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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 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 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 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 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 Influence of Combustion on DarcyBard Convection with Inherent Local Thermal Non-equilibrium Between Phases
This work deals with a DarcyBard convection problem in the presence of combustion and with local thermal non-equilibrium between the fluid and the solid phases. The effects of combustion and local thermal non-equilibrium on the onset of convection is studied in the linear and nonlinear regimes. Unlike all reported local thermal non-equilibrium problems reported so far, the present problem has a unique situation of having thermal non-equilibrium not only in the perturbed state but also in the basic state. Further, we observe that local thermal non-equilibrium does not, under any circumstance, lead to local thermal equilibrium except in an approximate sense when the combustion is quite weak. The effect of combustion is to advance the onset of convection compared to that in its absence. The effect of local thermal non-equilibrium is to reinforce the effect of combustion. In the presence of both these effects, sub-critical instability exists. The results are obtained numerically and have implication in practical porous medium convection problems. 2022, The Author(s), under exclusive licence to Springer Nature B.V. -
Study of hybrid nanofluid flow in a stationary cone-disk system with temperature-dependent fluid properties
Cone-disk systems find frequent use such as conical diffusers, medical devices, various rheometric, and viscosimetry applications. In this study, we investigate the three-dimensional flow of a water-based Ag-MgO hybrid nanofluid in a static cone-disk system while considering temperature-dependent fluid properties. How the variable fluid properties affect the dynamics and heat transfer features is studied by Reynoldss linearized model for variable viscosity and Chiams model for variable thermal conductivity. The single-phase nanofluid model is utilized to describe convective heat transfer in hybrid nanofluids, incorporating the experimental data. This model is developed as a coupled system of convective-diffusion equations, encompassing the conservation of momentum and the conservation of thermal energy, in conjunction with an incompressibility condition. A self-similar model is developed by the Lie-group scaling transformations, and the subsequent self-similar equations are then solved numerically. The influence of variable fluid parameters on both swirling and non-swirling flow cases is analyzed. Additionally, the Nusselt number for the disk surface is calculated. It is found that an increase in the temperature-dependent viscosity parameter enhances heat transfer characteristics in the static cone-disk system, while the thermal conductivity parameter has the opposite effect. The Author(s) 2024. -
Study of generator shaft behaviour during subsynchronous resonance using finite element method
Scientific research in electric power stations includes various online monitoring and control of equipments. Turbine and generator plays a key role in generating power. Frequency response analysis of the shaft which connects turbine and generator is used to detect the steady state response. It will enable the user to understand and design the system in such a way that it can withstand resonance, fatigue and other vibrations. Subsynchronous resonance which arises during line compensation by series capacitors increases oscillations in the turbine generator shaft system. The oscillations developed at low frequency causes physical damage to the shaft. There are several real time monitoring of the rotor shaft and turbine shaft misalignment by using laser technologies. The aim of this research paper is to use frequency response and modal analysis technique to detect the stress in the shaft and improve the design of it. A viscous damper is designed in the 3D model at the point of highly stressed area to control the resonance effect caused by series capacitors. 2020, Levrotto and Bella. All rights reserved. -
Study of Effect of Vipassana on Anxiety and Depression
International Journal of Psychology and Behavioral Sciences, Vol-2 (6), pp. 274-276. -
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 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 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 correlation between optical flux and polarization variations in BL Lac objects
Polarized radiation from blazars is one key piece of evidence for synchrotron radiation at low energy, which also shows variations. We present here our results on the correlation analysis between optical flux and polarization degree (PD) variations in a sample of 11 BL Lac objects using ?10 yr of data from the Steward Observatory. We carried out the analysis on long-term (?several months) as well as on short-term time-scales (?several days). On long-term time-scales, for about 85 per cent of the observing cycles, we found no correlation between optical flux and PD. On short-term time-scales, we found a total of 58 epochs with a significant correlation between optical flux and PD, where both positive and negative correlation were observed. In addition, we also found a significant correlation between optical flux and ?-ray flux variations on long-term time-scales in 11 per cent of the observing cycles. The observed PD variations in our study cannot be explained by changes in the power-law spectral index of the relativistic electrons in the jets. The shock-in-jet scenario is favoured for the correlation between optical flux and PD, whereas the anticorrelation can be explained by the presence of multizone emission regions. The varying correlated behaviour can also be explained by the enhanced optical flux caused by the newly developed radio knots in the jets and their magnetic field alignment with the large-scale jet magnetic field. 2022 The Author(s). -
