Browse Items (1473 total)
Sort by:
-
Study of Classical Be Stars in Open Clusters in the Galaxy
Classical Be stars (CBe) in open clusters older than 100 Myr are identifed. The objective is to study their characteristics relating to age, spectral type and evolutionary state. This study compliments with that of Mathew et al. (2008), where they identifed and characterized emission-line stars in open clusters younger than 100 Myr. For this research, we surveyed a sample of newline71 open clusters older than 100 Myr using the slitless spectroscopy technique, with the Himalayan Chandra Telescope (HCT). From the survey we identifed 15 CBe stars from the 11 open clusters older than 100 Myr. Out of 15 CBe stars, 13 CBe stars show Hand#945; in emission. Among these CBe stars, TYC 2679-432-1 is a new detection, belonging to Berkeley 50 open newlinecluster. Two more CBe stars, out of 15 CBe stars, are found to exhibit Hand#945; in absorption for the frst time. This suggests that they might be passing through a disc-loss episode (or transient phase). Apart from our survey of 71 old open clusters, we found additional 16 Be stars from 6 open clusters older than 100 Myr in the literature (which was not observed in the slitless survey due to observation limitations). In addition to HFOSC instrument mounted on HCT, we used Opto Mechanics Research (OMR) spectrograph newlinemounted on 2.3-m Vainu Bappu Telescope (VBT) for taking the slit spectra of CBe stars. We performed the optical spectroscopy of 16 classical Be stars in 11 open clusters older than 100 Myr. Among these 16 stars, 15 newlinewere identifed from our previous slitless spectroscopic study. The remaining one, LS III +47 37b is a new detection, which is confrmed through the present study. Our analysis also suggests that one out of these 16 stars, [KW97] 35-12 might be a weak Hand#945; emitter in nature showing Hand#945; EW of -0.5 Moreover, it is found that the Hand#945; EW for 15 stars is lt -40 in agreement with previous works. Interestingly, we did not observe any FeII emission line or even the OI 8446 emission feature in any of our sample stars. -
-
-
Study of Early-Type Emission -Line Stars in the Galaxy Using Lamost and GAIA
The study of massive emission-line stars (ELS), particularly those classifed as O, B, and A spectral types, is crucial in advancing our understanding of various fundamental aspects of astrophysics. They provide valuable insights into the accretion processes, the physical conditions of material around the star, and the stellar evolution. The frst theme of thesis focuses on identifying and characterising hot ELS newlinefrom large spectroscopic surveys such as LAMOST and Gaia DR3. Utilising the low-resolution spectroscopic database provided by LAMOST DR5, we employed an automated Python routine and identifed 3339 unique sources encompassing 4138 spectra that exhibited Hand#945; in emission. Since both HAeBe stars and Classical Ae/Be stars show Hand#945; emission, we differentiated them in terms of IR excess, from the analysis of 2MASS and WISE photometry. This analysis led to the identifcation of 1089 Classical Be stars, 233 Classical Ae stars, and 56 Herbig Ae/Be (HAeBe) stars. This homogeneous emission-line spectra will facilitate detailed investigations of the emission phenomenon, removing concerns related to data compilation newlinebiases from diverse sources. We also compared the astrophysical parameters obtained from Gaia DR3 with those derived from LAMOST spectra. The emission-line source classifcations in Gaia DR3 were reasonably consistent with those in the LAMOST OBA emission catalogue. Additionally, newlinewe used a larger dataset of ELS to update the empirical relation for converting pseudo-equivalent width to observed equivalent width in Gaia DR3, employing a second-degree polynomial relation and piece-wise linear ft newlineparameters. As the second theme of the thesis, we examined the X-Shooter spectral newlinedatabase to study the circumstellar medium of HAeBe stars through various spectral features and near-infrared excess. We observed that the ma-jority of massive and young stars exhibited emission in all higher-order HI newlinelines, while stars displaying only lower-order lines had effective temperatures below 12000 K and ages ranging from 5 to 10 Myr. -
Study of effect of modulations on the onset of rayleigh benard convection in a couple stress fluid
