The attitude control system of the satellite for earth observation ‘ Bhaskara ’

The satellite for earth observation (Bhaskara) launched on 7 June 1979 has two TV cameras and a passive microwave radiometer for earth resources survey and meteorological studies respectively. As it had to be a minimally modified version of the earlier spin-stabilised satelliteAryabhata, it is configured as a spin-stabilised satellite, with a spin rate of between 6 to 11 rev/min and a spin axis perpendicular to the orbital plane within 3°. The cold gas system ofAryabhata was modified to provide a low and controlled thrust for spin-up and spin-axis orientation operations. Two horizon-crossing sensors are used for automatic spin-axis control and attitude determination on ground. This paper presents a report on the system evolution, design, simulation and on-orbit performance ofBhaskara.     

State space formulation of ammonia reactor dynamics

The specific objective of this paper is to develop a state space model of a tubular ammonia reactor which is the heart of an ammonia plant in a fertiliser complex. A ninth order model with three control inputs and two disturbance inputs is generated from the nonlinear distributed model using linearization and lumping approximations. The lumped model is chosen such that the steady state temperature at the exit of the catalyst bed computed from the simplified state space model is close enough to the one computed from the nonlinear steady state model. The model developed in this paper is very useful for the design of continuous/discrete versions of single variable/multivariable control algorithms.     

Factors influencing mechanical and wear performance of dental composite: A review

With environmental concerns, the development of a posterior restorative as a true alternative to dental amalgam is needed. Hence in this regard, the development of dental composites have gained more and more research interest, as they have the advantages of better aesthetic look, better physical, mechanical and tribological properties over the other available dental materials. In this study, the physical, mechanical and wear behavior of dental composite materials have been reviewed. Numerous experimental results, analytical studies and their significance were also discussed for various dental composite materials. The wear performances of dental composite materials depend up on various factors including oral environment, resin chemistry, filler technology and interfacial bonding strength etc. In oral environment, the presence of medium such as water, air, heptane, ethanol, artificial saliva etc. and durations up to which composite is kept such as seven days, one month or three months etc. affect the performance of dental composite. Abrasion and erosion are two major causes for the wear behavior of dental composite / human teeth. The major issue, most significant challenges and effect of each of those factors affecting the performance of dental composite materials were also presented along with some recommendation.     

Effect of structure and composition on physico-mechanical properties of methyl methacrylate/alkyl (meth)acrylate copolymers

The paper describes the fabrication of cast acrylic sheets by copolymerizing a prepolymer syrup of methyl methacrylate (MMA) with varying amounts (1-20 wt.-%) of 2-ethylhexyl methacrylate (EHMA), n-hexyl methacrylate (HMA), 2-ethylhexyl acrylate (EHA) and n-hexyl acrylate (HA). The effect of structure of copolymer on light transmittance, density and mechanical properties was evaluated. Tensile strength and storage modulus decreased with increasing comonomer content. Strain increased significantly in the case of MMA/EHA and MMA/HA copolymer sheets having 15 and 20 wt.-% of comonomer. Softening temperature of copolymer samples was determined using dynamic mechanical thermal analysis. Heat deflection temperature under flexural load and softening temperature decreased with increasing comonomer content in copolymer.     

Mechanical and wear performance evaluation of tungsten added Co‐30Cr‐4Mo‐1Ni biomedical alloy with Taguchi optimization

Development of biomaterial with diverse properties (physical, mechanical, wear, and many others) for hip femoral head is one of the most exigent tasks. Improper material often causes component failure during functioning. Therefore, in this study, a series of implant materials containing tungsten of different weight percentages were fabricated by high temperature vacuum casting induction furnace and the physical, mechanical, and wear properties were examined. The proportions were varied from 0 wt. % ‐ 4 wt.% in a cobalt–chromium alloy (Co‐30Cr‐4Mo‐1Ni). The mechanical properties were tested by the micro‐hardness tester and the compression testing machine, while the wear performance was analyzed through a pin‐on‐disc tribometer under different operating conditions at room temperature. Initially in this study, steady state experimental analysis was performed to obtain the volumetric wear loss and coefficient of friction by varying sliding velocity and normal load respectively. Afterwards, the Taguchi experimental design has been conducted to obtain the optimum wear response. Lastly, scanning electron microscopy and atomic force microscopy were utilized to analyze the contour of wear mechanism and 3D surface topography. From the results obtained, it was found that Co‐30Cr‐4Mo‐1Ni‐2 W implant material provides the best combination of the properties for a given application.     

