Showing papers by "National Aerospace Laboratories published in 2009"
TL;DR: In this article, the authors reviewed the recent developments on lead-free piezo materials emphasizing on their preparation, structure-property correlation, etc., and concluded that some lead free compositions show stable piezoelectric responses even though they do not match the overall performance of PZT.
Abstract: Lead zirconate titanate (PZT) based piezoelectric materials are well known for their excellent piezoelectric properties. However, considering the toxicity of lead and its compounds, there is a general awareness for the development of environmental friendly lead-free materials as evidenced from the legislation passed by the European Union in this effect. Several classes of materials are now being considered as potentially attractive alternatives to PZTs for specific applications. In this paper, attempts have been made to review the recent developments on lead-free piezo materials emphasizing on their preparation, structure–property correlation, etc. In this context, perovskite systems such as bismuth sodium titanate, alkali niobates (ANbO3), etc. and non-perovskites such as bismuth layer-structured ferroelectrics are reviewed in detail. From the above study, it is concluded that some lead-free compositions show stable piezoelectric responses even though they do not match the overall performance of PZT. This has been the stimulant for growing research on this subject. This topic is of current interest to the researchers worldwide as evidenced from the large number of research publications. This has motivated us to come out with a review article with a view that it would give further impetus to the researchers already working in this area and also draw the attention of the others.
865 citations
TL;DR: In this article, an implicit pressure-based finite volume method is used for time-accurate computation of incompressible flow using second order accurate convective flux discretisation schemes, which is validated against measurement data for mean surface pressure, skin friction coefficients, the size and strength of the recirculating wake for the steady flow regime and also for the Strouhal frequency of vortex shedding and the mean and RMS amplitude of the fluctuating aerodynamic coefficients for the unsteady periodic flow regime.
326 citations
TL;DR: In this article, a composite oxide of alumina yttria doped cubic zirconia (AZY, (1−−−x)Al2O3-8-mol% ytria stabilized xZrO2 (x = 10wt%)) powder was used to enhance the microhardness, corrosion resistance and wear resistance properties of the Ni matrix.
110 citations
TL;DR: X-ray diffraction analysis has shown that the crystal structure of CZH was mainly composed of epsilon-zinc hydroxide and the modified CZh indicated the presence of zinc stearate, and FTIR spectrum also confirmed the existence of zincStearate in the composite film.
93 citations
TL;DR: In this paper, a superhydrophobic surface was obtained by embedding hydrophobically modified fumed silica (HMFS) particles in polyvinylidene fluoride (PVDF) matrix.
84 citations
TL;DR: Spectrally selective AlxOy/Al/AlXOy multilayer absorber coatings were deposited on copper (Cu) and molybdenum (Mo) substrates using a pulsed sputtering system as mentioned in this paper.
81 citations
TL;DR: In this paper, the authors discuss the experimental studies carried out towards the development of a flow-sensor in the optical domain to determine the presence of resin qualitatively, three different approaches have been attempted to detect the arrival of the resin flow front and these results are presented.
Abstract: Today, with cost becoming a very important factor, alternate processes for manufacturing composite structures, wherein the cost can be reduced but having similar properties to prepreg systems are being explored. National Aerospace Laboratories, India is developing the VERITy process, which is essentially a hybridization of the VARTM and the autoclave moulding process. One of the concerns in this process, especially when making large integrated co-cured structures with varying thickness, is the knowledge of the resin flow front location at all times. A network of flow-sensors, which could provide information on the flow front location, would be very useful. This paper discusses the experimental studies carried out towards the development of a flow-sensor in the optical domain to determine the presence of resin qualitatively. Three different approaches have been attempted to detect the arrival of the resin flow front and these results are presented.
71 citations
TL;DR: The codes provided predictions that were consistent with expectations based on the turbulence modelling used, which was k-e, k-w with vortex corrections, and an algebraic stress model, with good agreement with flight data.
