Showing papers by "Vikram Sarabhai Space Centre published in 1998"

On the development of instability criteria during hotworking with reference to IN 718

Abstract: A simple instability condition based on the Ziegler’s continuum principles as applied to large plastic flow, is developed for delineating the regions of unstable metal flow during hot deformation. It can be used for any type of the flow stress versus strain rate curve. This criterion has been validated using the flow stress data of IN 718 with microstructural observations. The optimum hot working conditions for the superalloy IN 718 are suggested based on the instability map.

Stress relaxation behavior of short pineapple fiber reinforced polyethylene composites

Abstract: Stress relaxation behavior of short pineapple fiber reinforced polyethylene composites in tension has been studied with special reference to the effect of fiber loading, fiber length, chemical trea...

Reactive compatibilization of a nitrile rubber/phenolic resin blend: Effect on adhesive and composite properties

Abstract: Resole phenolic resins containing various p-cresol (PC) to phenol (P) mol ratios were prepared and characterized. These phenolic resins were blended with nitrile rubber (NBR) and the measurements of adhesive joint strength, stress–strain properties, DSC, TGA, DMA, TEM, and SEM were performed using a 50 : 50 NBR/phenolic resin blend. It was observed that the adhesive joint strength and the mechanical properties of the blend enhanced significantly on incorporation of p-cresol into the phenolic resin, and the optimum p-cresol/phenol mol ratio was in the vicinity of 2 : 1. Observation of a more continuous phase and the increase in Tg of the rubber region in the blend indicated increased reactivity and compatibilization of NBR with phenolic resin as p-cresol was incorporated. The effect of silica filler on the properties of the nitrile rubber/phenolic resin blend was also studied without and with p-cresol modification and the results suggest that silica filler take not only the role of a reinforcing filler in the nitrile–phenolic–silica composite, but also a role as surface compatibilizer of the blend components. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 1187–1201, 1998

Instability map for hot working of 6061 Al-10 vol% metal matrix composite

Abstract: A simple condition for metallurgical instability, useful in the development of processing maps for analysing high-temperature forming of metals, is suggested following a criterion based on continuum principles as applied to large plastic flow proposed by Ziegler. It can be used for any type of flow stress versus strain rate curve. This criterion has been validated using the flow stress data of a 6061 Al-10 vol% metal matrix composite with microstructural observations. Optimum hot working conditions based on the instability map are suggested for this material.

Iterative method for the inversion of multiwavelength lidar signals to determine aerosol size distribution.

Abstract: Two iterative methods of inverting lidar backscatter signals to determine altitude profiles of aerosol extinction and altitude-resolved aerosol size distribution (ASD) are presented. The first method is for inverting two-wavelength lidar signals in which the shape of the ASD is assumed to be of power-law type, and the second method is for inverting multiwavelength lidar signals without assuming any a priori analytical form of ASD. An arbitrary value of the aerosol extinction-to-backscatter ratio (S1) is assumed initially to invert the lidar signals, and the ASD determined by use of the spectral dependence of the retrieved aerosol extinction coefficients is used to improve the value of S1 iteratively. The methods are tested for different forms of altitude-dependent ASD’s by use of simulated lidar-backscatter-signal profiles. The effect of random noise on the lidar backscatter signals is also studied.

A DSC Study on the Effect of RDX and HMX on the Thermal Decomposition of Phase Stabilized Ammonium Nitrate

Abstract: Phase stabilized ammonium nitrate(PSAN) was prepared by incorporating copper(II) diammine nitrate in the ammonium nitrate(AN) crystal lattice The effect of two energetic materials viz, RDX and HMX on the thermal decomposition of PSAN was investigated using DSC The decomposition temperatures of PSAN and RDX are almost in the same temperature range The decomposition of PSAN is endothermic and that of RDX is exothermic, while that of a 1:1 mixture of the two is exothermic with a net heat release in excess of that of the individual reactions, which is attributed to the combustion reaction among the decomposition products of PSAN and RDX Though HMX decomposes at a higher temperature, in presence of molten PSAN a substantial portion of it decomposes at a lower temperature along with PSAN with a net excess heat release, again due to the combustion reaction The kinetic parameters of the two exothermic decompositions were computed using two equations based on variable heating rate method viz, Kissinger and Ozawa equations The activation energy obtained from Ozawa method was refined by an iteration procedure There is a close agreement among the values obtained by the three methods The kinetic parameters for PSAN-HMX mixture are lower, probably due to the effect of its higher heat release

