We report the design concept and fabrication of MRI phantoms, containing blocks of aligned microcapillaires that can be stacked into larger arrays to construct diameter distribution phantoms or ...fractured, to create a “powder-averaged” emulsion of randomly oriented blocks for vetting or calibrating advanced MRI methods, that is, diffusion tensor imaging, AxCaliber MRI, MAP-MRI, and multiple pulsed field gradient or double diffusion-encoded microstructure imaging methods. The goal was to create a susceptibility-matched microscopically anisotropic but macroscopically isotropic phantom with a ground truth diameter that could be used to vet advanced diffusion methods for diameter determination in fibrous tissues. Two-photon polymerization, a novel three-dimensional printing method is used to fabricate blocks of capillaries. Double diffusion encoding methods were employed and analyzed to estimate the expected MRI diameter. Susceptibility-matched microcapillary blocks or modules that can be assembled into large-scale MRI phantoms have been fabricated and measured using advanced diffusion methods, resulting in microscopic anisotropy and random orientation. This phantom can vet and calibrate various advanced MRI methods and multiple pulsed field gradient or diffusion-encoded microstructure imaging methods. We demonstrated that two double diffusion encoding methods underestimated the ground truth diameter.
To account for the random orientation of ellipsoidal inhomogeneities, the averaging process over two Euler angles has been widely used in existing micromechanics models. However, the interaction ...among various inhomogeneity orientations is often overlooked and may also affect the prediction of composite elastic modulus. In this work, to consider the interaction among randomly oriented aggregates and fibers in cement concrete, a micromechanics model, the orientation interaction model (OIM), is proposed. An orientation average model (OAM) is also proposed, and by comparing OIM and OAM predictions, the effect of inhomogeneity orientation interaction on the predicted elastic modulus is shown. In both models, geometries of coarse aggregates and fibers are approximated using ellipsoids with three different semiaxes and cylinders, respectively, and their orientations are described using three Euler angles. Compared with existing random orientation models using two Euler angles, the proposed models can capture the isotropy of composites with randomly oriented asymmetric ellipsoidal inhomogeneities. By comparing the predictions of OIM, OAM, composite sphere models and the Mori-Tanaka method with spherical inhomogeneities, it is found that with the increase of the stiffness contrast between the matrix and inhomogeneities, the effects of the inhomogeneity geometry and orientation interaction become more apparent. The model predictions agree well with the experimental data. By changing the value range of three Euler angles based on the inhomogeneity orientation distribution, both models are also suitable for transversely isotropic and anisotropic composites containing several types of inhomogeneities.
•The proposed models apply to composites with asymmetric ellipsoidal inhomogeneities.•Every parameter in the proposed models has a physical meaning, and model calibration is not needed.•The proposed models apply to transversely isotropic and anisotropic composites with several types of inhomogeneities.
•Statistically–founded model for viscous flow through fibrous filter.•Validated by experiments in variety of technical filters.•Pressure–drop correlation for wide parameter range.•Representative ...domain size and effects of fibre orientation included.
Clean air or gas is critical e.g. in many industrial or health applications. Various filtration processes are employed to ensure that liquid droplets and/or solid particles, i.e. so–called aerosols, are removed efficiently and at low costs from the respective gas streams. Fibrous filters are often used in this context, because of their low cost, their high capture efficiency, and their low pressure drop. Hence, the performance of fibrous filter materials is judged based on their pressure drop and capture efficiency. A generalized model to describe and predict these two parameters would be helpful in developing optimal filter material.
In the current work, numerical investigations using the commercial CFD tool ANSYS CFX are engaged to predict the pressure drop caused by multiple randomly–oriented fibres in a 3D domain. The fibres are randomly generated until the desired filter solidity is met. Hereby, the generation algorithm ensures that no fibres intersect. The implementation is based on a fictitious–domain approach, which has the great advantage to perfectly decouple the mesh from the fibre generation, as fibres are implemented as volumes of infinitely–high flow resistance in the fixed mesh. Hence, there is no need for a case–specific and body–fitted mesh. The effect of the domain size is particularly investigated and we find that a domain size of more than 45 times the fibre diameter in all three directions, proves to give domain–size–independent results for the pressure drop. The boundary conditions are also carefully analyzed, and we find that particularly the symmetry planes parallel to the mean flow direction are not appropriate as the representative domain is cut out of the filter layer. Here, the introduction of a pseudo–plane proves to minimize non–physical flow fields. Moreover, using scanning microscopy, a sample fibrous filter is analysed to characterize the (random) fibre orientation. It is found, that within the filter plane an evenly–distributed fibre orientation angle α can be observed, while out of the filter plane the orientation angle β appears rather limited. This limitation is also implemented into the model. Finally, several simulations are performed for various operating conditions to infer an empirical correlation to predict the pressure drop. This correlation is validated using experimental data.
