Showing papers by "National Aerospace Laboratories published in 2004"

Density measurements using the Background Oriented Schlieren technique

Abstract: This paper describes the implementation of a novel technique called Background Oriented Schlieren that can produce quantitative visualization of density in a flow. This technique uses only a digital still camera, a structured background, and inverse tomographic algorithms which can extract two-dimensional slices from a three-dimen- sional flow. This has been applied to obtain the density field for an axisymmetric supersonic flow over a cone- cylinder model. Comparisons with cone tables show excellent agreement. List of symbols b wave angle h viewing angle q density (kg/m 3 ) k wavelength (m) e angle of deflection D diameter of cylinder f focal length of imaging lens G(k) Gladstone-Dale number j ffiffiffiffiffiffi 1 p

Raman spectroscopy studies on the thermal stability of TiN, crN, TiAlN coatings and nanolayered TiN/CrN, TiAlN/CrN multilayer coatings

Abstract: About 1.5-um-thick single-layer TiN, CrN, TiAlN coatings and nanolayered TiN/CrN, TiAlN/CrN multilayer coatings were deposited on silicon (111) substrates using a13; reactive direct current magnetron sputtering process. Structural characterization of the coatings was done using x-ray diffraction (XRD) and micro-Raman spectroscopy. All13; the coatings exhibited NaCl B1 structure in the XRD data. Raman spectroscopy data of as-deposited coatings exhibited two broad bands centered at 230x2013;250 and 540x2013;630 cmx2212;1.13; These bands have been assigned to acoustical and optical phonon modes, respectively. Thermal stability of the coatings was studied by heating the coatings in air in a13; resistive furnace for 30 min in the temperature range 400x2013;900 xB0;C. Structural changes as a result of heating were characterized using Raman spectroscopy and XRD. Raman13; data showed that TiN, CrN, TiN/CrN, TiAlN, and TiAlN/CrN coatings started to oxidize at 500, 600, 750, 800, and 900 xB0;C, respectively. To isolate the oxidation-induced spectral changes as a result of heating of the coatings in air, samples were also annealed in vacuum at 800 xB0;C under similar conditions. The Raman data of vacuum-annealed coatings showed no phase transformation, and intensity of the optical phonon mode increased and shifted to lower frequencies. The origin of these spectral changes is discussed in terms of defect structure of the coatings. Our results indicate that the thermal stability of nanolayered multilayer coatings is superior to the13; single-layer coatings.

Structure and properties of reactive DC magnetron sputtered TiN/NbN hard superlattices

Abstract: Nanostructured TiN/NbN superlattices at various modulation wavelengths (Λ) were deposited on silicon (111) and tool steel substrates using a reactive DC magnetron sputtering process. Structural characterization of the coatings was done using X-ray diffraction (XRD) in Bragg-Brentano θ–2θ geometry. All the multilayer coatings exhibited (111) preferred orientation in the XRD data. Appearance of satellite reflections (SR) along (111) principal reflection (PR) was used to check the quality of the superlattice coatings. The coatings exhibited superlattice structure for 106 A≥Λ≥30 A. The mechanical properties of the coatings were measured using a nanoindentation technique. The multilayer coatings exhibited hardness as high as 4000 kg/mm2 at a modulation wavelength of 48 A, which was ∼2.4 times the value of the rule-of-mixture. Thermal stability of the TiN/NbN multilayer coating was studied by vacuum annealing the coating for 30 min in the temperature range of 100–850 °C. Subsequent structural changes in the coating were measured by an in situ high-temperature X-ray diffractometer. Coating exhibited thermal stability up to 700 °C and subsequently showed reduction in the superlattice character. The corrosion behavior of 1.5 μm thick single layer TiN and NbN, and multilayer TiN/NbN (Λ=50 A) coatings deposited on tool steel substrates were investigated using potentiodynamic polarization in 0.5 M NaCl and 0.5 M HCl solutions under non-deaerated conditions. The microstructural changes, as a result of corrosion, were investigated by atomic force microscopy (AFM). The results indicated that the multilayer coatings exhibited superior corrosion resistance as compared to the single layer coatings.