Imide-modified polyurethanes, syntheses, thermal, and mechanical characteristics

Abstract: Polyurethanes based on polyethylene glycol and hydroxy-terminated polybutadiene, bearing pyromellitimide groups in the backbone, were synthesised via a sequential method. The diisocyanate and diamine telechelic precursor polymers, the polyamic acid intermediate and the polyimides were characterized by physicochemical and thermoanalytical techniques. The imide content could be varied in the chain through selection of the polyol of desired molecular weight or by a chain extension process of the diisocyanate precursor. The thermal and mechanical properties of the imide-modified polyurethane increased proportionate to the hard segment content, constituted by the imide and the urea groups. The presence of the imide group resulted in better thermomechanical profile and caused enhancement in glass transition temperature of the modified polyurethane vis-a-vis the unmodified resin. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 1483–1491, 1998

Conducting Fabrics as a Shield Against Electromagnetic Interference (EMI)

Abstract: The paper reports a method to design and develop a highly flexible EMI shielding material based on newly emerging technology of in situ grafting of conducting polymers onto insulating surfaces such as fabrics.

An experimental investigation of underexpanded jets from oval sonic nozzles

Abstract: Underexpanded jets from oval sonic nozzles were experimentally studied for various pressure ratios up to 20.3. The results revealed that the barrel type of shock structure was present only in the major axis plane except at low aspect ratios. The results also revealed that the jet spreading rate in the minor axis plane of the nozzle was much higher compared to that in the major axis plane, resulting in axis switching of jets. The cross sectional area of these jets were considerably higher compared to the axisymmetric jets indicating higher interface area for viscous mixing in the near field region and increased mixing in the far field region clear of shock structure.

Contraction of high eccentricity satellite orbits using K-S elements with air drag

Abstract: A new non–singular analytical theory for the contraction of high eccentricity satellite orbits under the influence of air drag is developed in terms of the K–S elements, utilizing a spherically symmetrical atmospheric model. The series expansions include up to sixth power in terms of an independent variable λ, used by King–Hele in his theory. Numerical experimentation establishes a supremacy of the present theory over that of King–Hele over a wide range of the involved orbital parameters. The theory can be used effectively for the orbital decay of geostationary transfer orbits and during the mission planning of aeroassisted orbital transfer orbits.

Meteor trail induced backscatter in MST radar echoes

Abstract: The amplitude variations and Doppler spectra of the 50 MHz radar echoes from meteor trails are investigated. The radar echoes from underdense meteor trails particulary during nighttime do not always fit the conventional specular reflection model. The echo was found to spread, sometimes, over as large as 8–10 km in range. In each range bin, the spread echo from the trail is produced with an altitude dependent time delay after the passage of the meteoroid producing the ionized trail. The Doppler spectra of the spread echo in each range bin is asymmetric with a high frequency wing comprised of narrow spectral peaks and successive spectral peaks amplitude decrease with frequency. These observations are interpreted as due to multiple reflections of the radar pulse between the striations generated parallel to the geomagnetic field along the length of the meteor trail. There is evidence to show that within the dynamo region the EW and NS components of the currents at lower and higher altitudes could be in opposite directions during the early morning hours of Sq electric field reversal.

Non-similar solutions for flow and heat transfer in a viscoelastic fluid over a stretching sheet

Abstract: The non-similar boundary layer equations for the flow of an incompressible second-order fluid over a stretching sheet are solved analytically to obtain the co-efficients of skin-friction and heat transfer.

Flow of a second-order fluid over a stretching surface having power-law temperature

Abstract: The important physical quantities such as the coefficients of skin-friction and heat transfer are obtained from the closed-form solutions for the boundary layer equations of the flow of a second-order fluid over a stretching surface having power-law temperature.

