Principal Physical Mechanisms Research Articles

Performance Analysis of Cavitating Flow in Centrifugal Pumps Using Multiphase CFD

A multi-phase CFD method is used to analyze centrifugal pump performance under developed cavitating conditions. The differential model employed is the homogeneous two-phase Reynolds-Averaged-Navier-Stokes equations, wherein mixture momentum and volume continuity equations are solved along with vapor volume fraction continuity. Mass transfer modeling is provided for the phase change associated with sheet cavitation. Using quasi-3D (Q3D) analysis, steady and time-dependent analyses were performed across a wide range of flow coefficients and cavitation numbers. Characteristic performance trends associated with off-design flow and blade cavitation are observed. The rapid drop in head coefficient at low cavitation numbers (breakdown) is captured for all flow coefficients. Local flow field solution plots elucidate the principal physical mechanisms associated with the onset of breakdown.

High yield non-axisymmetric atomization of nickel base superalloys : R.W.Christensen et al. (Valimet Inc. Stockton, USA.) Int. J. Powder Metall., vol 33, no 7, 1997, 37–50

A preliminary study has shown that certain nonaxisymmetric close-coupled atomization geometries demonstrate superior efficiency in the production of fine nickel base superalloy powder compared to conventional close-coupled atomization utilizing an axisymmetric annular gas orifice and melt nozzle, Test conditions range from creating minor disturbances in the gas flow to substantial changes to both the gas flow and geometry of the melt nozzle. Various tests assess the mechanisms by which this performance is achieved; these include local static and dynamic pressure measurements, flow visualization and water atomization tests. The principal physical mechanisms leading to yield improvement are thought to be atomization plume spreading, and improved melt film formation at the nozzle tip. The film formation and stabilization effects have a basis in a flow model developed previously for symmetric geometries. The highest improvement in powder yield occurs when the nonaxisymmetric nozzles are operated with atomization parameters that result in nonaxisymmetric atomization plumes. Recognition of these plume characteristics provides a basis for process control and improved yields of fine powder.

On the hollow-cathode effect: conventional and modified geometry

The hollow-cathode effect (HCE) is studied for conventional and modified hollow-cathode discharges (HCDs). HCDs are considered that operate at substantial current densities and moderate voltages (e.g. in a conventional HCD, >1 mA and <400 V, respectively), which are typical of the HCDs in common use. A simple kinetic model is developed and used to predict that the HCE exists even if a small fraction of the total ionization occurs in the cathode-fall regions (CF). The HCE results from ionization becoming more efficient in the negative glow (NG) so that no significant ionization is required to occur in the CF. Consequently, the size of the CF diminishes and a large increase in current takes place. The main features of the HCE are reproduced and good agreement with experiment is found. The results suggest that low-slope-resistance voltage-current characteristics are realized when the ionization in the CF is small. It is next demonstrated that the ions play an important role and that even a small increase in the ion flux from the NG to the CF can contribute to the HCE. The principal geometrical criteria for the conventional and modified HCDs are presented. A segmented HCD tube is considered to study an interesting transition from a conventional HCD through a modified HCD to an abnormal glow discharge. Finally, the principal physical mechanisms which determine the operating characteristics of the modified HCD are identified and discussed.

Seasonal phytoplankton dynamics in the eastern tropical Atlantic

The coastal zone color scanner (CZCS) that operated aboard the Nimbus 7 satellite provided extensive coverage of phytoplankton pigment concentrations in the surface waters of the eastern tropical Atlantic (ETA) from March 1979 to February 1980 and coincided with four major research cruises to this region. Total primary production within the ETA (5°N–10°S, 25°W–10°E) was determined from CZCS pigment estimates and an empirical algorithm derived from concurrent in situ data taken along 4°W that relates near‐surface chlorophyll concentration and integrated primary production. We estimated an average annual production for the ETA of 2.3 Gt C yr−1 with an associated 3.5‐fold seasonal variation in the magnitude of this production. We describe the principal physical mechanisms controlling seasonal phytoplankton dynamics within the ETA and propose that in addition to seasonal change in the thermocline depth, one must also consider changes in the depth of the equatorial undercurrent. An extensive validation effort indicates that the standard CZCS global products are a conservative estimate of pigment concentrations in ETA surface waters. Significant underestimates by the CZCS global products were observed in June and July which we attributed, in part, to aerosol correction errors and, more importantly, to errors caused by a significant reduction in the concentration of near‐surface dissolved organic matter that resulted from strong equatorial upwelling.

Influence of helium and impurities on the tensile properties of irradiated vanadium alloys

Helium preimplantation and neutron irradiation influence on the tensile properties of vanadium alloys in the irradiation/test temperature range of 500–800°C were investigated and principal physical mechanisms of the mechanical behaviour of the irradiated alloys were considered.

Gas flow and mixing in the airways.

