Department of Mechanical Engineeringhttp://gyan.iitg.ernet.in/handle/123456789/342022-01-22T01:06:00Z2022-01-22T01:06:00ZDevelopment of compact schemes for the biharmonic form of Navier-Stokes equations on compact nonuniform grids without transformationKumar, Pankajhttp://gyan.iitg.ernet.in/handle/123456789/19642021-11-26T21:38:44Z2021-01-01T00:00:00ZDevelopment of compact schemes for the biharmonic form of Navier-Stokes equations on compact nonuniform grids without transformation
Kumar, Pankaj
This work is concerned with developing compact finite difference schemes for simulating incompressible viscous flows governed by the Navier-Stokes (N-S) equations on non uniform grids without transformations. The main work is concerned with developing two efficient schemes, one for the steadystate form of the N-S equations and the other one, is for its transient counterpart. They are developed for the the pure stream function formulation of the N-S equations, and second order accurate in both time and
space. Apart from simulating several complex fluid flow situations for validating our schemes, emphasis is given more on simulating flow past bluff bodies, which finds wide ranging applications in laboratories and industries. In the process, numerical rate of convergence of the schemes and grid independence of the computed solutions are also established. Comprehensive analysis is carried out for the flow past stationary, rotating and oscillating circular cylinders in uniform flows for moderate Reynolds numbers (Re) and starting flow for the stationary case for high Res. For the stationary case, high quality simulations are accomplished for a wide range of Res ranging from
0 4 10 £ Re £10 in the laminar regime, including the periodic flow characterized by von Kàrmàn vortex street. The α, β, sub-α and sub- β phenomena, which are the trademark of the secondary and tertiary vortex dynamics associated with such flows, are studied in detail. However, the highlight of the thesis is the simulation of flow past sharp edges in uniform and accelerated flows, where we have considered the flat plate and a suspended wedge as our test cases. For the flat plate, several modes of vortex shedding patterns are established through a comprehensive FFT analysis. For the wedge, onset of turbulence is verified by the presence of coherent and three-fold structures in the flow. Moreover, the proposed schemes have also been utilized to accurately simulate heat transfer problems, including conjugate heat transfer problems in suddenly expanding channels. In all the cases, our numerical results are seen to be extremely close to the available numerical and experimental results.
Supervisor: Jiten C Kalita
2021-01-01T00:00:00ZModel based Analysis and Identification of Unbalance and Misalignment in Rotor Systems Levitated by Active Magnetic BearingsKumar, Prabhathttp://gyan.iitg.ernet.in/handle/123456789/19482021-11-26T21:39:09Z2021-01-01T00:00:00ZModel based Analysis and Identification of Unbalance and Misalignment in Rotor Systems Levitated by Active Magnetic Bearings
Kumar, Prabhat
The work presented in the thesis emphasizes mainly on the simultaneous identification of unbalance and misalignment faults as well as AMB stiffness parameters in a magnetically levitated rigid and flexible rotor system using a novel trial misalignment approach with model based identification algorithm. The proposed approach is aligned with the similar concept of trial unbalance in the rotor balancing. In real practice, the known trial misalignment between the rotor operating axis and AMBs axis can be provided by two innovative techniques i.e., physical trial misalignment (PTM) and virtual trial misalignment (VTM). At first, a rigid rotor with a disc at the middle levitated by two identical parallel misaligned AMBs is mathematically modelled. It is based on two translational displacements at AMB locations. The linearized form of force due to misaligned AMBs for the case of residual and additional trial misalignments has been obtained. The second model consists of both parallel and angular misalignments in a rigid rotor with two offset discs supported on two different isotropic AMBs. It is based on two translational and two rotational displacements at the rotor center of gravity. The third model is a FEM based model for a multidisc flexible rotor system equipped with multiple (anisotropic and different) misaligned AMBs. The last model is an extension of third model with integration of misaligned sensors along with misalignments in AMBs in a flexible rotor system. Moreover, the first three model is relying on PTM approach, whereas the last model is based on more reliable VTM approach. Equations of motion of the different models are developed and numerically solved to get time domain displacements at various rotor positions and controlling current responses at AMB locations. Further, a fast Fourier Transform (FFT) technique is utilized to obtain frequency domain responses from time domain signals. The magnitude and corrected phase of the responses are computed to get the real and imaginary components of frequency based signals. These are given as input in the novel trial misalignment based identification methodology for quantitative estimation of unknown unbalance and AMB misalignment parameters in a rotor-AMB system. The rotor unbalance parameters (disc eccentricities and phases), AMB residual misalignments and their force-displacement and force-current stiffness constants are the identified parameters from developed estimation methodology. Additionally, the residual offsets of sensors located at AMB locations are also identified using VTM approach in the final model. The identification algorithm has been tested against various levels of measurement noise and modelling errors and found to be robust.