In this thesis we study the linear and non-linear analyses of Rayleigh-Benard convection in a couple stress fluid. The effect of rotational modulation, temperature modulation and gravity modulation in the presence of external constraints like magnetic field and electric field are studied. The problems investigated in this thesis throw light on externally controlled convection in a couple stress fluid. The problems investigated in this thesis have possible applications in geophysics, astrophysics, oceanography engineering and experiment/ space situations with g-jitter connected with gravity simulation studies. with this motivation, we investigate in this thesis four problems and, their summary is given below one by one. (i)Linear and non-liner analyses of rotational modulation on Rayleigh-Benard convection in a couple stress fluid. The linear and non-linear analyses of Rayleigh-Benard convection in a couple stress fluid with rotational modulation is studied. The linear and non-linear analyses are, respectively based on normal mode technique and truncated representation of fourier series. The expression for Rayleigh number and correction Rayleigh number are obtained using regular perturbation method in the case of liner theory. The resulting no-autonomous lorenz model obtained in no-linear analysis is solved numerically using the Runge-Kutta-Fehlberg45 method to quantify the heat transport. The effect of rotational modulation is shown to be stabilizing there by leading to a situation of reduced heat transfer. The problem suggests an elegant method of controlling internal convection.(ii) Linear and non-linear analyses of gravity modulation on Rayleigh-Benard convection in a weakly electrically conducting couple strss fluid.The effect of time-periodic body force on the onset of Rayleigh-Benard convection in weak electrically conducting couple stress fluid is investigated. The stability of the horizontal fluid layer heated from below is examined by assuming time periodic body acceleration. -
STUDY OF EFFECT OF NON-LINEAR TEMPERATURE PROFILE AND CORIOLIS FORCE ON THE ONSET OF RAYLEIGH-B??NARD- MARANGONI MAGNETO CONVECTION
The effects of non-linear temperature gradients and rotation on the onset of Rayleigh ?? Benard ?? Marangoni magneto ?? convection in an electric conducting Newtonian fluid are studied using the Galerkin technique. In the case of Rayleigh-Benard convection, the eigenvalue is obtained for free-free, rigid-free and rigid-rigid velocity boundary combinations with isothermal and adiabatic temperature conditions on the boundaries. In the case of Marangoni and Rayleigh-Benard-Marangoni convection the eigenvalues are obtained for an upper free / adiabatic and a lower rigid / isothermal boundaries. A linear stability analysis is performed. The influence of various parameters on the onset of convection has been analyzed. The four non-linear temperature profiles are considered and their comparative influence on onset is discussed. -
Study of linear and non-linear analyses of rayleigh-benard chandrasekhar convection in micropolar fluid with saturated porous medium
Thermal instability of Chandrasekhar convection in a micropolar fluid, saturated porous layer is being investigated in this study. The model is confined between two horizon- tal plates of infinite length and separated by a fixed distance. A uniform temperature gradient is provided at the lower plate, while the upper plate is cooled. The study is analysed for both linear as well as non-linear cases to understand the stability, transfer of heat and mass within the model. The study is carried out in extending the model for Rayleigh-Benard and double diffusive convection with external constraints such as in-ternal heat, concentration source, gravity modulation, temperature modulation, coupled cross diffusion and throughflow. The outcome of the study is discussed in detail and presented pictorially through graphs. Linear and Non-linear Analyses of Double Diffusive Chandrasekhar Convection with Heat and Concentration Source in Micropolar Fluid with Saturated Porous Media under Gravity Modulation. A thermosolutal convective study is carried out in the presence of externally imposed magnetic field and gravity modulation together with heat and concentration source in a micropolar fluid. This problem is analysed for both linear and non-linear cases by as- suming the strength of heat and concentration source same. The expression for critical thermal Rayleigh number and correction thermal Rayleigh number are obtained using regular perturbation method to understand the stability of problem. The transfer of heat and mass transport is investigated by deriving the eighth order Lorentz equation. It is found that internal Rayleigh number and Darcy number speeds up the onset of instabil-ity while the coupling parameter and Chandrasekhar number has a reverse effect. Also, the transfer of mass is more compared to the transfer of heat. Linear and Non-linear Analyses of Double Diffusive Chandrasekhar Convection Coupled with Cross-diffusion in Micropolar Fluid over Saturated Porous Medium The problem aims to find the effects of coupled cross-diffusion in a micropolar fluid over a porous medium, subjected to double-diffusive-Chandrasekhar convection. The usual stability analysis has been employed to determine the critical thermal Rayleigh number. Non-linear analysis is carried out by deriving the Lorentz equations using trun- cated Fourier series representation. Heat and Mass transport are quantified by Nusselt and Sherwood number respectively. Analysis