53,966 research outputs found

    Mobile WiMAX: impact of mobility on the performance of limited feedback linear precoding

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    Constraints on scalar diffusion anomaly in three-dimensional flows having bounded velocity gradients

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    This study is concerned with the decay behaviour of a passive scalar θ\theta in three-dimensional flows having bounded velocity gradients. Given an initially smooth scalar distribution, the decay rate d/dtd/dt of the scalar variance is found to be bounded in terms of controlled physical parameters. Furthermore, in the zero diffusivity limit, κ0\kappa\to0, this rate vanishes as κα0\kappa^{\alpha_0} if there exists an α0(0,1]\alpha_0\in(0,1] independent of κ\kappa such that <<\infty for αα0\alpha\le\alpha_0. This condition is satisfied if in the limit κ0\kappa\to0, the variance spectrum Θ(k)\Theta(k) remains steeper than k1k^{-1} for large wave numbers kk. When no such positive α0\alpha_0 exists, the scalar field may be said to become virtually singular. A plausible scenario consistent with Batchelor's theory is that Θ(k)\Theta(k) becomes increasingly shallower for smaller κ\kappa, approaching the Batchelor scaling k1k^{-1} in the limit κ0\kappa\to0. For this classical case, the decay rate also vanishes, albeit more slowly -- like (lnPr)1(\ln P_r)^{-1}, where PrP_r is the Prandtl or Schmidt number. Hence, diffusion anomaly is ruled out for a broad range of scalar distribution, including power-law spectra no shallower than k1k^{-1}. The implication is that in order to have a κ\kappa-independent and non-vanishing decay rate, the variance at small scales must necessarily be greater than that allowed by the Batchelor spectrum. These results are discussed in the light of existing literature on the asymptotic exponential decay eγt\sim e^{-\gamma t}, where γ>0\gamma>0 is independent of κ\kappa.Comment: 6-7 journal pages, no figures. accepted for publication by Phys. Fluid

    Stability of plane Poiseuille-Couette flows of a piezo-viscous fluid

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    We examine stability of fully developed isothermal unidirectional plane Poiseuille--Couette flows of an incompressible fluid whose viscosity depends linearly on the pressure as previously considered in Hron01 and Suslov08. Stability results for a piezo-viscous fluid are compared with those for a Newtonian fluid with constant viscosity. We show that piezo-viscous effects generally lead to stabilisation of a primary flow when the applied pressure gradient is increased. We also show that the flow becomes less stable as the pressure and therefore the fluid viscosity decrease downstream. These features drastically distinguish flows of a piezo-viscous fluid from those of its constant-viscosity counterpart. At the same time the increase in the boundary velocity results in a flow stabilisation which is similar to that observed in Newtonian fluids with constant viscosity

    Linear degree growth in lattice equations

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    We conjecture recurrence relations satisfied by the degrees of some linearizable lattice equations. This helps to prove linear growth of these equations. We then use these recurrences to search for lattice equations that have linear growth and hence are linearizable

    Lunar nuclear power feasibility study

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    Based on review of literature and on limited examination of nuclear power systems now proposed for space applications, a nuclear fission reactor powered system should be seriously considered as the first large (order of 50 kWe or greater) power system to be placed on a lunar base. With relatively minor modifications, the major one being addition of a cooled side shield, the proposed 100 kWe product of the SP-100 Program could be adapted for use on a lunar base

    Modelling value-added tax in the presence of multiproduction and differentiated exemptions

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    We develop a framework for economy-wide modelling of value-added tax systems. Our framework models a number of complexities of VAT systems as they are implemented by tax agencies. In particular, we model multiple rates, multiple exemptions, multiple degrees of refundability across commodity users, and multi-product enterprises. A detailed VAT framework is important for correct modelling of VAT within a general equilibrium model. Such a framework is also of value in correctly representing the distribution of indirect tax payments within the database of a general equilibrium model, a prerequisite of accurate welfare analysis. We use the model to analyse the effects of simplifying Vietnam’s complex VAT system. We simplify the system by moving from three tax rates to one budget-neutral rate, while also removing many discretionary exemptions.value added tax; dynamic CGE model; Vietnam; indirect tax reform
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