Abstract: This article presents a comparison of the predictions of three RANS codes for flight conditions of the F-16XL aircraft which feature vortical flow. The three codes, ENSOLV, PMB and PAB3D, solve on structured multi-block grids. Flight data for comparison was available in the form of surface pressures, skin friction, boundary layer data and photographs of tufts. The three codes provided predictions which were consistent with expectations based on the turbulence modelling used, which was k- , k- with vortex corrections and an Algebraic Stress Model. The agreement with flight data was good, with the exception of the outer wing primary vortex strength. The confidence in the application of the CFD codes to complex fighter configurations increased significantly through this study.
65 citations
TL;DR: In this paper, ternary NiWP deposits were prepared using an alkaline citrate bath and the concentration of tungstate in the bath was varied from 5 to 40 g/L to obtain NiWP deposit containing various W and P contents.
49 citations
TL;DR: In this paper, the effect of cowl length, plug length, and plug contour variation on the performance and base pressure characteristics of a Mach 2.0 annular conical aerospike nozzle with and without freestream flow was studied.
Abstract: An experimental investigation has been carried out to study the performance and base pressure characteristics of a Mach 2.0 annular conical aerospike nozzle with and without freestream flow. The effect of cowl length, plug length, and plug contour variation on the nozzle performance and base pressure characteristics is studied. It is observed that the overexpansion shock from the internal nozzle, overexpansion shock on the spike surface, and the expansion fan from the cowl lip of the internal nozzle dominate the overall flowfield development. The presence of freestream flow reduces the nozzle performance by approximately 4% relative to static conditions. Base pressure characteristics are observed to be strongly influenced by the movement of these shocks on the plug surface, and their subsequent interaction with the inner shear layer controls the base-wake closure. Relative to the conical plug conflguration, the contoured plug shows considerably enhanced base pressure characteristics. Real-time pressure measurements on the spike reveal highly unsteady flow in the intermittent region of separation.
48 citations
TL;DR: In this paper, a solution combustion method was used to synthesize nano-composites of low-oxidation compounds such as CeO2 and CeAlO3 using mixture-of-fuels.
TL;DR: In this paper, a Si/SiC ceramic was prepared from cotton fabric by the reactive infiltration of liquid silicon into the carbon template, and a large density difference between the samples has been observed.
TL;DR: In this article, the authors discussed the nature of shock unsteadiness in an overexpanded thrust optimized parabolic nozzle, prevalent in various flow separation modes experienced during start up and shutdown.
Abstract: This paper discusses the nature of shock unsteadiness, in an overexpanded thrust optimized parabolic nozzle, prevalent in various flow separation modes experienced during start up \({(\delta P_{0} /\delta t > 0)}\) and shut down \({(\delta P_{0}/\delta t < 0)}\) sequences. The results are based on simultaneously acquired data from real-time wall pressure measurements using Kulite pressure transducers, high-speed schlieren (2 kHz) of the exhaust flow-field and from strain-gauges installed on the nozzle bending tube. Shock unsteadiness in the separation region is seen to increase significantly just before the onset of each flow transition, even during steady nozzle operation. The intensity of this measure (rms level) is seen to be strongly influenced by relative locations of normal and overexpansion shock, the decrease in radial size of re-circulation zone in the back-flow region, and finally, the local nozzle wall contour. During restricted shock separation, the pressure fluctuations in separation region exhibit periodic characteristics rather than the usually observed characteristics of intermittent separation. The possible physical mechanisms responsible for the generation of flow unsteadiness in various separation modes are discussed. The results are from an experimental study conducted in P6.2 cold-gas subscale test facility using a thrust optimized parabolic nozzle of area-ratio 30.
TL;DR: In this paper, polycrystalline Bi 1−−− x Sb x (x ǫ = 0.12, 0.10, 0, 0., and 0.15) semiconducting alloys were synthesized by mechanical alloying in order to achieve homogeneous thermoelectric materials with improved mechanical strength.
TL;DR: In this article, the effect of rivet loads on life estimation including the use of concepts such as by-pass stresses is discussed and the issue of fail safe and damage tolerant design of civil aircraft structures with riveted joints is addressed, especially the implication of unequal load distribution on the failures of such joints and it is suggested that these unequal rivet load distributions be catered for at the early design stage itself via finite element analysis and the possibility of an overarching safety factor could be considered that incorporates both ultimate load and damage tolerance conditions.
