National Aerospace Laboratories
National Aerospace Laboratories Institutional RepositoryNot a member yet
7121 research outputs found
Sort by
Metasurfaces for Stealth Applications: A Comprehensive Review.
No full textMetasurfaces are ultrathin, two-dimensional structures composed of periodic or quasi-periodic arrays of sub-wavelength scatterers. They possess the unique ability to comprehensively control the phase, amplitude and polarization of incident electromagnetic waves with added advantages such as ease of fabrication and less space consumption. On account of these factors, they are progressively replacing their three-dimensional counterparts, i.e. metamaterials in a wide gamut of fields
such as signal multiplexing, stealth technology, holographic imaging, planar optical devices, polarization transformation
devices and so on. Further, metasurfaces offer a strong and promising platform for aerospace applications due to their diversified functionalities and reduced weight penalties. Moreover, it has been widely used for the realization of thin, broadband and polarization independent radar absorbing structures (RAS). In this regard, this paper presents a concise review on the recent advancements in the field of metasurfaces specifically for stealth applications. Special emphasis has been laid on diffusion and coding metasurfaces due to their attractive properties towards the realization of low observable platforms. Furthermore, various types of metasurfaces as well as the different techniques used for the optimization of metasurfaces are also described in detail
Effect of lean primary-zone operation on emissions and stability of non-premixed combustors
No full textLean operation in the primary combustion zone of a gas turbine combustor is advantageous from NOx reduction point of view. The present work deals with the influence of lean primary zone operation on combustion performance, NOx emissions, and flame stability of a model gas turbine combustor with simplex atomizer. Air distribution is varied to operate the primary combustion zone from stoichiometric to leaner operating conditions (Φpri 1.04–0.61), at fixed fuel flow rate and overall air–fuel ratio. Combustor performance is quantified with the help of gas temperature profile at exit and species concentrations at the end of primary zone. It is found that a primary zone equivalence ratio of 0.79 is optimal for high combustor efficiency and low NOx emissions, with a stable flame. When the primary zone equivalence ratio approaches the lean blowout (LBO) limit (Φpri ∼ 0.5), intermittent low–frequency, high-amplitude pressure oscillations arise, especially at higher primary zone air flow rates. Inclusion of a metallic ring in the primary combustion zone widens the equivalence ratio for flame stability, with a small reduction in combustion efficiency. It also reduces the noise level and suppresses the intermittent high-amplitude oscillations close to LBO
Improving Structural Integrity of a Centrifugal Compressor Impeller by Blading Optimization
No full textThree-dimensional blading features of impellers like sweep and lean improve the aerodynamic performance of centrifugal compressors, especially in terms of pressure ratio, stall margin and efficiency. But, these blade geometrical features often impose challenges on mechanical design and structural integrity of the impeller. Modern small gas turbine engines demand high-work input impellers rotating at very high speeds, making the structural design even more difficult. The present study deals with improving the structural integrity of a centrifugal impeller of a compressor stage designed for 42,000 rpm and power input of 1200 kW. The baseline design of the impeller, arrived using an advanced three-dimensional design software, met the aerodynamic performance requirements but failed to satisfy structurally. Higher backward lean present in the blade was found to be contributing to severe bending stresses and deformation. Hence, three different impellers, with modified thickness, lean angle and wrap angle distributions, were designed. These designs were subjected to 3D CFD and FEM analysis. Structural analysis was carried out to study stresses and deformations for all configurations, followed by pre-stressed modal analysis to predict their natural frequencies and corresponding mode shapes. The design with modified wrap angle distribution not only met the aerodynamic requirements but also satisfied structural requirements. This paper elaborates the design evolution of the centrifugal impeller from the baseline design to a more structurally stable one
Correlating Stress Ratio Effects on the Fatigue Crack Growth Rate of a Nickel Base Superalloy IN718
No full textConstant amplitude fatigue crack growth rate tests were conducted on a nickel base superalloy IN718 at various stress ratios, R ranging from R = 0.1 to 0.7. Tests were conducted at room temperature and in lab air atmosphere. Tests were performed in a 100 KN computer-controlled servo-hydraulic test machine using compact tension specimens with sinusoidal waveform at 10 Hz. Crack length was monitored by compliance technique using COD gage. Increasing stress ratio was observed to increase crack growth rates and also decrease threshold stress intensity factor range, ∆Kth. Stress ratio effects on crack growth rates were correlated by using a two-parameter crack driving force, . This approach was observed to provide a reasonably good correlation which can further be employed in modeling crack growth behavior under service loads
Flow Separation Control on a NACA-4415 Airfoil at Low Reynolds Number
No full textAn experimental study using 3C-PIV was carried out for investigating the effect of adapting a passive flow control device similar to that found on the humpback whale flippers (tubercles) to the leading edge of a NACA-4415 airfoil at a Reynolds number of 120,000. The evolution of the mean velocity field on the NACA-4415 airfoil and with leading-edge tubercles are measured using stereo-particle image velocimetry at an angle of attack of 18°. The leading-edge tubercles exhibited attached flow up to 50% of the airfoil and reduced the height of the separated region over modified airfoil as compared to the baseline. This results in an improved aerodynamics performance by the modified airfoil
Development of vanadium impregnated flat absorber composite PEO coating on AA6061 alloy
No full textThe formation of black plasma electrolytic oxidation (PEO) coating on aluminium alloy using vanadyl sulphate as additive in a silicate-based electrolyte is investigated. A brief discussion on the mechanism of incorporation of vanadium in PEO coating starting with a cationic species of vanadium (i.e., VO2+) in solution is attempted. The coating formation with respect to morphology and composition is systematically studied as a function of process time. With increase in process time, incorporation of vanadium ions in the coating increases and the formation of black colour is attributed to the presence of V3+ and V4+ species in the coating. A 55 ± 5 μm thick flat absorber black coating with high solar absorptance (0.92) and high infrared emittance (0.88) is obtained with a process time of 10 min
High-temperature flexural strength of I-CVI processed Cf/SiC composites with variable interphases.
