Excess Velocity Research Articles

IMPEDIMENT TO INCIPIENT FLUIDIZATION IN WET BEDS OF POROUS NONSPHERICAL PARTICLES

ABSTRACT Fluidized-bed units are efficiently and advantageously employed for drying various wet and sticky particulate materials provided that the bed of such materials can be kept under a fluidized condition. The effect was explored with the incremental addition of water to the bed of nonspherical, porous particles of ceramsite and lignite fluidized with wet air. The limiting fluidization-defluidization point was determined by experiments in a cold model fluidized bed contained in a transparent glass column using wet and dry particles. On the basis of the data amassed, an empirical correlation was developed. This relationship makes it possible to predict the dimensionless excess gas velocity that keeps the wet bed fluidized, as a function of the relative amount of moisture in the bed and the dimensionless particle size.

LOW REYNOLDS NUMBER TURBULENT LIFTED FLAMES IN HIGH CO-FLOW

ABSTRACT This study presents the results of experiments designed to investigate flame lift-off behavior to nozzle velocity, co-flow velocity, fuel-type, and nozzle size for low Reynolds Number turbulent flows (in and near the hysteresis regime). Local excess jet velocities are computed using jet relations from Tieszen et al. The results show that the local excess jet velocity remains linear with respect to nozzle velocity through most of the hysteresis regime, even though flame lift-off height is not linear. This suggests a non-linear relation not captured by Kalghatgi (1984) for lift-off in the near field and hysteresis regime. Local excess jet velocities at the reattachment point were also computed for flames that are lifted more than three nozzle diameters above the burner. The results show that there is a minimum excess jet velocity for which a flame can stabilize. This minimum velocity is inversely proportional to the laminar burning velocity of the fuel squared. A new relation for lift-off height at the reattachment point for flames in the hysteresis region is derived and compared to experimental data.

Near-Optimal Low-Thrust Earth-Mars Trajectories via a Genetic Algorithm

A genetic algorithm is used to determine several types of Earth‐Mars trajectories, with the objective of maximizing payload delivered to Mars. The trajectories have three phases: Earth escape, heliocentric flight, and arrival into low Martian orbit. The actual planetary orbits are used with one approximation: Mars’s very small orbit inclination to the ecliptic plane is ignored. Impulses provided by chemical rockets are used for Earth departure and capture into orbit about Mars. The optimizer chooses the magnitude and direction of the impulse explicitly for the departure from Earth and implicitly, by choosing the hyperbolic excess velocity vector, for the arrival at Mars. The heliocentric flight uses low-thrust nuclear‐electric propulsion; cases with continuous thrust and cases in which a coasting, that is, a no-thrust phase, is allowed are both considered. The continuous time history of the thrust pointing angle is modeled using a sequence of cubic polynomials whose coefficients become parameters of the genetic algorithm’s chromosome. The fitness function of the genetic algorithm is dynamic; it first forces the spacecraft to arrive within the Martian sphere of influence. With that accomplished, it then emphasizes optimizing final mass.

Conservative special relativistic radiative transfer for multidimensional astrophysical simulations: Motivation and elaboration

Many astrophysical phenomena exhibit relativistic radiative flows. While velocities in excess of $v \sim 0.1c$ can occur in these systems, it has been common practice to approximate radiative transfer to $\cO(v/c)$. In the case of neutrino transport in core-collapse supernovae, this approximation gives rise to an inconsistency between the lepton number transfer and lab frame energy transfer, which have different $\cO(v/c)$ limits. A solution used in spherically symmetric $\cO(v/c)$ simulations has been to retain, for energy accounting purposes, the $\cO(v^2/c^2)$ terms in the lab frame energy transfer equation that arise from the $\cO(v/c)$ neutrino number transport equation. Avoiding the proliferation of such ``extra'' $\cO(v^2/c^2)$ terms in the absence of spherical symmetry motivates a special relativistic formalism, which we exhibit in coordinates sufficiently general to encompass Cartesian, spherical, and cylindrical coordinate systems.