Study of crop growth parameters using Airborne Imaging Spectrometer data

High-spectral-resolution data of the Airborne Imaging Spectrometer (AIS) developed by the Indian Space Research Organisation were analysed to check their potential for extraction of information concerning crop growth parameters. Reflectance spectra of wheat plots at different growth stages were generated to study the position of red edge. The shift of inflection wavelength towards longer wavelength was observed with increase in Leaf Area Index (LAI) and chlorophyll content. For the range of LAI from 0.08 to 3.16 and chlorophyll content from 2 to 39 (gm^-2), the shift of inflection wavelength was observed in the range of 713-723 nm. A linear relationship was observed between the inflection wavelength and LAI as well as chlorophyll content.     

Improved DGS parameter extraction method for the polarization purity of circularly polarized microstrip antenna

An improved method of parameter extraction of the most popular dumbbell shaped defected ground structure (DGS) is developed, which removes the drawbacks of the earlier approaches of parameter extraction methods. The proposed method is validated considering a circularly polarized (CP) microstrip antenna integrated with dumbbell shaped DGS for application of suppression of cross polarization. A set of fabricated laboratory prototypes, with and without DGS, is experimentally studied. The results show that the integration of DGS with antenna provides polarization purity of about 10 dB compared to the same patch without DGS at bore‐sight of the patch and enhances the return loss bandwidth about 40%. Good agreement among theory, simulated results, and the experimental results qualifies the proposed method as an effective tool for parameter extraction of the DGS. © 2016 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2016.     

Quantifying residual numerical diffusion in flux-corrected transport algorithms

Residual diffusion in fluid-dynamics calculations results from the finite order of approximation in the underlying linear algorithm, including the effect of smoothing sometimes added for numerical reasons, and, in the case of monotonicity-preserving algorithms such as flux-corrected transport (FCT), the nonlinear action of the flux limiter on steep profiles. Some widely used FCT algorithms contain a multiplicative constant that reduces the antidiffusion coefficient by 0.01%–0.1%. Replacing this constant with a smoothly varying function of velocity which equals unity when the Courant number vanishes causes the linear diffusion to go to zero when the flow velocity does. The use of a velocity-dependent antidiffusion coefficient minimizes numerical smearing of discontinuities and associated effects in the neighboring flow. Computational examples are presented. The residual diffusion for nonzero flow speeds is nonlinear and problem dependent. A method is presented for calibrating it in any given code in the context of a particular problem, and is applied to the FCT algorithms described here.     

Influence of vermiculite on performance of flyash-based fibre-reinforced hybrid composites as friction materials

Flyash-based fibre-reinforced hybrid phenolic composites filled with vermiculite were fabricated and characterized for their physical, thermal, mechanical and tribological performance. The performance were evaluated in terms of their friction-fade, friction-recovery, maximum disc temperature rise and wear behaviour on a Krauss friction tester conforming to the Regulation-90 as per the Economic Commission for Europe (ECE) norms. The fade behaviour has been observed to be optimally dependent on the flyash–vermiculite combination whereas the recovery remained broadly unaffected at ～112 ± 14%. Addition of vermiculite has contributed to the reduction in the maximum disc temperature rise whereas it enhanced the frictional amplitude, i.e. μ_max–μ_min. The wear behaviour remains closely related to the trend observed in fade. The addition of vermiculite has caused an increase in the post-braking onset of degradation temperature of the surface composition as compared to the pre-braking composition. The analyses of friction and wear performance of the composites were carried out and major factors influencing the tribo-performance were identified. Worn surface morphology investigation using scanning electron microscope has revealed that the addition of vermiculite alters the compositional interactions at the braking interface leading to flyash–vermiculite combination specific topographical attributes responsible for tribo-performance evolution.     

Validating LES-based flow and dispersion models

Validating LES-based flow and dispersion models for the purpose of predicting transient flow and dispersion phenomena is more demanding than validating RANS-based codes. Since the model output is no longer related to stationary or quasi-stationary boundary conditions, and since the model results are not meant to be used for predicting mean flow and dispersion patterns, an evaluation of the model based on mean results is not meaningful for most of the model-specific applications. A more sophisticated but also more complex validation approach based on statistically representative ensembles is required. By comparing frequency/probability distributions of flow and dispersion results with qualified reference data the reliability of complex models can be evaluated. Based on a careful comparison of FAST3D-CT simulation results with corresponding systematic wind tunnel data from the JU2003 experiments, an example for an application specific evaluation procedure for instantaneous puff dispersion modeling is given.