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Fatigue crack growth behaviour of a nickel base super alloy GTM720 under cold-TURBISTAN spectrum load sequence
No full textDamage tolerance concept is widely used in aerospace industry for designing structural components against fatigue. In this study, an attempt is made to predict fatigue crack growth behavior in an aero-engine material under a standard spectrum load sequence using a closure independent crack driving force parameter, K* and compare with experimental results. Initially, constant amplitude (CA) fatigue crack growth rate (FCGR) behavior of GTM720, a nickel based super alloy was determined at stress ratios, R = σmin/σmax ranging from R = 0.1 to 0.7. Then, empirical fatigue crack growth law was derived from this CA-FCGR data in terms of two parameter crack driving force, K*. Further, the fatigue crack growth behavior was predicted through cycle-by cycle approach in a pre-cracked compact tension (CT) specimen of GTM720 alloy under standard cold-TURBISTAN spectrum load sequence using K* as the crack driving force. Also, experimental fatigue crack growth behavior under the same load sequence was determined in a CT specimen and compared with predicted results. A fairly good correlation was observed with predicted and experimental results. Predicted fatigue crack growth life was conservative and the fatigue crack growth life ratio, Npred/Nexpt was about 0.92
Understanding of an effect of plenum volume of a low porosity bend skewed casing treatment on the performance of single-stage transonic axial flow compressor
No full textThe present study intends to improve the performance of transonic axial flow compressor stage and its operating range by implementing passive flow control technique, a Casing Treatment. A plenum chamber with two different volumes placed above the bend skewed slots was implemented. The objective was to understand the effect of plenum volume on the performance of transonic axial flow compressor retrofitted with bend skewed casing treatment. The porosity of the selected bend skewed casing treatment was 33%. A detailed steady-state CFD analysis has been carried out for the compressor operating at six different speeds. Axial location of the casing treatment above the rotor tip was chosen based upon the previous experiments reported in literature [1]. For the same axial location and porosity, plenum chamber depth was varied from zero depth to full plenum depth to understand effect of a plenum volume. The results were compared with baseline model with solid casing wall. Significant improvement in stall margin was observed at all rotational speeds. Minor deviation was observed in stage total pressure ratio with reduction in efficiency at design speed
Rotor Flow Analysis in the Presence of Fuselage Using Unsteady Panel Method
No full textPanel methods are known to be simple yet effective during initial design stages. With the advent of advanced CFD methods and high speed computers, there is a feeling that these methods are no longer needed. On the contrary, experience with many practical problems shows that their utility is significant. There are also some problems where panel methods may be even superior to CFD approaches. One such problem is that of flow field analysis of rotors in the presence of stationary fuselage. In this paper we make use of an unsteady panel method which is a simple extension of a steady panel method to assess the effect of presence of fuselage on the rotor wake. Another significant advantage an unsteady panel method offers is that it is particularly easy to ‘fly’ the rotor in the presence of fuselage and assess the wake. Typical flight scenarios include hover, forward flight, ascent and descent flights. Some specific applications include assessment of wake flow for weapon separation from helicopters and estimates of noise due to unsteady rotor loading
3D Computational Studies of Flapping Wing in Frontal Gusty Shear Flow
No full textThe present paper reports findings of the 3D computational studies of the effect of frontal gusty shear flow on the force patterns of a flapping wing. A rigid wing with semi-elliptical wing planform with asymmetric 1 DoF flapping kinematics was exposed to a gusty shear flow. The shear gradient of the flow was varied from −10 to +10 in steps of 5. Computation studies were carried out for Re = 150 which lies in the typical flight Reynolds number range of natural flyers like a fruit fly and anthropogenic flyers like a Pico Aerial Vehicle. 3D, unsteady, laminar, and incompressible Navier-Stokes equations were solved using finite volume formulation based commercial code ANSYS Fluent. Wing kinematics and gusty inflow conditions were modelled into the solver by User Defined Functions (UDFs). Wing motion was simulated using the dynamic meshing technique. The effect due to variation in the frontal inflow condition was studied quantitatively and qualitatively. Comparisons of the instantaneous and gust cycle averaged forces and moment coefficients about the wing root mid chord point and 3D phase space projections of the forces and moment coefficients was carried out. Qualitative studies were carried out by comparing the static pressure over both the surfaces of the wing and the vortex patterns near the flapping wing using λ2–criteria. It was observed from these studies that negative shear gradient resulted in a rise in the vertical force and moment and a minor reduction in the horizontal force. Positive shear gradient resulted in a minor rise in horizontal force and a significant reduction in vertical force and moment
