Showing papers by "Vikram Sarabhai Space Centre published in 2003"

Aerosol spectral optical depths over the Bay of Bengal: Role of transport

Abstract: Recent experiments have shown the potential role of air masses in transporting aerosols to locations far away from source regions. Despite the importance of the Bay of Bengal to Indian climate and monsoon, no serious aerosol observations are available for this region. Extensive aerosol optical depth estimates, made for the first time from an island location, Port Blair (11.63°N; 92.71°E) in the Bay of Bengal, during the Indian winter of 2002, are used to examine the impact of air trajectories in modifying the optical depths and their spectral dependences. The results are examined for their distinctiveness with respect to the origin as well as transport. It is seen that the trajectories arriving from the regions east of the station (South China, Thailand, Laos, Cambodia, Vietnam, Burma) are richer in aerosol abundance, more in the sub micron size range, than those arriving from the west, across the Indian landmass.

Cyclization reaction in poly(acrylonitrile/itaconic acid) copolymer: An isothermal differential scanning calorimetry kinetic study

Abstract: The kinetics of the cyclization reaction in the poly(acrylonitrile/itaconic acid) (PAN) copolymer in the temperature range 175–200°C was studied by isothermal differential scanning calorimetry (DSC). The reaction conforms to an nth-order autocatalytic model, with an overall order of 2. The kinetic parameters were derived by multiple regression analysis. The activation energy is 110.3 kJ/mol for the acid-catalyzed reaction and 153.2 kJ/mol for the autocatalytic part. The autocatalysis indicates the catalysis of the nitrile group cyclization by the initially formed pyrimidine groups. The kinetic parameters were used to predict the reaction profile at a given temperature. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 915–920, 2003

Synthesis and Properties of Polyarylene ether Nitrile Copolymers

Abstract: Polyether nitrite and its copolymers were synthesized by nucleophilic substitution reaction of 2,6-dichlorobenzonitrile with resorcinol and with varying mole proportions of resorcinol and hydroquinone. The polymers were characterized by different physico-chemical techniques. Thermogravimetric studies showed that all the polymers were stable up to 400°C with a char yield of above 48% at 900°C in N 2 atmosphere. The glass transition temperature was found to increase with increase in concentration of hydroquinone units in the polymer; however no trend was observed in their crystalline melt temperatures. Copolymer composition was determined using FT-IR technique. Crystallinity of the polymers was also studied using wide angle X-ray diffraction.

Influence of changes in the prevailing synoptic conditions on the response of aerosol characteristics to land- and sea-breeze circulations at a coastal station

Abstract: The changes in the response of near surfaceaerosol properties to land- and sea-breezecirculations, associated with the changes in the prevailing synoptic meteorological conditions, are examined for a tropical coastal station Aerosol properties are nearly similar in both the breeze regimes (land and sea) during seasons of marine airmass while they are distinct during seasons of continental airmass As the prevailing winds shift from continental to marine and the ambient weather changes from winter conditions to the humid monsoon season, the submicron mode, which dominated the aerosol mass-size distribution, is largely suppressed and the dominance of the super micron mode increases During periods of continental air mass (winter), the aerosol loading is significantly higher in the land-breeze regime, (particularly in the submicron range) but as the winds shift to marine, the loading initially becomes insensitive to the breeze regimes and later becomes higher in the sea-breeze regime, particularly in the super micron range

On the Solar EUV Deposition in the Inner Comae of Comets with Large Gas Production Rates

Abstract: [1] In this letter we have made a comparative study of degradation of solar EUV radiation and EUV-generated photoelectrons in the inner comae of comets having different gas production rates, Q, with values 1 × 1028, 7 × 1029, 1 × 1031, and 1 × 1032 s−1. We found that in higher-Q comets the radial profile of H2O+ photo-production rate depicts a double-peak structure and that the differences in sunward and anti-sunward photoionization rates are pronounced. We show that photoelectron impact ionization is an order of magnitude larger than photoionization rate near the lower photoionization peak in comets with Q > ∼1 × 1031 s−1. The present study reveals the importance of photoelectrons relative to solar EUV as the ionization source in the inner coma of high-Q comets.