related to the effect of various parameter are plotted and the result for the same are interpreted. As Dufour parameter increases, the diffusion of heat and solute takes place which increases the temperature difference and thereby delays the onset of convection by making the system stable. It is observed from the results that Dufour parameter and Soret parameter have opposite influence on the stability of the problem. Linear and Non-linear Analyses of Throughflow Effect on the Onset of Rayleigh-Benard-Chandrasekhar Convection in Micropolar Fluid with Porous Medium. The Rayleigh-Benard convection over a porous medium saturated with micropolar fluid is simulated via externally imposed magnetic effect. The stability of the system is ex- amined using linear analysis by the method of normal mode. The rate of heat and mass transport within the system is examined by deriving sixth order Lorentz equation using non-linear analysis. It is observed that by regulating the throughflow effect, the con- vection of the system can be controlled. Also, the pro-gravity and anti-gravity cases has a stabilizing effect on the system. From mathematical calculation, it can be seen that heat transfer rate is same for both pro-gravity and anti-gravity cases, because of the symmetric nature of the boundaries considered. Linear and Non-linear Analyses on the Effect of Time Periodic Boundary Temper- ature and Internal Heat Source in a Micropolar Fluid on the Onset of Rayleigh- Benard-Chandrasekhar Convection with Porous Medium The impact of temperature modulation at the boundaries over a surface containing voids that is soaked in micropolar fluid is investigated to understand the thermal instability of the convection exposed to magnetic effect and internal heating of the system. A small amplitude of perturbation is given to the system to analyse the thermal instability by performing a linear study through the method of regular perturbation, which yields Rayleigh number for unmodulated and modulated system. Three cases of study is car-ried out to inspect the vibrating temperature field at the boundaries, namely symmetric case where the temperature is modulated in-phase, asymmetric case where the temper-ature is modulated out-phase and the case where only the bottom wall temperature is modulated. The non-linear analysis is employed to find the heat in the system, using Lorentz model. The outcome of the study conveys that sub critical motion occurs dur- ing in-phase modulation while the out-phase modulation leads to a more stable system. Moreover, internal Rayleigh number hastens the onset of convection. -
Study of linear and non-linear analyses of Rayleigh-Benard-Chandrasekhar convecion in micropolar fluid with saturated porous medium
Thermal instability of Chandrasekhar convection in a micropolar fluid, saturated porous layer is being investigated in this study. The model is confined between two horizontal plates of infinite length and separated by a fixed distance. A uniform temperature gradient is provided at the lower plate, while the upper plate is cooled. The study is analysed for both linear as well as non-linear cases to understand the stability, transfer of heat and mass within the model. The study is carried out in extending the model for Rayleigh-Benard and double diffusive convection with external constraints such as in- ´
ternal heat, concentration source, gravity modulation, temperature modulation, coupled cross diffusion and throughflow. The outcome of the study is discussed in detail and presented pictorially through graphs. -
Study of Low-Mass Stars and Brown Dwarfs In Star Forming Regions of Diverse Environments
The formation and evolution of low-mass stars and brown dwarfs is an intricate process orchestrated by the environmental conditions in which they form. As a natural byproduct of this process, circumstellar disks are formed, whose dynamic relationship with the environment plays a pivotal role in determining the fate of the star and the timescale for planet formation. While low-mass stars are a dominant product of the star formation process, brown dwarfs occupy a unique position, bridging the gap between newlinelow-mass stars and planets. In this thesis, we have examined the low-mass stars and brown dwarfs in young clusters located at different distances with diverse UV radiation felds and stellar densities to decode the role of environment in shaping the IMF, its inand#64258;uence on disk evolution and to understand the formation of brown dwarfs. We have targeted three young star forming regions for the newlinestudy namely and#963; Orionis, twin clusters IC 1848-East and West, and IC 1396 using deep multi-wavelength photometry as well as near-IR spectroscopy with 4m and 10m class facilities. For the and#963; Orionis cluster we used deep CFHT-WIRCAM near-IR data and the novel water-band photometry technique along with Gaia DR3 to identify the candidate low-mass members. Spectroscopic follow-up observations