Triggering of flow instabilities by simulated sub/supercritical rayleigh heat addition in an aero-gas turbine afterburner
No full textMilitary aircraft employ aero-gas turbines fitted with afterburners to meet the requirements of rapid increase in thrust for flight operations that involve combat maneuvers. The airflow rate through an aero-gas turbine remains unchanged even after an afterburner is invoked to ensure that there is no disruption in the turbomachinery operating characteristics. Triggering of flow instabilities leading, in turn, to combustor instabilities could occur due to the incorrect Rayleigh heat addition in the constant area afterburner. In normal operation, the propelling nozzle should be correctly opened up to pass the increased volumetric flow rate of the heated air due to heat addition in the afterburner. Any mismatch could result in violent flow instabilities including possible fan stall in a turbofan. The processes of triggering instabilities by sub/supercritical Rayleigh heat addition have been characterized. The gas dynamic equivalence of secondary mass addition to heat addition has been analyzed and experimentally validated in a model afterburner combustion test rig. Consequently, sudden sub/supercritical heat addition in an afterburner with its corresponding propelling nozzle closure has been studied and characterized for the equivalent mass addition in a separate model afterburner simulation test rig
Probabilistic Flutter Analysis of a Cantilever Wing
No full textA probabilistic flutter analysis of geometrically coupled cantilever wing is carried out using first-order perturbation approach by considering bending and torsional rigidities as Gaussian random variables. The unsteadiness in the aerodynamic flow is modeled using Theodorsen’s thin airfoil theory. The probabilistic response of the wing is obtained in terms of mean, standard deviation, and coefficient of variation (COV) of real and imaginary parts of the eigenvalues at various free stream velocities. The perturbation results are also compared with Monte Carlo simulations. It is observed that the probabilistic response obtained from the perturbation approach is very accurate up to 7% COV in bending rigidity but in the case of torsional rigidity, it starts losing accuracy after 3%
Photocatalytic behavior of titania coatings fabricated by suspension and solution precursor plasma spray processes
No full textTitania (TiO2) powder is the most extensively employed photocatalyst for the purification of water. Nonetheless, the usage of conventional powder catalysts in water treatment has the limitation of settling of the catalyst during the reaction and the additional step of separation of the powders from the liquid. To overcome this limitation and extend the application of TiO2 for practical applications, coatings are promising. In the present study, a comparison of the photocatalytic property of titania coatings deposited by suspension plasma spray (SPS) and solution precursor plasma spray (SPPS) methods using similar plasma spray parameters is presented. The coatings are characterized by field emission scanning electron microscopy, profilometry, X-ray diffractometry, and Raman spectroscopy. The photocatalytic performance and efficiency of photodecomposition of the developed coatings are studied using methylene blue as the organic pollutant. The study demonstrates a higher photocatalytic activity and photodecomposition efficiency for the SPPS titania coating compared to SPS titania coatings. The improved performance is attributed to the presence of oxygen vacancies and is corroborated with X-ray photoelectron spectroscopy. The SPPS titania coatings hold promise for industrial-scale applications as it is economical, simple and does not involve multiple steps
Improved Corrosion Protection of Magnesium Alloys AZ31B and AZ91 by Cold-Sprayed Aluminum Coatings
No full textMagnesium (Mg) alloys have a high strength/
weight ratio, high dimensional stability, good machinabil-
ity, and the ability to be recycled. However, their poor
corrosion resistance in humid environments limits their
usage for exterior aerospace components. This study aims
to improve the corrosion resistance of two Mg alloys
(AZ31B and AZ91) by using aluminum coatings. The latter
have been deposited by a low pressure and temperature
cold spray process. An aluminum powder (60 wt%) with a
particle size ranging between 1 and 8
l
m and nickel
powder (40 wt%) with a particle size of about 70
l
m were
blended and used as feedstock powder. The coating
thickness was about 240
l
m. Its densification was achieved
by the in-situ hammering effect of the nickel particles. The
shot-peening effect also resulted in an enhanced coating
hardness. The microstructure, mechanical properties, and
corrosion resistance of the coatings have been investigated.
