Departure Velocity Research Articles

Development of numerical computational model for revolving metallic particles' behavior in GIS and its evaluation

It is widely accepted that gas-insulated switchgear (GIS) has proven to be reliable, compact and has high availability. However, metallic particles forced to fly and kept in motion in a high electric field can cause partial discharges which lead to a flashover of GIS. For a metallic wire particle less than 3 mm long, its revolving flight and horizontal migration behavior become remarkable. This has been confirmed using a high-speed framing video camera. For the revolving particle, we also found that the viscous drag which acts on it controls its flying behavior predominantly. We have formulated a time motion equation for a revolving metallic particle on the basis of statistical analysis of the time-resolved and digitized motion data obtained by a high-speed framing video camera and taking the drag as one of the major force terms. Numerical solution of the time-motion equation gives the maximum flight height-time curves, incidence, departure velocity, and migration velocity of the revolving particle against the grounded electrode with or without slope. Fairly good agreement has been confirmed between the measured and simulated dynamic behavior of the revolving particles. Other major parameters such as allowable maximum flight height, the climbing or descending speed and distance along the slope of the grounded electrode and the trapping factors of a particle trap have been revealed deductively through the simulation. This enables it to optimize the configuration and the operational performance of the particle trap. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 153(2): 28–38, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20122

Low thrust transfer optimisation of satellites formations to heliocentric Earth trailing orbits through a gradient restoration algorithm

When dealing with mission requirements calling for highly stable gravitational and thermal environments, one of the common options always considered is that of the heliocentric Earth trailing orbits (HETO). This is the case, for instance, of the Laser Interferometer Space Antenna (LISA), a joint ESA-NASA effort aimed at detecting gravitational radiation from deep-space sources, hence testing the fundamental gravitational theories. This article is about the low-thrust transfer trajectory optimisation for a three-satellites flotilla to a HETO. To this end, and after a brief presentation of the selected reference mission (LISA), a five impulses transfer strategy is conceived to build a solution guess for the ignition of the Sequential Gradient Restoration Algorithm (SGRA), which solves the full optimal control problem (FOCP) of finding the optimal low thrust profiles injecting each SC to its respective target orbit while minimising a given functional. An inequality path constraint on the Solar Aspect Angle (SAA) is imposed on the low-thrust vector of each SC. Finally several relevant conclusions are derived from the presented results, among them the direct relationship between the departure velocity from Earth, the final SC masses and the transfer duration.

Development of Numerical Computational Model for Revolving Metallic Wire Particles' Behavior in GIS and its Evaluation

It has been widely accepted that Gas Insulated Switchgear (GIS) has proven to be reliable, compact and has high availability. However, metallic particles forced to fly and kept in motion in high electric field, can cause partial discharges which lead to a flashover of GIS. For the metallic wire particle with the length less than 3mm, its revolving flight and horizontal migration behavior become remarkable. This has been confirmed by the high speed framing video camera. For the revolving particle, we also found that the viscous drag which acts on it controls its flying behavior predominantly. Authors have formulated time motion equation for a revolving metallic particle on the basis of the statistical analysis of the time-resolved and digitized motion data obtained by a high speed framing video camera and taking the drag as one of the major force term. Numerical solution of the time-motion equation gives the maximum flight height-time curves, incidence, departure velocity and migration velocity of the revolving particle against the grounded electrode with or without slope. Fairly well-agreements have been confirmed between the measured and simulated dynamic behavior of the revolving particles. The other major parameters such as allowable maximum flight height, the climbing or descending speed and distance along the slope of the grounded electrode and the trapping factors of a particle trap has been revealed deductively through the simulation. This enables it to optimize the configuration and the operational performance of the particle trap.