conducted with IRTF-SpeX validated the selection of the candidates by this technique with a 100% effcacy. We then compiled a comprehensive membership catalog for a mass range and#8764;19-0.004 Mand#8857;. The form of the stellar-substellar IMF was found to be consistent with other nearby star forming regions suggesting a lack of signifcant environmental inand#64258;uence. Further, we analysed the evolutionary class of the members based on the mid-IR slope of the SED to study their disk properties. We estimated the disk fraction of the low-mass sources to be consistent with other star form ing regions considering the age of the cluster. This showed that in this region, external photoevaporation does not play a major role in the inner disk evolution. -
Study of magnetoconvection with maxwell cattaneo law
This thesis deals with the study of Rayleigh-Bard-convection in a Newtonian fluid and micropolar fluid by replacing the classical Fourier law by non-classical Maxwell-Cattaneo heat flux law. The effects of second sound, non-uniform basic temperature gradients, suctioninjection-combination, temperature modulation and gravity modulation in newlinepresence of external constraints like magnetic field and rotation are studied. newlineThe problems investigated in this thesis throw light on externally controlled convection in Newtonian and micropolar fluids in the presence of Maxwell-Cattaneo law. The problems investigated in this thesis deal newlinewith practical problems with very large heat fluxes and/or short time duration. With this motivation, we investigate in this thesis five problems and their summary is given below. (i) Effects of Coriolis force and non-uniform basic temperature gradients on the onset of Rayleigh-Bard-Chandrasekhar newlineconvection with Maxwell-Cattaneo law The effect of non-uniform temperature gradient on RayleighBard-Chandrasekhar convection in a rotating Newtonian fluid with Maxwell-Cattaneo law is studied using the Galerkin technique. The eigenvalues is obtained for free-free, rigid-free and rigid-rigid velocity boundary combinations with isothermal and adiabatic boundaries. A linear stability analysis is performed. The influence of various parameters on the onset of convection has been analyzed. One linear and five non-linear temperature profiles are considered and their comparative influence on onset is discussed. It is found that the results are noteworthy at short times and the critical eigenvalues are less than the classical ones. It is shown that the system having magnetic field will delay in the onset newlineof instability. In general, it is observed that step function and inverted parabolic temperature profile are the most destabilizing and stabilizing profiles. -
Study of magnetoconvection with maxwell cattaneo law /
This thesis deals with the study of Rayleigh-Bénard-convection in a Newtonian fluid and micropolar fluid by replacing the classical Fourier law by non-classical Maxwell-Cattaneo heat flux law. The effects of second sound, non-uniform basic temperature gradients, suctioninjection-combination, temperature modulation and gravity modulation in
presence of external constraints like magnetic field and rotation are studied. The problems investigated in this thesis throw light on externally controlled convection in Newtonian and micropolar fluids in the presence of Maxwell-Cattaneo law. The problems investigated in this thesis deal with practical problems with very large heat fluxes and/or short time duration. With this motivation, we investigate in this thesis five problems and their summary is given below. (i) Effects of Coriolis force and non-uniform basic temperature gradients on the onset of Rayleigh-Bénard-Chandrasekhar convection with Maxwell-Cattaneo law The effect of non-uniform temperature gradient on RayleighBénard-Chandrasekhar convection in a rotating Newtonian fluid with
Maxwell-Cattaneo law is studied using the Galerkin technique. The eigenvalues is obtained for free-free, rigid-free and rigid-rigid velocity boundary combinations with isothermal and adiabatic boundaries. A linear stability analysis is performed. The influence of various parameters on the onset of convection has been analyzed. One linear and fiveix non-linear temperature profiles are considered and their comparative influence on onset is discussed. It is found that the results are noteworthy
at short times and the critical eigenvalues are less than the classical ones. It is shown that the system having magnetic field will delay in the onset of instability. In general, it is observed that step function and inverted parabolic temperature profile are the most destabilizing and stabilizing profiles. The range of values of the parameters of the problem for which oscillatory convection in the case of free-free isothermal boundary exists is also discussed. (ii) The effect of temperature modulation on the onset of RayleighBénard-Chandrasekhar convection using Maxwell-Cattaneo law The effect of imposed time-periodic boundary temperature (ITBT, also called temperature modulation) and magnetic field at the onset of Rayleigh-Bénard convection is investigated by making a linear analysis. The classical Fourier heat law is replaced by the non-classical MaxwellCattaneo law. The venezian approach is adopted in arriving at the critical Rayleigh number and wave number for small amplitude of ITBT. Three