This paper investigates the channel aging problem of mobile light-fidelity (LiFi) systems. In the LiFi physical layer, the majority of the optimization problems for mobile users are non-convex and ...require the use of dual decomposition or heuristics techniques. Such techniques are based on iterative algorithms, and often cause a high processing delay at the physical layer. Hence, the obtained solutions are rendered sub-optimal since the LiFi channels are evolving. In this paper, a proactive-optimization (PO) approach that can alleviate the LiFi channel aging problem is proposed. The core idea is to design a long-short-term-memory (LSTM) network that is capable of predicting posterior positions and orientations of mobile users, which can be then used to predict their channel coefficients. Consequently, the obtained channel coefficients can be exploited to derive near-optimal transmission-schemes prior to the intended service-time, which enables real-time service. Through various simulations, the performance of the designed LSTM model is evaluated in terms of prediction error and inference complexity, as well as its application in a practical LiFi optimization problem.
Display omitted
•Nonlinear stress–strain relation of random cellulose FRP was clarified by multiscale simulation.•Tensile strength fluctuated by 39.6 % and 4.20 % depending on fibre aspect ratio and ...aggregation.•The proposed parameter of fibre contribution was available for uniform design of cellulose FRP.•Fibre aspect ratio and aggregation belonged to secondary factors in term of fibre contribution.
Multiscale finite element analysis based on the asymptotic homogenisation theory was performed on cellulose-fibre-reinforced plastics. Polypropylene, a thermoplastic resin, was used as the matrix, and the effect of the fibre morphology on the nonlinear response of the material was clarified. A new indicator, that is, the proportion of the fibre contribution based on strain energy, was applied to quantify the effect of reinforcing fibres in improving the mechanical properties. Representative volume element models of randomly oriented fibre dispersion were constructed using a random sequential adsorption algorithm, and the effects of the fibre aspect ratio and aggregation on the nonlinear response under a tensile load were clarified. Consequently, it was deduced that the elastic properties and maximum tensile stress fluctuated by 26.8 % and 39.6 %, respectively, depending on the fibre aspect ratio, and 5.95 % and 4.20 %, respectively, owing to fibre aggregation. Furthermore, by introducing the fibre contribution proportion, we found that both the fibre aspect ratio and aggregation belong to the secondary factor, and their influence can be evaluated almost uniformly by the increase rate determined from the fibre orientation distribution of the primary factor.
•A micromechanical model is proposed to analysis of electro-elastic behavior of piezoelectric hybrid composites.•The benefits of carbon nanotubes into the traditional piezoelectric composites from a ...structural point of view is indicated.•Interphase region plays a crucial role in the effective properties of the piezoelectric hybrid composites.•Decreasing carbon nanotube diameter leads to an improvement in the electro-elastic behavior of piezoelectric hybrid composites.
This paper investigates the overall elastic and piezoelectric properties of unidirectional piezoelectric fiber-reinforced polymer composites containing randomly oriented carbon nanotubes (CNTs). To this end, a multi-procedure micromechanics approach based on the Mori-Tanaka model is proposed. In the first step, the elastic properties of a nanocomposite consisting of randomly distributed CNTs in the polymer matrix is modeled. The formation of the interphase region due to non-bonded van der Waals (vdW) interaction between the CNTs and the polymer is taken into account in the micromechanical simulation. In the second step, considering the nanocomposite as a matrix and piezoelectric fiber as reinforcement, the elastic and piezoelectric properties of CNT-piezoelectric fiber-reinforced hybrid composites are predicted. The effects of volume fractions of CNT and piezoelectric fiber, and CNT diameter, thickness and adhesion exponent of the interphase on the hybrid composite elastic and piezoelectric coefficients are examined. The results clearly highlight the benefits of CNTs into the conventional piezoelectric composites from a structural point of view. The overall electro-mechanical properties of the piezoelectric fiber-reinforced polymer composites can be significantly enhanced with adding CNTs. It is found that the interphase region can play a crucial role in the effective properties of the piezoelectric hybrid composites.