Synthesis and characterization of iron carbonyl‐modified hydroxyl‐terminated polybutadiene: A catalyst‐bound propellant binder for burn‐rate augmentation

Abstract: Iron was coordinately linked to the hydroxyl-terminated polybutadiene (HTPB) backbone using iron carbonyl via a ligand displacement reaction. The modified HTPB thus obtained was reddish brown in color and was characterized by GPC, FTIR, NMR, thermal, and propellant studies. No significant changes in the rheology, molecular weight, and molecular weight distribution were seen in the modified resin when the bonded Fe content was >0.8.0 wt %. However, the hydroxyl value of the resin decreased by 3–7 irrespective of the weight percent of the bonded Fe, and this was more likely due to the Fe-catalyzed oxidation of the CH2OH moiety, mostly to the CHO group. Apparently, this has not affected the cure characteristics of the binder, as demonstrated by the good mechanical properties of the gum stock and the propellant. The catalytic efficiency of the bonded Fe on the burn rate of the propellant was more efficient than was the free Fe added to the propellant. The aging characteristics of the resin for the bound iron content of ≤0.8 wt % was apparently good, as its viscosity and molecular weight did not undergo any drastic changes even after 18 months' storage under ambient conditions. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2813–2823, 2003

Graft Copolymerization of 2-Hydroxy Ethyl Methacrylate onto Chitosan with Cerium (IV) Ion. I. Synthesis and Characterization

Abstract: Graft copolymerization of 2-Hydroxy ethyl methacrylate (HEMA) on to chitosan was studied using cerium (IV) as the initiator. Optimization of the grafting was worked out by varying the reaction time and monomer concentration. Under controlled conditions, up to 685% grafting with a grafting yield of 92.4% was achieved. FTIR, thermal and XRD techniques were used to confirm the formation of the grafted copolymer. Grafting caused a marginal decrease in the mechanical strength in the dry conditions and a significant decrease under wet conditions for the resultant polymer. The products showed significantly improved swelling at pH 7.4 and pH 1.98 compared to the original chitosan. Grafted polymer showed enhanced Tg and decomposition temperature. The grafting also resulted in improved hydrophilicity as is evident from the contact angle studies of the films.

Variability of Mixed-Layer Heights over the Indian Ocean and Central Arabian Sea during INDOEX, IFP-99

Abstract: The upper air data collected from the balloon-borne GLASS Sondes launched from the Oceanic Research Vessel (ORV) Sagar Kanya during the Intensive Field Phase of the Indian Ocean experiment (INDOEX, IFP-99;SK-141 Cruise) are utilized forstudying the variability in the mixed-layer heights observed over the western tropical Indian Ocean and central Arabian Sea. During the entire cruise, typical daytime convective mixed-layer heights (roughly corresponding to 1400 LT) obtained from θV and q profiles, were observed to be in the range 200–900 m. Shallowmixed -layer heights are observed, in general, over the Inter-Tropical Convergence Zone (ITCZ). Over the central Arabian Sea, vertical profiles of θV and q demonstrate a double mixed-layer structure of the marine atmospheric boundary layer (MABL), which gradually disappears close to the Indian coastline.

Large amplitude free vibrations of beams: an energy approach

Abstract: The conservation of energy principle, which says that the total energy of a vibrating system, at any given instant of time is constant, is used to evaluate the large amplitude free vibration behaviour of beams with immovable ends. With an assumed space mode, which satisfies the geometric boundary conditions, the final governing equation is obtained. This equation is solved by a numerical integration scheme to obtain the ratio of frequencies of nonlinear to linear systems. Comparison of the present results with those of other researchers on this topic and a Galerkin approximation is also included.