Bubble Dynamic Parameters and Pool Boiling Heat Transfer on Plasma Coated Tubes in Saturated R-134a and R-600a

An optical method for measuring the bubble dynamic data subject to an isolated bubble model is presented at low heat flux q⩽1kW/m2; while the operating heat fluxes are up to 30 kW/m2. By simultaneous measurements of departure diameters, velocities, frequencies and nucleation site densities, the heat transfer contribution of an individual active site is evaluated. A single phase heat transfer correlation was used to model the present heat transfer data. The test specimens consisted of tubes with porous copper (Cu) and molybdenum (Mo) plasma coated surfaces. The porosity (ε), the thickness of the porous layer (δ), and the mean pore diameter (η) of the tested tubes are the following: 0.055⩽ε⩽0.057,100⩽δ⩽300μm, and 3⩽η⩽4μm. The tests were carried out using R-134a and R-600a as working fluid at a saturation temperature of 18°C and with low and moderate heat fluxes (⩽1 kW/m2) for boiling visualization and related measurements (⩽30 kW/m2).

Effects of heater surface orientation on the critical heat flux—I. An experimental evaluation of models for subcooled pool boiling

An experimental study of boiling at high heat fluxes in low-velocity subcooled forced convection boiling is presented, demonstrating the effects of subcooling and buoyancy orientation on the critical heat flux (CHF) and the bubble residence time. At the low velocity of 0.04 m s −1 used, the flow forces acting on the vapor are insignificant compared with buoyancy and the CHF behaves as in pool boiling, depending primarily on the orientation of the heater surface with respect to gravity. This dependence is related to the bubble residence time, which is found to be inversely proportional to the CHF for all heater orientations at a given operating condition. This relationship, that the product of the CHF and the corresponding bubble residence time is a constant, suggests that the mechanism for dryout is independent of the heater surface orientation, despite changes in the vapor departure velocity and the CHF with the orientation angle. Furthermore, increases in the bulk liquid subcooling substantially reduce the net rate of vapor generation such that the bubble residence times at the CHF are independent of subcooling. The work concludes by proposing the energy per unit area leaving the heater surface during the bubble residence time as the CHF mechanism, which is more generalized than the mechanism assumed in the macrolayer dryout models.

Statistical analysis of the surface circulation of the California Current

We use a set of mixed‐layer drifting buoy trajectories from the California Current (20°N–40°N) during 1985–1990 to obtain statistically reliable estimates of the mean currents, the mean variance field, and the geographically varying diffusivity, integral timescales, and integral space scales. Typical values for the diffusivity are 1.1–8.7 × 107 cm2 s−1, while the timescales and space scales are 2.1–7.1 days and 16–59 km, respectively. The variance field displays a strong westward gradient out to 125°W, and diffusivity shows a tendency to decrease toward the southwest part of the domain. Significant anisotropy is found in the variance field near the coastal boundary and at 30°N, 130°W, which is the region where the subarctic and northern subtropical fronts approach the California Current. The antisymmetric component of the diffusivity tensor indicates that cyclonic eddies dominate the mesoscale signature of drifters in this region. We seek simple parameterizations to relate the scales of motion of the random velocity field to the diffusivity by testing least squares fits to κ∞ ∝ u02T and κ∞ ∝ u0L, where u02 is the velocity variance. We found no cases for which these two hypotheses could be distinguished. For the meridional component the linear regressions are not successful, which suggests that the meridional departure velocities result from a flow regime that is significantly organized by, for example, waves or coherent structures. A subset of the drifters measured temperature along their tracks, and we use the resultant data to produce the first direct estimates of the horizontal eddy heat flux divergence based on Lagrangian estimates. In addition, we separately compute the “eddy diffusivity” parameterization of the eddy heat flux divergence, ∇ · 〈u′θ′〉 = ∇(κ∇Θ), using our diffusivity estimates and a sea surface temperature climatology. The two independent terms agree well, which provides a measure of reassurance about the diffusivity estimates. The eddy heat flux divergence in the California Current is very small (<5 W m−2) and does not appear to be significant in the long‐term heat budget of the upper ocean in this region.

