Showing papers by "National Aerospace Laboratories published in 2011"

Image Fusion Technique using Multi-resolution Singular Value Decomposition

Abstract: A novel image fusion technique based on multi-resolution singular value decomposition (MSVD) has been presented and evaluated. The performance of this algorithm is compared with that of well known image fusion technique using wavelets. It is observed that image fusion by MSVD perform almost similar to that of wavelets. It is computationally very simple and it could be well suited for real time applications. Moreover, MSVD does not have a fixed set of basis vectors like FFT, DCT and wavelet etc. and its basis vectors depend on the data set.Defence Science Journal, 2011, 61(5), pp.479-484, DOI:http://dx.doi.org/10.14429/dsj.61.705

Extended Kalman Filter vs. Error State Kalman Filter for Aircraft Attitude Estimation

Abstract: The Kalman filter (KF) is the optimal estimator that minimizes the mean square error when the state and measurement dynamics are linear in nature, provided the process and measurement noise processes are modeled as white Gaussian. However, in the real world, one encounters a large number of scenarios where either the process or measurement model (or both) are nonlinear. In such cases a class of suboptimal Kalman filter implementations called extended Kalman filters (EKF) are used. EKFs operate by linearizing the nonlinear model around the current reference trajectory and then designing the Kalman filter gain for the linearized model. Recently, an alternative approach has emerged for a certain class of problems where the error in the states is estimated using a Kalman filter, rather than the state itself. This error state KF (ErKF) approach, by deriving the error state dynamics, via the perturbation of the nonlinear plant, lends itself to optimal updates in the error states and optimal prediction and updates in the error state covariance. This is because the error state dynamics are linear, thereby satisfying a condition for optimal Kalman filtering. This paper offers a comparison between the EKF and ErKF via simulations and shows that the ErKF performance is robust to a variety of aircraft maneuvers performed. Furthermore, this paper shows that the ErKF, unlike the EKF, need not be repeatedly tuned with respect to the noise covariances in order to obtain acceptable estimation performance.

Fabrication of Superhydrophobic Nanocomposite Coatings Using Polytetrafluoroethylene and Silica Nanoparticles

Abstract: Superhydrophobic nanocomposite coatings were fabricated by incorporating hydrophobically modified silica (HMS) nanoparticles in polytetrafluoroethylene (PTFE) emulsion. Hydrophobicity of the coating was dependent on the concentration of HMS. Coatings containing optimum amounts of PTFE and HMS exhibited superhydrophobic property with high water contact angle (WCA) of 165∘ and low sliding angle <2∘. Scanning electron microscopic (SEM) studies have shown a binary surface topography composed of microbumps and nanoscale granules. The synergistic effect of the micro-nano-binary structure and low surface energy of PTFE was responsible for the superhydrophobicity of the coating. The method is simple and cost-effective and can be used for preparing self-cleaning superhydrophobic coatings on large areas of different kinds of substrates like glass, metal, and composites.

Impact of background error statistics on forecasting of tropical cyclones over the north Indian Ocean

Abstract: [1] While the quality of background error statistics (BES) is recognized as one of the key components of assimilation, considerable uncertainties exist in prescribing BES, especially since the prescription and impact of BES can also depend on the weather regime and geographical location. In this backdrop, it is necessary to quantify the impact of different BES for particular weather systems; this is particularly true for cyclones over the north Indian Ocean which have characteristics different from those over the Atlantic and the Pacific. The objective of this work is to assess the relative improvement in forecasting tropical cyclone track and intensity due to different BES. We have used global BES (GBES), computed from global model forecasts for 357 cases distributed over a period of one year and regional BES (RBES), generated from short-range forecasts with Weather Research and Forecasting (WRF) model for a 30 day period. From a series of assimilation experiments using the WRF three-dimensional variational (3D-Var) data assimilation system with different BES, and a number of parameters to quantify the impact of BES, it is shown that the use of RBES in WRF 3D-Var significantly improves prediction of track as compared to simulations with no assimilation or GBES. Further, the skill with RBES is comparable with, or better than, many operational skills, although a strict comparison is difficult due to differences in the events and the basins. While parallel and significant efforts are needed for the formulation and incorporation of BES in assimilation systems in general, this study quantifies relative advantages of using RBES in forecasting cyclones over the Indian Ocean.

