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.

Dual Band Polarization Insensitive Frequency Selective Surface Absorber

Abstract: This work presents dual band, polarization insensitive and angular stable radar absorber structure Here, proposed absorber unit cell has two resonator structures M and N Each resonator provides absorption at distinct frequencies in X and Ku band Simulation results in different cases show that the structure is polarization insensitive and angular stable up to 600 Calculated effective impedance plot and surface current distribution at the top and bottom surface supports the absorption mechanism at the resonance frequencies To verify the simulations, a prototype of absorber is fabricated on FR-4 substrate and tested in an anechoic environment

Parallel numerical simulation of compressible free shear layers in a scramjet engine

Abstract: This paper presents an accurate and reliable numerical method for simulating compressible free shear layers in a scramjet engine. A special attention is focused upon the proper modeling effort of the turbulence and temperature fluctuation effect in the Reynolds-averaged simulations. As the specific turbulence model, a two-equation k — e model is adopted instead of the widely-used algebraic model. To take the temperature fluctuation effect into account, a two-equation type transport model is solved and the modified Arrhenius type reaction rate equation is used. Numerical simulation results for four types of flows typically found in a scramjet engine are shown, and detailed discussion is given about the prediction capability of our numerical code and the critical problems concerning free shear layers in a scramjet engine such as the effect of flow compressibility on the free shear layer growth rate, key factors to affect the flame holding and the effect of turbulence temperature fluctuation on the combustion process.

Aircraft drag prediction and reduction. Addendum 1: Computational drag analyses and minimization; mission impossible?

Abstract: The Special Course on Aircraft Drag Prediction was sponsored by the AGARD Fluid Dynamics Panel and the von Karman Institute and presented at the von Karman Institute, Rhode-Saint-Genese, Belgium, on 20 to 23 May 1985 and at the NASA Langley Research Center, Hampton, Virginia, USA, 5 to 6 August 1985. The course began with a general review of drag reduction technology. Then the possibility of reduction of skin friction through control of laminar flow and through modification of the structure of the turbulence in the boundary layer were discussed. Methods for predicting and reducing the drag of external stores, of nacelles, of fuselage protuberances, and of fuselage afterbodies were then presented followed by discussion of transonic drag rise. The prediction of viscous and wave drag by a method matching inviscid flow calculations and boundary layer integral calculations, and the reduction of transonic drag through boundary layer control are also discussed. This volume comprises Paper No. 9 Computational Drag Analyses and Minimization: Mission Impossible, which was not included in AGARD Report 723 (main volume).

The fatigue and fracture behavior of micron-rubber and nano-silica particles modified epoxy polymer

Abstract: A thermosetting epoxy polymer was hybrid-modified by incorporating 9 wt.% of CTBN rubber microparticles and 10 wt.% of silica nanoparticles. The resin was poured into steel mould and cured to produce bulk epoxy polymer sheets from which fatigue test specimens were machined. The total fatigue life of the hybrid-modified epoxy polymer was determined by conducting constant amplitude fatigue tests with dog-bone shaped test specimens, at a stress ratio, R = σmin/σmax = 0.1, using a sinusoidal waveform at a frequency of 3 Hz. Further, the fatigue crack growth behavior of the hybrid-modified epoxy polymer, at a stress ratio, R = 0.1, was determined using a standard 50 mm wide compact tension specimen. The fatigue fracture surfaces were observed using a scanning electron microscope. The cyclic fracture toughness of the hybrid-modified epoxy polymer, estimated from the fracture surface analysis, correlated well with the reported values of the toughness; which was significantly greater than that of the neat epoxy polymer. The energy dissipating micromechanisms of, (i) rubber particle cavitation and plastic deformation of the surrounding material, and (ii) silica nanoparticle debonding followed by plastic void growth, were observed to be operative, resulting in an improved fracture toughness. The fatigue crack initiation and propagation lives were determined from the experimental data. The enhanced capability to withstand longer crack lengths, due to the improved toughness together with the retarded crack growth rate, were observed to enhance the total fatigue life of the hybrid-modified epoxy polymer.

Structural health monitoring of co-cured composite structures using FBG sensors

Abstract: Structural Health Monitoring (SHM) of aircraft structures, especially composite structures, has assumed increased significance on considerations of safety and costs. With the advent of co-cured structures, wherein bonded joints are replacing bolted joints there is a concern regarding skin-stiffener separation, which might not be detected unless a rigorous non-destructive testing (NDT) is done. It would hence be necessary to be able to detect and assess skin-stiffener separation in composite structures before it reaches the critical size. One of the health monitoring strategies is through strain monitoring using fibre optic strain sensors such as Fibre Bragg Grating (FBG) sensors. The first aspect that needs to be addressed is the characterization of the FBG sensors. Issues of embedment in composites have also to be addressed. Before evolving a damage detection strategy, the sensitivity of the structural strain to skin-stiffener separations must be clearly understood and quantified. This paper presents the analysis and experiments done with a composite test box to study the effect of skin-stiffener separation on the strain behaviour. The box consists of two skins stiffened with spars made of Bi-Directional (BD) glass-epoxy prepreg material. The spars are bolted to the skins and removing suitable number of bolts simulates 'de-bonds'. The strains of the healthy box are compared with the unhealthy box. The strains in the experiments are monitored using both strain gauges and Fibre Bragg Grating (FBG) sensors. The experimental results show that there is significant change in the measured strain near and away from the debond location. The finite element analysis of the box is done using ABAQUS and the analysis is validated with the experimental results. A neural network based methodology is developed here to detect skin-stiffener debonds in structures. A multi-layer perceptron (MLP) neural network with a feed forward back propagation algorithm is used to determine the size/severity of damage. The FE model is used to generate the neural network training data for various sizes of debonds. The results show that the network is able to predict the damage size well. The network is implemented for a specified load. However, it is seen that the damage size predicted is independent of the applied load and the network performance is dependent on the fidelity of the finite element model used to train the network.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.