No full textThe effect of single-layer pyrocarbon (PyC) and multilayered (PyC/SiC)n=4 interphases on the flexural strength of un-coated and SiC seal-coated stitched 2D carbon fiber reinforced silicon carbide (Cf/SiC) composites was investigated. The composites were prepared by I-CVI process. Flexural strength of the composites was measured at 1200 °C in air atmosphere. It was observed that irrespective of the type of interphase, the seal coated samples showed a higher value of flexural strength as compared to the uncoated samples. The flexural strength of 470 ± 12 MPa was observed for the seal coated Cf/SiC composite samples with multilayered interphase. The seal coated samples with single layer PyC interphase showed flexural strength of 370 ± 20 MPa. The fractured surfaces of tested samples were analyzed in detail to study the fracture phenomena. Based on microstructure-property relations, a mechanism has been proposed for the increase of flexural properties of Cf/SiC composites having multilayered interphase
Design of sliding mode flight control system for a flexible aircraft
No full textThe evolution of large transport aircraft is characterized by longer fuselages and larger wingspans, while efforts to decrease the structural weight reduce the structural stiffness. Both effects lead to more flexible aircraft structures with significant aeroelastic coupling between flight mechanics and structural dynamics, especially at high speed, high altitude cruise. The lesser frequency separation between rigid body and flexible modes of flexible aircraft results in a stronger interaction between the flight control system and its structural modes, with higher flexibility effects on aircraft dynamics. Therefore, the design of a flight control law based on the assumption that the aircraft dynamics are rigid is no longer valid for the flexible aircraft. This paper focuses on the design of a flight control system for flexible aircraft described in terms of a rigid body mode and four flexible body modes and whose parameters are assumed to be varying. In this paper, a conditional integral based sliding mode control (SMC) is used for robust tracking control of the pitch angle of the flexible aircraft. The performance of the proposed nonlinear flight control system has been shown through the numerical simulations of the flexible aircraft. Good transient and steady-state performance of a control system are also ensured without suffering from the drawback of control chattering in SMC
Strain sensor's network for low-velocity impact location estimation on carbon reinforced fiber plastic structures: Part-II.
No full textIdentification of low velocity impact (LVI) location in composite aircraft structures is seamless need for safe, reliable operation and maintenance of aerospace industry. To locate the LVI’s an optimized sensor network has designed using the strain response from fiber Bragg grating (FBG) & resistance strain gauge (RSG) sensor bonded to the composite structure. Strain scan (SS) algorithm has been developed to locate such events reported as Part-I. In this work, we have developed a novel algorithm based on weighted energy (WE) of the sensor response. The LVI’s has been carried out on composite structures & the locations of LVI’s have estimated using SS, WE & previously developed machine learning base support vector machine (SVM) algorithms. The WE and SS algorithms are based on proximity of events (closer to the sensor, higher the response), whereas LS-SVR is a data-driven approach. Further, we have compared the performance of the developed algorithms and algorithms cited in the literature using the performance index (PI), a measure of estimation efficiency as a function of the number of sensors, dimension/area of the structure, error & number of test cases. It is established that WE algorithm shown suprema performance over the other algorithm with 34 mm mean Euclidian distance error & PI value of 5.5
Measurement of Turbulent Characteristics at Low Reynolds Number and Comparison with CFD for Eppler 387
No full textTransition to turbulence is an important phenomenon which has been studied extensively in literature both experimentally as well as numerically. Development of turbulence characteristics in the laminar separation bubble has been studied experimentally using hot wire anemometer and numerically for two Reynolds number using two transition models. Experimentally measured turbulence characteristics are compared with numerical results, and the performance of the transition models has been brought out