1,511 research outputs found
On the two-dimensional stability of the axisymmetric Burgers vortex
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
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
ON THE COARSE GEOMETRY OF JAMES SPACES
International audienceIn this note we prove that the Kalton interlaced graphs do not equi-coarsely embed into the James space J nor into its dual J *. It is a particular case of a more general result on the non equi-coarse embeddability of the Kalton graphs into quasi-reflexive spaces with a special asymptotic stucture. This allows us to exhibit a coarse invariant for Banach spaces, namely the non equi-coarse embeddability of this family of graphs, which is very close to but different from the celebrated property Q of Kalton. We conclude with a remark on the coarse geometry of the James tree space J T and of its predual
On the three-dimensional temporal spectrum of stretched vortices
The three-dimensional stability problem of a stretched stationary vortex is
addressed in this letter. More specifically, we prove that the discrete part of
the temporal spectrum is only associated with two-dimensional perturbations.Comment: 4 pages, RevTeX, submitted to PR
Some factors affecting the efficiency of potato production, under Al–Ghab plain conditions, Syrian Arab Republic
ArticleData were collected by a field survey of 300 farmers from Al–Ghab region (Syria)
during 2014–2015. The non–parametric Data Envelopment was used in analyzing the Technical
efficiency. The relationship between farm size and production efficiency was considered.
Technical efficiency amounted about 53% and most of farms are operating at low level of
technical efficiency. The relationship between farm size and productivity efficiency is Non–
linear, it decreases from small to medium farm size and then increases as the size increase. Large
farms have the higher net farm income per thousand square metersand are the most efficient
technically followed by small and medium farm size. To disclose that factors causing the
technical efficiency, Two–limit Tobit Regression Model was used. The calculated results showed
that, Household Size, Occupation, Farm Size, Experience in Farming, Seed Type and
Membership are factors that cause the technical inefficiency potato farming at Al–Ghab region.
Therefore, the Syrian Planning Board and Decision Makers should take this results into account
when they draw their plans to improve farmer's skills by allocating more investment in farm
research and extension programmers
Edge-guided image gap interpolation using multi-scale transformation
This paper presents improvements in image gap restoration through the incorporation of edge-based directional interpolation within multi-scale pyramid transforms. Two types of image edges are reconstructed: 1) the local edges or textures, inferred from the gradients of the neighboring pixels and 2) the global edges between image objects or segments, inferred using a Canny detector. Through a process of pyramid transformation and downsampling, the image is progressively transformed into a series of reduced size layers until at the pyramid apex the gap size is one sample. At each layer, an edge skeleton image is extracted for edge-guided interpolation. The process is then reversed; from the apex, at each layer, the missing samples are estimated (an iterative method is used in the last stage of upsampling), up-sampled, and combined with the available samples of the next layer. Discrete cosine transform and a family of discrete wavelet transforms are utilized as alternatives for pyramid construction. Evaluations over a range of images, in regular and random loss pattern, at loss rates of up to 40%, demonstrate that the proposed method improves peak-signal-to-noise-ratio by 1–5 dB compared with a range of best-published works
Control of an Active Suspension System as a Benchmark for Design and Optimization of Restricted Complexity Controllers
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
Three-dimensional stability of Burgers vortices
Burgers vortices are explicit stationary solutions of the Navier-Stokes
equations which are often used to describe the vortex tubes observed in
numerical simulations of three-dimensional turbulence. In this model, the
velocity field is a two-dimensional perturbation of a linear straining flow
with axial symmetry. The only free parameter is the Reynolds number , where is the total circulation of the vortex and is
the kinematic viscosity. The purpose of this paper is to show that Burgers
vortex is asymptotically stable with respect to general three-dimensional
perturbations, for all values of the Reynolds number. This definitive result
subsumes earlier studies by various authors, which were either restricted to
small Reynolds numbers or to two-dimensional perturbations. Our proof relies on
the crucial observation that the linearized operator at Burgers vortex has a
simple and very specific dependence upon the axial variable. This allows to
reduce the full linearized equations to a vectorial two-dimensional problem,
which can be treated using an extension of the techniques developped in earlier
works. Although Burgers vortices are found to be stable for all Reynolds
numbers, the proof indicates that perturbations may undergo an important
transient amplification if is large, a phenomenon that was indeed observed
in numerical simulations.Comment: 31 pages, no figur
Perdeuterated cyanobiphenyl liquid crystals for infrared applications
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
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 µm 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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