TL;DR: In this paper, a stable binary pressure sensitive paint formulation was developed in which pyrene is covalently bonded to the polymer binder so that paint degradation is prevented, which is due to the loss of pyrene as a result of diffusion and sublimation from the coating.
Abstract: Pyrene-based pressure sensitive paints (PSP) have certain advantages due to their high pressure sensitivity and low temperature coefficient but their major drawback is the paint degradation under wind tunnel conditions. This is due to the loss of pyrene as a result of diffusion and sublimation from the coating. We have developed a novel stable binary PSP formulation in which pyrene is covalently bonded to the polymer binder so that paint degradation is prevented. The coating thus obtained is a siloxane-based hybrid organic-inorganic material. The pressure sensitive paint also contains an additional reference luminophore, which is insensitive to pressure but sensitive to intensity variations on the model surface. The second luminophore is incorporated in the paint to correct for the excitation intensity variations during the wind tunnel experiment. The temperature coefficient of second luminophore exactly cancels the temperature coefficient of the pressure sensitive luminophore, thereby resulting in a binary paint with negligible temperature sensitivity.
TL;DR: In this article, detailed characteristics of the tropical intraseasonal variability (TISV) in the MRI-20km60L AGCM that uses a variant of the Arakawa-Schubert cumulus parameterization are analyzed.
Abstract: This study documents the detailed characteristics of the tropical intraseasonal variability (TISV) in the MRI-20km60L AGCM that uses a variant of the Arakawa‐Schubert cumulus parameterization. Mean states, power spectra, propagation features, leading EOF modes, horizontal and vertical structures, and seasonality associated with the TISV are analyzed. Results show that the model reproduces the mean states in winds realistically and in convection comparable to that of the observations. However, the simulated TISV is less realistic. It shows low amplitudes in convection and low-level winds in the 30‐60-day band. Filtered anomalies have standing structures. Power spectra and lag correlation of the signals do not propagate dominantly either in the eastward direction during boreal winter or in the northward direction during boreal summer. A combined EOF (CEOF) analysis shows that winds and convection have a loose coupling that cannot sustain the simulated TISV as realistically as that observed. In the composited mature phase of the simulated MJO, the low-level convergence does not lead convection clearly so that the moisture anomalies do not tilt westward in the vertical, indicating that the low-level convergence does not favor the eastward propagation. The less realistic TISV suggests that the representation of cumulus convection needs to be improved in this model.
TL;DR: In this article, the influence of an inorganic additive cobalt on the incorporation of Si3N4 particles in a Ni matrix has been investigated and a correlation between the particle content, cobalt content, and structure was attempted.
TL;DR: In this paper, the life cycle of deep convective systems over the eastern tropical Pacific (30°N to 30°S, 180 to 90°W) was studied in terms of cloud types, as classified by a split window.
Abstract: The life cycle of deep convective systems over the eastern tropical Pacific (30°N to 30°S, 180 to 90°W) was studied in terms of cloud types, as classified by a split window (11 μm and 12 μm). Hourly split window image data of Geostationary Operational Environmental Satellite (GOES-W) from January 2001 to December 2002 was used in this study. Deep convective systems consist mostly of optically thick cumulus type clouds in the earlier stage and a cirrus type cloud area that increases with time in the later stage. During this analysis period and over the analysis area, the life stage of deep convective system, to a large extent, can be identified by computing the percentage of cirrus type clouds within the deep convective system from a single snap shot of the split window image. Coincident Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) observations were used to study the relationship between the percentage of cirrus type clouds within a deep convective system (i.e., its life stage) and the rainfall rate. It was found that the rainfall rate tends to be larger in the earlier stage of the life cycle when a smaller percentage of cirrus type cloud is present within the deep convection.
TL;DR: In this article, the effects of a meniscus on the oscillations of a viscous liquid filling a right circular cylindrical container by using the natural viscous complex eigenfunctions of the problem was studied.
TL;DR: In this paper, the effect of a wetting condition at the moving contact line on the frequency and damping of surface waves on an inviscid liquid in a circular cylinder was considered.