They showed that the aluminum had a face centered cubic
structure. Potentiodynamic polarization tests were per-
formed along with a combination of materials characteri-
zation techniques to assess the corrosion resistance of the
coatings when immersed in a 3.5 wt% NaCl solution for
long durations. The results revealed that the corrosion
resistance increased with the immersion time because of
the formation of a protective oxide layer on the surface.
These results were supported by elemental and structural
analyses. This study shows that cold-sprayed aluminum
coatings are a promising candidate for enhancing the
corrosion resistance of AZ31B and AZ91Mg alloys com-
pared to other thermal spray processes
An Efficient Approach to Initialization of Visual-Inertial Navigation System using Closed-Form Solution for Autonomous Robots
No full textThe visual-inertial navigation system using a single camera and IMU requires an accurate initialization without increasing the processing cost and complexity for real-time deployment. The processing cost in the existing solutions can be traced to the gyroscopic bias estimation using 1) Closed-form solutions (Martinelli, Int. J. Comput. Vis. 106(2), 138–152, 2014; Kaiser et al., IEEE Robot. Autom. Lett. 2(1), 18–25, 2017 and 2) Loosely coupled schemes using visual-inertial alignment (Mur-Artal and Tardós IEEE Robot. Autom. Lett. 2(2), 796–803 2017); Qin, IEEE Trans. Robot. 34(4), 1004–1020 2018). The complexity arises because of the non-linear nature of the system to estimate the gyro bias, which is solved either by directly solving the non-linear, non-convex problem or by decoupling the vision and IMU measurements using linear models. The termination conditions are based on condition number or covariance of the estimated variables, which varies from one experiment to another. The present paper seeks to improve the closed-form solution with higher accuracy and less processing cost per frame. The proposed method separates the gyroscope bias estimation from the closed-form solution using tightly coupled but linear models with reduced number of variables in the closed-form solution. This paper also addresses the problem inherent to the closed-form solutions, which requires sufficient motion in the initialization window with a minimum number of common features. Towards this, a novel method of propagating the past information into the present initialization window is presented. This reduces the total processing cost per frame by limiting the initialization window to 10 frames ≈ 1s without compromising the motion inside the window. We also present a common and intuitive termination criteria which is independent from the experiment scenario. This helps to increase the robustness in the initialization by removing erroneous solutions. The proposed method is evaluated with EuRoC Micro Aerial Vehicle (MAV) dataset (Burri et al. 2016) sequences. We compare the proposed method with a recently proposed loosely coupled method, which shows the improved accuracy, processing cost and robustness in the initialization
Experimental Evaluation of Laminated Carbon Composite Step Lap Repair Through Static and Fatigue Compression Loading
No full textThe static compression test is performed on various carbon fiber reinforced plastic (CFRP) epoxy-based laminate panels manufactured with the vacuum enhanced resin infusion technology (VERITy) process. These tests are conducted for establishing the compression strength and evaluation of the step-lap repaired method effectiveness. The barely visible impact damage (BVID) is simulated experimentally by impacting with low energy. The response of the pristine, impact-damaged, and step-lap repaired composite panels are captured experimentally through the online strain measurement during the testing at various locations. The predicted strain response from the finite element analysis is validated with that of the experiment for all of the specimen configuration. Reasonable good agreement is observed between the predicted and the experimental strain values in terms of their magnitude and the trend. It is observed that both of the strength and the stiffness are regained in the composite panels after the step-lap repair scheme is implemented. Constant amplitude fatigue behavior of pristine and step-lap repaired composite panels are tested under compression-compression fatigue loads. It is observed that the stiffness of both of these panels is not significantly degraded over a testing period of million fatigue cycles