National Aerospace Laboratories

About: National Aerospace Laboratories is a facility organization based out in Bengaluru, India. It is known for research contribution in the topics: Coating & Corrosion. The organization has 1838 authors who have published 2349 publications receiving 36888 citations.

Thermal and toughness property studies on a polybenzimidazole-modified epoxy resin system

Abstract: The effect of polybenzimidazole (Pill) on a silica-filled epoxy resin matrix has been investigated. Polybenzimidazole (PBI) was incorporated into a difunctional epoxy resin matrix to the extent of 10%, before being cured with an anhydride hardener. The effects of Pill on the curing reaction and glass transition temperature (Tg) and on the toughness of the cured epoxy matrix have been studied using differential scanning calorimetry (OSC), thermogravimetric ctric analysis (TGA) and a universal testing machine (Instron). The results indicate that the PBI modifier enhanced not only the glass transition temperature of the difunctional epoxy m.atrix but also its toughness, by its catalytic action. Further investigations have been carried out on the fractured specimens, using scanning electron microscopy (SEM) to support the enhanced toughness property of the epoxy matrix.

Exact solutions for Stokes flow in and around a sphere and between concentric spheres

Abstract: A general method is suggested for deriving exact solutions to the Stokes equations in spherical geometries. The method is applied to derive exact solutions for a class of flows in and around a sphere or between concentric spheres, which are generated by meridional driving on the spherical boundaries. The resulting flow fields consist of toroidal eddies or pairs of counter-rotating toroidal eddies. For the concentric sphere case the exact solution when the inner sphere is in instantaneous translation is also derived. Although these solutions are axisymmetric, they can be combined with swirl about a different axis to generate fully three-dimensional fields described exactly by simple formulae. Examples of such complex fields are given. The solutions given here should be useful for, among other things, studying the mixing properties of three-dimensional flows.

Effect of Thermal Aging on the Crystal Structural Characteristics of Poly(tetra fluoro ethylene)

Abstract: The residual effects of cumulative thermal aging on the crystal structural characteristics of the fluoro carbon poly(tetra fluoro ethylene) (PTFE) have been studied by X-ray diffraction methods. The initial hexagonal arrangement of the PTFE chains in a 157 helical conformation is left unaffected by the exposures to temperatures (T), up to and beyond its melting point, Tm. The unit cell registers a residual anisotropic volume expansion. The anisotropy arises from the enhanced enlargement of the basal plane dimension a compared with the axial dimension c. Conformational changes contributing to the observed increase in the chain length have been examined. Enhancement of residual crystallinity of samples aged at T's lt; Tm suggests that the selective thermal aging could be used as an effective tool to improve the initial crystallinity of commercially available PTFE. The activation energy for 50% enhancement in initial crystallinity has been estimated as 53.9 kJ mol-1. Aging at 400xB0;C, a temperature above Tm, is accompanied by markedly different features viz., deterioration in crystallinity and other structural characteristics. The overall behavior of thermally aged PTFE bears a marked similarity to many polyamides. POLYM. ENG. SCI., 47:1724-1729, 2007. xA9; 2007 Society of Plastics Engineers

Reactive flow in scramjet external nozzle

Abstract: The combustion and thrust characteristics in the singleexpanded ramp nozzle (SERN) were studied numerically with two-dimensional Navier-Stokes equations and k-o turbulence model. The external nozzle is one of the important parts of the scramjet engine, since the external nozzle gives a major portion of thrust of scrarnjet engine. One of the critical problems of external nozzle is the unburned gas mixture entering the external nozzle from combustor. The mixing and combustion processes in the external nozzle were studied and the effects of entrance condition of unburned gas mixture on the thrust characteristics are estimated.