cases of oscillating temperature field are examined: (a) symmetric, so that the wall temperatures are modulated in-phase, (b) asymmetric, corresponding to out-of-phase modulation and (c) only the lower wall is modulated. The temperature modulation is shown to give rise to sub-critical motion. The shift in the critical Rayleigh number is calculated
as a function of frequency and it is found that it is possible to advance or delay the onset of convection by time modulation of the wall temperatures. It is shown that the system is more stable when the boundary temperatures are modulated out of phase.x
(iii) The effect of gravity modulation on the onset of RayleighBénard-Chandrasekhar convection using Maxwell-Cattaneo law The effect of gravity modulation and magnetic field at the onset of Rayleigh-Bénard-Chandrasekhar convection is investigated by making a regular perturbation technique. The stability of the horizontal fluid layer heated from below is examined by assuming time-periodic body acceleration called g-jitter, which normally occurs in satellites and in vehicles connected with microgravity simulation studies. The venezian
approach is adopted in arriving at the critical Rayleigh number and wave number for small amplitude of gravity modulation. The shift in the critical Rayleigh number is calculated as a function of frequency of modulation. It is observed that gravity modulation leads to delayed convection. (iv) The effect of suction-injection-combination (SIC) on the onset of Rayleigh-Bénard-Chandrasekhar convection in a micropolar fluid with Maxwell-Cattaneo law The effect of suction-injection-combination (SIC) on the onset of Rayleigh-Bénard-Chandrasekhar convection in a micropolar fluid with Maxwell-Cattaneo law is studied using the Galerkin technique. The eigenvalue is obtained for free-free, rigid-free and rigid-rigid velocity boundary combinations with isothermal and adiabatic on the spinvanishing boundaries. A linear stability analysis is performed. The influence of various micropolar fluid parameters on the onset of convection has been analyzed. It is found that the effect of Prandtl number on the stability of the system is dependent on the SIC beingxi pro-gravity or anti-gravity. A similar Pe-sensitivity is found in respect of the critical wave number. The problem suggests an elegant method of external control of internal convection. (v) The effect of non-uniform temperature gradients on RayleighBénard-Chandrasekhar convection in a micropolar fluid with
Maxwell-Cattaneo law The effect of non-uniform temperature gradient on RayleighBénard-Chandrasekhar convection in a micropolar fluid with MaxwellCattaneo law is studied using the Galerkin technique. The eigenvalue is obtained for free-free, rigid-free and rigid-rigid velocity boundary combinations with isothermal and adiabatic on the spin-vanishing boundaries. A linear stability analysis is performed. The influence of various micropolar fluid parameters on the onset of convection has been
analyzed. Six different non-uniform temperature profiles are considered their comparative influence on onset is discussed. It is observed that the micropolar fluid layer heated from below is more stable compared to the classical Newtonian fluid layer. -
Study of Nanolayered Structure of Commercially Available Carbon Materials and Soot
Developments in the modern world periodically call for the discovery or invention of new and exotic materials. In the present situation, to develop unique and novel materials, which move beyond the barriers of the physical limits of the amount of micro- miniaturization possible as well as the current technology and take advantage of the opportunities not yet imagined, is not at all a need but a necessity. The advent of Nano technology of carbon allotropes is a giant leap towards this goal. The starting of the era of carbon nanomaterials traces back to 1985 when the fullerenes with a foot ball structure were accidently discovered. From then on, the field of carbon nanotechnology was in the constant limelight on account of the amazing properties displayed by the various allotropes of carbon. These properties are dependent mainly on the type of hybridization present in the nanostructures, which categorizes them to amorphous or crystalline. Also, there exist some structures which are the combination of these two and are termed as nanocrystalline or turbostratic structures. The discovery of graphene, which has a turbostratic structure and is the thinnest material known and the strongest ever measured, with outstanding properties such as highest