particles of a certain sieve size are more susceptible to confinement in the channels than their lower aspect-ratio counterparts due to their reduced mass per particle. This was suggested from froth sampling experiments where drag and drainage were balanced in one case, and more drag dominated in the other.

Strong effect of liquid Fe–S on elastic wave velocity of olivine aggregate: Implication for the low velocity anomaly at the base of the lunar mantle

The origin of the seismic low velocity zone (LVZ) observed at the base of the lunar mantle is important for understanding the nature of the lunar mantle. Liquid Fe–S is considered as a possible candidate of the cause of the LVZ. However, effect of liquid Fe–S on elastic wave velocity of mantle rock has been considered to be minor, according to theoretical estimation based on the high dihedral angle of liquid Fe–S in olivine aggregate observed in previous microstructural analysis of polished sample cross sections. Here we carry out direct measurements of elastic wave velocities of liquid Fe–S-bearing olivine aggregate and report strong reduction of the elastic wave velocities in the presence of liquid Fe–S. The effects of liquid Fe–S in olivine aggregates are much more pronounced than the theoretical estimation. Three-dimensional X-ray microtomography analysis reveals that the geometry of liquid Fe–S strongly depends on the size, with larger liquid Fe–S blobs having low aspect ratios, which may predominantly affect elastic wave velocity. Since smaller spherical Fe–S blobs are much more abundant than the larger elongated blobs, conventional dihedral angle analyses based on scanning electron microscope images tend to statistically over-sample smaller blobs and under-sample larger blobs. Our results suggest that the LVZ at the base of the lunar mantle can be explained by the presence of 4.3–5.0 vol.% liquid Fe–S. The LVZ may be a hidden reservoir of highly siderophile elements (HSEs), which may explain the low abundance of HSEs in the lunar mantle compared to the Earth.

Comparison of Velocity and Reynolds Stress Distributions in a Straight Rectangular Channel with Submerged and Emergent Vegetation

Vegetation in rivers and streams plays an important role in preventing erosion and improving bank stability. Comparison between emergent vegetation (bank vegetation) and submerged vegetation, in terms of velocity and Reynolds stress distributions, for the same aspect ratio and flow discharge, has received limited attention in the literature. This study investigates the velocity and Reynolds stress, as well as the log law for submerged and emergent vegetation in a laboratory flume and compares the results for a different set up with different sediment size and aspect ratio but the same discharge. The results indicate that the influence of submerged vegetation on the secondary currents generation is less than emergent vegetation. In addition, the log law application is valid for both submerged and emergent vegetation cases, however, it is valid up to y/h = 0.75 for emergent vegetation (in which the vegetation cover in banks is partly out of the water) but up to y/h = 0.25 for vegetation bank. For both submerged and emergent vegetation, Reynolds stress distribution presents a convex form but with a different turning point. Comparison of the results with those in an artificial pool over submerged vegetation and low aspect ratio (<5) keeps almost the same form for velocity and Reynolds stress distributions but decreases the turning point in Reynolds stress distribution. For the submerged vegetation cover, the location of zero shear stress superposes that of maximum velocity, but for the emergent vegetation approaching the bank vegetation and shifting the maximum velocity towards the bed, the location of zero shear stress approaches the bed.

Examination and Prediction of the Lift Components of Low Aspect Ratio Rectangular Flat Plate Wings

An investigation of the lift components present over low aspect ratio rectangular wings is presented. Wing aspect ratios ranging from 0.5 to 3 are examined using published experimental results and analytic analysis methods. The methods are based on the fundamental decomposition of Polhamus; that is, lift is attributed to a potential flow lift component coupled with a vortical lift component stemming from the leading and side edges of the flat plate wing. The analysis suggests a low sensitivity to Reynolds numbers spanning three orders of magnitude and brings into doubt the realization of a leading edge vortex lift component for wings with unswept leading edges under steady state conditions. The analytic prediction method of Purvis is shown to provide close accord with all experimental data sets when lift contributions caused by a leading edge vortex are excluded.

