The influence of trigonometric sine, square and triangular wave-types of time-periodic gravity-aligned oscillations on Rayleigh–Bénard convection in Newtonian liquids and in Newtonian nanoliquids is studied in the paper using the generalized Buongiorno two-phase model. The five-mode Lorenz model is derived under the assumptions of Boussinesq approximation, small-scale convective motion and some slip mechanisms. Using the method of multiscales, the Lorenz model is transformed to a Ginzburg–Landau equation the solution of which helps in quantifying the heat transport through the Nusselt number. Enhancement of heat transport in Newtonian liquids due to the presence of nanoparticles/nanotubes is clearly explained. The study reveals that all the three wave types of gravity modulation delay the onset of convection and thereby to a diminishment of heat transport. It is also found that in the case of trigonometric sine type of gravity modulation heat transport is intermediate to that of the cases of triangular and square types. The paper is the first such work that attempts to theoretically explain the effect of three different wave-types of gravity modulation on onset of convection and heat transport in the presence/absence of nanoparticles/nanotubes. Comparing the heat transport by the single-phase and by the generalized two-phase models, the conclusion is that the single-phase model under-predicts heat transport in nanoliquids irrespective of the type of gravity modulation being effected on the system. The results of the present study reiterate the findings of related experimental and numerical studies.
In this article we study the combined effect of internal heating and time-periodic gravity modulation on thermal instability in a closely packed anisotropic porous medium, heated from below and cooled from above. The time-periodic gravity modulation, considered in this problem can be realized by vertically oscillating the porous medium. A weak non-linear stability analysis has been performed by using power series expansion in terms of the amplitude of gravity modulation, which is assumed to be small. The Nusselt number has been obtained in terms of the amplitude of convection which is governed by the non-autonomous Ginzburg?CLandau equation derived for the stationary mode of convection. The effects of various parameters such as; internal Rayleigh number, amplitude and frequency of gravity modulation, thermo-mechanical anisotropies, and Vadász number on heat transport has been analyzed. It is found that the response of the convective system to the internal Rayleigh number is destabilizing. Further it is found that the heat transport can also be controlled by suitably adjusting the external parameters of the system. 相似文献
A linear stability analysis is performed for mono-diffusive convection in an anisotropic rotating porous medium with temperature-dependent
viscosity. The Galerkin variant of the weighted residual technique is used to obtain the eigen value of the problem. The effect
of Taylor–Vadasz number and the other parameters of the problem are considered for stationary convection in the absence or
presence of rotation. Oscillatory convection seems highly improbable. Some new results on the parameters’ influence on convection
in the presence of rotation, for both high and low rotation rates, are presented. 相似文献
The excited states of single metal atom (X = Co, Al and Cu) doped boron nitride flake (MBNF) B15N14H14-X and pristine boron nitride (B15N15H14) are studied by time-dependent density functional theory. The immediate effect of metal doping is a red shift of the onset of absorption from about 220 nm for pristine BNF to above 300 nm for all metal-doped variants with the biggest effect for MBNF-Co, which shows appreciable intensity even above 400 nm. These energy shifts are analysed by detailed wavefunction analysis protocols using visualisation methods, such as the natural transition orbital analysis and electron-hole correlation plots, as well as quantitative analysis of the exciton size and electron-hole populations. The analysis shows that the Co and Cu atoms provide strong contributions to the relevant states whereas the aluminium atom is only involved to a lesser extent. 相似文献
The Rayleigh-Benard situation in Boussinesq-Stokes suspensions is investigated using both linear and non-linear stability analyses. The linear and non-linear analyses are based on a normal mode solution and minimal representation of double Fourier series, respectively. The effect of suspended particles on convection is delineated against the background of the results of the clean fluid. The realm of non-linear convection warrants the quantification of heat transfer and this has been achieved on the Rayleigh-Nusselt plane. Possibility of aperiodic convection is discussed. 相似文献
Density functional theory (DFT) and time‐dependent density functional theory (TDDFT) calculations were performed with the basis sets 6‐31G for DFT and 6‐31G(d), 6‐31+G(d,p) for TDDFT on pure graphene nanoribbon (GNR) C30H14 and metal‐decorated C29H14‐X (MGNRs; X=Ni, Fe, Ti, Co+, Al+, and Cu+). The metal/carbon ratio (X:C 3.45 %) and the doping site were fixed. Electronic properties, that is, the dipole moment, binding energy, and HOMO–LUMO gaps, were calculated. The absorption and emission properties in the visible range (λ=400–720 nm) were determined. Optical gaps, absorption and emission wavelengths, oscillator strengths, and dominant transitions were calculated. Pure graphene was found to be the most stable form. However, of the MGNRs, C29H14?Co+ and C29H14?Al+ were found to be the most and least stable, respectively. All GNRs were found to have semiconducting nature. The optical absorption of pure graphene undergoes a shift on metal doping. Emission from the pure graphene followed Kasha′s rule, unlike the metal‐doped GNRs. 相似文献
Optical properties of (6,5) single-walled carbon nanotubes (SWCNTs) of 1 and 2?nm lengths were studied with the help of time-dependent density functional theory and transition density matrix based analysis. Electronic band gap of all SWCNTs is found to be in the range 0.82–1.67?eV. The peak absorptions occur around 600?nm for 1?nm nanotubes and get broader as the length increases to 2?nm. The natural transition orbital analysis was applied to understand the electron delocalisation upon absorption. Finally, the exciton sizes were determined and found to be in the range 6.80–7.25?Å for 1?nm SWCNTs, and 6.82–11.56?Å for 2?nm SWCNTs, which were further used in the electron delocalisation analysis. All the excitons were found to be Frenkel in nature. This study illustrates how the excited state properties in SWCNTs can vary upon length change and improves the understanding of electronic excitations in SWCNTs, which would be beneficial in photovoltaic applications. 相似文献
Transport in Porous Media - We investigate the Küppers–Lortz (KL) instability in the rotating Brinkman–Bénard convection problem by assuming that there is local thermal... 相似文献
Journal of Thermal Analysis and Calorimetry - In the paper, we make linear and nonlinear stability analyses of Rayleigh–Bénard convection in a Newtonian nanoliquid-saturated... 相似文献