100 research outputs found
Reduced-order modeling and dynamics of nonlinear acoustic waves in a combustion chamber
For understanding the fundamental properties of unsteady motions in combustion chambers, and for applications of active feedback control, reduced-order models occupy a uniquely important position. A framework exists for transforming the representation of general behavior by a set of infinite-dimensional partial differential equations to a finite set of nonlinear second-order ordinary
differential equations in time. The procedure rests on an expansion of the pressure and velocity fields in modal or basis functions, followed by spatial averaging to give the set of second-order equations in time. Nonlinear gasdynamics
is accounted for explicitly, but all other contributing processes require modeling. Reduced-order models of the global behavior of the chamber dynamics, most importantly of the pressure, are obtained simply by truncating the
modal expansion to the desired number of terms. Central to the procedures is a criterion for deciding how many modes must be retained to give accurate results. Addressing that problem is the principal purpose of this paper. Our
analysis shows that, in case of longitudinal modes, a first mode instability problem requires a minimum of four modes in the modal truncation whereas, for a second mode instability, one needs to retain at least the first eight modes. A second important problem concerns the conditions under which a linearly stable system becomes unstable to sufficiently large disturbances. Previous work has given a partial answer, suggesting that nonlinear gasdynamics alone cannot produce pulsed or 'triggered' true nonlinear instabilities; that suggestion is now theoretically established. Also, a certain form of the nonlinear energy
addition by combustion processes is known to lead to stable limit cycles in a linearly stable system. A second form of nonlinear combustion dynamics with a new velocity coupling function that naturally displays a threshold character
is shown here also to produce triggered limit cycle behavior
Design/Development of Mini/Micro Air Vehicles through Modelling and Simulation: Case of an Autonomous Quadrotor
Design and development of an autonomous quadrotor micro aerial vehicle is undertaken following a systematic approach. A fairly detailed model was constructed and simulations were then carried out with the purpose of refining the baseline design, building a controller, and testing the flying qualities of the vehicle on a ground-based flight simulator. Following this, a smooth transition to rig and flight testing has been enabled in a cost- and time-effective manner, meeting all the design requirements.Defence Science Journal, 2011, 61(4), pp.337-345, DOI:http://dx.doi.org/10.14429/dsj.61.108
Modeling and dynamics of nonlinear acoustic waves in a combustion chamber
Future combustors designed for better efficiency and lower pollutant emission are expected to operate
closer to their stability boundary, thereby increasing the risk of encountering combustion
instability. Onset of combustion instability leads to limit cycle oscillations in the acoustical fluctuations
that can often reach amplitudes large enough to cause severe damage. Active control
strategies are, therefore, being considered to prevent combustion instabilities, but their development
requires nonlinear models that can faithfully capture the combustor system dynamics. A
framework for the approximate analysis of the nonlinear acoustics in a combustion chamber exists,
which includes all relevant linear contributions and also second order gasdynamic nonlinearities.
Nonlinear combustion effects in the form of pressure and velocity coupling models have also been
incorporated into the analysis with the aim of capturing the phenomenon of triggered instability,
where the acoustical fluctuations are linearly stable to small perturbations, but show a limit cycle
behavior for larger perturbations. However, several questions such as those relating to 1) modal
truncation of the equations for the acoustic dynamics, 2) absence of triggered limit cycles in the
formulation with only second order gasdynamic nonlinearities, and 3) the form of the velocity coupling
function, including the need for a threshold character, have not been satisfactorily resolved.
In this paper, we address some of these questions on modeling and dynamics of acoustic waves in
combustion chambers, using the approximate analysis, that have remained unanswered over the
years
Idiopathic multicentric Castleman disease: a mysterious case of generalized lymphadenopathy
Castleman disease is a syndrome with significant clinico-pathological overlap between malignancy, autoimmune causes and infectious etiologies. It is a spectrum and can vary in extent from unicentric to multicentric disease with generalized lymphadenopathy, organ involvement, constitutional symptoms and cytopenias, and in severity from non-severe to severe disease with TAFRO symptoms. Idiopathic multicentric Castleman disease (iMCD) is a diagnosis of exclusion after multiple causes as per diagnostic criteria are excluded. Treatment varies between the disease severity types with anti-IL 6 antibodies for non-severe disease to cytotoxic chemotherapy agents for severe disease with TAFRO symptoms. We hereby report a case of a non-severe type of iMCD with a prolonged course and delayed arrival at the diagnosis, owing to the rarity of this condition, which stresses the need for a reduced threshold to consider MCD, early in the differential diagnosis. Interestingly there were positive auto-antibodies and elevated IgG4 levels in this patient, but applying strict criteria helps to distinguish the diagnosis
Novel Redundant Sensor Fault Detection and Accommodation Algorithm for an Air-breathing Combustion System and its Real-time Implementation
Failure of sensors used to provide a feedback signal in control system can cause serious deterioration in performance of system, and even instability may be observed. Based on knowledge of aircraft engine systems, the main cause of fault in such air-breathing combustion systems (ACS) with no rotating parts is due to the pressure sensors. Fast online detection of faults before the error grows very large and accommodation is critical to the success of the mission. However, at the same time, it is necessary to avoid false alarms. Hence, early detection of small magnitude faults with acceptable reliability is very challenging, especially in the presence of sensor noise, unknown engine-to-engine variation and deterioration and modeling uncertainty. This paper discusses the novel fault detection and accommodation (FDA) algorithm based on analytical redundancy based technique for ACS.Defence Science Journal, 2010, 60(1), pp.61-75, DOI:http://dx.doi.org/10.14429/dsj.60.10
Bifurcation and nonlinear analysis of a time-delayed thermoacoustic system
In this paper, of primary concern is a time-delayed thermoacoustic system, viz. a horizontal Rijke tube. A continuation approach is employed to capture the nonlinear behavior inherent to the system. Unlike the conventional approach by the Galerkin method, a dynamic system is naturally built up by discretizing the acoustic momentum and energy equations incorporating appropriate boundary conditions using a finite difference method. In addition, the interaction of Rijke tube velocity with oscillatory heat release is modeled using a modified form of King’s law. A comparison of the numerical results with experimental data and the calculations reported reveals that the current approach can yield very good predictions. Moreover, subcritical Hopf bifurcations and fold bifurcations are captured with the evolution of dimensionless heat release coefficient, generic damping coefficient and time delay. Linear stability boundary, nonlinear stability boundary, bistable region and limit cycles are thus determined to gain an understanding of the intrinsic nonlinear behaviors
Bifurcation analysis of inertia coupled roll manoeuvres of airplanes
The extended bifurcation analysis (EBA) technique has been recently proposed for the bifurcation analysis of dynamical systems with constrained equilibrium states. In this paper, the EBA technique is used to investigate non-linear behaviour of an aircraft in constrained roll manoeuvres. A comparative performance analysis of the aircraft in three roll manoeuvres, viz. zero sideslip roll, velocity vector roll, and roll manoeuvre with a linear aileron-rudder interconnect (ARI), has been carried out. Selective numerical simulations are carried out to supplement the information obtained from bifurcation analysis
Low distortion micropower filterless digital class D amplifier
This report investigates the concept, design and implementation of a filterless Class D amplifier, for digital hearing instrument applications. The traditional Class D amplifier requires a low pass filter to eliminate the switching currents and the high frequency components. The filter accounts for a major part in the cost of the Class D amplifier, and thus by eliminating the filter the cost can be minimized and the product can be made competitive.Master of Science (Integrated Circuit Design
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