Applicability of Boussinesq approximation in a turbulent fluid with constant properties

The equations of motion describing buoyant fluids are often simplified using a set of approximations proposed by J. Boussinesq one century ago. To resume, they consist in assuming constant fluid properties, incompressibility and conservation of calories during heat transport. Assuming fulfilment of the first requirement (constant fluid properties), we derive a set of 4 criteria for assessing the validity of the two other requirements in turbulent Rayleigh-B\'enard convection. The first criterion $\alpha \Delta \ll 1$ simply results from the incompressibility condition in the thermal boundary layer ($\alpha$ and $\Delta$ are the thermal expansion coefficient and the temperature difference driving the flow). The 3 other criteria are proportional or quadratic with the density stratification or, equivalently with the temperature difference resulting from the adiabatic gradient across the cell $\Delta_{h}$. Numerical evaluations with air, water and cryogenic helium show that most laboratory experiments are free from such Boussinesq violation as long as the first criterion is fulfilled. In ultra high Rayleigh numbers ($Ra>10^{16}$) experiments in He, one of the stratification criteria, scaling with $\alpha \Delta_{h}$, could be violated. This criterion garanties that pressure fluctuations have a negligible influence both on the density variation and on the heat transfer equation through compression/expansion cycles. Extrapolation to higher $Ra$ suggests that strong violation of Boussinesq approximation could occur in atmospheric convection.Comment: Submitted to Phys.Fluids (oct 2007

Are the welfare losses from imperfect targeting important?

The authors evaluate the size of the welfare losses from using alternative “imperfect” welfare indicators as substitutes for the conventionally preferred consumption indicator. They find that whereas the undercoverage and leakage welfare indices always suggest substantial losses, and the poverty indices suggest substantial losses for the worst performing indices, their preferred welfare index based on standard welfare theory suggests much smaller welfare losses. They also find that one cannot reject the hypothesis that the welfare losses associated with using the better performing alternative indicators are zero. In the case of their preferred welfare index, this reflects the fact that most of the targeting errors, i.e., exclusion and inclusion errors, are highly concentrated around the poverty line so that the differences in welfare weights between those receiving and not receiving the transfers are insufficient to make a difference to the overall welfare impact.Welfare economics. ,Poverty. ,Consumption (Economic theory). ,

On the targeting and redistributive efficiencies of alternative transfer instruments

The paper shows how the so-called distributional characteristic of a policy instrument can be additively decomposed into two components; one that captures the targeting efficiency of the instrument, the other its redistributive efficiency. Using these measures, the paper provides an interpretation of the commonly used leakage and undercoverage rates (and other indices based on these concepts) within standard welfare theory. An empirical application of the decomposition approach to Mexican data is presented.Welfare economics Mathematical models ,Mexico ,

On the targeting and redistributive efficiencies of alternative transfer instruments

The paper shows how the so-called distributional characteristic of a policy instrument can be additively decomposed into two components; one that captures the targeting efficiency of the instrument, the other its redistributive efficiency. Using these measures, the paper provides an interpretation of the commonly used leakage and undercoverage rates (and other indices based on these concepts) within standard welfare theory. An empirical application of the decomposition approach to Mexican data is presented.Welfare economics Mathematical models ,Mexico ,

Exact charged black-hole solutions in D-dimensional f(T) gravity: torsion vs curvature analysis

We extract exact charged black-hole solutions with flat transverse sections in the framework of D-dimensional Maxwell-f(T) gravity, and we analyze the singularities and horizons based on both torsion and curvature invariants. Interestingly enough, we find that in some particular solution subclasses there appear more singularities in the curvature scalars than in the torsion ones. This difference disappears in the uncharged case, or in the case where f(T) gravity becomes the usual linear-in-T teleparallel gravity, that is General Relativity. Curvature and torsion invariants behave very differently when matter fields are present, and thus f(R) gravity and f(T) gravity exhibit different features and cannot be directly re-casted each other.Comment: 24 pages, 3 figures. arXiv admin note: text overlap with arXiv:1110.402

Improving the photovoltaic response of a poly(3-octylthiophene)/n-Si heterojunction by incorporating double-walled carbon nanotubes

Poly(3-octylthiophene)/n-Si heterojunction solar cells were studied with and without incorporation of double-walled carbon nanotubes (DWCNs) in the polymer layer. The performance of the device improves significantly by the incorporation of DWCNs. We report a power conversion efficiency, open circuit voltage, short-circuit current density and fill factor of 0.49%, 0.53 V, 5.9 mA cm−2 and 0.15 respectively for an un-optimized cell containing DWCNs. Reference cells without DWCNs show a much lower performance. DWCN incorporation yields better hole transport, easy exciton splitting and suppression of charge recombination, thereby improving photovoltaic action. DWCN seems a promising material for improving hole transport in organic solar cells

Improving photovoltaic response of poly„3-hexylthiophene…/n-Si heterojunction by incorporating double walled carbon nanotubes

Poly(3-hexylthiophene)/n-Si heterojunction solar cells were studied with and without incorporation of double walled carbon nanotubes (DWCNs) in the polymer layer. Performance of the device improves by manyfold by incorporation of DWCN. The authors report power conversion efficiency, open circuit voltage, short-circuit current density, and fill factor of 0.026%, 0.446 V, 0.3398 mA/cm2, and 0.17, respectively, for an unoptimized cell containing DWCN. Reference cells without DWCNs show much lower performance. DWCN incorporation yields better hole transport, easy exciton splitting, and suppression of charge recombination, thereby improving photovoltaic action. DWCN seems promising materials for improving hole transport in organic solar cells

Spontaneous symmetry breaking in three-Higgs-doublet S3S_3-symmetric models

The talk summarises work done by the authors consisting of a detailed study of the possible vacua in models with three Higgs doublets with $S_3$ symmetry and without explicit CP violation. Different vacua require special regions of the parameter space which were analysed in our work. We establish the possibility of spontaneous CP violation in this framework and we also show which complex vacua conserve CP. In our work we discussed constraints from vacuum stability. The results presented here are relevant for model building.Comment: 11 pages, no figures. Prepared for the proceedings of DISCRETE2016: the Fifth Symposium on Prospects in the Physics of Discrete Symmetries, 28 November-3 December 2016, University of Warsaw, Poland, to appear in the Journal of Physics: Conference Series (JPCS