NO Reduction Over Noble Metal Ionic Catalysts

Abstract: In last 40 years, catalysis for NOx removal from exhaust gas has received much attention to achieve pollution free environment. CeO2 has been found to play a major role in the area of exhaust catalysis due to its unique redox properties. In last several years, we have been exploring an entirely new approach of dispersing noble metal ions in CeO2 and TiO2 for redox catalysis. We have extensively studied Ce1−xMxO2−δ (M = Pd, Pt, Rh), Ce1−x−yAxMyO2−δ (A = Ti, Zr, Sn, Fe; M = Pd, Pt) and Ti1−xMxO2−δ (M = Pd, Pt, Rh, Ru) catalysts for exhaust catalysis especially NO reduction and CO oxidation, structure–property relation and mechanism of catalytic reactions. In these catalysts, lower valent noble metal ion substitution in CeO2 and TiO2 creates noble metal ionic sites and oxide ion vacancy. NO gets molecularly adsorbed on noble metal ion site and dissociatively adsorbed on oxide ion vacancy site. Dissociative chemisorption of NO on oxide ion vacancy leads to preferential conversion of NO to N2 instead of N2O over these catalysts. It has been demonstrated that these new generation noble metal ionic catalysts (NMIC) are much more catalytically active than conventional nano crystalline noble metal catalysts especially for NO reduction.

Influence of scandium on an Al–2% Si alloy processed by high-pressure torsion

Abstract: High-pressure torsion (HPT) was used to process Al–2% Si and Al–2% Si–0.25% Sc alloys for up to five turns and the mechanical properties of the processed materials were evaluated using the ball indentation technique (BIT). The results show that the presence of Al3Sc precipitates is effective in producing higher strength levels and greater grain refinement in the Al–2% Si–0.25% Sc alloy. The introduction of scandium reduces the grain size of the Al–2% Si alloy from ∼0.38 to ∼0.15 μm after 5 turns of HPT and the corresponding maximum tensile strength is increased from ∼325 to ∼375 MPa. The grain and substructure formation in the Al–2% Si alloy is similar to aluminum with dislocation cell formation and a reasonably recovered microstructure whereas in the Al–2% Si–0.25% Sc alloy it is non-homogeneous with arrays of non-equilibrium boundaries and dislocation tangles within the grains.

Dynamic characteristics of epoxy hybrid nanocomposites

Abstract: An experimental study is conducted on epoxy composite, reinforced with nano alumina and Multiwall CNTs (0.5 and 1.0 wt%) in monolithic and hybrid forms to evaluate its stiffness and damping properties. Among monolithic reinforcements, Multiwall CNTs (1.0 wt %) have shown a significant increase in loss modulus, whereas nano alumina (0.5 wt%) has increased the storage modulus of the composite. With hybrid reinforcements, there is further increase in storage modulus (0.5 wt%) observed; however, a slight reduction in loss modulus (1.0 wt%) is noticed in comparison with CNT-reinforced nanocomposite. Based on these experimental observations, a two-layer hybrid composite configuration, consisting of nano alumina (0.5 wt% as layer 1) and Multiwall CNTs (1.0 wt% as layer 2) is fabricated and tested. The obtained results have indicated significant and simultaneous increase in storage and loss modulus, which brings out the importance of using the hybrid reinforcement configurations.

Effect of process parameters on growth rate and diameter of nano-porous alumina templates

Abstract: Anodic aluminium oxide (AAO) template with hexagonal shaped nano-pores with high aspect ratio was fabricated by two-step anodization processes from high purity aluminium foil. It was observed that pore dimensions were affected by anodizing voltage, electrolyte temperature and the duration of anodization time. The vertical growth rate of the pores (10–250 nm/min) was found to vary exponentially with anodizing voltage; however, it exhibits linear increment with the electrolyte temperature. The measured pore diameter (50–130 nm) shows a linear variation with anodizing voltage. The bottom barrier oxide layer was etched out by pore widening treatment to obtain through holes.

Multi-resolution image fusion by FFT

Abstract: Multi-resolution image analysis by Fast Fourier Transform (MFFT) algorithm has been presented and evaluated for pixel level image fusion. The idea is to apply simple and proven technique of FFT to image fusion. The performance of this algorithm is compared with that of well known wavelet based image fusion technique. It is observed that the performance of image fusion by MFFT is slightly better than wavelet based image fusion algorithm. It is computationally very simple and it could be well suited for real time applications.

Optimization of Helicopter Rotor Using Polynomial and Neural Network Metamodels

Abstract: This study aims to determine optimal locations of dual trailing-edge flaps and blade stiffness to achieve minimum hub vibration levels in a helicopter, with low penalty in terms of required trailing-edge flap control power. An aeroelastic analysis based on finite elements in space and time is used in conjunction with an optimal control algorithmtodeterminethe flaptimehistoryforvibrationminimization.Usingtheaeroelasticanalysis,itisfoundthat the objective functions are highly nonlinear and polynomial response surface approximations cannot describe the objectives adequately. A neural network is then used for approximating the objective functions for optimization. Pareto-optimal points minimizing both helicopter vibration and flap power are obtained using the response surface and neural network metamodels. The two metamodels give useful improved designs resulting in about 27% reduction in hub vibration and about 45% reduction in flap power. However, the design obtained using response surface is less sensitive to small perturbations in the design variables.