Process window for electron beam melting of Ti–42Nb wt.%

Pre-alloyed β-phase Ti˗42Nb alloy was successfully produced for the first time by E-PBF. The study focuses on the determination of the processing parameter window by varying the beam current, beam speed, layer thickness, and line offset to achieve the defect-free manufacturing of new material with desired properties. Overall, 49 regimes were investigated. The Ti˗42Nb powder were characterized using the DSC/TG, XRD, and SEM/EDX analyses to evaluate its suitability for E-PBF manufacturing. The alloys with the best-built quality fall into the narrow zone between the line energies of 0.30 and 0.34 J/mm. The predicted optimal process parameters were I = 4 mA, v = 700–800 mm/s, h = 100 μm, U = 60 kV, and t = 100 μm. Detailed microstructural characterization was carried out to gain insights into the fundamental mechanisms that govern the behavior of the studied alloys. TEM identified the α'' martensitic phase nucleation occurred preferentially at the β grain boundaries. Un-melted ellipsoidal NbC (∼10 μm) particles were detected with no preferential segregation sites. EBSD revealed coarse microstructures and <001> fiber texture, as well as epitaxial grain growth of columnar grains of about 300 μm. The optimal regime demonstrated a texture composed of a high amount of low aspect ratio grains (50%), which yielded a microindentation hardness of 3.0 GPa and a low elastic modulus of 68 GPa. Hence, these results provide opportunities to design novel alloys to be of interest for biomedical applications. Moreover, this study extends the scope of AM by establishing the process parameter window that yields a material with favorable mechanical properties.

Effect of aspect ratio of synthetic Wollastonite nanofibers on mechanical, thermal and flammability properties of polyoxymethylene nanocomposites

AbstractPolyoxymethylene (POM) nanocomposites were reinforced with 1 phr of synthetic wollastonite nanofibers (SWN) to investigate the effect of the aspect ratio of SWN on the mechanical, thermal and flammability properties. The tensile strength, Young's modulus, and impact strength of POM nanocomposites demonstrated an increasing trend as the aspect ratio of SWN increased. POM nanocomposites incorporated SWN with an aspect ratio of about 20 (POM/SWN‐20) exhibited higher tensile strength (2%), Young's modulus (3%) and impact strength (50%) as compared to that of about 5 (POM/SWN‐5). A higher aspect ratio of SWN also contributed to the better thermal stability of POM nanocomposites, in which the initial degradation temperature of POM/SWN‐20 was about 6°C higher than POM/SWN‐5. Interestingly, SWN that possessed a lower aspect ratio performed better in flame retardancy, as indicated by the increase in the time to ignition and the decrease in the peak heat release rate. This present study demonstrates the relationship between the aspect ratio of filler (SWN) with the polymer matrix (POM), which is crucial in producing nanocomposites that have great potential in automotive applications.

Nonlinear thermal post-buckling analysis of rectangular FG plates using CUF

In this study, a numerical analysis examines the thermal post-buckling response of rectangular functionally graded (FG) plates. Since the post-buckling is analyzed in a thermal environment, temperature distribution across the thickness is computed by an accurate numerical method. Here, for the geometrically nonlinear analysis because of assuming large displacement, FEM is utilized according to Carrera Unified Formulation (CUF), a precise formulation and higher-order deformation theory. The principle of virtual work is employed to achieve the nonlinear equilibrium equations. The governing equations are solved by operating the arc-length method. The influence of parameters such as geometric aspect ratio, length-to-thickness ratio, boundary conditions, type of temperature distribution, and volume fraction index on the FG plates’ temperature-deflection path has been investigated. In addition, a comparison among the present study method with another reference about thermal post-buckling and also linear buckling analysis (LBA) is performed. Nonlinear post-buckling analysis shows that the bifurcation point occurs only at fully clamped boundary conditions and low aspect ratios. It proved the results of LBA in the mentioned state are verifiable. The outcomes confirm the usefulness of utilizing CUF on thermal post-buckling since the temperature curves in the figures are at a lower level compared to the similar reference.

