1,075 research outputs found

    Structure and stability of non-symmetric Burgers vortices

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    We investigate, numerically and analytically, the structure and stability of steady and quasi-steady solutions of the Navier–Stokes equations corresponding to stretched vortices embedded in a uniform non-symmetric straining field, ([alpha]x, [beta]y, [gamma]z), [alpha]+[beta]+[gamma]=0, one principal axis of extensional strain of which is aligned with the vorticity. These are known as non-symmetric Burgers vortices (Robinson & Saffman 1984). We consider vortex Reynolds numbers R=[Gamma]/(2[pi]v) where [Gamma] is the vortex circulation and v the kinematic viscosity, in the range R=1[minus sign]104, and a broad range of strain ratios [lambda]=([beta][minus sign][alpha])/([beta]+[alpha]) including [lambda]>1, and in some cases [lambda][dbl greater-than sign]1. A pseudo-spectral method is used to obtain numerical solutions corresponding to steady and quasi-steady vortex states over our whole (R, [lambda]) parameter space including [lambda] where arguments proposed by Moffatt, Kida & Ohkitani (1994) demonstrate the non-existence of strictly steady solutions. When [lambda][dbl greater-than sign]1, R[dbl greater-than sign]1 and [epsilon][identical with][lambda]/R[double less-than sign]1, we find an accurate asymptotic form for the vorticity in a region 11. An iterative technique based on the power method is used to estimate the largest eigenvalues for the non-symmetric case [lambda]>0. Stability is found for 0[less-than-or-eq, slant][lambda][less-than-or-eq, slant]1, and a neutrally convective mode of instability is found and analysed for [lambda]>1. Our general conclusion is that the generalized non-symmetric Burgers vortex is unconditionally stable to two-dimensional disturbances for all R, 0[less-than-or-eq, slant][lambda][less-than-or-eq, slant]1, and that when [lambda]>1, the vortex will decay only through exponentially slow leakage of vorticity, indicating extreme robustness in this case

    On the two-dimensional stability of the axisymmetric Burgers vortex

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    The stability of the axisymmetric Burgers vortex solution of the Navier–Stokes equations to two-dimensional perturbations is studied numerically up to Reynolds numbers, R=Gamma/2pinu, of order 104. No unstable eigenmodes for azimuthal mode numbers n=1,..., 10 are found in this range of Reynolds numbers. Increasing the Reynolds number has a stabilizing effect on the vortex

    Modelling and Optimization of the Air Operational Manoeuvre

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    Increasing complexity of the operational environment and advanced technology implementation in combat will probably lead to a serious limitation of human performance in all operational domains and activities in the future. With except of the clear indications, that tactical robotics will outperform human soldiers in many routine tasks on the battlefield, the area of operational decision making (resistible for decades to some automation) seems to be slowly approaching to the same stage. Presented article discusses the fundamental theory of optimization of the air operational maneuver and present the approach to the solution. The solution is highly theoretical and uses a modelling and simulation as an experimental platform to the visualization and evaluation of solution. The problem of air operational maneuver is specific in this case by many variables imposed on initial parametrization of the task (starting and destination point could not be known at the beginning, only \u201cair operational\u201d area should be selected) and very wide search of possible courses of action and the best \u201cmulti criteria\u201d choice identification

    Control of an Active Suspension System as a Benchmark for Design and Optimization of Restricted Complexity Controllers

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    A benchmark problem for restricted complexity controller design is introduced. The objective is to design the lowest-order controller which meets the control specifications for an active suspension system. The input-output data of the plant are provided on the benchmark site and the final controllers are evaluated using the closed-loop data. Thirteen solutions proposed to solve the benchmark problem are briefly presented and classified in terms of methodology and compared with respect to their complexity and performance

    The World in Transition: A Comparative Analysis of Youth Perceptions in China and America

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    Many scholars have argued, over the next two decades China will rise while the United States begins a relative decline. Today’s youth will eventually become the next generation of leaders and policymakers. Thus, understanding youth’s perceptions of these trends can help provide insight into how they will shape the future Sino-U.S. relationship and their state’s standing in the world. This research found that the youth in China were optimistic toward their country’s prospects in the international community compared to their American counterparts. The relative optimism among the Chinese has made them more willing to participate in global affairs and institutions compared to American youth. Increased media exposure regarding the Sino-U.S. relationship correlated with greater negative perceptions of their counterpart’s country between both the Americans and Chinese. However, Americans and Chinese youth that had learned more than one language had greater positive perceptions and willingness to cooperate with their counterpart’s country

    The Practice of Immigration Health in Complex Emergency Situations - A Case Study of Kosovo from March to July 1999

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    The need to rapidly transport refugees, displaced as a result of evolving complex humanitarian emergencies creates challenges for those refugee-receiving nations that require formaI immigration medical screening of these populations. Balancing the need to expediently resettle the refugees with these legislative and regulatory medical requirements can be logistically and operationally difficult. During the 1999 Kosovo crisis, the Humanitarian Evacuation Programme from the Former Yugoslav Republic of Macedonia rapidly moved large numbers of Kosovar Albanian refugees to nations with existing formal immigration medical screening requirements. This paper describes the successful management and delivery of immigration health services during this complicated international event.La nĂ©cessitĂ© de transporter rapidement les rĂ©fugiĂ©s dĂ©placĂ©s Ă  cause d'urgences humanitaires complexes Ă  Ă©volution rapide reprĂ©senteun dĂ©fi pour les nations receveuses de rĂ©fugiĂ©s requĂ©rant un tamisage mĂ©dical routinier de sa population immigrante. Des points de vue logistique et opĂ©rationnel, il peut s'avĂ©rer fort difficile pour ces Ă©tats de concilier la nĂ©cessitĂ© de relocaliser rapidement les rĂ©fugiĂ©s avec les exigences de leurs lois et rĂšglements en matiĂšre mĂ©dicale et sanitaire. Lors de la crise du Kosovo de 1999, le Programme d'Évacuation Humanitaire de l'ancienne rĂ©publique yougoslave de MacĂ©doine dĂ©plaça rapidement un grand nombre de rĂ©fugiĂ©s albanais kosovars vers des nations requĂ©rant formellement un tamisage mĂ©dical de sa population immigrante. Cet article dĂ©crit la gestion et la distribution rĂ©ussies de services de santĂ© Ă  l'immigration au cours de cet Ă©vĂ©nement international complexe

    Perdeuterated cyanobiphenyl liquid crystals for infrared applications

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    Perdeuterated 4'-pentyl-4-cyanobiphenyl (D5CB) was synthesized and its physical properties evaluated and compared to those of 5CB. D5CB retains physical properties similar to those of 5CB, such as phase transition temperatures, dielectric constants, and refractive indices. An outstanding feature of D5CB is that it exhibits a much cleaner and reduced infrared absorption. Perdeuteration, therefore, extends the usable range of liquid crystals to the mid infrared by significantly reducing the absorption in the near infrared, which is essential for telecom applications

    Submicron silicon powder production in an aerosol reactor

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    Powder synthesis by thermally induced vapor phase reactions is described. The powder generated by this technique consists of spherical, nonagglomerated particles of high purity. The particles are uniform in size, in the 0.1–0.2 ”m size range. Most of the particles are crystalline spheres. A small fraction of the spheres are amorphous. Chain agglomerates account for less than 1% of the spherules
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