Dispersion and Slip Casting of Hydroxyapatite

Abstract: The dispersibility in deionized water of hydroxyapatite (HA) synthesized by a high-temperature (1000°C) solid-state reaction between tricalcium phosphate and calcium hydroxide was investigated as a function of the pH of the medium and the quantity of two dispersing agents (A 5 inorganic, B 5 organic)added to the slips. Although pH modification had a negligible effect on dispersibility, both of the dispersing agents produced a good dispersion at considerably higher concentrations (>2 wt% of HA). At optimum amounts (2–4 wt%) of the dispersing agents, the slips showed near-Newtonian flow behavior up to 45 wt% solids loading and non-Newtonian behavior at >50 wt%. By the optimal addition of dispersing agents and conditioning by ball milling, 60–67 wt% (32–39 vol%) solids-loaded HA slips could be cast into plaster molds to produce 50%–58% dense green bodies, which, in turn, sintered to 90%–94% density in the temperature range 1300°–1400°C. The sintered HA exhibited a three-point flexural strength of 40–60 MPa and a homogeneous microstructure, with interspersed microporosities.

Low-velocity Impact Characterization of Glass, Carbon and Kevlar Composites Using Repeated Drop Tests

Abstract: Low-velocity ( 1.2-2.4 m/s) repeated drop weight impact tests were carried out on advanced composites (glass, carbon, kevlar in epoxy matrix - LY 556, HT 972) using an instrumented impact test machine (DYNATUP 8250). The incident energies ranged from 3.5 to 15 J (constant impactor mass 5.42 kg). Number of drops to failure (Nf) data were obtained for the above with simultaneous recording of the load-time and energy-time traces. The extent of impact damage for glass and kevlar composites was interpreted using final delamination area maps (tracings). Results for all the three composites showed that with an increase in drop numbers, the peak load (PL) steadily decreased while the total energy (Et) increased, until failure. Also, as the incident energy (Ein) was varied in arithmetic progression, the number of drops to failure (Nf) varied in harmonic progression. Finally it was concluded that repeated drop tests with final delamination area maps assist in understanding the impact damage tolerance of polymer composites

Thermally induced vibration control of composite plates and shells with piezoelectric active damping

Abstract: A coupled piezoelectric field is modelled with an expansion strain in the numerical formulation to analyse piezohygrothermoelastic laminated plates and shells. Finite element actuator and sensor equations are derived using a nine-noded field consistent shallow shell element. Thermally induced vibration control is attempted using piezoelectrically developed active damping. The influence of piezoelectric anisotropy on active damping is evaluated, adopting a simple modelling technique. With 40% reduced actuation capability in the lateral direction, the directionally active lamina is observed to be equally efficient in controlling the vibration. In general, the directionally active lamina is efficient if the primary actuation direction is oriented along the fibre direction or in the direction of bending. The directional actuation appeared to be more effective in the velocity feedback control for cantilevered plates and shells. However, in the simply supported case, a balanced actuation effort is required to provide better controllability, which can be achieved by tailoring the directional actuation. The importance of geometric curvature for the actuator performance is also highlighted.

Compression behavior of VC0.85 up to 53 GPa

Abstract: Samples of VC0.85 sandwiched between aluminum disks were compressed in a diamond anvil cell and X-ray diffraction patterns recorded at high pressures up to 53 GPa using synchrotron radiation. The presence of aluminum in the cell rendered the sample pressure nearly hydrostatic and also served as the pressure standard. No phase transformation was observed up to the highest pressure. The measured unit cell volume versus pressure data gave 258 ± 11 GPa and 4.5 ± 0.6 for the bulk modulus and the pressure derivative, respectively.

