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Transport properties of AgPb16SbTe18 prepared by the inclusion of nano AgSbTe2 into PbTe matrix.
No full textFor the present investigation, AgPb16SbTe18 material was prepared by the inclusion of AgSbTe2 nanopowder into PbTe matrix, employing mechanical alloying route. This material was characterized for phase formation, microstructure, and transport properties. The microstructure showed densely populated grains with a grain size ~ 80 nm which plays an important role in controlling the transport properties. A remarkable enhancement in Seebeck coefficient (~ − 500 µV at 450 K) value of AgPb16SbTe18 sample was observed which is attributed to the phonon scattering and the potential barrier, encountered by the charge carriers at the nanostructured grain boundaries. The other important findings are enhancement in electrical conductivity and reduction in thermal conductivity which directly enhance the figure of merit of the material and make it promising for thermoelectric power generator application in the mid-temperature zone
Parameter identification of Bouc–Wen type hysteresis models using homotopy optimization.
No full textStructural members exhibit hysteretic behavior under cyclic loading. Among the hysteresis models available in the literature, the differential model proposed by Bouc-Wen is most widely used, owing to its robustness. This model involves many parameters that define the shape of the hysteresis loops. Estimating these unknown parameters is an identification problem that can be tackled by optimization algorithms by using prediction error as the objective function. Stochastic methods like simulated annealing and genetic algorithms can help find global minima but at a high computational cost. Here, the homotopy technique is employed to identify the unknown parameters. The efficiency of this technique in identifying the parameters of the Bouc–Wen model is demonstrated with examples. The present approach is then compared with global optimization methods, such as genetic algorithms and particle swarm optimization techniques. Numerical results confirm that the homotopy method is superior in terms of computational effort and convergence efficiency
Analytical Estimation of Radiation Mode Radar Cross Section (RCS) of Phased Arrays
No full textA novel analytical method has been proposed to compute radiation mode RCS of phased arrays. The array RCS is computed by tracing the path of an impinging EM signal as it travels through the array system. The scattered field contribution from each component of array and its feed network is expressed in terms of the reflection and transmission coefficients. The individual contributions are then coherently superimposed to arrive at total array RCS. The proposed method can be used to obtain the radiation mode RCS of any conventional phased array or low profile patch array with high impedance surface (HIS)-based substrate or superstrate. Results are shown for dipole array with parallel feed network, corporate-fed patch array with conventional metallic ground plane and HIS-based hybrid ground plane. This approach is applicable for estimation and control of radiation mode RCS of any type of phased array
Effects of numerical anti-diffusion in closed unsteady flows governed by two-dimensional NavierStokes equation
No full textNumerical methods producing acceptable results for a long time abruptly blow up, without providing any indication of localized onset of sudden numerical instability. This has been identified as focusing problem in literature. It is noted that the scale selection of error does not depend on the relevant excited physical space-time scales. While this has been encountered in weather prediction studies, it is not widely reported from the solution of Navier-Stokes equation (NSE). Recently, in “Focusing phenomenon in numerical solution of two-dimensional Navier-Stokes equation, In: Pirozzoli S., Sengupta T. (eds) High-Performance Computing of Big Data for Turbulence and Combustion, CISM International Centre for Mechanical Sciences (Courses and Lectures), vol 592. Springer, Cham (2019)”, focusing was demonstrated for a steady fluid flow and its mechanism was identified from global spectral analysis (GSA) of 2D convection diffusion equation (CDE). Focusing was shown to be due to the anti-diffusion caused by the discretization of diffusion term for the chosen numerical scheme. The present work consolidates the one-to-one correspondence between numerical anti-diffusion of 2D CDE and focusing for unsteady flows by solving flow inside a 2D lid driven cavity (LDC) for the Reynolds number of 10,000. We also present a method to remove numerical anti-diffusion using multi-dimensional filters. Detailed analysis of space-time discretization with filters is also provided to explain the cure of focusing
Microstructural evolution in Al–Mg–Sc alloy (AA5024): Effect of thermal treatment, compression deformation and friction stir welding.