room temperature electrical conductivity; high mechanical robustness etc was a ground breaking one. These remarkable properties open up a wide range of potential applications ranging from clean energy to nano-electronics to bio-medical devices. Thus, it is a necessity to explore and characterize various effective sources of these nanomaterials. The present study is an attempt to investigate such efficient, easily available and cost-effective precursors. Soot, also known as black carbon, is a fine-grained solid residue that results from incomplete combustion of hydrocarbons and is a widely used precursor for the production of carbon nanomaterials. Carbon soot is a major component of smoke from the combustion of carbon-rich organic fuels and hydrocarbons and hence has a vast number of sources. In the study presented here soot obtained from the thermal decomposition of commercially available kerosene, diesel oil, paraffin wax and lubricant oil is investigated. Nanostructure of the commercially available carbon black is also studied. Various techniques such as Micro Raman spectroscopy, Fourier Transform Infrared Spectroscopy (FT-IR), X-ray diffraction (XRD), High Resolution Scanning Electron Microscopy (HR-SEM), Electron dispersive spectroscopy (EDS) and elemental analysis are employed for the structural and morphological characterization of the samples. Raman scattering is used as a probe to study the disorder in the carbon skeleton materials. The intensity ratio of the D and G modes occurring in the spectra is proportional to the number of rings at the edge of the grain and also indicates the quality of the sample. FT-IR spectroscopy is used to characterize qualitatively the functional groups of carbon materials. XRD is the most common analytical technique used for determining the structure of ordered and disordered carbons from the positions of the diffraction peaks at 2?? angle. The structural parameters like the size of the ordered grains along c and a axis (Lc and La), the average spacing of the crystallographic (002) planes (d002) can be determined through Scherrer equations. SEM micrographs give the surface morphology of the nanomaterials present and the EDS analysis gives the abundance of the microscopic constituents. Elemental composition of the samples can be derived from the elemental analysis using CHNS (Carbon Hydrogen Nitrogen Sulphur) analyser. The present study shows that all the samples investigated obeys the Tuinstra-Koening relation and posses a nanocrystalline structure. The ratio of the defect and graphite bands is found to be very low, especially in the case of diesel soot which has a value very much lower than those reported in the earlier studies, indicating high quality and a low amount of disorder in the samples. HR-SEM micrographs clearly indicate that the carbon nanostructured present in the samples are in the form of non-uniform nanospheres with diameter varying between 26-100 nm. The characteristic diffraction peak of graphene corresponding to (100) diffraction is observed in the x-ray diffraction profiles of all the samples. The interlayer spacing determined in all the samples lies very close to that of graphite. The H/C atomic ratio from the CHNS analysis is found to be very low and confirms the nanocrystalline structure of the materials. The graphite band position in the IR spectra indicates that the nanospheres formed are to be composed more of crystalline graphitic carbon. From the EDS analysis it is evident that all the samples have very high carbon content and are free from impurities and thus concludes that the materials and methods used in the present study for the synthesis of carbon nanospheres possessing a nanocrystalline structure are efficient and cost effective and are good precursors for graphene. -
Study of Rayleigh-Benard Dynamical System Involving Newtonian and Nanofluids in Rectangular and Cylindrical Enclosures
Analyzing and#64258;uid and#64258;ow behavior in the presence of temperature gradients subjected to internal and external forces in diand#64256;erent geometries is essential for optimization newlineprocesses for various engineering applications, guiding the design of more efcient thermal systems. This thesis focuses on investigating the Rayleigh-Bard convection problems occupying rectangular and cylindrical enclosures. The linear and newlineweakly nonlinear analyses are carried out that reveal the results on regular convection, heat transport and chaotic motion for each of the problems. Steady and newlineunsteady states of the Rayleigh-Bard system are studied using the Lorenz model. The dynamical system is investigated to look for possible chaotic motion. Fluid systems can exhibit chaotic behavior, and understanding the chaotic nature of these and#64258;ows is essential for accurate predictions of their evolution over time. In view of this, the regular, chaotic, and periodic natures of the dynamical system is thoroughly analyzed. Further, the