Comparison of the oxygen sensor performance of some pyrene derivatives in silicone polymer matrix

Abstract: Pyrene and its derivatives find application as fluorescent probes for monitoring the oxygen concentrations in air and various fluids Photophysical characteristics of some commercially available derivatives of pyrene in silicone polymer matrix were studied in order to find out whether they could be employed as a substitute for pyrene in the pressure sensitive paint (PSP) The derivatives under study were: 1-pyrenyl methanol (PMO), 1-pyrenyl butanol (PBO), 1-pyrene butyric acid (PBA), and 1-pyrene acetic acid (PAA) Distinct differences were observed in the fluorescence emission spectra of the silicone coatings of derivatives depending on the functional group attached to pyrene The oxygen quenching sensitivity (OQS) of these probes were evaluated and compared to that of pyrene It was found that the fluorescence spectra of all silicone coatings of pyrene derivatives exhibited monomer and excimer peaks The excimer emission of PBO in silicone matrix was oxygen sensitive whereas the coatings of the other three pyrene derivatives exhibited poor oxygen quenching

Estimation of Attitudes from a Low Cost Miniaturized Inertial Platform using Kalman Filter based Sensor Fusion Algorithm

Abstract: Due to costs, size and mass, commercially available inertial navigation systems are not suitable for small, autonomous flying vehicles like ALEX and other UAVs. In contrast, by using modern MEMS (or of similar class) sensors, hardware costs, size and mass can be reduced substantially. However, low-cost sensors often suffer from inaccuracy and are influenced greatly by temperature variation. In this work, such inaccuracies and dependence on temperature variations have been studied, modelled and compensated in order to reach an adequate quality of measurements for the estimation of attitudes. This has been done applying a Kaiman Filter-based sensor fusion algorithm that combines sensor models, error parameters and estimation scheme. Attitude estimation from low-cost sensors is first realized in a Matlab/Simulink platform and then implemented on hardware by programming the micro controller and validated. The accuracies of the estimated roll and pitch attitudes are well within the stipulated accuracy level of ±5‡ for the ALEX. However, the estimation of heading, which is mainly derived from the magnetometer readings, seems to be influenced greatly by the variation in local magnetic field

Bending Behavior of Hybrid-Actuated Piezoelectric Sandwich Beams

Abstract: The finite element analysis is presented for hybrid-actuated piezoelectric sandwich beam structures. The hybrid actuation is modelled by incorporating a transversely polarized, d31-based extension actuation lamina and an axially polarized, d15-based shear actuation lamina. Further the bending behavior of sandwich beams are evaluated for various boundary conditions with segmented actuators. The active stiffening effect is assessed through bending deflection behavior. The extension and shear actuators are collocated as well as non-collocated along the length of beam to see the combined actuation effort. It is observed that for the clamped-free case, the actuation effect is augmented with collocated actuators; however this trend is not followed in the other cases. Interestingly, the non-collocated actuators show better cumulative actuation effort for different boundary conditions except in the hinged-hinged case, where shear actuation appears to be predominant. As extension and shear actuations have distinctive features, both can be employed in a non-collocated fashion for better control action.

Nanoindentation and atomic force microscopy measurements on reactively sputtered TiN coatings

Abstract: Titanium nitride (TiN) coatings were deposited by d.c. reactive magnetron sputtering process. The films were deposited on silicon (111) substrates at various process conditions, e.g. substrate bias voltage (VB) and nitrogen partial pressure. Mechanical properties of the coatings were investigated by a nanoindentation technique. Force vs displacement curves generated during loading and unloading of a Berkovich diamond indenter were used to determine the hardness (H) and Young’s modulus (Y) of the films. Detailed investigations on the role of substrate bias and nitrogen partial pressure on the mechanical properties of the coatings are presented in this paper. Considerable improvement in the hardness was observed when negative bias voltage was increased from 100–250 V. Films deposited at |V B| = 250 V exhibited hardness as high as 3300 kg/mm2. This increase in hardness has been attributed to ion bombardment during the deposition. The ion bombardment considerably affects the microstructure of the coatings. Atomic force microscopy (AFM) of the coatings revealed fine-grained morphology for the films prepared at higher substrate bias voltage. The hardness of the coatings was found to increase with a decrease in nitrogen partial pressure.