Studies on the Control of Shock Wave-Boundary layer Interaction Using Steady Microactuators

Abstract: unswept compression ramp in a Mach 2 flow to study their effect on the unsteady pressure loads due to the interaction. Flow visualization and fluctuating pressure measurements were obtained in the separated shock wave/boundary layer interaction produced by the ramp. Results show that the array of microjets issuing in the supersonic crossflow creates a shock which effectively reduces the Mach number in front of the separation shock. This causes the separation shock to weaken, and move upstream by as much as 4δo from its mean undisturbed location. The mean pressure distribution was altered due to the microjet control with a maximum reduction of 7% on the wall surface downstream of the separation shock and 25% on the ramp surface. The wall pressure fluctuations were also reduced significantly in the separation region with control where the intermittent region was also reduced by more than 40% with the applied control. The pressure spectra show that the unsteadiness on the wall and the ramp surfaces has been significantly reduced with the applied control.

The effect of micron-rubber and nano-silica particles on the fatigue crack growth behavior of an epoxy polymer

Abstract: A thermosetting epoxy polymer was hybrid-modified by incorporating 9 wt.% of CTBN rubber micro particles and 10 wt.% of silica nano-particles. The unmodified and the hybrid-modified resins were poured into steel moulds and cured to produce bulk epoxy polymer sheets from which standard compact tension test specimens were machined. Fatigue crack growth tests were conducted using a 50 kN servo-hydraulic test machine, with the following test parameters: stress ratio, R = σmin/σmax = 0.1, sinusoidal waveform and frequency, ν = 3 Hz. The crack length was monitored by a compliance technique. The fracture surfaces were observed in a high resolution scanning electron microscope. The fatigue crack growth rate of the hybrid epoxy polymer was observed to be significantly lower than that of the unmodified epoxy polymer. The threshold stress intensity factor range, ΔKth, of the epoxy polymer was observed to increase by the addition of micron-rubber and nano-silica particles. The energy dissipating mechanisms viz, (i) cavitation of the rubber microparticles followed by plastic-deformation and void growth of the epoxy and, (ii) silica nanoparticle debonding followed by plastic-deformation and void growth of the epoxy, were observed to be operative and contribute for the reduced crack growth rate in the hybrid epoxy polymer.

Control of vortex shedding from a circular cylinder using imposed transverse magnetic field

Abstract: Purpose – The purpose of this paper is to simulate the flow of a conducting fluid past a circular cylinder placed centrally in a channel subjected to an imposed transverse magnetic field to study the effect of a magnetic field on vortex shedding at different Reynolds numbers varying from 50 to 250.Design/methodology/approach – The two‐dimensional incompressible laminar viscous flow equations are solved using a second‐order implicit unstructured collocated grid finite volume method.Findings – An imposed transverse magnetic field markedly reduces the unsteady lift amplitude indicating a reduction in the strength of the shed vortices. It is observed that the periodic vortex shedding at the higher Reynolds numbers can be completely suppressed if a sufficiently strong magnetic field is imposed. The required magnetic field strength to suppress shedding increases with Reynolds number. The simulation shows that the separated zone behind the cylinder in a steady flow is reduced as the magnetic field strength is in...

Experimental Studies on a Rudimentary Four Wheel Landing Gear

Abstract: Experiments were carried out on a rudimentary four-wheel landing-gear arrangement to understand the flow physics of this configuration. Measurements carried out included oil-flow visualization, steady surface pressures and two-dimensional particle image velocimetry measurements in the near wake at a Reynolds number based on wheel diameter of 1 � 106. The surface visualization demonstrated the significantly asymmetric behavior of the flow on both the front and back wheel. The surface-pressure data show that separation is earlier on the wing side as opposed to the ground side for both wheels, caused by the presence of the vertical strut. The data show that the truck and strut are dominant contributors to the total drag. The off-body measurements in the near wake document the confluent wakes from the wheel and the horizontal strut leading to a three-dimensional unsteady wake. The data presented here provide a comprehensive picture, both quantitatively and qualitatively, of the flow over a rudimentary landing gear and are useful for validation of computational fluid dynamics efforts in addition to understanding the flow physics of such configurations.