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

This paper investigates the transitional flow dynamics behind a passively flapping airfoil supported by nonlinear springs in the presence of gusty inflow. The fluid-structure interaction (FSI) framework is composed of an incompressible Navier-Stokes solver weakly coupled with a two degree-of-freedom (dof) nonlinear structural model. The fluid-elastic system shows a rich bifurcation behavior in terms of successive Hopf bifurcations in uniform flow condition as the mean wind speed is increased. Presence of gusty fluctuations in the inflow makes the dynamics more complex through transitional states that we refer to as ‘intermittency’ between different dynamical states. A regular intermittent state between quasi-periodic dynamics and low amplitude aperiodic response has been observed when the FSI system is subjected to a time harmonic gust in terms of sinusoidal fluctuation. A parametric study has been carried out for various amplitudes and frequencies of the sinusoidal fluctuation to demarcate the transitional regimes. Thereafter, the system is subjected to random gusts modeled as Ornstein-Uhlenbeck process and ‘on-off’ and ‘burst’ type intermittent dynamics have been observed for long time-scale and short time-scale input fluctuations respectively. The intermittent states have been characterized through time series analyses tools and the corresponding flow-field dynamics has been investigated in detail. © 2021, Springer Nature Switzerland AG.

Department of Applied Mechanics

World University root node

Notes on Numerical Fluid Mechanics and Multidisciplinary Design

Transitional Flow Dynamics Past a Passively Flapping Airfoil in Gusty Flow

This study investigates the transitional wake dynamics of a simultaneously pitching-heaving airfoil in the low Reynolds number regime. The transition from Kármán to reverse Kármán wakes and the subsequent wake deflection are known to take place entirely in the periodic regime as the dynamic heave velocity (proportionally, the amplitude-based Strouhal number) is gradually increased. However, further increase in may result in the loss of dynamic stability of the deflected vortex street and an eventual transition to chaos. This phenomenon has largely remained unexplored in the existing literature. The present study attempts to fill this gap and aims to establish a dynamic link between the near-and far-field wake transitions. It is shown that the deflected jet undergoes a switching of deflection direction in the far-field as the near-field encounters a quasi-periodic transition with the increase in. The quasi-periodic behaviour of the near-field is seen to get interspersed with intermittent aperiodic windows, resulting in a complete reversal of the deflection direction through flipping of the immediate vortex couple. Eventually, the wake topology becomes fully chaotic through a series of rapid aperiodic jet-switching. To the best of the authors' knowledge, this is the first study that investigates the role of this aperiodic jet-switching in ushering in chaos in the wake of a pitch-heave flapping system. The mechanism of jet-switching and the role of leading-edge vortex are also revealed. Flow field dynamics during these changes and the underlying vortex interactions are analysed using nonlinear dynamical tools. © 2021 The Author(s). Published by Cambridge University Press.

Journal of Fluid Mechanics

Dynamic interlinking between near- and far-field wakes behind a pitching-heaving airfoil

In order to obviate the requirement of drift dynamics in adaptive dynamic programming, integral reinforcement learning (IRL) has been proposed as an alternate formulation of Bellman equation. However control coupling dynamics is still needed to obtain closed-form expression of optimal control effort. In addition to this, initial stabilizing controller and two sets of neural networks (NN) (known as Actor-Critic) are required to implement IRL scheme. In order to remedy these, this paper presents a critic-only IRL controller coupled with an experience replay (ER)-based identifier to solve optimal tracking control problem for an unknown continuous-time systems under actuator constraints. The presented control architecture is shown to yield tighter residual sets for state tracking error and error in NN weights. The simulation results establish the efficacy of the presented control scheme on continuous-time systems. © 2021 Informa UK Limited, trading as Taylor & Francis Group.

