Department of Mechanical Engineering

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In recent times, shock tube flows have been widely employed in many micro-scale devices in the fields of propulsion technology, micro-heat engines, particle delivery systems, and so on. The very small length scales in such micro-shock tubes make the flow physics more complicated compared to the ordinary macro-shock tubes. The major differences in the flow features are the profound influences of wall effects and rarefaction effects. The rarefaction effect alters the boundary layer structure by imparting additional velocity and thermal gradients to the wall-bounded fluid. These phenomena can strongly affect the micro-shock tube flow characteristics such as shock-contact wave speeds, wave propagations, hot and cold zone properties. The main objective of the present work is to produce a detailed understanding on the wave propagation characteristics in a micro-shock tube under rarefied conditions using computational fluid dynamics methods. The shock-contact interface movement under different operating conditions such as Knudsen number and pressure ratio are investigated in detail and compared with the macro-scale shock tube flows. The difference between the numerical and analytical works and their cause is identified and discussed. The results obtained show that the shock strength attenuates rapidly for micro-shock tubes compared to macro-shock tubes. The shock-contact propagation and the distance between them in a micro-shock tube have a strong dependence on rarefaction effects. The more the rarefaction effects are, lesser will be the shock-contact distance. The shock-contact distance decreases as the pressure ratio increases. A strong attenuation in shock strength can also be observed as the rarefaction increases. © IMechE 2013.

World University root node

Proceedings of the Institution of Mechanical Engineers, Part G: Journal of Aerospace Engineering

Computational analysis of the wave motions in micro-shock tube flow

We present a two-dimensional computational flow solver for simulation of two-way interactions between moving rigid bodies and two-fluid flows. The fluids are assumed to be incompressible and immiscible. The two-step projection method along with Graphics Processing Unit (GPU) acceleration is employed to solve the flow equations. The fluid-solid interaction is captured by using the fictitious domain method. A consistent mass and momentum scheme is implemented, which allows for simulation of multiphase flows characterized by large density ratios. The evolution of interfaces in the three-phase system is tracked by using the volume-of-fluid method with two scalar functions, representing the solid domain and one of the fluids. A geometrical approach is employed to reconstruct the interfaces in cells containing three phases and capture the intersection of phase interfaces (triple point). The performance and accuracy of the flow solver are assessed through a set of canonical test cases. Then, it is used to simulate the interactions between a free-floating buoy and waves generated by a bottom-hinged paddle in a wave tank. © 2014 Elsevier Ltd.

Computers and Fluids

Computational simulation of the interactions between moving rigid bodies and incompressible two-fluid flows

This paper investigates the dynamic behaviour of serial chains with degrees-of-freedom (DOF) as large as 100,000. A recursive solver called Recursive Dynamic Simulator (ReDySim), based on the Newton–Euler formulation and the Decoupled Natural Orthogonal Complement (DeNOC) matrices, was used to simulate the dynamics of these systems. Planar, as well as spatial motions of chains with moderate- (DOF≤1,000), large- (1,000<DOF≤10,000), and huge- (DOF>10,000) DOF were simulated. The results were validated by several means, such as comparisons with reported results wherever available, results obtained from commercial software, energy checks, etc. The study shows that ReDySim is capable of analysing serial chains of huge-DOF with acceptable numerical accuracy. The scheme is found to be numerically stable as well as computationally efficient, owing to the linear-time computation of the joint accelerations. Numerical studies were conducted to establish the theoretical basis for better performance of the proposed ReDySim solver. With the demonstrated capabilities of ReDySim, it may be found suitable for a large number of applications involving serial systems with huge-DOF. © 2013, Springer Science+Business Media Dordrecht.