inand#64258;uence of various parameters on the indicators of chaos is explored. Additionally, the thermal performance of the system is looked into by introducing nanoparticles/nanotubes into the base and#64258;uid. newlineWith the aformentioned motivation, we now present the abstract of each of the problems considered in this thesis one-by-one. 1. Impact of boundary conditions on Rayleigh-Bard convection: stability, heat transfer and chaos In the frst problem of the thesis, discussed in Chapter 3, a comparison is made newlinebetween the results of Rayleigh-Bard convection problem for diand#64256;erent boundary combinations, namely, rigid-rigid-isothermal, rigid-free-isothermal and free-free isothermal boundaries for a Newtonian and#64258;uid. The linear and weakly-nonlinear analyses reveal that the onset of regular and chaotic motions in the case of rigid-freeisothermal boundaries happens later than that of free-free isothermal boundaries but earlier than rigid-rigid-isothermal boundaries.+ -
Study of single and two component convection in micropolar liquid
In this thesis, we study linear and non-linear analysis of RayleighBard and double diffusive convection in a micropolar liquid. The effect of non-uniform basic temperature gradient, non-uniform basic concentration gradient, temperature modulation at the boundary and gravity modulation are studied. newlineThe problem investigated in this thesis through a light on externally controlled internal convection in a micropolar liquid. The problems investigated in this thesis have possible application in geophysics, newlineastrophysics, oceanography engineering and in space situations with gjitter connected with gravity stimulation study. With this motivation, we investigate in this thesis four problems and their summary is given below one by one. (i) EFFECT OF GRAVITY MODULATION ON HEAT TRANSFER BY RAYLEIGH-BARD CONVECTION IN A MICROPOLAR LIQUID newlineThe vertical oscillation, or g-jitter or gravity modulation, is known to appear in the situation of the satellite. In the laboratory, Rayleigh-Bard system subjected to time-periodic vertical oscillations may be useful in regulating the onset of convection and heat transfer. This aspect is also in newlinefocus in the thesis. In this problem the effect of time-periodic body force or grtavity modulation on the onset of Rayleigh-Bard convection in a micropolar liquid is investigated. The linear and non-linear analyses are performed. The linear theory is based on normal mode analysis and perturbation method. The expression for correction Rayleigh number is obtained as a function of frequency of modulation and other micropolar liquid parameters. The non-linear analysis is based on the truncated Fourier series representation. The resulting non-autonomous Lorenzvii model is solved numerically to quantify the heat transport. It is observed that the gravity modulation leads to delayed convection and reduced heat newlinetransfer. (ii) LINEAR AND WEAKLY NON- LINEAR STABILITY ANALYSIS OF DOUBLE-DIFFUSIVE CONVECTION IN A MICROPOLAR LIQUID. -
Study of single and two component convection in micropolar liquid /
In this thesis, we study linear and non-linear analysis of RayleighBénard and double diffusive convection in a micropolar liquid. The effect of non-uniform basic temperature gradient, non-uniform basic concentration gradient, temperature modulation at the boundary and gravity modulation are studied.
The problem investigated in this thesis through a light on externally controlled internal convection in a micropolar liquid. The problems investigated in this thesis have possible application in geophysics, astrophysics, oceanography engineering and in space situations with gjitter connected with gravity stimulation study. With this motivation, we investigate in this thesis four problems and their summary is given below one by one.
(i) EFFECT OF GRAVITY MODULATION ON HEAT
TRANSFER BY RAYLEIGH-BÉNARD CONVECTION IN A
MICROPOLAR LIQUID The vertical oscillation, or g-jitter or gravity modulation, is known to appear in the situation of the satellite. In the laboratory, Rayleigh-Bénard system subjected to time-periodic vertical oscillations may be useful in regulating the onset of convection and heat transfer. This aspect is also in
focus in the thesis. In this problem the effect of time-periodic body force or grtavity modulation on the onset of Rayleigh-Bénard convection in a micropolar liquid is investigated. The linear and non-linear analyses are performed. The linear theory is based on normal mode analysis and perturbation method. The expression for correction Rayleigh number is obtained as a function of frequency of modulation and other micropolar
liquid parameters. The non-linear analysis is based on the truncated Fourier series representation. The resulting non-autonomous Lorenzvii model is solved numerically to quantify the heat transport. It is observed that the gravity modulation leads to delayed convection and reduced heat transfer.