Evaluation of recursive methods for aircraft parameter estimation

Abstract: Recursive parameter estimation consists of extracting aerodynamic stability and control derivatives using the flight measured input/output data in a real-time mode. It is useful in applications such as on-board verification of recorded data, iterative experiment design, verification of flight control laws, and reconfiguration of control system following damages. Three recursive algorithms, namely the Recursive Least Squares (RLS), Extended Kalman Filter (EKF) and Discrete Fourier Transform (DFT) are formulated and a comparative study has been carried out in the off-line environment using simulated and flight data for longitudinal and lateral-directional modes of aircraft motion. The results from the standard iterative Output Error Method (OEM) form the basis for evaluating the performance of these recursive methods. All the recursive methods discussed in this document gave results matching with those from OEM. The overall performance of DFT method was found to be better than that of the RLS and EKF methods. Also, DFT is computationally simpler and unlike the other two methods, there is no requirement of any tuning parameter. From this aspect, the DFT technique seems to be preferable for online identification purposes.

Application of an imaging spectropolarimeter to agro-environmental sciences

Abstract: Hyperspectral analysis of solar rays reflected from the Earth’s surface is expected to play an important role in future Earth observation. Two imaging liquid crystal tunable filter (LCTF) spectropolarimeters for the visible and near-infrared wavelength bands have been developed by NAL over the past several years for such analysis. In order to realize the practical application of these optical sensors, efforts are currently under way to develop them into sensor packages for airborne observation systems. This paper first presents the concept and architecture of an optical observation system using an LCTF spectropolarimeter which is sensitive to radiation in the 650-1100 nm near-infrared wavelength band, along with its construction. The results of a farm observation conducted using a visible wavelength LCTF imaging spectropolarimeter are then presented by the spectral images of the observed areas as an example of a preliminary application to agro-environmental sciences. The results of a second farm observation conducted using a near-infrared LCTF imaging spectropolarimeter are presented by spectral images of an observed crop specimen, and radiances of solar rays reflected from the specimen are also shown. Finally, the applicability of the LCTF spectropolarimeter to agriculture observation is summarized based on the results of these agricultural observations.

Drag Reduction from Square-Base Afterbodies at High Speeds

Abstract: Experiments have been carried out in the 0.5-m base flow wind tunnel at high speeds evaluating the drag reduction potential of a family of square base afterbodies including jet flow at the base. Direct afterbody total drag measurements have been made on square bases as well as conventional axisymmetric afterbodies with conical and circular arc boat tails having the same annular base area and jet flow parameters. The results show conclusively that, in the Mach-number range of 0.95‐1.60, the square-base afterbodies have globally minimum drag in the range of jet pressure ratio studied; the total drag reduction observed is about 10‐12% relative to the circular arc afterbodies, which can be of significant value in design applications. Certain broad flow features on square-base afterbodies are discussed based on surface-pressure measurements and surface flow-visualization studies. Nomenclature Ab = annular base area Ab/A f = ratio of annular base area to forebody area, 0.23 A f = forebody cross-sectional area A j = area of jet at exit A j/A f = ratio of jet exit area to forebody area, 0.30 C D = afterbody total drag coefficient; drag force/(q∞∗ A f ) CDb = base-drag coefficient; base force/(q∞∗ A f ) C Dβ = boat-tail profile drag coefficient; boat-tail drag force/(q∞∗ A f ) C p = afterbody surface-pressure coefficient; ( p − p∞)/(0.5γ p∞ M 2 ∞ ) Cpb = base-pressure coefficient; ( pb − p∞)/(0.5γ p∞ M 2 ∞) D = forebody diameter of the model, 127 mm db = base diameter d j = nozzle-exit diameter M∞ = freestream Mach number Poj/ p∞ = jet pressure ratio; ratio of stagnation pressure of jet to the freestream static pressure pb = base pressure p∞ = freestream static pressure q∞ = dynamic pressure (0.5γ p∞ M 2 ∞) β = boat-tail angle, see Fig. 3