Multibody System Dynamics

Dynamics of serial kinematic chains with large number of degrees-of-freedom

The present work is an experimental study of an airfoil flapping and simultaneously moving forward in quiescent water in a tank. The focus here is to study using flow visualizations and PIV how the flexible flap attached at the trailing edge of a rigid airfoil affects the flow generation. The parameters that were varied were the frequency and amplitude of flapping and forward speed. The Strouhal number based on lateral excursion of the trailing edge is kept around 0.3 in all the experiments. We also briefly present the flow in the absence of free-stream velocity or forward speed (Strouhal number = ∞). © 2014: The Royal Institution of Naval Architects and IIT Madras.

RINA, Royal Institution of Naval Architects - International Conference on Computational and Experimental Marine Hydrodynamics, MARHY 2014

Falpping flexible foil propulsion

Thrust-generating flapping foils are known to produce jets inclined to the free stream at high Strouhal numbers St = fA/U∞, where f is the frequency and A is the amplitude of flapping and U∞ is the free-stream velocity. Our experiments, in the limiting case of St → ∞ (zero free-stream speed), show that a purely oscillatory pitching motion of a chordwise flexible foil produces a coherent jet composed of a reverse Bénard-Kármán vortex street along the centreline, albeit over a specific range of effective flap stiffnesses. We obtain flexibility by attaching a thin flap to the trailing edge of a rigid NACA0015 foil; length of flap is 0.79 c where c is rigid foil chord length. It is the time-varying deflections of the flexible flap that suppress the meandering found in the jets produced by a pitching rigid foil for zero free-stream condition. Recent experiments (Marais et al., J. Fluid Mech., vol. 710, 2012, p. 659) have also shown that the flexibility increases the St at which non-deflected jets are obtained. Analysing the near-wake vortex dynamics from flow visualization and particle image velocimetry (PIV) measurements, we identify the mechanisms by which flexibility suppresses jet deflection and meandering. A convenient characterization of flap deformation, caused by fluid-flap interaction, is through a non-dimensional 'effective stiffness', EI∗ = 8 EI/(ρVTEmax2 sf cf3/2), representing the inverse of the flap deflection due to the fluid-dynamic loading; here, EI is the bending stiffness of flap, ρ is fluid density, VTEmax is the maximum velocity of rigid foil trailing edge, sf is span and cf is chord length of the flexible flap. By varying the amplitude and frequency of pitching, we obtain a variation in EI∗ over nearly two orders of magnitude and show that only moderate EI∗ (0.1 ≲ EI∗ ≲ 1) generates a sustained, coherent, orderly jet. Relatively 'stiff' flaps (EI∗ ≳ 1), including the extreme case of no flap, produce meandering jets, whereas highly 'flexible' flaps (EI∗ ≲ 0.1) produce spread-out jets. Obtained from the measured mean velocity fields, we present values of thrust coefficients for the cases for which orderly jets are observed. © 2014 Cambridge University Press.

Journal of Fluid Mechanics

Flexibility in flapping foil suppresses meandering of induced jet in absence of free stream

Present study focuses on the diffuse damage, its possible direction of growth and eventually its effect in the growth of matrix cracking. A detailed homogenization based micro- mechanics is used to study the local stress fields at the crack front of diffuse damage in unit cell RVE (representative volume element). Based on the state of this stress field, macro stresses which are responsible for the growth of diffuse damage along the circumference and away from circumference of fiber will be predicted. Study will focus on the coupling effect of three damages i.e fiber breakage, diffuse damage, matrix cracking. Based on these studies a continuum mechanics based (meso) damage model, developed by previous studies, has been improved by incorporating the coupling effect of these damages. © 2012 AIAA.

Collection of Technical Papers - AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference

Effect of coupling of damage mechanisms in the degradation model of composite laminates

A simple continuum damage model is developed based on an understanding of the effect of the damage mechanisms on effective material response parameters. A detailed micromechanical analysis, using the mathematical theory of homogenization is conducted, to understand the coupled influence of damage mechanisms and volume fraction on the degradation of the primary stiffness coefficients. The study further explains the generation of additional residual strains due to damage, through the degradation of the coefficient of thermal expansion. Using these studies, a macrolevel free energy expression is developed, and based on this expression, constitutive equations for macrolevel stresses and thermodynamic forces, corresponding to damage parameters, are derived. Further, a thermodynamically consistent, simplified model for evolution of damage is developed. The simplified model developed is effective in capturing the observed diffuse damage response of a coupon of any unidirectional composite, in the absence of laminate level delamination. © 2012 Elsevier Ltd.