A fast algorithm to solve viscous two-phase flow in an axisymmetric rocket nozzle

Abstract: SUMMARY A numerically fast algorithm has been developed to solve the viscous two-phase flow in an axisymmetric rocket nozzle. A Eulerian‐Eulerian approach is employed in the computation to couple the gas‐particle flow. Turbulence closure is achieved using a Baldwin‐Lomax model. The numerical procedure employs a multistage time-stepping Runge‐Kutta scheme in conjunction with a finite volume method and is made computationally fast for the axisymmetric nozzle. The present numerical scheme is applied to compute the flow field inside JPL and AGARD nozzles. # 1998 John Wiley & Sons, Ltd. Nozzle flow field analysis constitutes an important area of research and development work in aerospace engineering, since the performance of a launch vehicle depends on the propulsive power of the nozzle. Optimum thermal insulation of the nozzle wall also demands accurate estimation of the heat transfer rate to the nozzle wall. The aluminium oxide particles in the exhaust contribute to an inefficiency in the expansion process in the propulsive nozzle. This inefficiency is attributed to velocity and thermal lag between the gas and particles. It is necessary to know the behaviour of the two-phase flow expanding through the nozzle in order to evaluate the motor performance. Since the 1970s, numerical simulation of the gas‐particle two-phase nozzle has been extensively studied with the development of numerical schemes. Chang 1 has solved the unsteady two-fluid equations in conservation form using MacCormack’s scheme to predict the gas and particle fields in an axisymmetric convergent‐divergent nozzle. Crowe 2 provides a review of numerical models for dilute gas‐particle flows. The main advantage of the two fluid model is that the numerical procedures already established for single-phase flow can be used for two-phase flow. However, the major drawbacks of this scheme are numerical diffusion of the particle phase and higher computer storage and computer time requirements for multisize particles. A numerical study of the gas‐particle flow in a solid motor nozzle has been made by Hwang and Chang 3 using MacCormack’s explicit scheme in

Nonlinear oscillations of thick asymmetric cross-ply beams

Abstract: The axial and transverse motion of asymmetrically laminated cross-ply beams is coupled due to (i) the non-coincidence of reference and mid-planes and (ii) finite amplitudes. To derive the equations that govern the nonlinear oscillatory behavior of these laminated beams, a spatial deformation satisfying geometric constraints is substituted in the kinetic potential and Hamilton's principle is employed. This leads to three coupled nonlinear second order ordinary differential equations. These equations have quadratic nonlinear terms in addition to usual linear and cubic terms. The existence of these terms does not allow a straight forward solution in the presence of axial and rotatory inertia for nonlinear frequencies/periods. The objective of the present paper is to propose two possible solution methods for computing nonlinear frequencies/periods of asymmetrically laminated cross-ply beams. In both the methods, energy of positive and negative deflection cycles is maintained equal. A few numerical examples are solved to validate the methods. The effect of axial and rotatory inertia on the large amplitude oscillatory behavior is investigated.

Determination of Pressure Dependence of Burning Rate in Solid Motors Using Ultrasonic Technique

Abstract: The instantaneous burning rate of an aluminized composite solid propellant in a ballistic evaluation motor was measured continuously using an ultrasonic pulse-echo technique. The motor was designed to operate over a range of pressure from 2.5 to 4.5 MPa during its burning period. Hence, the burning-rate data could be obtained at different pressures directly from a single motor Ž ring itself, and the data were used to determine the pressure dependence of the burning rate. Thus, the technique is shown to overcome the need for carrying out several strand tests or ballistic evaluation motor tests, each at a different pressure, for characterizing a solid propellant burning rate.

Numerical investigation of viscous flow over a hemisphere-cylinder

Abstract: A numerical analysis has been performed to investigate turbulent compressible flow over a hemisphere-cylinder body at zero incidence in the Mach number range of 0.8–2.0. The numerical code solves the Navier-Stokes equations using finite volume technique in conjunction with multistage Runge-Kutta time-stepping method. Comparisons have been made with the available experimental data such as shadowgraph pictures, shock stand-off distance, shock position and surface pressure distribution. They are found in good agreement. A separated flow on the hemisphere-cylinder junction is noticed between Mach numbers 0.8 and 0.9. It is observed from the velocity vector plots that the flow appears to become parallel to the body in the vicinity of the stagnation point of the hemisphere at supersonic Mach number.