International Journal of Control

Simultaneous identification and optimal tracking control of unknown continuous-time systems with actuator constraints

The ability of autonomous agents like Unmanned Aerial Vehicles (UAVs) and spaceships to avoid collision with static and dynamic obstacles while navigating through an obstacle-cluttered environment is very critical for overall success in any mission. Over the years, There have been significant efforts culminating into some well-established collision avoidance algorithms like Velocity Obstacle (VO), Optimally Reciprocal Collision Avoidance, Nonlinear Velocity Obstacle, Probabilistic Velocity Obstacle (PVO), Collision Cone, etc. to achieve collision avoidance. However, in their basic set-ups several of them assume constant velocity model for the obstacles and does not take into consideration the dynamic constraints of the agent. In the limited setting, when agent-obstacles are alike in terms of planning and control, collision avoiding methods like Acceleration Velocity Obstacle (AVO) do take into account the agent’s and obstacles’ dynamic constraints. Still they are not robust to sensor noise and environmental uncertainty. The increased involvement of UAVs in our day to day lives and the availability of once expensive, and ever-improving computational power, has necessitated an algorithm to be able to plan for safe dynamically constrained navigation in an environment with uncertainty in the motion of the obstacles, sensor noise and uncertainty beyond the occluded regions. To this end, as an expansion over the PVO and AVO methods, a novel motion planner-Indicator probabilistic Acceleration Velocity Obstacle (IPAVO) method-is developed in this paper for smooth, collision-free navigation of the agent with acceleration constraints in an uncertain, unstructured environments with an element of anticipation for the future environment. Effectiveness of the developed IPAVO algorithm for safe collision-free smooth navigation of the agent is investigated in simulation studies in two environments with obstacles having constant velocity aother with obstacles maneuvering on a trajectory. Then in a hybrid environment consisting of the linear motion obstacle and dynamic maneuvering obstacles, an Extensive simulation studies is performed to elucidate the performance of the proposed IPAVO algorithm on four crucial parameters-target interception time, computational cycle time, minimum obstacle distance, and overall control effort under variable obstacle density. The algorithm deftly handles the environmental uncertainty and with all these desired features, IPAVO has the potential of real-time implementation. © 2021, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.

AIAA Scitech 2021 Forum

Indicator probabilistic acceleration velocity obstacles for dynamic collision avoidance in uncertain environments

The high strain rate tensile response of titanium-based fiber metal laminates (FMLs), consisting of layers of titanium Ti-6Al-4V alloy sheet and glass fiber reinforced composites, is examined. A hand layup method is used to fabricate four different layups of FMLs, exhibiting the same thickness of the total metal layer. A split Hopkinson tensile bar apparatus is used to load titanium and composite under a high strain rate to obtain baseline data. High-speed digital image correlation is used to measure the strain directly on the specimen gage region. The elastic-plastic response of FMLs up to maximum stress is predicted by the classical laminated plate model and orthotropic plasticity model. This is followed by a behavior considering the mechanics of delamination. The results show that the layup sequence of titanium-based FMLs considerably affects the failure behavior of composites following ultimate strength. This strength increases at high strain rates and seems higher for titanium-based FMLs than aluminium-based FMLs. This is primarily caused by the rate-dependent response of the titanium and composite. The failure strain of glass fiber reinforced epoxy (GFRP) constituent, failure strain, and toughness of FMLs are affected by isolating composite layers by metallic layers within FMLs and are found to be rate sensitive. Isolation of composite layers from one another by metallic layers results in more progressive failure of FMLs. The proposed models are validated with experiments of aluminium-based FMLs available in the literature. © The Author(s) 2021.

Journal of Composite Materials

Effect of high strain rate on tensile response and failure analysis of titanium/glass fiber reinforced polymer composites

A simple two-dimensional elasticity solution is presented here for flexure of an infinite sandwich strip with rigidly clamped ends; it is based on the superposition of a strong form solution for a simply supported strip under transverse loading and a Ritz solution for end-loading. Results useful for future comparisons are presented in tabular form. On the basis of these results, the accuracy of classical plate theory and first-order shear deformation theory is critically examined. Finally, the difference between rigidly clamped end conditions and a softer version of clamped conditions is highlighted with reference to far-field response. © The Author(s) 2021.

Journal of Sandwich Structures and Materials

Flexure of a rigidly clamped orthotropic sandwich plate strip – An elasticity solution using superposition method

Viscous effects on high enthalpy flow over a double-wedge are studied using computational fluid dynamics (CFD). Local thermodynamic equilibrium (LTE) conditions are assumed to apply. A finite volume based compressible flow solver is developed for the purpose. Effects of LTE based properties of air on the resulting flow field are studied in detail. © 2021, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.