Composite Structures

Micromechanics based diffuse damage model for unidirectional composites

Rotary wing aeroelasticity is a highly complex phenomenon involving coupling between flexible blade dynamics and unsteady aerodynamics including stall and unsteady wake effects. In this paper, a low-cost computational aeroelastic model including the structural coupling from geometric parameters and nonlinearities associated with structural modeling and dynamic stall, applicable to steady, level forward flight, has been developed. The differential equations of motion are solved in time domain in a sequential manner to obtain the response of all the blades in the rotor system, the dynamic inflow variables, and the sectional loads at every time step. A fourth-order Runge-Kutta integration scheme has been adopted for solving the differential equations. Iterations are carried out until convergence is achieved in blade response and helicopter trim. The effect of blade geometric parameters such as pretwist, hinge offset, and torque offset on aeroelastic response of a helicopter rotor system is investigated numerically. It is shown that the structural coupling from blade geometric parameters significantly influences the rotor blade response and loads. © 2013 American Society of Civil Engineers.

Journal of Aerospace Engineering

Influence of blade geometric parameters on aeroelastic response of a helicopter rotor system

Because of their potential for application in both civil and military sectors aswellas the challenge in designingatiny flying vehicle, micro aerial vehicles have attracted enormous attention. Several configurationsof micro aerial vehicles are under development, namely fixed wing, rotary wing, and flapping wing. Under rotary-wing micro aerial vehicles, coaxial contrarotating configurations with a stabilizer bar seem to provide excellent stability behavior in pitch and roll degrees of freedom. This paper presents the results of an experimental study performed on a coaxial-contrarotor helicopter modeltoevaluate the performanceofthe rotor system using different rotor blades. The thrust and power of the rotors in independent and coaxial configurations were observed. The percentage thrust loss in the coaxial configuration was seen tobedependentonthe ratioof the upper- and lower-rotor revolutions per minuteas wellasthe spacing between the rotors. A theoretical analysis using a nonuniform inflow model was developed, which corroborated very well with the experimental data. Using low-Reynolds-number drag coefficients for the airfoil, the estimated power consumption of the rotors was found to match very well with the experimental data. A closed-form solution using a uniform inflow model to estimate the thrust performance of a coaxial rotor is proposed, which also matches with the experimental observations. © 2013 by Puneet Singh and C. Venkatesan. Published bythe American Institute of Aeronautics and Astronautics, Inc.

Journal of Aircraft

Experimental performance evaluation of coaxial rotors for a micro aerial vehicle

In the present study, drying experiments on mixed-mode solar dryer with cylindrical potato samples have been performed for a wide range of process variables. The variables investigated are absorbed thermal energy, air flow rate, food product loading density and sample thickness. A mathematical framework is developed to estimate several dryer performance indicators namely: drying efficiency, SEC (specific energy consumption), CO2 emissions mitigation, carbon credits earned and amount of different fossil fuels saved due to use of solar drying. The fossil fuels investigated in the present study are coal, diesel, natural gas and LPG (liquefied petroleum gas). The results of investigation indicate that for all drying test conditions, the given dryer is capable to mitigate the maximum CO2 emissions with the replacement of coal by solar energy. Larger values of absorbed energy and load density cause increased SEC and CO2 mitigation potential whereas reverse trend is observed for sample thickness. However, the influence of air flow rate on these parameters is found to be quite different. In order to establish functional relationship between SEC and process variables, a correlation is developed using Levenberg-Marquart algorithm. The statistical error analysis reveals that the proposed correlation is capable of satisfactory prediction of experimental results. © 2013 Elsevier Ltd.