Department of Aerospace Engineering

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

A parametric study is carried out to establish the effect of different geometric parameters on the design of cruciform specimen and a suitable geometry is proposed for studying the biaxial behaviour of glass CSM reinforced polyester composites. Cruciform specimens are fabricated and subjected to equi-biaxial loading using an inplane biaxial test rig. The experimental observations confirmed the predictions of numerical analysis that the failure is governed by raised stresses near the transition zone rather than the biaxial stresses in the gauge area.

Department of Applied Mechanics

ECCM 2012 - Composites at Venice, Proceedings of the 15th European Conference on Composite Materials

Suitability of cruciform specimens for characterising biaxial behaviour of composite laminates

Although increase in burn rates of AP was observed in literature due to doping of K+, it was never considered a candidate for burn rate enhancement as K+ was considered as an impurity in AP. This paper uses this knowledge to improve the burn rates of a composite solid propellant. Successive recrystallization of AP is used to increase the potassium content in AP. This paper also demonstrates the effective use of activated charcoal to reduce the burn rate pressure index, which increased due to the presence of recrystallized AP in a propellant. Addition of known burn rate modifiers like iron oxide and copper chromite to the propellant made from recrystallized AP is also done and it is seen that these burn rate modifiers acted in tandem with recrystallized AP to increase propellant burn rates. An effective way has been found to obtain high burn rates with low pressure index. © 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

GENERAL

47th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit 2011

Development of a high burn rate non - Aluminized composite propellant with low burn rate pressure index

Although composite propellant has been studied for a long time, the role of density of the composite propellant has not been addressed adequately in the literature. This paper examines the role of density of the propellant with regards to burn rate and burn rate pressure index. Propellants were prepared in the four different size (15, 70, 200 and 1000 g) mixers with identical input of ingredients to examine the effect of mixer size on density and burn rate of the composite solid propellant. It is noticed that density of the propellant changes significantly with change in the mixer size from 15 g to 1000 g. This is because for the smaller mixers the surface area to volume ratio is large and the actual percentage of aluminum and ammonium perchlorate (especially coarse) that goes into the propellant reduces. High burn rate with decrease in mixer size is also accompanied with increase in burn rate pressure index. These high burn rate and burn rate pressure index with decrease in mixer size is because of larger fraction of coarse AP particles as compared to fine AP particles remaining attached to the mixer and decreases the coarse to fine AP ratio in the cured propellant. 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Effect of mixer size on density and burn rate of composite solid propellant

A novel 'stack' model of a rate-independent polycrystal, which extends the 'ALAMEL' model of Van Houtte et al. (2005) is proposed. In the 'stack' model, stacks of N neighboring 'ALAMEL' domains collectively accommodate the imposed macroscopic deformation while deforming such that velocity and traction continuity with their neighbors is maintained. The flow law and consistency conditions are derived and an efficient solution methodology based on the linear programming technique is given. The present model is applied to study plastic deformation of an idealized two-dimensional polycrystal under macroscopically imposed plane-strain tension and simple shear constraints. Qualitative and quantitative variations in the predicted macroscopic and microscopic response with N are presented. The constraint on individual 'ALAMEL' domains diminishes with stack size N but saturates for large N. Computational effort associated with the present model is analyzed and found to be well within one order of magnitude greater than that required to solve the classical Taylor model. Furthermore, implementation of the consistency conditions is found to reduce computation time by at least 50%. © 2010 Elsevier Ltd. All rights reserved.

World University root node

International Journal of Plasticity

A 'stack' model of rate-independent polycrystals

This paper presents a complete analysis of a three dimensional retro-PN guidance law, suited for intercepting targets that are of higher speeds than the interceptors. It proposes a somewhat counter-intuitive guidance law that uses a negative navigation constant (as against the usual positive one) to show that this modification makes it possible to achieve collision conditions that were inaccessible to the standard law. An analysis for three dimensional engagements is presented in a modified polar coordinate system and complete capturability results are obtained. Simulation results are given to support the theoretical findings. © 2011 IEEE.