Supervisor: Rajiv Tiwari
2021-01-01T00:00:00ZModel based identification of internal and external damping in a cracked rotorRoy, Dipendra Kumarhttp://gyan.iitg.ernet.in/handle/123456789/19402021-09-03T11:09:01Z2020-01-01T00:00:00ZModel based identification of internal and external damping in a cracked rotor
Roy, Dipendra Kumar
The aim of the work is to develop an identification algorithm to estimate various rotor faults present in a rotor bearing system. The methodology considered both external and internal damping where the external damping mainly occurs due to bearings and the internal (rotating) damping, which appears due to rubbing of free surfaces and material hysteresis during shaft rotation in dynamic conditions. Internal damping is also one of the causes of instability in rotors, when the rotor system rotates above critical speeds. Experimental estimation of the internal damping is a challenging task as compared to the external damping in a cracked rotor system. Hence, the main contribution of the present work is to distinctly estimate the internal and external damping in a cracked rotor-bearing system both theoretically and experimentally.
In the process of development of identification procedure in the form of regression equation for the estimation of internal and external damping other important unknown system parameters, like the unbalance and loss of stiffness of shaft due to the crack, are also identified. This reduces the estimation error of main parameters to be estimated. For this purpose, initially a simple two and four degrees-of-freedom (DOFs) rotor model has been considered, which includes two translational co-ordinates and two translational as well as rotational co-ordinates respectively. In four DOFs modeling, effect of the gyroscopic couple is also considered due to an offset disc. Here, to overcome the practical difficulty of measuring rotational DOFs accurately, the dynamic condensation has been carried out in the development of identification algorithm. In both the modeling of the rotor system, the crack model is based on the assumption of a switching crack, which gives excitation at multiple harmonics both in forward and backward whirl. Full spectrum responses are obtained through fast Fourier transform (FFT) of numerically generated displacement responses and are utilized in the developed identification procedure to estimate rotor system parameters, which is found to be robust even in the presence of measurement noise in system responses and bias errors in system parameters. For the application of the developed identification procedure in the physical system, experimentally measured are processed through the same methodology as done in the numerically generated signal. Residual bow
in the shaft affects the 1X component of the frequency domain response (full spectrum), which is removed by utilizing slow run response of the rotor system. Moreover, ambiguity in phase while using FFT to get full spectrum is tackled by using multi-frequency reference signal. Estimates from experimental measurements are found to be consistent even at different rotor speeds.
Finally, to model a more realistic rotor system, the current work has been extended for cracked flexible-rotor flexiblebearing with multiple-disc rotor system using finite element modelling considering four-DOF per node. Herein, apart from internal damping, bearing stiffness and damping coefficients have also been estimated using the developed
identification algorithm with the help of full spectrum. To check the robustness of the algorithm random noise is added to the time domain responses and estimate the fault parameters associated with the rotor system.
