Department of Mechanical Engineering

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We have conducted laboratory experiments to model important aspects of plumes in mantle convection. We focus on the role of the viscosity ratio U (between the ambient fluid and the plume fluid) in determining the plume structure and dynamics. We build on previous studies to highlight the role of viscosity contrast in determining the morphology of mantle plumes and provide detailed visualizations and quantitative information on the convection phenomenon. In our experiments, we are able to capture geophysical convection regimes relevant to mantle convection both for hot spots (when U>1) and plate-subduction (when U<1) regimes. The planar laser induced fluorescence (PLIF) technique is used for flow visualization and characterizing the plume structures. The convection is driven by compositional buoyancy generated by the perfusion of lighter fluid across a permeable mesh and the viscosity ratio U is systematically varied over a range from 1/300 to 2500. The planform, near the bottom boundary for U=1, exhibits a well-known dendritic line plume structure. As the value of U is increased, a progressive morphological transition is observed from the dendritic-plume structure to discrete spherical plumes, accompanied with thickening of the plumes and an increase in the plume spacing. In the vertical section, mushroom-shaped plume heads at U=1 change into intermittent spherical-blob shaped plumes at high U, resembling mantle plume hot spots in mantle convection. In contrast, for low values of U(1/300), the regime corresponds to subduction of plates in the mantle. In this regime, we observe for the first time that plumes arise from a thick boundary with cellular structure and develop into sheet-plumes. We use experimental data to quantify these morphological changes and mixing dynamics of the plumes at different regimes of U. We also compare our observations on plume spacing with various models reported in the literature by varying the viscosity ratio and the buoyancy flux. © 2016 Elsevier Ltd

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Chemical Engineering Science

The role of viscosity contrast on plume structure in laboratory modeling of mantle convection

A modified enthalpy porosity formulation is introduced to capture melting and solidification of pure substances. When melting and solidification of pure substances are addressed by the fixed grid based volume averaging technique, it is possible to obtain two equivalent and interchangeable mathematical formulations of the energy conservation equation if the governing equation is expressed in terms of temperature as the primary dependent variable. Between these two formulations, only one form is shown to provide physically consistent numerical solutions when very large difference in specific heats for liquid and solid phases is involved. A modified enthalpy updating scheme is proposed to predict the solid/liquid fraction during melting and solidification process of pure substances having large difference in phase specific heats. The results from the proposed scheme are validated with the existing results from literature involving numerical prediction of freezing of water. The physical consistency of the simulation results obtained by solving two interchangeable forms of energy conservation equation is tested and compared considering a case study involving melting of ice. While one of the conservation forms fails to predict the melting process, the other conservation form successfully predicts physically consistent result. The proposed formulation is capable of predicting melting and solidification of all pure substances including those with large difference in phase specific heats such as water and paraffin wax. © 2016 Elsevier Ltd

International Communications in Heat and Mass Transfer

Enthalpy porosity model for melting and solidification of pure-substances with large difference in phase specific heats

This paper presents impact modeling of a multi-Arm robotic system mounted on a service satellite while capture of tumbling orbiting objects. A robotic system with multiple arms would be capable of capturing multiple objects simultaneously. Further when satellite is in broken state or does not have provision for grapple and tumbling, the interception is very difficult. In such cases, interception using multi-Arm robotic system can be appealing as this will increase the probability of grasp in comparison to single-Arm robot. In this paper, three phases of the capturing operation, namely, approach, impact and post impact have been modeled. In the approach phase, the end-effectors' velocities are designed same as that of the grasping point on the target in order to avoid high impact forces. But in practice, there will be a nonzero relative velocity between the end effector and the grapple point, leading to an impact. In the impact phase, a framework is developed to estimate the changes in the generalized velocities caused by the impact. In post impact phase, these velocities are used as an initial condition for the post impact dynamics simulations of the combined robotic system and target object. Efficacy of the framework is shown using a dual-Arm robot mounted on a service satellite performing capturing operation for two tumbling objects. © 2017 ACM.

ACM International Conference Proceeding Series

Impact modeling and estimation for multi-arm space robot while capturing tumbling orbiting objects

The present paper describes an investigation aimed at evaluating the microstructural and dry sliding adhesive wear characteristics of Fe–xCr–4Mn–3C hardfacing alloys applied through shielded metal arc welding. The effect of chromium addition on the microstructure of hardfacings was carried out by using optical microscope, field emission scanning electron microscope and X-ray mapping. Dry sliding wear tests were performed on a pin-on-disc wear tribometer. From the experimental results, it was observed that the primary carbides were refined and increased with the increase of chromium content. The morphology of carbides revealed that the primary carbides were rod shaped. The increased chromium content was also found to be beneficial to enhance hardness and wear resistance of hardfacings. The correlation between hardness and wear resistance exhibited the reliability of hardness as an indicator of the wear performance of hardfacings. © 2016, The Indian Institute of Metals - IIM.

Transactions of the Indian Institute of Metals

On the Microstructure and Wear Behavior of Fe–xCr–4Mn–3C Hardfacing Alloys

In this paper the effect of thermal fatigue on mechanical behaviour of the heat affected zone on ferritic side of bimetallic weld was investigated. The bimetallic weld coupons between ferritic steel SA516 Grade 70 and stainless steel 304L were fabricated using TIG (Tungsten Inert Gas) welding process. In this investigation, an experimental test rig was developed and used to simulate the thermal fatigue conditions at laboratory scale. The thermal fatigue factors selected were heating time, notch radius and number of cycles. © 2017 Trans Tech Publications, Switzerland.