To survey the current state of scientific knowledge of gas flow and mixing in pulmonary airways, especially at high frequencies. Results from the authors' own laboratory studies and Western bioengineering literature on respiratory fluid mechanics. This survey concentrates on understanding the principal physical mechanisms that underlie the enhancement of airway gas transport in high-frequency oscillation. The results of experimental, computational, and mathematical studies are described. The topic was covered by seven presenters at the Münster Meeting on High Frequency Ventilation, January 31 to February 2, 1993. Six of these presentations are summarized in the six sections of this paper under the following headings: Introductory Survey; Three-Dimensional Numerical Simulation of Inspiratory and Expiratory Flows in Small Airways; Computational and Experimental Models of High-Frequency Oscillation; Unsteady Gas Mixing in Airways; Gas Dispersion From the Lagrangian Viewpoint; and Soluble Gas Mass Transfer in Tubes and Airways. The dominant mechanism for the enhancement of gas transport along airways at high frequency is likely to lie in the coupling of the secondary motions caused by airway curvature with the oscillatory longitudinal flow. However, laboratory experiments and theoretical analyses have led only to an accurate simulation of the phenomena in idealized geometries, not in real lungs.

Electromagnetic generation of ultrasound in ferromagnets

This review discusses the principal physical mechanisms responsible for direct electromagneticacoustic conversion in ferromagnets. It is shown that in a wide range of frequency, magnetic field, and temperature ultrasound is generated via the inductive and magnetoelastic interactions. In the latter mechanism features are observed that are due to the displacement of domain walls and spin flip, and are also observed at various phase transitions. A detailed comparison of the theory of electromagnetic excitation of ultrasound with experimental data obtained for 3d and 4f magnets also showed that this phenomenon can be used as a precise method of constructing magnetic phase diagrams of magnets and determining their homogeneous and inhomogeneous exchange interaction constants, and the magnetic anisotropy and magnetostriction constants.

Nonlinear saturation of single-mode trapped electron instability

The trapped electron instability, like other types of drift waves, may appear as a single mode in many linear plasma devices. Recognizing the significant variation of density gradient over the mode localization region, a nonlocal linear analysis in slab model yields eigenfunctions for Weber-like equations. Using three-wave, nonresonant, two-dimensional mode coupling based on these Weber-like eigenfunctions, two mode-coupling equations for the fundamental and higher-order radial harmonics are derived. The solution of these coupled equations yields the nonlinear saturation level of the unstable mode. Mode coupling to the damped higher-order radial modes is the principal physical saturation mechanism of the unstable fundamental mode.

Nonlinear saturation of single-mode trapped ion instability

The trapped ion instability appears as a single mode in the Columbia Linear Machine and is expected to appear as such in tandem mirrors. Using three-wave, nonresonant mode coupling of radially nonlocal eigenfunctions, two mode-coupling equations for the unstable fundamental and stable first-order radial harmonics are derived and solved. Mode coupling to the damped higher-order radial harmonics is the principal physical saturation mechanism of the unstable fundamental mode.

Effect of static normal stresses on energy dissipation with interconnected bending and torsional vibrations of rods

1. The effect of the static stresses σc on the energy dissipation depends on the principal physical mechanism of damping, the ratio of the variable stresses σα and τα, and on the sign of σc. 2. The principal physical mechanism determines the qualitative nature of the change in energy dissipation when σc is superimposed (it is analogous to the change in energy dissipation with simple vibrations). 3. The effect of σc on the damping properties of the material (\(\overline \delta\)) decreases, approaching unity, with increasing logarithmic decrement of the vibrations and with variation of the value of K [an exception is steel 1Kh18N9T for which the effect of σc on the energy dissipation is practically nil (\(\overline \delta \approx 1\))]. 4. The level of the energy losses with interconnected vibrations reacts insignificantly on the sign of σc. The greatest influence is found when K = ∞.

Transistor Internal Parameters for Small-Signal Representation

The joint IRE-AIEE Task Group 28.4.7, on Transistor Internal Parameters, was organized with the following objectives: 1) To formulate a small-signal equivalent-circuit representation of a transistor, the parameters of which emphasize separately the principal physical mechanisms of the device. 2) To recommend symbols for these parameters consistent with accepted usage and other standards. 3) To exhibit the relationship between the equivalent-circuit representation described in item 1 above and those simplified representations commonly employed in circuit analysis or design. 4) To propose and discuss methods of determining these parameters from electrical measurements at the terminals. This report summarizes the work of the group on the first three of these objectives.

A PRELIMINARY QUANTITATIVE ANALYSIS OF PRECIPITATION MECHANISMS

Abstract The two principal physical mechanisms for the formation of precipitation are considered to be the Bergeron-Findeisen ice-crystal process and the collision of particles of unlike size in the gravitational field. Quantitative estimates of the growth of precipitation elements by the two processes are made. It is found that the ice-crystal effect is much more rapid than the collision process in the initial stages of growth. When the particles are of a mass comparable to drizzle drops, the two processes are equally effective; for larger particles the accretion process becomes more rapid than the ice-crystal effect. For typical mid-latitude conditions it is concluded that neither process, acting alone, is capable of producing rain drops of the customarily observed size. It appears that there is required some combination of the two processes such as the clumping of ice crystals to form snow flakes, the growth of ice crystals which melt and then grow by accretion, or the growth of graupel by the simultan...