Seismic performance of short precast columns with UHPC grouted sleeve connections: An experimental and numerical study

Grouted sleeve connections have been developed in various precast structures such as columns in buildings and bridges. When they are used in precast columns, the aspect ratio of the column is reduced owing to the shifted plastic hinge phenomenon. In contrast, short columns with aspect ratios of less than three are susceptible to shear failure under earthquakes. This study investigated the effect of UHPC (ultra-high-performance concrete) grouted sleeve connections on the seismic performance of precast columns. One cast-in-place column (CIP) and two precast columns (PCGS1 and PCGS2) with an original aspect ratio of 3.11 were manufactured. The two precast columns were designed with grouted sleeve connections at different positions, which were supposed to have lower actual aspect ratios according to the shifted plastic-hinging phenomenon. Cyclic loading tests were conducted on all the columns to study their seismic performance. The damage progression and seismic responses were analyzed experimentally. Shifted plastic hinging was theoretically calculated, and a numerical model was developed using OpenSees platform. From the acquired results, the precast column PCGS1, which was connected by grouted sleeve connections at the bottom, failed in shear failure. The other two columns failed in flexure-shear failure. The seismic performance of precast column PCGS1 was significantly reduced through a comparison of the hysteretic responses. A calculation method was proposed to obtain the critical height of the grouted sleeve connections, which helped the short columns avoid shear failure during earthquakes. The numerical analysis showed that the location of grouted sleeves had a negligible effect on seismic performance when placed above the critical height. The increase in the compression ratio and rebar strength positively affected the bearing capacity but negatively influenced the ductility.

Performance of Savonius Turbines with Tubercles Inspired by Humpback Whales

Recent studies found that tubercles had been integrated into various applications beyond nature to enhance performance. Therefore, the present work investigates the performance of a Savonius turbine with various tubercle configurations at a wind speed of 7 m/s which corresponds to a Reynolds number of 148,000. The Savonius turbines with tubercle features were initially designed and inspired by the distribution of tubercles on the flippers of humpback whales. However, only the models with the best performance were chosen for fabrication and tested in the wind tunnel. The new turbine designs have 33% of the blade without tubercles, with turbine model 1 (D1) having a large tubercle beginning in the midspan and tapering towards the endplates, while turbine model 2 (D3) applies the 33% without tubercles in the midspan of the blade. The results demonstrated that turbine model 1 had the highest maximum Cp of 0.19 at the tip speed ratio of 0.78 and indicated that the spanwise location of tubercles in which it is integrated into the turbine is a significant geometric arrangement to improve the performance. The new turbine models designed with tubercles performed significantly better than the baseline model, with 46.15% and 23.08% improvements, respectively. The flow visualization for both models with tubercle configurations showed smaller wake sizes that represent reduced drag and eventually produced higher performance in comparison with the baseline. The results provide insights into the implementation of the tubercles concept on a Savonius turbine with a low aspect ratio.

Mechanical behavior of splice joints for bridge columns connected by grouted sleeves

Grouted sleeves are widely adopted in accelerated bridge construction (ABC), and connecting column segments by splice joints. As a part of column structure in prefabricated bridges, mechanical behavior of splice joint under seismic load is paid less attention. 4 scaled precast columns connected by grouted sleeves (PCCGS) with low aspect ratio under cyclic lateral loading were performed, giving the damage mode, force and deformation ability of splice joints. The splice joint of specimen was then sliced to investigate the inner failure at the joint surface and obtain bond-slip of contact between steel and grout. Finite element model (FEM) with refined mesh was established to reveal the damage mechanism of splice joint and lateral force and displacement of column top. A computational model was proposed to predict the force-displacement of splice joint, innovatively considering bond and slip between rebar and sleeve. The FE model and computing model are helpful to conduct seismic designing of PCCGS.

Effective electromechanical properties and energy harvesting response in PMN-0.3PT/PDMS flexible piezoelectric composites: a combined experimental and theoretical study