Abstract: We consider the effect of a wetting condition at the moving contact line on the frequency and damping of surface waves on an inviscid liquid in a circular cylinder. The velocity potential x3C6; and the free surface elevation x3B7; are sought as complex eigenfunction expansions. The x3C6; eigenvalues are the classical ones whereas the x3B7; eigenvalues are unknown and have to be computed so as to satisfy the wetting condition on the contact line and the other free surface conditions x2013; these turn out to be complex in general. A projection of the latter conditions on to an appropriate basis leads to an eigenvalue problem, for the complex frequency x3A9;, which has to be solved iteratively with the wetting condition. The variation of x3A9; with liquid depth h, Bond number Bo, capillary coefficient x3BB; and static contact angle x3B8;c0 is explored for the (1, 0), (2, 0), (0, 1), (3, 0) and (4, 0) modes. The damping vanishes for x3BB; = 0 (pinned-end edge condition) and x3BB;=x221E; (free-end edge condition) with a maximum in the interior while the frequency decreases with increasing x3BB;, approaching limiting values at the endpoints. A comparison with the analytic results of Miles (J. Fluid Mech., vol. 222, 1991, p. 197) for the no-meniscus case and the experimental results of Cocciaro, Faetti, amp; Festa (J. Fluid Mech., vol. 246, 1993, p. 43), where a meniscus is present, is good. The study provides a simple procedure for calculating the inviscid capillary damping associated with the moving contact line in a circular cylinder of finite depth with meniscus effects also being considered.
TL;DR: Si/SiC ceramic composite was prepared by infiltration of liquid silicon into carbon preforms that was made from cotton fabric and phenolic resin This composite was subjected to the chemical vapour infiltration (CVI), using methyltrichlorosilane as a precursor gas.
01 Oct 2009
TL;DR: In this article, the CAWAPI project participants have contributed steady and unsteady viscous simulations of a full-scale, semi-span model of the F-16XL aircraft.
Abstract: Nine groups participating in the CAWAPI project have contributed steady and unsteady viscous simulations of a full-scale, semi-span model of the F-16XL aircraft. Three different categories of flight Reynolds/Mach number combinations were computed and compared with flight-test measurements for the purpose of code validation and improved understanding of the flight physics. Steady-state simulations are done with several turbulence models of different complexity with no topology information required and which overcome Boussinesq-assumption problems in vortical flows. Detached-eddy simulation (DES) and its successor delayed detached-eddy simulation (DDES) have been used to compute the time accurate flow development. Common structured and unstructured grids as well as individually-adapted unstructured grids were used. Although discrepancies are observed in the comparisons, overall reasonable agreement is demonstrated for surface pressure distribution, local skin friction and boundary velocity profiles at subsonic speeds. The physical modeling, be it steady or unsteady flow, and the grid resolution both contribute to the discrepancies observed in the comparisons with flight data, but at this time it cannot be determined how much each part contributes to the whole. Overall it can be said that the technology readiness of CFD-simulation technology for the study of vehicle performance has matured since 2001 such that it can be used today with a reasonable level of confidence for complex configurations.
11 May 2009
TL;DR: In this article, the aeroacoustic characteristics of an over-expanded annular aerospike nozzle have been studied and compared with a convergent divergent nozzle for a given pressure ratio of 2.57 for the overexpanded condition.
Abstract: The aeroacoustic characteristics of an over-expanded annular aerospike nozzle have been studied. The overall sound pressure levels of the aerospike nozzle are compared with the levels obtained from a convergent divergent nozzle for a given pressure ratio of 2.57 for the over-expanded condition. It is observed that the over-expanded annular aerospike nozzle generates screech tones at this NPR and in general shows higher OASPL levels than the over-expanded convergent-divergent nozzle at peak radiation direction.
TL;DR: This paper examined the impact of altitude boost of the TRMM satellite from 350 km to 400 km in August 2001 and found that the boost affects the annual cycle in light rain rates (total, convective, and stratiform) with more frequent occurrence before (after) the boost.