Among the challenges of realizing the full potential of light-fidelity (LiFi) cellular networks are user mobility, random device orientation, and blockage. In this paper, we study the impact of those ...challenges on the performance of LiFi networks in an indoor environment using measurement-based channel models, unlike existing studies that rely on theoretical channel models. In our paper, we adopt spatial modulation (SM) and consider two configurations for the user equipment (UE). A multi-directional receiver (MDR) structure is proposed, in which the PDs are located on different sides of the UE, e.g., a smartphone. This configuration is motivated by the fact that conventional structures exhibit poor performance in the presence of random device orientation and blockage. In fact, we show that the MDR outperforms the benchmark structure by over 10 dB at bit-error ratio (BER) of <inline-formula> <tex-math notation="LaTeX">3.8\times 10^{-3} </tex-math></inline-formula>. Moreover, an adaptive access point (AP) selection scheme for the SM is considered, where the number of APs is chosen adaptively in an effort to achieve the lowest energy requirement for a target BER and spectral efficiency. The user performance with random orientation and blockage in the entire room is evaluated for sitting and walking activities, for which the orientation-based random waypoint (ORWP) mobility model is invoked. Furthermore, we demonstrate that the proposed adaptive technique with SM outperforms the conventional spatial multiplexing system. We also study the performance of the underlying system on the uplink channel where we apply the same techniques used for the downlink channel. It is shown analytically that the multi-directional transmitter (MDT) with adaptive SM is highly energy efficient.
The thermo-stability of microstructure during isothermal annealing at 900 °C and 1000 °C in a cold-worked Cu-Al2O3 composite with 0.1 wt.% Al content and its effect on resistance to softening were ...investigated in this paper. The results reveal that the microstructure following cold deformation consists of a Cu matrix with a refined grain size and high-density dislocations, accompanied by dispersed Al2O3 nanoparticles exhibiting an extremely low volume fraction. During isothermal annealing at 900 °C, the Al2O3 nanoparticles can strongly restrict the migration of the dislocations and suppress the recrystallization of the fine-grained Cu matrix by the Zener pinning effect. Furthermore, the presence of pinned dislocations facilitates the formation of sub-grain boundaries comprising high-density dislocations. Consequently, the Cu-Al2O3 composite with 0.1 wt.% Al content exhibits remarkable thermo-stability in its microstructure due to the incorporation of Al2O3 nanoparticles, resulting in a significantly elevated softening temperature of up to 1000 °C and thereby demonstrating excellent resistance against softening. However, the observed phenomenon of softening after isothermal annealing at 1000 °C for 5 hours can be attributed to extensive recrystallization growth that promotes twin boundary formation, primarily caused by the weakening Zener pinning effect resulting from Oswald ripening of Al2O3 and rod-like Al2O3 formation.
Direct electron transfer (DET)-type bioelectrocatalytic waves of bilirubin oxidase (BOD)-catalyzed O2 reduction and NiFe hydrogenase (H2ase)-catalyzed H2 oxidation are very small and un-detectable ...using glassy carbon (GC) electrodes, respectively; however, clear catalytic waves are observed when the enzymes are adsorbed on Ketjen black-modified GC (KB-GC) electrodes, in which KB provides mesopores for DET-type bioelectocatalysis. To explain the phenomena, we focus on the curvature effect of mesoporous structures on long range electron transfer kinetics and simulate steady-state voltammograms catalyzed by model redox enzymes adsorbed with a random orientation on planar and mesoporous electrodes based on a three-dimensional model. In the simulation, we assume a spherical enzyme with a radius of r, an active site located at a certain distance from the center of the enzyme, and a spherical pore with a radius of Rp in mesoporous electrodes in which the enzyme is trapped and adsorbed. The simulation reveals that mesoporous electrodes provide platforms suitable for DET-type bioelectrocatalysis of enzymes when Rp becomes close to r. Such curvature effects of mesoporous electrodes become especially notable for larger sized enzymes. Furthermore, the simulation reproduces the experimental data of BOD- and H2ase-catalyzed DET-type waves by considering the crystal structures of the enzymes. This work will open a route to improve the kinetic performance of the DET-type bioelectrocatalysis that has become very important in its practical application to a variety of bioelectrochemical devices.
PDF