In this work, lead magnesium niobate-lead titanate (PMN-0.3PT) and polydimethylsiloxane (PDMS)-based flexible piezoelectric-polymer composites are designd and developd for efficient mechanical energy harvesting through a combined experimental-theoretical approach. A solid-state reaction method was employed to synthesize PMN-0.3PT piezo-ceramic, which was subsequently used for the fabrication of -PMN-0.3PT/PDMS piezoelectric-polymer 0–3 composite with different volume fractions, = 0.03, 0.25, and 0.50 of PMN-0.3PT reinforcement. Uniformly distributed PMN-0.3PT particles were found to retain their structural symmetry across the volume fractions and are well adhered to the PDMS matrix. The effective electromechanical properties of the composites were measured and compared with model predictions employing the finite element method and Eshelby–Mori–Tanaka-based micromechanical models. Considering that flexibility is a critical design parameter, we propose a new figure-of-merit term that would consider both electromechanical conversion as well as the mechanical flexibility of the material. We show that at = 0.5, PMN-0.3PT/PDMS 0–3 composite yields an optimum combination of energy harvesting performance and flexibility. Our study further demonstrates that the orientation of the PMN-0.3PT particles does not significantly influence the effective elastic and dielectric properties at low and moderate PMN-PT content, attributed to the lower aspect ratio of the reinforcement particles. The piezoelectric charge coefficient showed small yet finite change with increasing reinforcement content. A maximum current density, 35 nA cm−2, and electric field, 90 V cm−1, was obtained with a cyclic compressive stress of 0.22 MPa (force, 50 N) at 5 Hz, in a piezoelectric generator, based on = 0.5.

Aspect ratio effect on flow past an elliptical cylinder near a moving wall

In this paper, flow over an elliptical cylinder located near a moving wall is investigated numerically to ascertain the effect of aspect ratio (AR) on flow characteristics in terms of wake pattern, separation and stagnation points, and hydrodynamic forces. The two-dimensional simulations span both steady and unsteady flow regimes, where the effect of AR is analyzed in conjunction with other important parameters, such as gap ratio (GR∈ [0.6, 1.2]), Reynolds number (Re ∈ [5, 150]), and angle of attack (AOA∈ [−45°, 45°]). At a low AR = 0.1, significant differences in the wake pattern and critical Re for steady to unsteady wake transition are observed in GR-Re space in comparison with previous results for high AR cylinders. This results from an influence of AR on vorticity convection into the wake. Additionally, the effect of variation in AR from a flat plate (AR = 0) to a circular cylinder (AR = 1) on the wake pattern is discussed and presented in AR-Re space at a low GR. The locations of separation and stagnation points are shown to result from an interplay of AR and wall suppression depicting a low sensitivity to Re with low AR or high wall suppression effect. Subsequently, the effect of AR in conjunction with AOA is analyzed where counterclockwise inclination is observed to reduce wall suppression, while a competing effect arising from the contrasting movements of cylinder bottom and rear side governs the wake pattern for clockwise inclination that is explicit at a certain AR. Eventually, the variations in drag and lift forces with AR for different AOA are characterized.

Finite element analysis of unreinforced masonry walls with different bond patterns

Masonry is the oldest building material, yet it is also the least understood due to the non-linear and composite nature of masonry, which consists of brick units, mortar, and unit-mortar contact. In this paper, the response of a two-dimensional masonry wall with a window opening subjected to an in-plane lateral pushover loading is simulated by varying the interface properties of brick such as crushing, elastic, cracking, and shear properties. The simplified micro-modeling technique with the Engineering Masonry model for bricks and linear stiffness properties for the interfaces in the bed and head joints is employed to investigate the geometric nonlinear behavior of the masonry wall. The pushover curves obtained from the numerical simulations indicate that there is a significant influence on the lateral load response of the wall due to elastic, crushing, and shear parameters while the cracking parameters have less impact on the ductile capacity of the structure. Moreover, the study is also extended to examine the effect of bond patterns such as English, Stretcher, Flemish, and Header bond with varied aspect ratios of 1,1.5 and 0.75. In all four bond patterns, it was observed that the walls with lower aspect ratios exhibited higher strength. Further, in comparison to the other bond patterns, walls with the Flemish bond pattern demonstrated higher strengths at both lower and higher aspect ratios.

Geochemical and morphological characterization of particles originating from tunnel construction

