29,039 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
Brain-computer interfacing in rehabilitation
No full textBrain–computer interfacing (BCI) systems involve controlling a computer using brain signals detected by electroencephalography (EEG). Signal processing software uses the EEG signal to control a cursor or application, such as word processing (Birbaumer et al., 1999 and Pfurtscheller et al., 1993). The field of BCI research is at a relatively early stage of producing reliable, robust systems that are widely accessible for everyday use. Several BCI research groups are developing systems to enable communication and environmental control for people with severe disabilities. A more recent area of exploration with BCI is for investigating mechanisms of normal function, dysfunction and recovery, as well as aiding diagnosis and re-training of function. The generation and control of EEG signals for driving a BCI system require training of the user. Methods include imagery tasks, evoked potentials and operant conditioning (for reviews see (Curran and Stokes, 2003 E. Curran and M. Stokes, Brain Cog 51 (2003), pp. 326–335.Curran and Stokes, 2003 and Kübler et al., 2001)). Signal processing techniques continue to be refined (James and Hesse, 2005) and are improving the accuracy and reliability of BCI technology but translation into routine clinical use is limited by several factors influencing accessibility and compliance. Surface or implanted recording devices can be used and for transient use in most areas of rehabilitation, surface electrodes are appropriate. An important aim of BCI research is to bridge the gap between major technological advances and the relatively limited success in practical applications. More clinical disciplines are encouraged to become involved in BCI research to achieve this aim.<br/
Vorticity moments in four numerical simulations of the 3D Navier–Stokes equations
Get PDFThe issue of intermittency in numerical solutions of the 3D Navier–Stokes equations on a periodic box [0,L]3 is addressed through four sets of numerical simulations that calculate a new set of variables defined by Dm(t)=(ϖ−10Ωm)αm for 1≤m≤∞ where αm=2m/(4m−3) and [Ωm(t)]2m=L−3∫V|ω|2mdV with ϖ0=νL−2. All four simulations unexpectedly show that the Dm are ordered for m=1,…,9 such that Dm+1<Dm. Moreover, the Dm squeeze together such that Dm+1/Dm↗1 as m increases. The values of D1 lie far above the values of the rest of the Dm, giving rise to a suggestion that a depletion of nonlinearity is occurring which could be the cause of Navier–Stokes regularity. The first simulation is of very anisotropic decaying turbulence; the second and third are of decaying isotropic turbulence from random initial conditions and forced isotropic turbulence at fixed Grashof number respectively; the fourth is of very-high-Reynolds-number forced, stationary, isotropic turbulence at up to resolutions of 40963
Shelley Stokes-Hammond interview, 15 September 2017
Get PDFShelley Stokes-Hammond is the oldest daughter of Louis Stokes. She is a graduate of The Ohio State University and Goucher College. She is a historic preservationist, author and public relations manager at Howard University. This 2017 interview was collected as part of a yearlong, community-wide commemoration of the 50th anniversary of Carl Stokes\u27 election as mayor of Cleveland
Shelley Stokes-Hammond interview, 15 September 2017
No full textShelley Stokes-Hammond is the oldest daughter of Louis Stokes. She is a graduate of The Ohio State University and Goucher College. She is a historic preservationist, author and public relations manager at Howard University. This 2017 interview was collected as part of a yearlong, community-wide commemoration of the 50th anniversary of Carl Stokes\u27 election as mayor of Cleveland
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
A sequential regularization method for time-dependent incompressible Navier--Stokes equations
Get PDFThe objective of the paper is to present a method, called the sequential regularization method (SRM), for the nonstationary incompressible Navier--Stokes equations from the viewpoint of regularization of differential-algebraic equations (DAEs), and to provide a way to apply a DAE method to partial differential-algebraic equations (PDAEs). The SRM is a functional iterative procedure. It is proved that its convergence rate is , where is the number of the SRM iterations and is the regularization parameter. The discretization and implementation issues of the method are considered. In particular, a simple explicit-difference scheme is analyzed and its stability is proved under the usual step-size condition of explicit schemes. It appears that the SRM formulation is new in the Navier--Stokes context. Unlike other regularizations or pseudocompressibility methods in the Navier--Stokes context, the regularization parameter in the SRM need not be very small and the regularized problem in the sequence may be essentially nonstiff in time direction for any . Hence the stability condition is independent of even for explicit time discretization. Numerical experiments are given to verify our theoretical results
Publication in BMC Research Notes: Shifts in soil and plant functional diversity along an altitudinal gradient in the French Alps
No full textAuthors: Stokes A., Angeles G., Barois I., Bounous M, Cruz-Maldonaldo N, Decaëns T, Freschet G., Gabriac Q, Hernandez D., Jimenez L., Ma J, Mao Z, Marin-Castro B, Merino-Martin L, Mohamed A, Reverchon F, Selli L., Sieron K., Weemstra M., Roumet
NAVIER–STOKES EQUATIONS ON THE β-PLANE
Get PDFMathematical 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
Integral representation of a solution to the Stokes-Darcy problem
Get PDFWith 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
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