Structure of a Bathtub Vortex : Importance of the Bottom Boundary Layer

A bathtub vortex in a cylindrical tank rotating at a constant angular velocity [omega] is studied by meansof a laboratory experiment, a numerical experiment and a boundary layer theory. The laboratory and numerical experiments show that two regimes of vortices in the steady-state can occur depending on [omega] and the volume flux Q through the drain hole: when Q is large and [omega] is small, a potential vortex is formed in which angular momentum outside the vortex core is constant in the non-rotating frame. However, when Q is small or [omega] is large, a vortex is generated in which the angular momentum decreases with decreasing radius. Boundary layertheory shows that the vortex regimes strongly depend on the theoretical radial volume flux through the bottomboundary layer under a potential vortex : when the ratio of Q to the theoretical boundary-layer radial volume flux Qb (scaled by 2π R2([omega] ν)12 ) at the outer rim of the vortex core is larger than a critical value (of order 1), the radial flow in the interior exists at all radiiand Regime I is realized, where R is the inner radius of the tank and ν the kinematicviscosity.When the ratio is less than the critical value, the radial flow in the interior nearlyvanishes inside a critical radius and almost all of the radial volume flux occurs only in the boundary layer,resulting in Regime II in which the angular momentum is not constant with radius. This criterion is found to explain the results of the laboratory and numerical experiments very well

Modeling two-phase flow in a swirl combustor. Final report. A. R. A. P. report No. 310

A phenomenological model of flow in a cyclone coal combustor-gasifier is presented. The model predicts how four performance parameters which measure the carbon conversion efficiency, ash separation, pressure drop, and heat losses depend on such design variables as chamber pressure, size and shape, flow rates, and stoichiometry, along with coal particle size and composition. Correct prediction of the performance parameters requires accurate analysis of the coal particle combustion, particle mechanics and fluid mechanics, all of which are coupled in this confined vortex flow. The model is based on assimilating simple approximations to the most important mechanisms governing flow and combustion in the chamber and thus necessarily involves empirical coefficients. Model verification by comparison with laboratory results from Pittsburgh Energy Research Center shows reasonable agreement, but due to the empiricism required in the model further tests are warranted. Model derivation and sensitivity analysis pinpoint the phenomena which are most critical to the performance of a cyclone combustor. Methods for reducing the empirical requirement of the model are discussed and recommendations made for model improvement

The NPS-FEL Injector Upgrade

The Naval Postgraduate School (NPS) has begun the design and assembly of the NPS Free-Electron Laser (NPS-FEL). As part of this effort, the original DC gunbased injector system from the Stanford Superconducting Accelerator has been moved to NPS and is being refurbished and upgraded to operate as a photoinjector. Design work has begun on a new, SRF, quarter-wave resonator based cavity that can serve as either an energy booster or photocathode gun. The overall NPS-FEL design parameters are for 40- MeV beam energy, 1 nC bunch charge, and 1 mA average beam current, built as an energy-recovery linac in its final configuration [1]. As we move towards this goal, the injector system will be incrementally upgraded to add photocathode capability, have a higher final beam energy, and improve the beam brightness, to meet the needs of the overall experimental program

Modelisation du kilometre inferieur de l'atmosphere

SIGLECNRS-CDST / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Design and operation of a superconducting quarter-wave electron gun

The article of record as published may be found at: http://dx.doi.org10.1103/PhysRevSTAB.14.053501Superconducting radio-frequency electron guns are viewed by many as the preferred technology for generating the high-quality, high-current beams needed for future high power-free-electron lasers and energy recovery linacs. All previous guns of this type have employed elliptical cavities, but there are potential advantages associated with other geometries. Here we describe the design, commissioning, and initial results from a superconducting radio-frequency electron gun employing a quarter-wave resonator configuration, the first such device to be built and tested. In initial operation, the gun has generated beams with bunch charge is excess in 78 pC, energy of 469 keV, and normalized rms emittances of about 4.9 um. Currently, bunch charge is limited by the available drive laser energy, and beam energy is limited by x-ray production and the available rf power. No fundamental limits on beam charge or energy have been encountered, and no high-field quenching events have been observed.Office of Naval Research and the High Energy Laser Joint Technology Office.Approved for public release; distribution is unlimited