6,498 research outputs found
The scenario of two-dimensional instabilities of the cylinder wake under EHD forcing: A linear stability analysis
We propose to study the stability properties of an air flow wake forced by a dielectric barrier discharge (DBD) actuator, which is a type of electrohydrodynamic (EHD) actuator. These actuators add momentum to the flow around a cylinder in regions close to the wall and, in our case, are symmetrically disposed near the boundary layer separation point.
Since the forcing frequencies, typical of DBD, are much higher than the natural shedding frequency of the flow, we will be considering the forcing actuation as stationary.
In the first part, the flow around a circular cylinder modified by EHD actuators will be experimentally studied by means of particle image velocimetry (PIV). In the second part, the EHD actuators have been numerically implemented as a boundary condition on the cylinder surface. Using this boundary condition, the computationally obtained base flow is then compared with the experimental one in order to relate the control parameters from both methodologies.
After validating the obtained agreement, we study the Hopf bifurcation that appears once the flow starts the vortex shedding through experimental and computational approaches. For the base flow derived from experimentally obtained snapshots, we monitor the evolution of the velocity amplitude oscillations. As to the computationally obtained base flow, its stability is analyzed by solving a global eigenvalue problem obtained from the linearized Navier–Stokes equations. Finally, the critical parameters obtained from both approaches are compared
Thermal conductivity of Mg-doped CuGeO_3 at very low temperatures: Heat conduction by antiferromagnetic magnons
Thermal conductivity \kappa is measured at very low temperatures down to 0.28
K for pure and Mg-doped CuGeO_3 single crystals. The doped samples carry larger
amount of heat than the pure sample at the lowest temperature. This is because
antiferromagnetic magnons appear in the doped samples and are responsible for
the additional heat conductivity, while \kappa of the pure sample represents
phonon conductivity at such low temperatures. The maximum energy of the magnon
is estimated to be much lower than the spin-Peierls-gap energy. The result
presents the first example that \kappa at very low temperatures probes the
magnon transport in disorder-induced antiferromagnetic phase of spin-gap
systems
Cooperative ordering of gapped and gapless spin networks in CuFeGeO
The unusual magnetic properties of a novel low-dimensional quantum
ferrimagnet CuFeGeO are studied using bulk methods, neutron
diffraction and inelastic neutron scattering. It is shown that this material
can be described in terms of two low-dimensional quantum spin subsystems, one
gapped and the other gapless, characterized by two distinct energy scales.
Long-range magnetic ordering observed at low temperatures is a cooperative
phenomenon caused by weak coupling of these two spin networks.Comment: 4 pages, 4 figure
Neutron Diffraction Study of the Pressure-Induced Magnetic Ordering in the Spin Gap System TlCuCl
Neutron elastic scattering measurements have been performed under the
hydrostatic pressure in order to investigate the spin structure of the
pressure-induced magnetic ordering in the spin gap system TlCuCl. Below the
ordering temperature K for the hydrostatic pressure
GPa, magnetic Bragg reflections were observed at the reciprocal lattice points
{\mib Q}=(h, 0, l) with integer and odd , which are equivalent to
those points with the lowest magnetic excitation energy at ambient pressure.
This indicates that the spin gap closes due to the applied pressure. The spin
structure of the pressure-induced magnetic ordered state for GPa was
determined.Comment: 4 pages, 3 figures, 3 eps files, jpsj2.cls styl
New connection formulae for some q-orthogonal polynomials in q-Askey scheme
New nonlinear connection formulae of the q-orthogonal polynomials, such
continuous q-Laguerre, continuous big q-Hermite, q-Meixner-Pollaczek and
q-Gegenbauer polynomials, in terms of their respective classical analogues are
obtained using a special realization of the q-exponential function as infinite
multiplicative series of ordinary exponential function
Enhancing the spectral gap of networks by node removal
Dynamics on networks are often characterized by the second smallest
eigenvalue of the Laplacian matrix of the network, which is called the spectral
gap. Examples include the threshold coupling strength for synchronization and
the relaxation time of a random walk. A large spectral gap is usually
associated with high network performance, such as facilitated synchronization
and rapid convergence. In this study, we seek to enhance the spectral gap of
undirected and unweighted networks by removing nodes because, practically, the
removal of nodes often costs less than the addition of nodes, addition of
links, and rewiring of links. In particular, we develop a perturbative method
to achieve this goal. The proposed method realizes better performance than
other heuristic methods on various model and real networks. The spectral gap
increases as we remove up to half the nodes in most of these networks.Comment: 5 figure
BiCuVO: a new narrow-band spin-gap material
A new spin-ladder family material BiCuVO is studied by means of the
magnetic susceptibility, heat capacity and neutron inelastic scattering
measurements on powder sample. Singlet ground state and a finite spin gap are
confirmed by thermal-activated type susceptibility and by distinct peak at 16
meV in spin excitation. Triple narrow band structure in spin excitation
spectrum, probably due to complex crystal structure, is observed and the
possibility of weakly-interacting spin-cluster system is discussed
Correlations and Omori law in Spamming
The most costly and annoying characteristic of the e-mail communication
system is the large number of unsolicited commercial e-mails, known as spams,
that are continuously received. Via the investigation of the statistical
properties of the spam delivering intertimes, we show that spams delivered to a
given recipient are time correlated: if the intertime between two consecutive
spams is small (large), then the next spam will most probably arrive after a
small (large) intertime. Spam temporal correlations are reproduced by a
numerical model based on the random superposition of spam sequences, each one
described by the Omori law. This and other experimental findings suggest that
statistical approaches may be used to infer how spammers operate.Comment: Europhysics Letters, to appea
Structural Critical Scattering Study of Mg-Doped CuGeO3
We report a synchrotron x-ray scattering study of the diluted spin-Peierls
(SP) material Cu_(1-x)Mg_xGeO_3. We find that for x>0 the temperature T_m at
which the spin gap is established is significantly higher than the temperature
T_s at which the SP dimerization attains long-range order. The latter is
observed only for xx_c the SP correlation length
quickly decreases with increasing x. We argue that impurity-induced competing
interactions play a central role in these phenomena.Comment: 5 pages, 4 embedded eps figures, to appear in PR
- …