Supervisor: Rajiv Tiwari
2020-01-01T00:00:00ZCharacteristics of cavities during start transient and established flow Conditions at supersonic mach numberPandian, Shttp://gyan.iitg.ernet.in/handle/123456789/19382021-09-03T11:08:47Z2020-01-01T00:00:00ZCharacteristics of cavities during start transient and established flow Conditions at supersonic mach number
Pandian, S
Supersonic combustion ramjet engine (scramjet) is being considered as a viable propulsion system for hypersonic cruise missiles, transport passenger vehicles for long-range operation, faster inter-continental travel and in the first stage of Two Stage To Orbit (TSTO) vehicle of low cost access to space. Some of the programs to demonstrate scramjet technology include Kholod of Russia, SCRAM, X43, X51 of USA, Hyshot and Scram Space Experiments by Australia. Recently, India has successfully demonstrated
scramjet powered flight technology in 2016. The major challenges in the design of scramjet engine are; cowl opening mechanism for air intake under high dynamic pressure, material selection to endure high temperature effects and supersonic combustion. Although the concept of scramjet engine appeared to be straightforward, achieving supersonic combustion remains a formidable task due to the presence of
chemical kinetics, high temperature and pressure, equivalence ratio, mixing rate, etc. Among which perhaps, mixing, flame holding and sustained combustion in high-speed supersonic flows, are the key problems due to compressibility effects. In order to overcome these challenges, several doable solutions have been proposed by many researchers amongst cavity assisted (with respect to length-to-depth ratio- L/D) supersonic combustion is proposed due to its simplicity and ability to reduce total pressure losses and drag as compared to other active or passive devices. For some L/D ratios, cavity shear layer is influenced by the acoustic feedback mechanism resulted in oscillation which can be used for efficient mixing whereas, in other L/D ratios, the cavities are acoustically stable with large recirculation zone that can be utilized for flame holding. In this background, it is proposed to concentrate on the geometrical modifications of the cavities to study either the mixing or the flame holding purposes and understand cavity flow physics. This is the theme of the thesis where the flow physics of various fundamental cavities are experimentally studied at supersonic speed. The core objective of the work involves fundamental studies of shallow rectangular cavities of various L/D ratios ranging from 1 to 10 to characterize its behavior at supersonic speed.
The background of the work is with respect to scramjet engine, its design issues and importance of cavity flow field in terms of mixing and flame holding at supersonic Mach numbers. With a detailed review of the literature pertaining to high speed mixing layer flows, different injection strategies, the cavity flow physics (in terms of oscillation and its suppression mechanisms) are presented. The fabrication of the convergent divergent nozzle, test section and various cavity geometries along with the calibration of nozzle along with the instrumentation, measurement schemes and the data reduction are integral part of experimental setup and data processing. The transient starting process and associated pressure spectra of the cavities along with mode switching phenomena are elaborately presented using Shadowgraph images and unsteady pressure sensors. The transitional cavities (1 ≤ L/D ≤ 3) are studied using time resolved Schlieren images (125000 fps with the shutter speed of 3.5 microseconds) and unsteady pressure measurements. The modes/tones from FFT are compared with modified Rossiter relations and the flow features around the cavity and their dynamics are captured. Further, shear layer vortex dynamics and wave propagation inside the cavity for an elapsed time of 8μs are studied. They are correlated to mode/tone of the cavity. From unsteady signals, coherence, cross correlation, spectrogram and wavelet transform are derived to understand the physics of the transitional cavities. The studies on the open
cavities (L/D = 4 and 5) deal with the characteristics such as vortex motion inside the cavity, waves dynamics as a consequence of shear layer vortex both inside and outside the cavity. The mode switching phenomena are studied through high-speed Schlieren images and unsteady pressure measurements. The studies further brought out the flow physics through derived parameters like coherence, cross correlation, spectrogram and wavelet transform. Similarly, flow features and analyzes of transition from open to close rectangular cavities (6 ≤ L/D ≤ 10) are studied and presented. The flow chart highlighting
the overview of the thesis is shown in following figure.
Supervisors: Niranjan Sahoo and Desikan SLN
2020-01-01T00:00:00Z