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Study of classical Be stars in open clusters older than 100 Myr
We performed the slitless spectroscopic survey to identify classical Be (CBe) stars in open clusters older than 100 Myr. Observing a sample of 71 open clusters, we identified 13 CBe stars in 11 open clusters, one of, which (TYC 2679-432-1) is a new detection. The 13 CBe stars show both H? in emission and IR excess, which confirm that they possess gaseous circumstellar discs. Two more CBe stars are found to exhibit H? in absorption for the first time, indicating that might be passing through disc-less episode presently. The spectral-type estimation is done for all these 15 stars and it is noticed that they belong to B0.5B8 type. Moreover, we found that the distribution of our sample is peaking near late B-types as expected. 2021, Indian Academy of Sciences. -
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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 chaos in the Darcy-Bard convection problem with Robin boundary condition on the upper surface
Possibility of chaos is studied in Darcy-Bard convection using the Dirichlet and the Robin boundary condition at the lower and upper boundaries, respectively. Comparison is made with the results of Dirichlet (classical-Darcy-Bard convection, CDBC) and Neumann boundary condition (Barletta-Darcy-Bard convection, BDBC). It is found that the cell size at onset is bigger in the case of BDBC compared to the generalized-Darcy-Bard convection (GDBC) and much bigger compared to CDBC. The critical-Darcy-Rayleigh number of BDBC is found to be the least and that of CDBC is the largest. Nonlinear-stability-analysis is performed leading to the scaled-generalized-Vadasz-Lorenz model (SGVLM). In deriving this model, help is sought from a local-nonlinear-stability-analysis that yields the form of the convective-mode. The SGVLM is shown to be dissipative and conservative, with its bounded solution trapped within an ellipsoid. Onset of chaos and its characteristics are studied using the Hopf-Rayleigh-number, the Lorenz-butterfly-diagram, and the plot of the amplitude of the convective-mode vs the control-parameter, R, which is the eigenvalue. Chaos sets in earlier in CDBC and much later in BDBC when compared to that in GDBC. Beyond the onset of chaos is seen a sequence of chaotic and periodic motions, with the latter sometimes being present for an extended period. 2024 Author(s). -
Study of candidate Be stars in the Magellanic Clouds using near-infrared photometry and optical spectroscopy
Mennickent et al. and Sabogal et al. identified a large number of classical Be (CBe) candidates (?3500) in the Large and Small Magellanic Clouds (LMC and SMC) based on their photometric variability using the OGLEII data base. They classified these stars into four different groups based on the appearance of their variability. In order to refine and understand the nature of this large number of stars, we studied the infrared properties of the sample and the spectroscopic properties of a subsample. We cross-correlated the optical sample with the IRSF-MCPS catalogue to obtain the J, H, K s magnitudes of all the four types of stars (?2500) in the LMC and SMC. Spectra of 120 stars belonging to the types 1, 2 and 3 were analysed to study their spectral properties. Among the four types, the type 4 stars are the dominant group, with ?60 and ?65 per cent of the total sample in the LMC and SMC, respectively. The near-infrared (NIR) colour-colour diagrams suggest that the type 4 stars in the LMC have a subclass, which is not found in our Galaxy or in the SMC. This subclass is ?18 per cent of the type 4 sample. The main type 4 sample which is ?49 per cent of the total sample has NIR properties similar to the Galactic CBe stars and the SMC type 4 stars. Though the new subclass of type 4 stars have high E(B-V) ? 0.75, they are not located close to regions with high reddening. The type 3 stars (?6 per cent and 7.3 per cent in the LMC and SMC) are found to have large H? equivalent width (EW) in the SMC and some are found to have large NIR excess. This small fraction of stars are unlikely to be CBe stars. Three stars among the type 3 stars in the LMC are found to be double periodic variables. The type 2 stars are found in larger fraction in the SMC (?14.5 per cent), when compared to the LMC (?6 per cent). The spectroscopic and the NIR properties suggest that these could be CBe stars. The type 1 stars are relatively more in the LMC (?24 per cent) when compared to the SMC (?13 per cent). The SMC type 1 stars have relatively large H? EW and this class has properties similar to CBe stars. The spectroscopic sample of type 1 stars which show H? in emission and are confirmed as CBe stars are more abundant in the SMC by a factor of 2.6. If the effect of metallicity is to cause more CBe stars in the SMC, when compared to the LMC, then type 1, type 2 and type 4 stars follow this rule, with an enhancement of 2.6, 2.4 and 1.3, respectively. 2012 The Authors Monthly Notices of the Royal Astronomical Society 2012 RAS.