Unidirectional fiber–polymer composites: Swelling and modulus anisotropy

The solvent swelling of unidirectional rubber–fiber composites was studied. The amount of matrix swelling was constrained to the extent that would be predicted from the thermodynamic theories of elasticity and polymer–solvent interaction. The geometry of swelling was found to be orthotropic in nature. A simple trigonometric function was derived to relate linear deformation due to swelling to the angle which the direction of its measurement makes with the fiber direction. The validity of the derivation was demonstrated experimentally. Considering swelling to be the imposition of tensile forces of equal magnitude in all directions, and considering a swelling-induced linear deformation to be analogous to a tensile compliance, a simple set of relationships between elastic parameters and their direction of measurement was derived: where E_θ, G_θ, v_θ, and η_θ are Young's modulus, shear modulus, Poisson's ratio, and the shear coupling ratio measured in a longitudinal transverse plane at an angle with the fiber direction, respectively, and E_L, G_LT, and θ_LT are the longitudinal Young's modulus, the longitudinal transverse shear modulus, and the longitudinal transverse Poisson ratio, respectively. Further simplifying the case of combined transverse isotropy and special orthotropy was the conclusion that 1/G_LT = 1/E_T + (1 + 2v_LT)/E_L. The relationships for G and E were experimentally demonstrated.     

Non-linear panel flutter for temperature-dependent functionally graded material panels

The non-linear flutter and thermal buckling of an FGM panel under the combined effect of elevated temperature conditions and aerodynamic loading is investigated using a finite element model based on the thin plate theory and von Karman strain-displacement relations to account for moderately large deflection. The aerodynamic pressure is modeled using the quasi-steady first order piston theory. The governing non-linear equations are obtained using the principal of virtual work adopting an approach based on the thermal strain being a cumulative physical quantity to account for temperature dependent material properties. This system of non-linear equations is solved by Newton–Raphson numerical technique. It is found that the temperature increase has an adverse effect on the FGM panel flutter characteristics through decreasing the critical dynamic pressure. Decreasing the volume fraction enhances flutter characteristics but this is limited by structural integrity aspect. The presence of aerodynamic flow results in postponing the buckling temperature and in suppressing the post buckling deflection while the temperature increase gives way for higher limit cycle amplitude.     

Chemorheological characterization of thermosetting polyurethane formulations containing different chain extender contents

Chemorheological analyses of thermosetting polyurethane (PU) formulations containing various amounts of chain extender were carried out. In the first part of the present research, effect of chain extender content on the gel time and its dependency with temperature were investigated for PU formulations. Gelation temperature (T_gel) and gel time (t_gel) of reactive systems were determined using temperature sweep and time test measurements under dynamic linear viscoelastic deformations. The results of temperature sweep experiments for the samples containing various chain extender weight percents showed that the gelation temperature declined by increasing the chain extender content of PU formulations. The analysis made on gel times determined in a wide temperature range indicated that the activation energy for the gelation process declined and 1/t_gel, as a measure of the rate of gelation process, dramatically increased with increasing chain extender concentration. In the second part of this study, calculation of viscoelastic characteristics at the gel point was performed. The results showed that the relaxation exponent (n) declined with increasing chain extender, while the fractal dimension increased. These results confirmed that the gel structures became more elastic and with a tighter structure by increasing the chain extender concentration in PU formulations. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Numerical simulation of two phase refrigerant flow through non-adiabatic capillary tubes using drift flux model

Journal of Mechanical Science and Technology - This research presents a drift flux model to simulate steady state refrigerant flow through horizontal lateral straight capillary tube-suction line...     

Leaf cuticle and wax ultrastructure of genus Alnus Mill. in Hyrcanian forests of Iran

The genus Alnus had three taxa in the old floral resources of Iran. Recently, five new taxa were added to the flora based on morphological studies. Individual samples were collected from 25 sites throughout three provinces covering Hyrcanian forests of Iran and classified into the eight mentioned taxa and then prepared for micro-morphological studies. Four groups of taxa including A. subcordata var. subcordata, A. subcordata var. villosa, A. orientalis, and A. glutinosa subsp. antitaurica had a specific structure. Similar structure was also found between two new species of A. djavanshirii and A. dolichocarpa, which were distinguished from the other groups. Simple trichomes with variation in size and density were observed among the taxa. Alnus subcordata var. villosa and A. glutinosa subsp. antitaurica had the densest trichomes on dorsal and ventral leaf surfaces. The results of this study show the suitability of ultrastructure features as a micro-morphological marker for solving the taxonomical ambiguities of the genus Alnus in Hyrcanian forests.     

Temperature of a photovoltaic module under the influence of different environmental conditions – experimental investigation

The climate changes affect photovoltaic (PV) module temperature significantly. The module temperature is one of the most important factors that influence the PV module efficiency and a deep analysis of PV module temperature will aid in better understanding of the environmental influences on the PV module performance. The module temperature depends on many parameters such as solar radiation, ambient temperature, air humidity, speed and direction of the wind, PV module orientation, dust and sand deposition on PV module, and PV module materials. An experimental research was conducted to investigate the effect of these factors on the PV module temperature in the Renewable Energy Laboratory of the Graduate University of Advanced Technology in Iran. The results of this study highlighted that the deposited dust over the PV module surface increases the module temperature and this consequently decreases the PV module power. It was also revealed that a combination of the temperature increase and the incident solar radiation decrease due to the dust deposition over the PV module enhances significantly the module power reduction.     