Abstract: Global and regional interannual variations of rainfall characteristics over the tropics were examined by applying empirical orthogonal function (EOF) analysis to TRMM PR 3A25 data from December 1997 to December 2007. The TRMM PR 3A25 data and other TRMM datasets detect the interannual variation of rainfall over the tropics, in concert with the SST change, which is closely related to the El Nino/La Nina cycle, including the pseudo-El Nino periods in 2002 and 2004. In addition, we examined the impact of the altitude boost of the TRMM satellite from 350 km to 400 km in August 2001 and found that the boost affects the annual cycle in light rain rates (total, convective, and stratiform). A baseline shift occurs in the annual cycle of light convective rain rate (convRainH) with more (less) frequent occurrence before (after) the boost. In addition, amplitude changes were observed in the annual cycle of light stratiform and total rain rates with more (less) frequent occurrence before (after) the boost. However, for the interannual variation, the coherent spatio-temporal structure of the El Nino/La Nina cycle is evident, and the interannual variation does not indicate any boost impact. In contrast, for heavy convRainH, the number events decreases after the boost, especially over land; however, it is difficult to conclude that this reduction is related to the boost.
TL;DR: It is concluded that fusion of IRST and radar would improve the tracking performance and reduce the positional uncertainty compared to individual trackers.
Abstract: Tracking algorithms for IRST and radar are implemented and their performance is checked with simulated data. Detailed mathematical expressions given could be useful for implementation. Performance evaluation metrics have been presented to check the tracking algorithm performance. Two fusion schemes have been presented and their performances evaluated with simulated data. It is concluded that both fusion schemes performed alike with the second fusion scheme giving slightly better results. From the results, it is also concluded that fusion of IRST and radar would improve the tracking performance and reduce the positional uncertainty compared to individual trackers. Defence Science Journal, 2009, 59(2), pp.175-182 , DOI:http://dx.doi.org/10.14429/dsj.59.1506
TL;DR: In this article, an overview of some recent numerical simulations of stationary crack tip fields in elastic-plastic solids is presented, which serve to illustrate several important features associated with stress and strain distributions near the crack tip and provide the foundation for understanding the operative failure mechanisms.
Abstract: In this paper, an overview of some recent numerical simulations of stationary crack tip fields in elastic-plastic solids is presented. First, asymptotic analyses carried out within the framework of 2D plane strain or plane stress conditions in both pressure insensitive and pressure sensitive plastic solids are reviewed. This is followed by discussion of salient results obtained from recent computational studies. These pertain to 3D characteristics of elastic-plastic near-front fields under mixed mode loading, mechanics of fracture and simulation of near-tip shear banding process of amorphous alloys and influence of crack tip constraint on the structure of near-tip fields in ductile single crystals. These results serve to illustrate several important features associated with stress and strain distributions near the crack tip and provide the foundation for understanding the operative failure mechanisms. The paper concludes by highlighting some of the future prospects for this field of study.
TL;DR: A new bi-luminophore system for optical sensing of temperature is described, fabricated by embedding europium thenoyltrifluoroacetonate and perylene in polystyrene matrix and displaying a fully reversible response to temperature.
Abstract: A new bi-luminophore system for optical sensing of temperature is described. The coating was fabricated by embedding europium thenoyltrifluoroacetonate (EuTTA) and perylene in polystyrene (PS) matrix. The luminescence emission of EuTTA was sensitive to temperature whereas perylene emission was temperature-insensitive and was used as a reference. Both luminophores were excited in the UV region of about 330 to 380 nm. The fluorescence emission of perylene and EuTTA occured at 474 nm and 615 nm respectively. The temperature sensitivities of both luminophores were influenced by (i) the type of polymer, and (ii) the concentration of luminophore in the matrix. Combining EuTTA and perylene in polystyrene matrix, a new bi-luminophore temperature sensing coating was developed. The temperature sensitivity of this coating was −1.80%/°C in the temperature range of 5 °C to 50 °C. The emission characteristics of this temperature sensitive coating displayed a fully reversible response to temperature.
TL;DR: In this article, the friction and wear behavior of polystyrene reinforced with steel powder (polymer-metal), alumina powder, polymer-ceramic and a mix of steel and alumina powders was investigated under dry sliding conditions using a pin-on-disc tribometer.
TL;DR: In this article, the design and development of an advanced linear reciprocating tribometer to study the performance of lubricants and slide way surfaces is presented, where the principle of operation consists of sliding test samples against a stationary counter surface under load and recording the parameters of interest.