Spray impingement on a hot surface in reacting stagnation flows

We report experimental and theoretical results concerning spray impingement on a hot wall in a laminar stagnating flow, both with and without a flat premixed flame present near the solid surface. Water droplets with diameters up to 200 μm are transported in a fuel-lean methane-air stream from a circular duct toward a stainless-steel plate. When the flame is present, it heats the wall to a temperature around 1250 K and causes the smaller droplets to disappear whereas the larger droplets cross the flame and impinge on the hot wall. Influences of the wall temperature on droplet impingement are studied by extinguishing the flame and allowing the plate to cool gradually to room temperature. Trajectories of the water droplets in the spray, illuminated by a Ar + laser, are recorded by a camera. Measurements of droplet diameter and of two components of velocity are made by a phase Doppler particle analyzer. Velocity profiles of the gas without spray are also measured with the phase Doppler particle analyzer by seeding the flow with MgO particles. The experimental results provide fundamental information on droplet vaporization in the flame, on the relationship between arrival and departure velocities of droplets striking the wall, and on the influence of wall temperature on droplet breakup. Simple theoretical models are developed to describe droplet collision dynamics at the wall and droplet trajectories in the stagnation flow. Comparisons between theory and experiment reveal how droplets respond to the flame and to the hot wall.

Gas-liquid turbulent two phase flow along vertical flat plate with gas evolution.

Gas-liquid turbulent two phase flow along a vertical flat plate with gas evolution was numerically simulated with a two-fluid model and Plandtl’s mixing length theory using the SIMPLER method. The calculation domain was assumed to be two-dimensional and gas bubbles to be evolved uniformly from the plate surface. The computed velocity profiles were affected by void fraction on the wall and the bubble diameters, but not by the bubble departure velocity from the plate surface. Both the gas and liquid velocity profiles and the computed wall shear stress with the present model were qualitatively and quantitatively coincident with those measured using LDV for the vertical flat plate electrode with H2 or O2 gas evolution.

Inflation in an exponential-potential scalar field model.

A spatially flat cosmological scalar field $\ensuremath{\Phi}$ model with the scalar field potential $\ensuremath{\propto}\mathrm{exp}(\ensuremath{-}\frac{\ensuremath{\Phi}}{\sqrt{p}})$, $pg1$, provides a simple class of inflationary cosmologies (which includes the usual exponential expansion inflation) that may be used as an analytical testing ground to help understand the predictions of the inflation model of the very early Universe. We divide the evolution of this model into three distinct epochs: scalar-field dominance and conventional radiation and baryon dominance; in each epoch we only account for irregularities in the dominant form of matter. We present closed-form solutions of the (synchronous gauge) relativistic linear perturbation equations that govern the evolution of inhomogeneities. These classical solutions, augmented with quantum-mechanically motivated initial conditions and joining conditions to match the expressions for the irregularities at the scalar-field-radiation and radiation-baryon transitions, are used to estimate the large-time form of the spectrum of energy-density irregularities, of the local departure velocity from homogeneous expansion, of large-scale fluctuations in the microwave background temperature, and of the gravitational-wave energy density. The inflation epoch results agree with those found from a purely quantum-mechanical analysis. Depending on the value of $p$ this model can have more large-scale power than the usual scale-invariant spectrum (at the expense of less small-scale power) and would seem to be marginally better at forming large-scale structure than the canonical model; however, the decrease in small-scale power serves to exacerbate the problem of late galaxy formation. As the model approaches the exponential expansion inflation limit, the power spectrum tends towards the scale-invariant form, although, in this limit the numerical prefactor diverges. We find that transverse peculiar velocity perturbations are not generated. Normalizing by fitting to the observed large-scale departure velocity, we find that models which stop inflating around ${10}^{7}$-${10}^{16}$ GeV are not obviously observationally inconsistent.

Impulsive Interplanetary Transfers for Prescribed Launch Date

A method of solution, suitable also for hand computation, to determine all of the twoand threedimensional ballistic interplanetary transfer trajectoiies which exist for a chosen launch date is outlined. Eccentricities of the planetary orbits have been taken into account. A new optimality condition tha t isolates those transfer orbits requiring a minimum of characteristic velocity is also derived. Numerical results of some Earth-to-Mars transfers for arbitrarily selected departure dates are summarized in a series of curves tha t display characteristic velocity expenditures and departure velocity orientation for trips of various durations. When the initial location of the planets is not specified a priori, an optimal mode of transfer exists for every transfer angle chosen. Specification of planetary configuration at departure acts as a constraint on the motion and causes optimal trajectories to occur only for discrete values of transfer angle. Unlike the coplanar case, threedimensional transfers give rise to two regions of minimal velocity, separated by a narrow inaccessible belt of transfer angles in the vicinity of 180°, which correspond to departures at right angles to the ecliptic plane.