Proceedings of the IEEE Conference on Decision and Control

Analysis of a retro-PN guidance law

This paper presents a fully coupled multi-scale finite element model for the description of the dissipative mechanical response of wood cell walls under large strains. Results show the ability of the present model to capture the main phenomenological responses found typically in wood at the microscopic scale. In addition, the structural and mechanical concepts involved in wood cells are exploited further in order to design new wood inspired composites. Numerical tests are conducted in prototypes of bio-inspired composites and demonstrate substantial gains in terms of resistance to failure and in the control of the overall flexibility/stiffness balance in the material. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

Proceedings of SPIE - The International Society for Optical Engineering

Computational multi-scale constitutive model for wood cell wall and its application to the design of bio-inspired composites

Bifurcation analysis is conducted on experimental data obtained from a simple setup comprising of ducted, laminar premixed conical flames to investigate the features of nonlinear thermoacoustic oscillations. It is observed that as the bifurcation parameter is varied, the system undergoes series of bifurcations leading to characteristically different nonlinear oscillations. Through the application of nonlinear time series analysis on pressure and flame (CH* chemiluminescence ) intensity time traces, these oscillations are characterised as periodic, aperiodic or chaotic oscillations and subsequently the nature of the obtained bifurcations is explained based on dynamical systems theory. Nonlinear interaction between the flames and the acoustic modes of the duct is clearly reflected in the high speed flame images acquired simultaneously with pressure and flame intensity measurements. Copyright © 2011 by ASME.

Proceedings of the ASME Turbo Expo

Experimental studies of bifurcations leading to chaos in a laboratory scale thermoacoustic system

An experimental investigation of the non-normal nature of thermoacoustic interactions in an electrically heated horizontal Rijke tube is performed. Since non-normality and the associated transient growth are linear phenomena, the experiments have to be confined to the linear regime. The bifurcation diagram for the subcritical Hopf bifurcation into a limit cycle behavior has been determined, after which the amplitude levels, for which the system acts linearly, have been identified for different power inputs to the heater. There are two main objectives for this experimental investigation. The first one deals with the extraction of the linear eigenmodes associated with the acoustic pressure from experimental data. This is accomplished by the Dynamic Mode Decomposition (DMD) technique applied in the linear regime. The non-orthogonality between the eigenmodes is determined for various values of heater power. The second objective is to identify evidence of transient perturbation growth in the system. The total acoustic energy in the duct has been monitored as the thermoacoustic system has been initialized by linear combinations of the two dominant eigenmodes. Transient growth, on the order of previous theoretical studies, has been found, and its parameter dependence on amplitude ratio and phase angle of the initial eigenmode components has been determined. This study represents the first experimental confirmation of non-normality in thermoacoustic systems. © 2011 by S. Mariappan, R. I. Sujith and P. J. Schmid.

Non-normality of thermoacoustic interactions: An experimental investigation

Non-normality can cause transient growth of perturbations in thermoacoustic systems with stable eigenvalues. This can cause low-amplitude perturbations to grow to amplitudes high enough to make nonlinear effects significant, and the system can become nonlinearly unstable, even though it is stable under classical linear stability. In this paper, we have demonstrated that this feature can lead to the failure of the traditional controllers that were designed on the basis of classical linear stability analysis. We have also shown in a simple model that it is possible to prevent 'nonlinear driving' by controlling transient growth, using linear controllers. The analysis is performed in the context of a horizontal Rijke tube. © 2011 Cambridge University Press.

Journal of Fluid Mechanics

Non-normality and its consequences in active control of thermoacoustic instabilities

During an attempt to work on a stratified flow problem envisaged as a sequel of the paper by Sameen &amp; Govindarajan (2007), it was found that the original paper contained errors in §§ 3.4 and 4.3 due to a factor of iα, which was inadvertently missed in two places in the code (i) in the buoyancy term due to the use of vertical velocity and streamfunction interchangeably, and (ii) in the apportionment between kinetic and potential energy in the Gmax calculation. Because of this, there were significant differences in the effect of Grashof number on stability. Figure 1 is the modified figure 9 of the original paper, for Pr =7 and ΔT = 25 K. The Poiseuille–Rayleigh–Bénard mode appears at Gr = 39.12 and is seen not to merge with the Poiseuille mode, unlike the conclusion made earlier. This modification applies at any Prandtl number from 10−2 to 102. The corrected versions of figures 17 and 21, showing Gmax contours for different Pr at Gr = 0 and different Gr for Pr = 1, are plotted in figures 2 and 3, respectively. The large growth reported at β = 0 was thus erroneous. The other main conclusions of the paper, that Prandtl number changes transient growth qualitatively, but not the least stable eigenmode, whereas viscosity stratification, which has a huge impact on exponential growth/decay, does not change transient growth much, remain the same. The secondary instabilities also remain unchanged. The stability equations (3.2) to (3.4) in the paper should read (for explanation, please refer to Sameen &amp; Govindarajan 2007)