9,758 research outputs found
Stokes diagnostics of simulated solar magneto-convection
No full textWe present results of synthetic spectro-polarimetric diagnostics of radiative MHD simulations of solar surface convection with magnetic fields. Stokes profiles of Zeeman-sensitive lines of neutral iron in the visible and infrared spectral ranges emerging from the simulated atmosphere have been calculated in order to study their relation to the relevant physical quantities and compare with observational results. We have analyzed the dependence of the Stokes-I line strength and width as well as of the Stokes-V signal and asymmetries on the magnetic field strength. Furthermore, we have evaluated the correspondence between the actual velocities in the simulation with values determined from the Stokes-I (Doppler shift of the centre of gravity) and Stokes-V profiles (zero-crossing shift). We confirm that the line weakening in strong magnetic fields results from a higher temperature (at equal optical depth) in the magnetic flux concentrations. We also confirm that considerable Stokes-V asymmetries originate in the peripheral parts of strong magnetic flux concentrations, where the line of sight cuts through the magnetopause of the expanding flux concentration into the surrounding convective donwflow
Intermittency and regularity issues in 3D Navier-Stokes turbulence
No full textTwo related open problems in the theory of 3 D Navier-Stokes turbulence are discussed in this paper. The first is the phenomenon of intermittency in the dissipation field. Dissipation-range intermittency was first discovered experimentally by Batchelor and Townsend over fifty years ago. It is characterized by spatio-temporal binary behaviour in which long, quiescent periods in the velocity signal are interrupted by short, active ‘events’ during which there are violent fluctuations away from the average. The second and related problem is whether solutions of the 3 D Navier-Stokes equations develop finite time singularities during these events. This paper shows that Leray’s weak solutions of the three-dimensional incompressible Navier-Stokes equations can have a binary character in time. The time-axis is split into ‘good’ and ‘bad’ intervals: on the ‘good’ intervals solutions are bounded and regular, whereas singularities are still possible within the ‘bad’ intervals. An estimate for the width of the latter is very small and decreases with increasing Reynolds number. It also decreases relative to the lengths of the good intervals as the Reynolds number increases. Within these ‘bad’ intervals, lower bounds on the local energy dissipation rate and other quantities, such as || u (·, t )|| ∞ and ||∇ u (·, t )|| ∞ , are very large, resulting in strong dynamics at sub-Kolmogorov scales. Intersections of bad intervals for n ≧1 are related to the potentially singular set in time. It is also proved that the Navier-Stokes equations are conditionally regular provided, in a given ‘bad’ interval, the energy has a lower bound that is decaying exponentially in time.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46170/1/205_2005_Article_382.pd
Effects of graphite nodules on crack growth behaviour of austempered ductile iron
No full textAustempered ductile iron (ADI) is a candidate material for camshafts, where however, the early stages of fatigue damage are of major concern during service. A fundamental microstructurally based assessment of the mechanisms of fatigue failure is important. An ADI microstructure austenitised at 900°C and austempered at 390°C has been investigated in detail. Crack initiation and growth behaviour was assessed under three-point bend testing conditions. Primary initiation events occurred exclusively at pores with further micro-crack initiation occurring at decohered graphite nodules in the monotonic plastic zone ahead of the advancing dominant macro-crack tip. Lifetime was however determined by propagation behaviour rather than coalescence events. The changes in the as-cast microstructure generated by this heat treatment have resulted in improved fatigue crack propagation performance due to the reduction in eutectic carbides and the relatively high quantity of retained austenite compared with previous studies, giving rise to greater crack path tortuousity and shielding
When is a Stokes line not a Stokes line?