(ii) LINEAR AND WEAKLY NON- LINEAR STABILITY
ANALYSIS OF DOUBLE-DIFFUSIVE CONVECTION IN A
MICROPOLAR LIQUID The linear and non-linear stability analysis of double diffusive convection in a micropolar liquid layer heated and saluted below and cooled from above is studied. The linear and non-linear analyses are respectively based on normal mode technique and truncated
representation of Fourier series. The influence of various parameters on the onset of convection has been analyzed in the linear case. The resulting autonomous Lorenz model obtained in non-linear analysis is solved numerically to quantify the heat and mass transforms through Nusselt and Sherwood number. It is observed that the increase in coupling parameter, micropolar heat conduction parameter and solutal Rayleigh number
increases the heat and mass transfer. (iii) THE EFFECT OF NON - UNIFORM TEMPERATURE / CONCENTRTION DISTRIBUTION ON THE ONSET OF DOUBLE-DIFFUSIVE CONVECTION IN A MICROPOLAR LIQUID The effect of non-uniform temperature/concentration distribution on the onset of double diffusive convection in a micropolar liquid layer heated and soluted below and cooled from above between two parallel
plates of infinite extend separated by a thin layer is studied using linear stability analysis based on normal mode technique. The eigen value is obtained for free-free, rigid-free, rigid-rigid, velocity boundary conditions with isothermal temperature boundary conditions using Galerkian method. It is observed that by choosing the appropriate non-uniformviii temperature or concentration gradient it is possible to advance or delay
the onset of double diffusive convection. (iv) EFFECT OF TEMPERATURE MODULATION ON THE ONSET OF DOUBLE – DIFFUSIVE CONVECTION IN A MICROPOLAR LIQUID
The effect of temperature modulation on the onset of double-diffusive convection in a micropolar liquid is investigated by making a linear stability analysis. The stability of a horizontal layer of fluid heated from below is examined when, in addition to a steady temperature difference between the walls of the layer, a time-periodic sinusoidal perturbation is applied to the wall temperatures. The Venezian approach is adopted in arriving at the critical Rayleigh and wave numbers for small amplitude
temperature. -
-
Study of substituion effectson structure and properties of high temperature superconductors and isostructure compounds
The thesis mainly describes the investigation of the structural formation of higher order members of bismuth system of superconductors Bi1.6sPb0.35Sr2CazCu4Oy (n = 4, 2234 phase), Bi1.6sPb0.35Sr2Ca4CusOy (n = 5, 2245 phase) and Bi1.65Pb0.35Sr2CasCu,Oy (n = 9, 2289 phase). The samples were synthesized by solid state reaction technique. Micro-structural and morphological features of the synthesized samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDS). The XRD pattern of all the samples revealed the presence of Bismuth 2212 and 2223 phases of which 2223 phase was found to be the predominant. Superconducting transition temperature (Tc) of the samples measured by self-inductance method and dc four probe method showed Tc value around 110 K. There was no signature of the formation of 2234, 2245 or 2289 phases in this investigation. newlineFormation of Bi-2245 compound was further investigated by preparing the sample in a new matrix route. The Tc on set of this sample was found to be 127 K which was the highest reported ever in bismuth system superconductors. The complete replacement of copper by nickel in bismuth system superconductor Bi2SraCu06 (2201) was ttempted by preparing the sample in air by solid state reaction method under open and closed environment. Morphological and microstructural features of the synthesized sample Bi2Sr2Ni0g was investigated by X-ray diffraction, SEM and EDAX. The analysis of X-ray diffraction pattern revealed that nickel can replace copper completely and form a single phase Bi2Sr2Ni06 only when prepared in a closed environment in air. This phase formation of BizSr2NiOo was reported first time. -
Study of substitution effects on structure and properties of high temperature superconductors and isostructure compounds
The branch of physics which deals with the properties of solid materials and their constituent particles such as protons, neutrons and electrons is known as solid state physics. There are wide ranges of physical properties of solids. Some of the materials are very good conductors of heat and electricity whereas others are bad conductors. -
Study of suction and injection combination on the onset of double-diffusive convection in a micropolar fluid
The effect of suction and injection combination (SIC) on the onset of double-diffusive convection in a micropolar fluid is studied us- ing linear stability theory. The eigenvalue is obtained for free-free, rigid-free, rigid-rigid velocity boundary combinations with isother- mal or adiabatic on the spin-vanishing boundaries numerically using Galerkin Technique. The influence of various micropolar fluid param- eters on the onset of convection has been analyzed. It is found that by adjusting SIC it is possible to control the double-diffusive convection in a micropolar fluid. It is also observed that the effect of Prandtl number on the stability of the system is dependent on the SIC being pro-gravity or anti-gravity.