No full textEffect of thermal annealing and deformation in both compression and friction stir welding conditions on microstructural evolution in the cold rolled Al-4.36Mg-0.26Sc-0.09Zr (wt.%) alloy was investigated. To evaluate the thermal stability of the alloy, differential scanning calorimetry and static annealing experiments were carried out as a function of temperature. Microhardness measurements and quasi-static compression testing were performed on the as-received and annealed alloy samples. Friction stir welding was carried out on the as-received alloy sheet in butt configuration at two different tool traverse speeds of 250 and 500 mm/min. Upon annealing, the alloy showed continuous recrystallization with transformation of the elongated grain structure possessing strong rolling texture to coarse equiaxed microstructure with random orientation. The annealed alloy exhibited reduced hardness and compressive strength at room temperature. Detailed microstructural investigation of hot compression deformed and friction stir welded samples revealed formation of subgrain structure and followed by fine recrystallized grains with nearly random orientation. The analysis suggests that continuous dynamic recrystallization involving progressive subgrain rotation is the possible mechanism for microstructural changes occurring during hot deformation of Al–Mg-Sc alloy
Parameter Identification in Nonlinear Mechanical Systems with Noisy Partial State Measurement Using PID-Controller Penalty Functions
No full textDynamic models of physical systems often contain parameters that must be estimated from experimental data. In this work, we consider the identification of parameters in nonlinear mechanical systems given noisy measurements of only some states. The resulting nonlinear optimization problem can be solved efficiently with a gradient-based optimizer, but convergence to a local optimum rather than the global optimum is common. We augment the dynamic equations with a morphing parameter and a proportional–integral–derivative (PID) controller to transform the objective function into a convex function; the global optimum can then be found using a gradient-based optimizer. The morphing parameter is used to gradually remove the PID controller in a sequence of steps, ultimately returning the model to its original form. An optimization problem is solved at each step, using the solution from the previous step as the initial guess. This strategy enables use of a gradient-based optimizer while avoiding convergence to a local optimum. The efficacy of the proposed approach is demonstrated by identifying parameters in the van der Pol–Duffing oscillator, a hydraulic engine mount system, and a magnetorheological damper system. Our method outperforms genetic algorithm and particle swarm optimization strategies, and demonstrates robustness to measurement noise
A novel rate based methodology for creep fatigue life estimation of superalloys
No full textIn this present work, we present an accumulated inelastic strain rate based methodology to predict creep fatigue life of two different superalloys i.e. Haynes 282 and IN 718. The evolution of differential strain rate during creep and fatigue respectively motivated the present work. As creep is the rate controlling damage process in creep fatigue interaction, present mean strain rate based approach considers creep strain rate as a kinetic variable controlling the process. The other variable considered for this approach is accumulative cyclic strain during Creep-fatigue interaction. Our prediction method is purely based upon a relative accumulation rate of creep and fatigue strain. This method correlates the creep fatigue life with rate of interaction in strain-controlled regions successfully from the data of creep fatigue tests on those superalloys at two temperatures, 650 deg C and 760 deg C. We found suitable microstructure sensitive constants in a lifing model
A multi-segment morphing system for a micro air vehicle using shape memory alloy actuators.
No full textA configurable multi-segment morphing system for a micro air vehicle (MAV) is presented in this study. One of the novelties is the development of an adaptive control allocation algorithm that provides fast, simultaneous and
independent operation of four morphing segments using shape memory alloy (SMA) actuators. The SMA operation is time-staggered in microsecond resolution to ensure that only one SMA draws power from the MAV battery at a time. The other novelties are the in-flight measurement of morphing angle using dual flex-sensors and morphing of leading edges such that the ‘morphing-line’ is diagonal (45º) to the MAV’s lateral axis. The system was implemented on an open source autopilot controller and operated using the MAV battery. It was ground-tested under propeller ON conditions and a droop rate of 35º/s and ability to track a 1 Hz sinusoidal variation of droop angle were realised
Speech interface for controlling micro air vehicle.
No full textThis paper focuses on the development of the speech interface for controlling a Micro Air Vehicle (MAV). A speech interface in such control applications will have two distinct modules. One is the Automatic Speech Recognition (ASR) module and the other is the Natural Language Processing (NLP) module. The ASR is developed using the models built using CMU Sphinx toolkit. The NLP scheme is proposed and developed using Natural Language Toolkit (NLTK). Understanding of the speech is very important in such kind of control applications. The NLP outcome is used to invoke the Ground Control Station (GCS) commands. The results are validated in a Flight Gear simulator using Mission Planner GCS configured for MAV
Low RCS Microstrip Patch Array with Hybrid High Impedance Surface Based Ground Plane
No full textFor a radiating structure such as dipole/patch array mounted on an aerospace platform, the radiation mode radar cross section (RCS) plays a significant role compared to the structural mode RCS. Thus the estimation and control of array RCS without degrading its radiating characteristics poses a challenge for an antenna engineer. In this paper, a novel design of a low profile 4-element patch array with hybrid HIS-based ground plane is presented to demonstrate both in-band and out-of-band structural RCS reductions. A significant broadband reduction in structural RCS has been achieved from 1GHz to 80GHz. The radiation mode RCS of the patch array is computed and controlled through optimized design parameters without degrading the radiation characteristics. The computed array RCS shows that even radiation mode RCS can be reduced except in operating frequency range