No full textDuring the course of a Stokes phenomenon, an asymptotic expansion can change its form as a further series, prefactored by an exponentially small term and a Stokes multiplier, appears in the representation. The initially exponentially small contribution may nevertheless grow to dominate the behaviour for other values of the asymptotic or associated parameters.We introduce the concept of a higher order Stokes phenomenon, at which a Stokes multiplier itself can change value. We show that the higher order Stokes phenomenon can be used to explain the apparent sudden birth of Stokes lines at regular points, why some Stokes lines are irrelevant to a given problem and why it is indispensible to the proper derivation of expansions that involve three or more possible asymptotic contributions. We provide an example of how the higher order Stokes phenomenon can have important effects on the large time behaviour of linear partial differential equations.Subsequently we apply these techniques to Burgers equation, a non-linear partial differential equation developed to model turbulent fluid flow. We find that the higher order Stokes phenomenon plays a major, yet very subtle role in the smoothed shock wave formation of this equation
Dispelling the Myths Behind First-author Citation Counts
No full textWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Integral representation of a solution to the Stokes-Darcy problem
No full textWith methods of potential theory we develop a representation of a solution of the coupled Stokes-Darcy model in a Lipschitz domain for boundary data in H-1/2
NAVIER–STOKES EQUATIONS ON THE β-PLANE
No full textMathematical analysis has been undertaken for the vorticity formulation of the two dimensional Navier–Stokes equation on the β-plane with periodic boundary conditions. This equation describes the flow of fluid near the equator of the Earth. The long time behaviour of the solution of this equation is investigated and we show that, given a sufficiently regular forcing, the solution of the equation is nearly zonal. We use this result to show that, for sufficiently large β, the global attractor of this system reduces to a point. Another result can be obtained if we assume that the forcing is time-independent and sufficiently smooth. If the forcing lies in some Gevrey space, the slow manifold of the Navier–Stokes equation on the β-plane can be approximated with O(εn/2) accuracy for arbitrary n = 0, 1, · · · , as well as with exponential accuracy
Mennine-Stokes Collection; no.07728
No full textSepia image of Henry B. Ailman and Henry M. Meridith posed standing in front of their bank, Meredith & Ailman Bankers. with two unidentified men. Poised at left side of photograph are two boys, and posed at right side of photograph stands an unidentified man in front of the C. G. Kidd____ Wholesaler in Cigars, Tobacco and Smokers ____ storefront. Image mounted on a gray matte board.Master file: image/tiff; 111,656 KB; Computer Hardware: Intel Pentium (R) 4 3.20 GHz/ 1.99 GB RAM manufactured by Dell; Operating system: Windows XP 2002; Creation software: Adobe Photoshop CS2 version 9.0.2; Scanner: flatbed reflective scanner Microtek 1000XL; Scanner software: Microtek SilverFast Ai 6.4.2r2b; Scanned by Jackie Becker on 2009-10-23
Effects of carbides on fatigue characteristics of austempered ductile iron
No full textCrack initiation and growth behavior of an austempered ductile iron (ADI) austenitized at 800 °C and austempered at 260 °C have been assessed under three-point bend fatigue conditions. Initiation sites have been identified as carbides remaining from the as-cast ductile iron due to insufficient austenization. The number of carbides cracking on loading to stresses greater than 275 MPa is critical in determining the failure mechanism. In general, high carbide area fractions promote coalescence-dominated fatigue crack failure, while low area fractions promote propagation-dominated fatigue crack failure. Individual carbides have been characterized using finite body tessellation (FBT) and adaptive numerical modeling (SUpport vector Parsimonious ANalysis Of VAriance (SUPANOVA)) techniques in an attempt to quantify the factors promoting carbide fracture. This indicated that large or long and thin carbides on the whole appear to be susceptible to fracture, and carbides that are locally clustered and aligned perpendicular to the tensile axis are particularly susceptible to fracture
Proper general decomposition (PGD) for the resolution of Navier–Stokes equations
No full textIn this work, the PGD method will be considered for solving some problems of fluid mechanics by looking for the solution as a sum of tensor product functions. In the first stage, the equations of Stokes and Burgers will be solved. Then, we will solve the Navier–Stokes problem in the case of the lid-driven cavity for different Reynolds numbers (Re = 100, 1000 and 10,000). Finally, the PGD method will be compared to the standard resolution technique, both in terms of CPU time and accuracy.Région Poitou-Charente
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