Guaranteed performance in reaching mode of sliding mode controlled systems

Abstract: Conventionally, the parameters of a sliding mode controller (SMC) are selected so as to reduce the time spent in the reaching mode. Although, an upper bound on the time to reach (reaching time) the sliding surface is easily derived, performance guarantee in the state/error space needs more consideration. This paper addresses the design of constant plus proportional rate reaching law-based SMC for second-order nonlinear systems. It is shown that this controller imposes a bounding second-order error-dynamics, and thus guarantees robust performance during the reaching phase. The choice of the controller parameters based on the time to reach a desirable level of output tracking error (OTE), rather than on the reaching time is proposed. Using the Lyapunov theory, it is shown that parameter selections, based on the reaching time criterion, may need substantially larger time to achieve the OTE. Simulation results are presented for a nonlinear spring-massdamper system. It is seen that parameter selections based on the proposed OTE criterion, result in substantially quicker tracking, while using similar levels of control effort.

Prediction of Fatigue Crack Growth After Single Overload in an Aluminum Alloy

Abstract: An analytical method using crack driving force parameter ∆K ∗ was developed to account for the single overload interaction effects on the constant amplitude fatigue crack growth behavior in D16 (2024-T3 equivalent) aluminum alloy. Crack growth acceleration and retardation were accounted for by using residual stress intensity concept. Fatigue crack growth behavior was predicted using this method for a thin SE(T) specimen subjected to 1) constant amplitude load and 2) constant amplitude load interspersed with single tensile overloads at certain intervals of crack length. Fatigue crack growth tests were performed with these load sequences, and the experimental crack growth results obtained were compared with predictions made by use of the proposed analytical method. All of the fatigue crack growth tests were performed in a servohydraulic test machine. Crack length was measured by the traveling microscope/cellulose acetate replication method. A fairly good correlation was obtained between the experimental and predicted fatigue crack growth curves under the applied loading sequence.

Low velocity impact damage tolerance studies on glass-epoxy laminates - Effects of material, process and test parameters

Abstract: An effort has been made to study the effects of variable incident impact energy and laminate thickness (obtained by varying the number of fabric layers at constant moulding pressure and also by varying the pressure for a fixed number of fabric layers), on the low velocity impact damage tolerance of GFRP composite laminates (woven E-glass fabric in a room temperature cure epoxy system LY556/HY951) Repeated drop tests were conducted using an in-house built drop weight impact tester Number of drops to failure (Nf), was considered as the impact damage tolerance index Correlations between Nf and composite material parameters (fiber weight fraction - Wf, laminate thickness - t, number of fabric layers - L) and a test parameter (incident impact energy - E) were found out and the bounding limits for these were defined Critical values of impact energy (Ec) and laminate thickness (tc) were defined (for the particular resin - hardener system chosen for the study) which can quite possibly be used as designer's guidelines A new ratio, (t/Wf), was introduced for the study of composite systems where both fiber weight fraction and laminate thickness get affected upon alteration of a process parameter (the pressure - P as in this case)

Comparative Study of Different Pressure-Sensitive-Paint Image Registration Techniques

Abstract: Image alignment plays an important role in pressure-sensitive-paint (PSP) data processing. A systematic comparative13; study of two commonly used approaches, namely, image transformation using image-warping transforms13; before ratioing and, alternatively, image mapping on to three-dimensional model space (using resection methodology13; prior to ratioing) prior to ratioing, is attempted here. PSP results along with conventional static port data as13; on a generic aircraft model at a freestream Mach number of 0.6 and incidence of 10 deg (taken from earlier work)13; are utilized to critically assess the merit of the preceding two approaches. The analyses show that image-warping13; methods can result in larger errors in regions of strong spatial gradients of pressure and on bodies that are significantly13; three dimensional. Further, the resolution obtained from the resection approach is significantly greater (of13; the order of less than a pixel) compared to the transform methods.

The no-slip boundary condition in fluid mechanics

Abstract: A moving fluid in contact with a solid body cannot have velocity relative to the body. Even though the question whether there is slip has been satisfactorily resolved now, it was a difficult and controversial problem. In the first part of this article several basic ideas and details related to this problem are discussed. The concluding part of the article will trace the development of ideas leading to the resolution of this important question. Extreme cases where fluid does slip will also be discussed.