The influence of inclined plates on expansion behaviour of solid suspensions in a liquid fluidised bed—a computational fluid dynamics study

A significant increase in the particle sedimentation rate can be achieved by introducing inclined plates into conventional fluidised beds. In turn, high suspension densities are possible at fluidisation velocities in excess of the particle terminal velocity. The installation of the inclined plates, however, alters the dynamic characteristics of the fluidised bed, in particular, impacting upon the expansion behaviour of the suspension. In the present work a Computational Fluid Dynamics (CFD) approach was employed to investigate the influence of inclined plates on the expansion behaviour of solids suspensions in liquid fluidised beds. The model is based on the solution of the Eulerian multiphase equations for up to two different particle sizes with a continuous phase of water. The momentum equations treat hindered settling behaviour via the inclusion of a volume fraction dependent drag law. The computational model was validated against our experimental data and compared with the predictions of a kinematic model developed in one of our earlier works. In general the predictions made by both the CFD and the kinematic models were found to be in good agreement with the experimental results.

The influence of inclined plates on expansion behaviour of solid suspensions in a liquid fluidised bed — A computational fluid dynamics study

A significant increase in the particle sedimentation rate can be achieved by introducing inclined plates into conventional fluidised beds. In turn, high suspension densities are possible at fluidisation velocities in excess of the particle terminal velocity. The installation of the inclined plates, however, alters the dynamic characteristics of the fluidised bed, in particular, impacting upon the expansion behaviour of the suspension. In the present work a Computational Fluid Dynamics (CFD) approach was employed to investigate the influence of inclined plates on the expansion behaviour of solids suspensions in liquid fluidised beds. The model is based on the solution of the Eulerian multiphase equations for up to two different particle sizes with a continuous phase of water. The momentum equations treat hindered settling behaviour via the inclusion of a volume fraction dependent drag law. The computational model was validated against our experimental data and compared with the predictions of a kinematic model developed in one of our earlier works. In general the predictions made by both the CFD and the kinematic models were found to be in good agreement with the experimental results.

Ski Jump Hydraulics

Ski jumps are a major element of each dam spillway because these are the only structures able to accomplish satisfactory energy dissipation for takeoff velocities in excess of some 20 m / s. This research aims to add to several hydraulic problems with ski jumps that have not yet been systematically solved so far. Based on an experimental campaign, the following problems were addressed: ~1! pressure head maximum and pressure distribution along a circular-shaped flip bucket; ~2! takeoff characteristics for a certain bucket deflection and a relative bucket curvature including the jet trajectories of both the lower and the upper nappes; ~3! impact characteristics in a prismatic tailwater channel with details of shock wave formation and height of recirculation depth; ~4! energy dissipation across the ski jump, from the upstream channel to downstream of jet impact; and ~5! choking flow conditions by the flip bucket. These results demonstrated the significant effect of the approach Froude number, the relative bucket curvature and the bucket angle. The results allow immediate application to the design of ski jumps in hydraulic engineering.

Ultrasonic Velocity Studies of Drug Parvon-spas in Mixed Alcohol–Water Solvent Systems at 25°C

Ultrasonic velocities and densities of the drug Parvon-spas in binary mixtures of water with methanol (MeOH), ethanol (EtOH), and propan-1-ol (1-PrOH) have been measured over the complete solvent composition range at 10 mol% intervals at 25°C. Various acoustic parameters such as the acoustic impedance (Z), adiabatic compressibility (β), intermolecular free length (Lf), relative association (R.A.), molar volume (Vm), and molar sound velocity (Rm) have been calculated. In addition, excess functions, i.e., excess adiabatic compressibility (βE), excess intermolecular free length (LfE), excess molar volume (VE), excess ultrasonic velocity (UE), and excess acoustic impedance (ZE) for these three solvent mixtures in the absence and presence of the drug have been calculated. A different behavior of these parameters in these alcohol systems has been discussed in terms of the length of the alcohol molecule, the molecular volume, as well as inter/intramolecular interactions of these molecules.