Global impedance attenuation network for multistage OTAs driving a broad range of load capacitor

Advanced multistage amplifiers suffer from load-dependent stability issues, which limit the load capacitor range they can drive. In this work, the concept of global impedance attenuation (GIA) network is introduced to improve an amplifier's stability in the presence of significant load capacitor variations. Composed of multiple parallel resistor-capacitor (RC) branches, the equivalent high-frequency output impedance of gain stages is shaped by the GIA network such that a desired frequency spectrum is obtained over a wide range of load capacitor. The parasitic poles at the output of the gain stages are nullified by the proposed network, thereby simplifying the amplifier's transfer function and reducing the minimum load capacitor it can drive. The idea is applied to design a three-stage operational transconductance amplifier (OTA) with cascode global impedance attenuation (CGIA). Small-signal analysis shows that the OTA is stable regardless of the load capacitor, and it can drive very small to ultra-large load capacitors. This feature is verified by the post-layout simulations of a CGIA amplifier in 0.18-μm complementary metal-oxide semiconductor (CMOS) process. The core occupies a die area of 0.0053 mm² while consuming a static current of 10.97 μA from 1.8-V voltage supply. The unity-feedback configuration is unconditionally stable for any load capacitor higher than 10 pF. To the best of our knowledge, this corresponds to the widest range of load capacitance reported for prior-art three-stage amplifiers.     

Simple, fast and accurate two-diode model for photovoltaic modules

This paper proposes an improved modeling approach for the two-diode model of photovoltaic (PV) module. The main contribution of this work is the simplification of the current equation, in which only four parameters are required, compared to six or more in the previously developed two-diode models. Furthermore the values of the series and parallel resistances are computed using a simple and fast iterative method. To validate the accuracy of the proposed model, six PV modules of different types (multi-crystalline, mono-crystalline and thin-film) from various manufacturers are tested. The performance of the model is evaluated against the popular single diode models. It is found that the proposed model is superior when subjected to irradiance and temperature variations. In particular the model matches very accurately for all important points of the I-V curves, i.e. the peak power, short-circuit current and open circuit voltage. The modeling method is useful for PV power converter designers and circuit simulator developers who require simple, fast yet accurate model for the PV module.     

A critical evaluation of EA computational methods for Photovoltaic cell parameter extraction based on two diode model

Due to its ability to handle nonlinear functions regardless of the derivatives information, evolutionary algorithms (EA) are envisaged to be very effective for extracting parameter of photovoltaic (PV) cell. This paper presents critical evaluation of the parameters extraction of two diode PV model using three EA methods, namely Genetic Algorithm (GA), Particle Swarm Optimization (PSO) and Differential Evolution (DE). For DE, two variations are proposed: (1) boundary based differential evolution (B-DE) and (2) penalty based differential evolution (P-DE). The performance of each method is evaluated based on several factors: accuracy and consistency of solution; speed of convergence; computational efficiency and the required number of control parameters. Comparisons are carried out using synthetic data and are validated by six PV modules of different types (multi-crystalline, mono-crystalline, and thin-film) from various manufacturers. Information derived from these critical evaluations can be useful to determine the best computational method to build an efficient and accurate PV system simulator.     

A comparison of the effect of supplementation and sunlight exposure on serum vitamin D and parathyroid hormone: A systematic review and meta-analysis

Abstract

Background: Supplementation and getting sunlight exposure are two treatments for vitamin D deficiency. However, studies reported conteroversial findings regarding the efficacy of these two methods.

Objective: To compare the effect of oral vitamin D supplementation with sunlight exposure on serum vitamin D and parathyroid hormone (PTH).

Methods: A computer-based literature search through PubMed, Scopus and Google scholar search engines was conducted until April 2019 to find clinical trials which compared the effect of oral vitamin D supplementation with sunlight exposure on serum vitamin D and PTH. Means for serum 25-hydroxy vitamin D3 (25(OH) D3) and PTH concentration were extracted. A subgroup analysis was used to detect potential sources of inter-study heterogeneity. Mean differences (MD) were analyzed using a random-effects model (the DerSimonian-Laird approach).

Results: A total of seven papers were included in the meta-analysis. Pooled analysis showed that vitamin D supplementation significantly elevated levels of serum 25(OH) D3 in comparison with sunlight exposure (MD: 8.56nmol/l, 95%CI: 4.15, 12.97, T² = 40.32%, H² = 9.45%, P for heterogeneity p?<?0.001). Also, the difference between the effect of vitamin D supplementation and sun exposure was lower in studies which used UVB radiation compared with studies which applied direct sunlight (MD: 11.65?nmol/l, 95%CI: 7.02, 16.28; P for between subgroup heterogeneity = 0.001).

Conclusion: Vitamin D supplementation was more effective than sun exposure at increasing serum 25(OH) D3. The difference between efficacy of vitamin D supplementation and sun exposure was lower in studies which used long-term sun exposure or applied UVB treatment instead of direct sunlight.