Forces and torques on a sphere moving near a dihedral corner in creeping flow

The low-Reynolds-number flow past a sphere moving near a right dihedral corner made by a stationary and a tangentially sliding wall is considered. Using the superposition principle, the arbitrary motion of the sphere is decomposed into simple elementary motions. Fully-resolved spectral-element simulations are carried out in the frame of reference translating and rotating with the particle such that the velocity on the particle’s surface vanishes. Forces and torques on the sphere are obtained as functions of the particle position near the corner. The data obtained are fitted by closed-form expressions which take into account symmetries of the problem, exact solutions, and asymptotic solutions from lubrication theory. The correlations obtained can easily be implemented in larger-scale one-way-coupled particulate-flow simulations to correct the particle motion near dihedral corners where mere point-particle models break down

Low-Prandtl-number B\'enard-Marangoni convection in a vertical magnetic field

The effect of a homogeneous magnetic field on surface-tension-driven B\'{e}nard convection is studied by means of direct numerical simulations. The flow is computed in a rectangular domain with periodic horizontal boundary conditions and the free-slip condition on the bottom wall using a pseudospectral Fourier-Chebyshev discretization. Deformations of the free surface are neglected. Two- and three-dimensional flows are computed for either vanishing or small Prandtl number, which are typical of liquid metals. The main focus of the paper is on a qualitative comparison of the flow states with the non-magnetic case, and on the effects associated with the possible near-cancellation of the nonlinear and pressure terms in the momentum equations for two-dimensional rolls. In the three-dimensional case, the transition from a stationary hexagonal pattern at the onset of convection to three-dimensional time-dependent convection is explored by a series of simulations at zero Prandtl number.Comment: 26 pages, 9 figure

Observation of Orbitally Excited B_s Mesons

We report the first observation of two narrow resonances consistent with states of orbitally excited (L=1) B_s mesons using 1 fb^{-1} of ppbar collisions at sqrt{s} = 1.96 TeV collected with the CDF II detector at the Fermilab Tevatron. We use two-body decays into K^- and B^+ mesons reconstructed as B^+ \to J/\psi K^+, J/\psi \to \mu^+ \mu^- or B^+ \to \bar{D}^0 \pi^+, \bar{D}^0 \to K^+ \pi^-. We deduce the masses of the two states to be m(B_{s1}) = 5829.4 +- 0.7 MeV/c^2 and m(B_{s2}^*) = 5839.7 +- 0.7 MeV/c^2.Comment: Version accepted and published by Phys. Rev. Let

Measurement of the ttbar Production Cross Section in ppbar collisions at sqrt s = 1.96 TeV in the All Hadronic Decay Mode

We report a measurement of the ttbar production cross section using the CDF-II detector at the Fermilab Tevatron. The analysis is performed using 311 pb-1 of ppbar collisions at sqrt(s)=1.96 TeV. The data consist of events selected with six or more hadronic jets with additional kinematic requirements. At least one of these jets must be identified as a b-quark jet by the reconstruction of a secondary vertex. The cross section is measured to be sigma(tbart)=7.5+-2.1(stat.)+3.3-2.2(syst.)+0.5-0.4(lumi.) pb, which is consistent with the standard model prediction.Comment: By CDF collaboratio

Search for chargino-neutralino production in ppbar collisions at sqrt(s) = 1.96 TeV

We present the results of a search for associated production of the chargino and neutralino supersymmetric particles using up to 1.1 fb-1 of integrated luminosity collected by the CDF II experiment at the Tevatron ppbar collider at a center-of-mass energy of 1.96 TeV. The search is conducted by analyzing events with a large transverse momentum imbalance and either three charged leptons or two charged leptons of the same electric charge. The numbers of observed events are found to be consistent with standard model expectations. Upper limits on the production cross section are derived in different theoretical models. In one of these models a lower limit on the mass of the chargino is set at 129 GeV/c^2 at the 95% confidence level.Comment: To be submitted to Phys.Rev.Let

The mechanics of particle accumulation structures in thermocapillary flows

The motion of small particles suspended in a cylindrical thermocapillary liquid bridge is considered. Owing to geometry and surface stresses the streamlines gather near the cylindrical free surface and provoke particle–free-surface collisions. We show numerically that tracers which are perfect but of finite size can accumulate on closed trajectories. A simple model is proposed to explain the attraction of particles to the closed trajectory based on the flow topology in the vicinity of a closed streamline which comes sufficiently close to the free surface and on particle–free-surface collisions which transfer particles among different streamlines

Particle-depletion dynamics in axisymmetric thermocapillary flows

The removal of suspended particles from the interior of a thermocapillary liquid bridge via a finite-particle-size effect restricting the particle motion near the free surface is analyzed in the framework of a model flow. The particle depletion occurs on the same short time scale as does the particle accumulation in experiments. Furthermore, the time scale diverges in a similar manner for decreasing particle size. The dependence of the time scale for particle accumulation on the particle size is explained in terms of a diverging return time to the free surface for those finite-size particles which are subject to the particle-free surface-interaction

On the different manifestations of particle accumulation structures (PAS) in thermocapillary flows

Particle de-mixing in flows in liquid-bridges driven by the Marangoni effect is investigated using primarily analytical models of the flow. The mechanism of particle–free-surface collisions is shown to explain the formation of experimentally observed particle depletion zones. This mechanism causes a mapping (or transfer) of particles moving on certain streamlines to other streamlines resulting in creation of a distinct depletion zone. Moreover, we demonstrate line-like particle accumulation along a chaotic streamline corresponding to SL2-PAS which is closed by a trajectory segment which is created by particle–free-surface interaction. The resulting limit cycle is stable due to the combined properties of the bulk transport and gathering at the free surface

Thermocapillary convection in liquid bridges with a deformed free surface

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