RCS gas-jet interaction in the Hypersonic Flight Experiment, HYFLEX

Abstract: The RCS gas-jet interaction experiment in the Hypersonic Flight Experiment, HYFLEX, was performed for the purpose of evaluating a method to estimate the jet interaction effects in flight condition, based on wind tunnel tests and/or CFD calculations. In this paper, flight data on surface pressure distribution measurement around a RCS thruster are presented in supersonic to hypersonic speed ranges, being compared with several wind tunnel test results and CFD calculations. Three correlation parameters of the jet interaction phenomena ( pressure ratio, mass flow ratio, and momentum ratio ) are examined in the comparisons. The results show that jet momentum ratio to external flow is generally a suitable correlation parameter in the case of the experimental yaw thruster in laminar boundary layer condition. On the other hand, mass flow ratio is estimated to be a better parameter than momentum ratio for turbulent boundary layer condition. NOMENCLATURE A area, m 2 b body width, m Ci rolling moment coefficient, L/qSb Ca yawing moment coefficient, N/qSb Cp pressure coefficient (p-p ~>)/q IB body length, m K jet thrust amplification factor, Eq.(4) L rolling moment, N-m rh mass flow rate, kg/s M Mach number MFR jet mass flow ratio to freestream, Eq.(2) MTR jet momentum ratio to freestream, Eq.(3) N yawing moment, N-m PRR jet static pressure ratio to freestream, Eq.(l) p static pressure, Pa q dynamic pressure, Pa Re Reynolds number based on body length * Senior Researcher, Aerodynamics Division. Member AIAA. t Researcher, Computational Sciences Division. Member AIAA. t Laboratory Head, Aerodynamics Division. Member AIAA. Copyright © 1997 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Re Reynolds number based on body length S vehicle planform area, m 2 T temperature, K; thrust, N V velocity, m/s X,Y,Z coordinates fixed to vehicle, m (see Fig.l) a angle of attack, deg A increment due to jet interaction y specific heat ratio Subscripts: 0 stagnation condition CG vehicle center of gravity VAC vacuum condition j condition of jet at nozzle exit °° freestream condition INTRODUCTION HYFLEX Hypersonic Flight Experiment was planned as the first Japanese lifting hypersonic flight in order to acquire flight data needed for the development of the Japanese operational unmanned orbiting plane, HOPE ' . The HYFLEX flight was successfully conducted on Feb. 12, 1996, using a small lifting body-type vehicle 2 . After the vehicle was accelerated by a two-stage solid rocket motor and separated at an altitude of 107 km with a velocity of 3.9 km/s, the vehicle glided down with guidance and control. Flight data acquired in HYFLEX consist of data on hypersonic aerodynamics, thermal protection system, and guidance and control. As an important item of the measurements on aerodynamics , RCS gas-jet interaction measurement was performed because estimation of aerodynamic interaction effects between RCS gas-jet and external flow have been one of key technologies in hypersonic reentry vehicle development. The experiment was aimed at evaluating a method to estimate the interaction effects in flight condition, based on ground-based data. In this paper, surface pressure distribution data affected by the gas-jet interaction phenomena are presented in comparison with wind tunnel test results simulating several candidates for jet interaction correlation parameter. In order to extract a suitable correlation parameter, the parameters as candidates American Institute of Aeronautics and Astronautics are examined in respect to interaction effects on surface pressure distribution, aerodynamic moment, and net thrust of thruster. CFD calculations are also conducted for validating the Navier-Stokes code used to solve the gas-jet interaction flowfield. REACTION CONTROL SYSTEM (RCS) Attitude of the HYELEX vehicle is controlled by coordinating an aerodynamic control surface called 'eleven' and a three-axis gas-jet reaction control system (RCS) . In low dynamic pressure region just after the vehicle separation, the vehicle is controlled only by the three-axis RCS. After reaching certain dynamic pressure, the control law changes to a combined attitude control by the eleven and the yaw RCS thrusters, continuing down to Mach 2. Figure 1 shows the vehicle configuration and arrangement of the RCS thrusters. Table 1 shows reference values of the vehicle for nondimensionalizing aerodynamic force and moment. The RCS thrusters are divided into two categories with different functions. The first category consists of six thrusters for controlling the vehicle attitude. The thrusters are located on thruster module bulges installed on the vehicle body base area in order to avoid unexpected gas-jet interaction effects on the attitude control. The second one is for investigating aerodynamic interaction phenomena between a RCS gas-jet and an external stream. Two experimental yaw thrusters which are installed in a symmetrical position on rear part of vertical stabilizing fins are used only for the purpose. The jet gas supplied to all of the thrusters is nitrogen without heating. Dimensions and performance of the experimental yaw thrusters are summarized in Table 2. The nozzle has a conical shape with a semi-vertex angle of 20 deg. The expansion ratio of it is equal to 57. The nozzle center axis is parallel to the vehicle Y-axis, so that it is not perpendicular to the outer surface of the stabilizing fin. Theoretical Mach number of the jet flow at the nozzle exit is 6.09. Stagnation pressure in the settling chamber of the nozzle poj is always regulated to a constant pressure. Stagnation temperature TOJ gradually decreases from room temperature with the decrease in nitrogen pressure in an accumulating tank. ONBOARD MEASUREMENT APPARATUS AND PROCEDURE In order to examine the gas-jet interaction effects, surface pressure distribution was measured around the experimental yaw thruster during the flight. As shown in Fig.2, twelve pressure orifices are located only on the left-side stabilizing fin, which have three-row arrangement nearly parallel to the leading edge of the fin. As observed in oil flow tests in hypersonic wind tunnels, the rows are nearly parallel to stream line of the external flow on the fin at angles of attack in the flight. Considering unexpectedly strong jet interaction effects, a wide area around the thruster is covered by the orifices. High accuracy pressure transducers with frequency outputs are used for each of the orifices. After the 20Hz-sampled outputs are sent to a PCM telemeter package through a signal conditioner with A/D converters, the data are transmitted by telemetry to the ground. During the flight, an onboard computer calculates gas-jet mass flow ratio to freestream and flight Mach number based on the estimated velocity and altitude, assuming the US Standard Atmosphere model. When the mass flow ratio or the flight Mach number becomes equal to specified values determined prior to the flight, the experimental thruster injects nitrogen gas for a period of 0.5 sec. Ten injections were planned during the flight. Both of the experimental yaw thrusters on the left and right fins are injected together to cancel yawing moment which may disturb the vehicle altitude control. Therefore, variations of aerodynamic moment due to the jet interactions can not be known directly from the flight data. Figure 3 shows the experimental yaw thruster injection timing in the real flight. In accordance with the test plan, ten injections of jet ( injection #1 to #10 ) were certainly performed from Mach 14 down to 3. The first five injections were conducted at an angle of attack of 49 deg and the remains at an angle of 30 deg. Various correlation parameters have been proposed for the gas-jet interaction phenomena 5 . In the flight data analysis of the Space Shuttle Orbiter, several correlation parameters were extensively compared" . In HYFLEX, three correlation parameters defined in the following equations were examined as candidates for a suitable correlation parameter. PL Pressure ratio Mass flow ratio (1)