Materials Science Forum

Analysis of mechanical behaviour of heat affected zone on ferritic side of bimetallic welds under thermal fatigue conditions

The bimetallic welds (BMWs) between ferritic low alloy steels and austenitic stainless steel are used widely in steam generators of the power plants. The adoption of these welds in wide industrial applications provides feasible solutions for the flexible design of the products by using each material efficiently and economically. The present paper is an effort towards studying the development of austenitic stainless steel buttering filler material for bimetallic weld joint. The work aims at the design and development of buttering layer electrode coatings for shielded metal arc welding process using extreme vertices design methodology suggested by McLean and Anderson to study the effect of electrode coating ingredients on the buttering layer metal composition and delta ferrite content to prevent solidification cracking. © 2017 Trans Tech Publications, Switzerland.

Investigations on design and formulation of buttering layer electrode coatings for bimetallic welds

Fin and tube heat exchangers are being used in several industrial applications by means of novel design and optimized performance. Improvements in geometric design may deliver energy efficient and cost-effective heat exchanger performance with reduced weight. In this paper, a systematic study on a cross-flow type fin and tube heat exchanger design for a waste heat recovery application is conducted. The geometric profile of the fin is characterized by a dimensionless design variable named aspect ratio which is parametrically varied to obtain different profiles. Two cases, case-I, and case-II with constant fin-base thickness and variable fin-base thickness, respectively, are studied considering the ‘rectangular’ fin profile as a ‘reference’. Heat transfer and pressure loss characteristics based on the parametric fin profiles are investigated numerically for a Reynolds number range from 5000 to 13,000 using computational fluid dynamics. The numerical results obtained for the reference fin profile are verified with the experimental correlations. Dimensionless parameters such as Nusselt number, Euler number, and efficiency index are calculated to predict the overall performance of the heat exchanger design. Results show that the overall performance improves by approximately 6.76% and 17.33% with a reduction in net unit weight by approximately 27.58% and 6.82% on changing the aspect ratio from 1.0 to 0.1 in case-I and case-II, respectively. In addition, feasible fin profile for a waste heat recovery application is predicted using Pareto optimality in each case. The study, therefore, helps to reveal the potential of different fin profiles in furnishing enhanced overall performance along with the net material cost benefits to the design engineers and manufacturers. © 2017 Elsevier Ltd

Applied Thermal Engineering

Implications of fin profiles on overall performance and weight reduction of a fin and tube heat exchanger

An analytical model to study the whistler turbulence spectrum and inertial range spectral scalings related with the electric and magnetic field spectra in a weakly non-collisional magnetized plasma is developed. In the present model, the dispersion relation of whistler wave propagating along the background magnetic field is exploited to derive the inertial range scaling laws corresponding to the electric field and magnetic field fluctuations. The model is based on the concept of Iroshnikov-Kraichnan inertial range magnetohydrodynamic turbulence. The present phenomenological turbulence scaling model suggests the energy spectra associated with the whistler wave electric field fluctuations and magnetic field fluctuations pursue -1 and -3 power-laws, respectively. Moreover, the results obtained by the present analytical model reasonably able to explain and compare the observational energy spectra in the wave number domain. The observational and analytical results support the whistler dominated turbulence with the similar spectral index -1 of the electric field spectra, whereas, the magnetic field spectral indices in the wave number domain are found and -3 for the observational and analytical approach, respectively. © 2017 The Royal Swedish Academy of Sciences.

Physica Scripta

Inertial range spectrum of field-aligned whistler turbulence

The aim of the present study is to develop a relatively simple flight dynamic model which should have the ability to analyse trim, stability and response characteristics of a rotorcraft under various manoeuvring conditions. This study further addresses the influence of numerical aspects of perturbation step size in linearised model identification and integration timestep on non-linear model response. In addition, the effects of inflow models on the non-linear response are analysed. A new updated Drees inflow model is proposed in this study and the applicability of this model in rotorcraft flight dynamics is studied. It is noted that the updated Drees inflow model predicts the control response characteristics fairly close to control response characteristics obtained using dynamic inflow for a wide range of flight conditions such as hover, forward flight and recovery from steady level turn. A comparison is shown between flight test data, the control response obtained from the simple flight dynamic model, and the response obtained using a more detailed aeroelastic and flight dynamic model. © Copyright Royal Aeronautical Society 2017.

Aeronautical Journal

Rotorcraft control response using linearised and non-linear flight dynamic models with different inflow models

Investigation of critical speeds and nonlinear free vibration analysis of a highly flexible rotor system have been studied. The rotary inertia and gyroscopic effect combined with inextensible geometric condition for pinned-guided shaft element have been taken into account to develop the governing equation of motion. The closed-form mathematical expressions have been derived for determining both linear and nonlinear natural frequencies and their behavioral patterns have been simultaneously demonstrated through time histories, FFTs and Poincare's maps upon changing the control parameters. The nonlinear natural frequencies have been found to be higher by about 4–6% as compared to the findings obtained via linear analysis. For lower spin speed of the shaft, both linear and nonlinear forward natural frequencies have been found to be dominant. The influence of rotary inertia on natural frequencies by considering Euler and Rayleigh beam elements, separately has been reported. The system response has been observed to be either periodic or quasi-periodic depending on the spin speed. Outcomes which were not explored earlier enable significant theoretical understanding of free vibration analysis and whirling speeds of rotating system which are of great practical importance for investigating further dynamic performance. © 2017