923 research outputs found
Jump at the onset of saltation
We reveal a discontinuous transition in the saturated flux for aeolian
saltation by simulating explicitly particle motion in turbulent flow. The
discontinuity is followed by a coexistence interval with two metastable
solutions. The modification of the wind profile due to momentum exchange
exhibits a second maximum at high shear strength. The saturated flux depends on
the strength of the wind as
Core of the Magnetic Obstacle
Rich recirculation patterns have been recently discovered in the electrically
conducting flow subject to a local external magnetic termed "the magnetic
obstacle" [Phys. Rev. Lett. 98 (2007), 144504]. This paper continues the study
of magnetic obstacles and sheds new light on the core of the magnetic obstacle
that develops between magnetic poles when the intensity of the external field
is very large. A series of both 3D and 2D numerical simulations have been
carried out, through which it is shown that the core of the magnetic obstacle
is streamlined both by the upstream flow and by the induced cross stream
electric currents, like a foreign insulated insertion placed inside the
ordinary hydrodynamic flow. The closed streamlines of the mass flow resemble
contour lines of electric potential, while closed streamlines of the electric
current resemble contour lines of pressure. New recirculation patterns not
reported before are found in the series of 2D simulations. These are composed
of many (even number) vortices aligned along the spanwise line crossing the
magnetic gap. The intensities of these vortices are shown to vanish toward to
the center of the magnetic gap, confirming the general conclusion of 3D
simulations that the core of the magnetic obstacle is frozen. The implications
of these findings for the case of turbulent flow are discussed briefly.Comment: 14 pages, 9 figures, submitted to Journal of Turbulenc
On the analogy between streamlined magnetic and solid obstacles
Analogies are elaborated in the qualitative description of two systems: the
magnetohydrodynamic (MHD) flow moving through a region where an external local
magnetic field (magnetic obstacle) is applied, and the ordinary hydrodynamic
flow around a solid obstacle. The former problem is of interest both
practically and theoretically, and the latter one is a classical problem being
well understood in ordinary hydrodynamics. The first analogy is the formation
in the MHD flow of an impenetrable region -- core of the magnetic obstacle --
as the interaction parameter , i.e. strength of the applied magnetic field,
increases significantly. The core of the magnetic obstacle is streamlined both
by the upstream flow and by the induced cross stream electric currents, like a
foreign insulated insertion placed inside the ordinary hydrodynamic flow. In
the core, closed streamlines of the mass flow resemble contour lines of
electric potential, while closed streamlines of the electric current resemble
contour lines of pressure. The second analogy is the breaking away of attached
vortices from the recirculation pattern produced by the magnetic obstacle when
the Reynolds number , i.e. velocity of the upstream flow, is larger than a
critical value. This breaking away of vortices from the magnetic obstacle is
similar to that occurring past a real solid obstacle. Depending on the inlet
and/or initial conditions, the observed vortex shedding can be either symmetric
or asymmetric.Comment: minor changes, accepted for PoF, 26 pages, 7 figure
Kestabilan Solusi Numerik Sistem Berderajat Kebebasan Tunggal Akibat Gempa Dengan Metode Newmark (Studi Kasus: Menghitung Respons Bangunan Baja Satu Tingkat)
Metode Newmark merupakan salah satu prosedur numerik yang biasa digunakan untuk menganalisa respon struktur terhadap beban gempa. Metode ini mempunyai dua parameter penting yaitu β dan , yang menetapkan variasi dari percepatan terhadap selang waktu dan menentukan karakteristik kestabilan dan akurasi dari metode tersebut. Apabila dipakai nilai γ = dan β = , artinya digunakan prinsip metode percepatan rata-rata. Sedangkan apabila dipakai nilai γ = dan β = , maka digunakan prinsip metode percepatan linear. Dan seperti metode numerik yang lain pada umumnya, kedua prinsip ini masing-masing juga mempunyai tingkat kestabilan dan akurasi yang berbeda-beda. Kestabilan dan ketelitian/akurasi proses numerik akan terjaga apabila dipakai nilai selang waktu (Δt) yang relatif kecil. Tujuan dari penelitian ini adalah mencari seberapa kecil nilai Δt yang harus digunakan untuk mendapatkan respon struktur yang stabil dan akurat. Struktur dimodelkan menjadi sistem berderajat kebebasan tunggal (SDOF) dan dikenakan beban impuls setengah gelombang sinus. Perhitungan respons menggunakan kedua prinsip di atas, masing-masing dilakukan variasi untuk nilai Δt dan periode (T). Prosedur ini dilakukan dengan bantuan program MS Excel. Hasil perhitungan menunjukkan untuk rata-rata prosentase perbedaan nilai hasil simpangan ≤ 1 %, kurang lebih diperlukan rata-rata nilai Δt = 0,007 T bila menggunakan prinsip percepatan linear, dan Δt = 0,005 T bila menggunakan prinsip percepatan rata-rata. Dengan kata lain, metode percepatan linear lebih efisien dalam mendapatkan hasil yang akurat dibandingkan metode percepatan rata-rata. Sebaliknya, metode percepatan linear mempunyai syarat nilai Δt tertentu agar proses numerik dapat dikatakan stabil (conditionally stable). Ketika digunakan Δt > 0,551 T respons yang dihasilkan oleh metode percepatan linear semakin lama semakin besar seiring pertambahan waktu meskipun adanya efek redaman dan ciri khas dari beban impuls dimana respons yang dihasilkan seharusnya semakin lama semakin kecil. Metode ini dapat dikatakan stabil ketika digunakan Δt < 0,551 T. Sedangkan metode percepatan rata-rata tetap stabil untuk berapa pun nilai Δt yang digunakan
Regularity for eigenfunctions of Schr\"odinger operators
We prove a regularity result in weighted Sobolev spaces (or
Babuska--Kondratiev spaces) for the eigenfunctions of a Schr\"odinger operator.