Wind Speed dependence of Air-Sea Exchange parameters over the Indian Ocean during INDOEX, IFP-99

Abstract: . Air-Sea exchange of momentum, heat and moisture over the oceanic surface plays an important role in understanding several processes spanning various scales of atmospheric and oceanic motions. The present study provides estimates of air-sea exchange parameters along the cruise track of the Intensive Field Phase of Indian Ocean Experiment (INDOEX, IFP-99) conducted on board Oceanic Research Vessel (ORV) Sagar Kanya during 20 January–12 March 1999 for a large region of the Indian Ocean. The study is aimed at acquiring a better understanding of the wind speed dependence of air-sea interaction parameters, such as roughness lengths for wind (z0), temperature (z0t) and hu-midity (z0q), which play a key role in the determination of the air-sea exchange coefficients and interface fluxes across the tropical oceans. The variation of drag coefficient (CD), sensible heat and water vapor exchange coefficients (CH and CE), are also discussed in relation to the wind speed. An empirical relation is derived between the estimated values of drag coefficients and the observed values of wind speeds for the hitherto data-sparse regions over the tropical Indian Ocean. Key words. Oceanography: physical (air-sea interaction) Meteorology and atmospheric dynamics (ocean-atmosphere interaction) – Oceanography: physical (marine meteorology)

Rheological cure characterization of phosphazene–triazine polymers

Abstract: Cyclomatrix phosphazene–triazine network polymers were synthesized by co-curing a blend of tris(2-allylphenoxy), triphenoxy cyclotriphosphazene (TAP), and tris(2-allylphenoxy) s-triazine (TAT) with bis(4-maleimido phenyl) methane (BMM). The co-curing of the three-component resin was investigated by dynamic mechanical analysis using rheometry. The cure kinetics of the Diels–Alder step was studied by examining the evolution of the rheological parameters, such as storage modulus (G′), loss modulus (G″), and complex viscosity (η*), for resins of varying compositions at different temperatures. The curing conformed to an overall second-order phenomenological equation, taking into account a self-acceleration effect. The kinetic parameters were evaluated by multiple-regression analysis. The absence of a definite trend in the cure process with blend composition ratio was attributed to the occurrence of a multitude of competitive reactions whose relative rates depend on the reactant ratio and the concentration of the products formed from the initial phase of reaction. The cure was accelerated by temperature for a given composition, whereas the self-acceleration became less prominent at higher temperature. Gelation was accelerated by temperature. The gel conversion decreased with increase in maleimide concentration and, for a given composition, it was independent of the cure temperature. The activation energy for the initial reaction and the crosslinking process were estimated for a composition with a maleimide-to-allyl ratio of 2 : 1. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 908–914, 2003

Post-buckling of cantilever columns having variable cross-section under a combined load

Abstract: Post-buckling of a cantilever column is examined under a combined load consisting of a tip-concentrated load and a distributed axial load, through dynamic formulation. The formulation of the problem is based on the moment–curvature relationship. The two-point boundary value problem described by the governing equations is dependent on the frequency parameter and the two load parameters. The buckling loads are those loads at which the eigencurve, namely, the load versus frequency curve of the column meets the load axis. A simple and reliable iterative procedure to convert the two-point boundary value problem into an initial value problem is followed and solved the non-linear differential equations utilizing a fourth-order Runge–Kutta integration scheme. To demonstrate the potentiality of the adopted numerical scheme, linear vibration frequencies of truncated, tapered cantilever wedges and cones are determined and compared with the published analytical and test results. Buckling and post-buckling loads of a simply supported stepped column are obtained and compared with the published test results. The loads and deflections of non-uniform cantilever columns are obtained for various slopes at the tip. The interaction of load parameters for a free–free truncated conical column has also been examined. The numerical results indicate that the path represented by the two load parameters turns out to be nearly a straight line.