Numerical simulation of viscous effects on high enthalpy flow over a double-wedge geometry

The demand for light-weight materials, exhibiting superior strength and higher toughness, has increased attention to cryogenic temperature rolling as it produces bi-modal microstructures with improved mechanical properties. The samples were cryorolled to a final thickness of 1 mm after thickness reductions of 50%, 66.66%, 75%, and 80%. The ballistic behavior of cryorolled AA 6082 aluminum alloy with different rolling strains/thickness reductions was investigated in this study. The cryorolled Al alloys were subjected to normal high-speed impact using dome-shaped projectiles at velocities between 50 and 250 m/s. The 66.66% thickness reduced samples exhibited higher impact resistance, especially at higher impact velocities. The maximum energy absorption of 106.43 J was observed for 66.66% thickness reduced sample at an impact velocity of 234.38 m/s. This study suggests the existence of an optimal value of cryogenic rolling strain for superior impact resistance, possibly due to an ideal combination of coarse and fine grains in the resulting bimodal microstructure. © 2021, ASM International.

Department of Engineering Design

Journal of Materials Engineering and Performance

Effect of Different Cryogenic Rolling Strains on the Impact Properties of 6082 Aluminum Alloy

Interaction of polydisperse droplets in a turbulent air flow features prominently in a wide range of phenomena, such as warm rain initiation as an example. In the current study, we present an experimental investigation on the effects of initial droplet field characteristics on the maximum droplet size growth. By performing experiments in a vertically oriented air flow facility, the air flow turbulence was able to be controlled through the mean flow velocity and an active turbulence generator. The initial droplet field characteristics (droplet diameter range of 0-120 m) were varied using spray nozzles of different flow numbers. Based on quantitative measurements of the droplet size distribution at various spatial locations using phase Doppler interferometry (PDI), we estimated the droplet size growth rate as a function of turbulence intensity, initial droplet number density and initial mean droplet size. For each (,), we observed the occurrence of an optimum turbulence intensity, with the corresponding maximum droplet size growth rate being. Two different trends were observed. When and were simultaneously increased and decreased, respectively, their competing influences resulted in small variations in. In contrast, when was held constant with a corresponding Stokes number smaller than unity, there existed a threshold above which increased rapidly with. These trends were then understood through long-distance microscopy (LDM) measurements. Beyond the aforementioned threshold, the fraction of uncorrelated small-sized <![CDATA[(St droplet pairs was found to rapidly increase with. Further detailed analysis of droplet tracking in the LDM images identified that the velocity fluctuations in the small-sized droplet pairs being induced by close encounters with inertial droplets was the underlying mechanism for the rapid increase of with. This mechanism potentially explains how droplet collisions can be enhanced in small droplets if the droplet field is sufficiently polydisperse. ©

Experimental study of the effects of droplet number density on turbulence-driven polydisperse droplet size growth

In this work, the local correlation-based one-equation transition model (Menter, F.R., Smirnov, P.E., Liu, T. and Avancha, R., A one-equation local correlation-based transition model. Flow, Turbulence and Combustion, vol. 95, no. 4, pp. 583–619, 2015.) is transformed into a zero-equation transition model. The new model provides an attractive choice in terms of quick implementation of a transition model in existing turbulent flow solvers with Menter's shear-stress transport (SST) turbulence model, as it only introduces three extra source terms in the transport equation of turbulent kinetic energy. The model is validated against a set of benchmark flat-plate test cases: T3 series and SK, and also subsonic flows past two different airfoils: Aerospatiale A-airfoil (Re=2.1million) and E387 (Re=0.2million), and finally applied to a transonic flow over 3D DLR-F5 wing (Re=1.5million). Results show that the proposed model produces similar transition prediction as the one-equation transition model, with a reduced computational effort. The computations are performed with an in-house finite-volume solver for compressible turbulent flows on block-structured grids. © 2020 Elsevier Ltd