More precisely, let K_{a}^{m}(\mathbb{R}^{3N}) be the weighted Sobolev space
obtained by blowing up the set of singular points of the Coulomb type potential
V(x) = \sum_{1 \le j \le N} \frac{b_j}{|x_j|} + \sum_{1 \le i < j \le N}
\frac{c_{ij}}{|x_i-x_j|}, x in \mathbb{R}^{3N}, b_j, c_{ij} in \mathbb{R}. If u
in L^2(\mathbb{R}^{3N}) satisfies (-\Delta + V) u = \lambda u in distribution
sense, then u belongs to K_{a}^{m} for all m \in \mathbb{Z}_+ and all a \le 0.
Our result extends to the case when b_j and c_{ij} are suitable bounded
functions on the blown-up space. In the single-electron, multi-nuclei case, we
obtain the same result for all a<3/2.Comment: to appear in Lett. Math. Phy
The brain decade in debate: II. Panic or anxiety? From animal models to a neurobiological basis
This article is a transcription of an electronic symposium sponsored by the Brazilian Society of Neuroscience and Behavior (SBNeC). Invited researchers from the European Union, North America and Brazil discussed two issues on anxiety, namely whether panic is a very intense anxiety or something else, and what aspects of clinical anxiety are reproduced by animal models. Concerning the first issue, most participants agreed that generalized anxiety and panic disorder are different on the basis of clinical manifestations, drug response and animal models. Also, underlying brain structures, neurotransmitter modulation and hormonal changes seem to involve important differences. It is also common knowledge that existing animal models generate different types of fear/anxiety. A challenge for future research is to establish a good correlation between animal models and nosological classification.Universidade Federal do Paraná Departamento de Farmacologia Laboratório de Fisiologia e Farmacologia do Sistema Nervoso CentralUniversity of Hawaii Department of NeurobiologyUniversity of Hawaii Department of PsychologyUniversidade de São Paulo Faculdade de Filosofia Ciências e Letras de Ribeirão Preto Departamento de PsicobiologiaUniversidade de São Paulo Faculdade de Medicina de Ribeirão Preto Departamento de FisiologiaUniversidade de São Paulo Faculdade de Medicina de Ribeirão Preto Departamento de NeuropsiquiatriaUniversidade Federal de Santa Catarina Departamento de FarmacologiaCentral Nervous System Research Department Sanofi SynthelaboAston University Institute of Pharmaceutical SciencesHoffmann-La Roche Ltd.Universidade Federal de São Paulo (UNIFESP) Escola Paulista de Medicina Departamento de PsicologiaUniversity of Leeds Department of Psychology Ethopharmacology LaboratoryUniversidade Federal do EspÃrito Santo Centro de Biomedicina Departamento de Ciências FisiológicasUNIFESP, EPM, Depto. de PsicologiaSciEL
Numerical Computations with H(div)-Finite Elements for the Brinkman Problem
The H(div)-conforming approach for the Brinkman equation is studied
numerically, verifying the theoretical a priori and a posteriori analysis in
previous work of the authors. Furthermore, the results are extended to cover a
non-constant permeability. A hybridization technique for the problem is
presented, complete with a convergence analysis and numerical verification.
Finally, the numerical convergence studies are complemented with numerical
examples of applications to domain decomposition and adaptive mesh refinement.Comment: Minor clarifications, added references. Reordering of some figures.
To appear in Computational Geosciences, final article available at
http://www.springerlink.co
Regularized Linear Inversion with Randomized Singular Value Decomposition
In this work, we develop efficient solvers for linear inverse problems based
on randomized singular value decomposition (RSVD). This is achieved by
combining RSVD with classical regularization methods, e.g., truncated singular
value decomposition, Tikhonov regularization, and general Tikhonov
regularization with a smoothness penalty. One distinct feature of the proposed
approach is that it explicitly preserves the structure of the regularized
solution in the sense that it always lies in the range of a certain adjoint
operator. We provide error estimates between the approximation and the exact
solution under canonical source condition, and interpret the approach in the
lens of convex duality. Extensive numerical experiments are provided to
illustrate the efficiency and accuracy of the approach.Comment: 20 pages, 4 figure
Pharmacological Alterations of Anxious Behaviour in Mice Depending on Both Strain and the Behavioural Situation
A previous study comparing non-emotive mice from the strain C57BL/6/ByJ with ABP/Le mice showed ABP/Le to be more anxious in an open-field situation. In the present study, several compounds affecting anxiety were assayed on ABP/Le and C57BL/6/ByJ mice using three behavioural models of anxiety: the elevated plus-maze, the light-dark discrimination test and the free exploratory paradigm. The compounds used were the full benzodiazepine receptor agonist, chlordiazepoxide, and the antagonist, flumazenil, the GABAA antagonist, bicuculline, the full 5-HT1A agonist 8-OH-DPAT, and the mixed 5-HT1A/5-HT1B agonist, RU 24969. Results showed the effect of the compounds to be dependent on both the strain and the behavioural task. Several compounds found to be anxiolytic in ABP/Le mice had an anxiogenic effect on C57BL/6/ByJ mice. More behavioural changes were observed for ABP/Le in the elevated plus-maze, but the clearest findings for C57BL/6/ByJ mice were observed in the light-dark discrimination apparatus. These data demonstrate that anxious behaviour is a complex phenomenon which cannot be described by a single behavioural task nor by the action of a single compound
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