Impactor mass effects in glass-epoxy composites subjected to repeated drop tests

Abstract: Experimental studies were carried out to understand the impactor mass effects on glass-epoxy composites using an instrumented impact machine (DYNATUP 8250) under low-velocity test conditions. Repeated drop tests were carried out at pre-determined energy levels for selective mass height combinations. Number of drops to failure (N1) was chosen as the damage criterion. Analysis of test results clearly showed that, at low incident energies, heavier impactors caused more damage to the laminates as reflected by their lower number of drops to failure. This effect diminished gradually at relatively higher incident energies.

Bending behavior of piezo-hygrothermo-elastic smart laminated composite flat and curved plates with active control

Abstract: The actuation and sensing behavior of piezo-hygrothermo-elastic flat and curved plates with active control is presented. A finite element procedure involving coupled piezoelectric field with hygrothermal strain is derived using first order shear deformation theory and is implemented in a nine-noded Lagrangian plate element. The accuracy of the element to model the piezoelectric, pyroelectric, hygroelastic, and thermoelastic behaviors of flat and doubly curved plates is validated with standard benchmark problems. The directional actuation that represents the piezoelectric anisotropy is introduced in the analysis and a comparison is made with isotropic actuation to control the thermal and moisture induced deformation in the laminated plates. Numerical studies are carried out with different fiber orientations to capture the influence of piezoelectric anisotropy on the actuation and sensing characteristics of the active lamina. The directionally actuated piezoelectric lamina that has a reduced piezoelectric capability in transverse direction is efficient, if properly tailored along the fiber directions. However, it is observed that the isotropic actuation has the potential to control the thermal and moisture developed deflection in angle-ply and cross-ply laminates. The actuator lamina is efficient if placed on the top of curved laminates in controlling the deflection.

Corrosion Behaviour of TiN/a-C Superhard Nanocomposite Coatings Prepared by a Reactive DC Magnetron Sputtering Process

Abstract: Nanocomposite coatings of TiN/a-C were prepared on tool steel substrates using a multitarget reactive DC magnetron sputtering process at various TiN layer thicknesses (0.6x2013;2.8 nm). The a-C layer thickness was approximately 0.45 nm. Structural characterisation of the coatings was done by X-ray diffraction (XRD). Incorporation of an a-C phase in TiN matrix reduced crystallite size of the coatings, as13; revealed by XRD and atomic force microscopy. XRD data showed that the nanocomposite coatings exhibited {111} texture and the average crystax2013;llite size was ca. 7.5x2013;9.0 nm. Nanoindentation data showed that 1.5 xB5;m thick nanocomposite coatings exhibited a maximum hardness of 5100 kg mm-2. The potentiodynamic polarisation of 1.5 xB5;m thick coatings in 0.5 MHCl solution indicated that the nanocomposite coatings exhibited superior corrosion protection of the tool steel substrate as compared to the single layer TiN coatings of similar thicknesses. Enhancement in the corrosion behaviour of the nanocomposite coatings has been attributed to small crystallite size and dense microstructure. Potentiodynamic polarisation studies conducted on ca. 100 nm thick nanocomposite coatings revealed that for a given a-C layer thickness the corrosion current decreased with a decrease in TiN layer thickness. This was supported by scanning electron microscopy (SEM) studies on the corroded samples. The SEM micrographs showed that density and diameter of the corrosion pits were smaller for nanocomposite coatings as compared to single layer TiN coatings of similar thicknesses.

Prediction of tropical cyclone genesis using a vortex merger index

Abstract: We propose a new method for detecting tropical cyclone genesis at an early stage by supposing merger of mesoscale midlevel vortices to be a common precursor event The merger event serves as a selection mechanism and is a possible explanation of why only a small fraction of cloud clusters which meet the necessary conditions actually develop into tropical cyclones The detection procedure uses satellite IR images which are available in near real-time After tests using data for the Bay of Bengal basin for the years 1999-2001, a real-time test was conducted for the post-monsoon 2002 and pre-monsoon 2003 seasons We found that the method was successful in detecting the formation of tropical cyclones 04B (2002) and 01B (2003) about 48 hours before they reached storm strength, and no storm escaped detection Copyright 2004 by the American Geophysical Union