Rock particles from drilling and blasting during tunnel construction (DB particles) are released to the aquatic environment where they may cause negative toxicological and ecological effects. However, there exists little research on the difference in morphology and structure of these particles. Despite this DB particles are assumed to be sharper and more angular than naturally eroded particles (NE particles), and in consequence cause greater mechanical abrasion to biota. Moreover, morphology of DB particles is assumed to depend on geology, thus depending on where construction takes place different morphologies may be emitted. The objectives in the current study were to investigate the morphological differences between DB and NE particles, and the influence of mineral and elemental content on DB particles. Particle geochemistry and morphology were characterized by inductively coupled plasma mass spectrometry, micro-X-ray fluorescence, X-ray diffraction, environmental scanning electron microscope interfaced with energy dispersive X-ray, stereo microscope, dynamic image analysis and coulter counter. DB particles (61–91% < 63 μm) collected from five different tunnel construction locations in Norway were 8–15% more elongated (lower aspect ratio) than NE particles from river water and sediments, although their angularity was similar (solidity; diff 0.3–0.8%). Despite distinct mineral and elemental characteristics between tunnel construction locations, DB morphology was not explained by geochemical content since only 2–2.1% of the variance was explained. This suggests that particle formation mechanisms during drilling and blasting are more influential of morphology than mineralogy, when working in granite-gneiss terrain. When tunnelling in granite-gneiss terrain, particles with greater elongation than natural particles may enter aquatic systems.

Production of nanocellulose using controlled acid hydrolysis from large-scale production of micro-fibrillated cellulose derived from oil palm empty fruit bunches

Nanocellulose is generally known as a versatile material, which is suitable for various applications due to its unique physicochemical properties, including light weight, ease of tunable surface functionalization, and excellent mechanical properties. This research aims to characterize and synthesize nanocellulose produced from acid hydrolysis of large-scaled micro-fibrillated cellulose (MFC) derived from oil palm empty fruit bunches by varying concentrations of H2SO4 from 20 to 35 v/v %. The obtained large-scaled MFC had a density of 1.01 kg/m3 and was dominantly composed of 71% cellulose. After acid hydrolysis of MFC, there were gradual changes in the colour of the obtained cellulose nanocrystals (CNC) dispersion from light white to a darker color with the increase of acid concentration, in which the over hydrolysis occurs at 35% of acid concentration. The use of 25-30% H2SO4 showed the optimum condition to avoid over-hydrolysis and resulted in bright white color of CNC dispersion with excellent stability at zeta potential value of -74.2±0.1 to -88.4±0.2 mV. It was supported by Fourier transform infrared (FTIR) due to the presence of negatively charged sulfonyl and hydroxyl groups upon CNC formation to offer excellent dispersion stability. Based on transmission electron microscope (TEM), rod-like shape CNC with a low aspect ratio of 11.8 at the dimension of 12.8 ± 6.7 nm in width and 151.9 ± 38.3 nm in length was successfully produced. Based on X-ray diffraction (XRD) analysis, the crystallinity of the sample was 76%.

Effect of marginal variation of aspect ratio and frequency with kinematic asymmetry on flow field around flapping rigid wing in hover

Phase-locked two dimensional particle image velocimetry (PIV) measurements are used to capture flow field around rigid flat plate wings undergoing main flapping motion (hover) with water as fluid medium in quiescent condition. Experiments are conducted using asymmetric upper-lower stroke single degree of freedom main flapping motion. Two different aspect ratio (AR) 1.5 and 1.0 rectangular wings at 1.5 Hz and 2.0 Hz flapping frequency and chord based Reynolds number of the order of 104 are studied. To achieve the desired aspect ratio, wing span is varied while wing chord remains fixed. Flow features for downstroke and upstroke are comparable for all four cases investigated. This includes vortex shedding from wingtip during wing stroke, vorticity intensification caused due to vortex stretching which occurs more dominantly for AR = 1.0, occurrence of KH instability in shear layer formed due to wing sweep and added mass effect due to the flow dragged by flapping wing. Vortex filamentation and fragmentation phenomena are observed during downstroke and upstroke respectively due to faster downstroke and slower upstroke. Rossby number, which is the ratio of inertial to Coriolis force is significant for rotational flapping wing. With increase in Rossby number there is a reduction in the spanwise flow over wing which influences strength of wingtip vortex at beginning of a stroke. It is found that spanwise flow for AR = 1.5 is stronger than AR = 1.0 and consequently the strength of wingtip vortex is higher for higher aspect ratio. However, peak vorticity is higher for AR = 1.0 due to vortex stretching effect which is expected to be more dominant at lower AR due to three dimensional effects. Effect of marginal variation in wing aspect ratio, flapping frequency with kinematic asymmetry are studied using velocity field, vorticity (ωz) contours, λ2 criterion, peak vorticity variation and its spatial distribution as well as kinetic energy variation of the flow field. Kinetic energy distribution and its connection with wake capture is explored.