Light-Directed Movement of Polymer Microstructures

Light has been used to induce photochemical changes in the surface chemistry of porous polymer microstructures giving rise to a substantial change in volume. When illumination is asymmetric, this results in light-directed motion of the structure. Swellable trimethylolpropane trimethacrylate cross-linked poly(2-hydroxylethyl methacrylate) microstructures were constructed by azo-bis-isobutyronitrile photopolymerization using a 20 x 0.5 NA microscope objective and 365 nm laser excitation. Structures were aminiated with glycine and protected with the photolabile group 4-nitroveratryloxycarbanyl (NVOC). Addition of NVOC resulted in a volume increase >10% when performed in the solvent N,N'-dimethylformamide. Photochemical cleavage of NVOC using asymmetric illumination of a cone-shaped microstructure with a 365 nm laser induced polymer shrinkage in excess of 4% at the base of the cone and resulted in a maximum velocity of 1 mm/s at the tip of the cone. Symmetric illumination gave rise to displacement of solvent from the microstructure due to shrinkage with a velocity in excess of 0.01 mm/s. This system could in principle be used for light-directed movement of micromechanical systems, optical control of microfluidics, or light activated chemical delivery.

Vertical ion motion observed with incoherent scatter radars in the polar lower ionosphere

Vertical ion velocities in excess of a few tens m s−1 have been found in the lower ionosphere (95–105 km) at high latitudes. Statistics using data from the European Incoherent Scatter (EISCAT) Tromsø UHF radar are performed to understand general character of vertical motions in the lower ionosphere for geomagnetically quiet conditions. We estimate the probability distribution function for the real vertical ion velocity, taking into account the measurement uncertainty. This calculation suggests that while most vertical ion velocities have amplitudes smaller than 20 m s−1 at 100 km level, the magnitudes larger than 20 m s−1 are observed as statistical exception. To understand if these large vertical motions in the lower ionosphere are associated with thermospheric motions, differences between ion and neutral wind velocities are calculated using data from experiments with the Sondrestrom incoherent scatter radar on 5 and 12 September 2003. The vertical ion velocities show magnitudes larger than 20 m s−1 below 103 km where the ion velocity is considered to be equal to the neutral wind velocity during the experiments according to calculations of the velocity difference.

Adiabatic release measurements in aluminum from 240-to500-GPa states on the principal Hugoniot

Adiabatic release measurements were performed on aluminum (6061-T6) from ∼240-to500-GPa(∼2.4–5Mbar) states on the principal Hugoniot. Using a magnetically accelerated flyer plate technique, capable of launching macroscopic aluminum flyer plates (approximately 12×25mm in lateral dimension and ∼300μm in thickness) to velocities in excess of 20km∕s, direct impact experiments were performed on a low shock impedance, 200-mg∕cc silica aerogel to determine the aerogel Hugoniot in the pressure range of ∼30–75GPa. Release experiments were then performed in which the aerogel was mounted onto an aluminum base plate, and the initial shock in the base plate was transmitted to the aerogel sample. Given the measured aerogel shock velocities from the release experiments and the measured aerogel Hugoniot from the direct impact experiments, release states on release adiabats of aluminum were ascertained. The results were compared to several equation of state models for aluminum, and a rigorous statistical analysis was performed. The statistical analysis enabled the suitability of these various models in describing the release response of hot, liquid states to be determined. This study enhances our understanding of the release response of aluminum for high-pressure states on the Hugoniot and lends confidence to the use of aluminum as a standard material in impedance-matching experiments.

Nonimpact Lunar Transfer Trajectories Using the Pseudostate Technique

An algorithm is developed for the design of one-way direct-transfer trajectories to the moon that achieve a specified approach trajectory periapsis altitude and inclination, at a specified time of arrival. A variant of the pseudostate technique is employed to derive the analytical transfer trajectory design. A novel and simple matching strategy that synchronizes the geocentric two-body transfer trajectory with the selenocentric approach trajectory is devised. This matching strategy avoids computation of the complicated and conventional state transition matrices. Two methodologies are presented for the computation of the nonimpact approach trajectory characteristics from the excess velocity vector. These methods and the matching strategy are incorporated in the algorithm to ensure nonimpact. The use of this nonimpact algorithm for quick mission design analysis of orbiter missions is demonstrated. Further, in order to achieve the approach trajectory characteristics accurately, a biased targeting technique is proposed. This semi-analytical technique uses the nonimpact algorithm and numerical integration and helps obtain fast and near-exact solutions for the three-body problem of generating lunar transfer trajectory design.

High-Resolution Spectroscopy of some Active Southern Stars

High-resolution échelle spectra of 42 nearby southern solar-type stars have been obtained, in a search for young, single, active, and rapidly rotating sun-like stars suitable for Doppler imaging and Zeeman Doppler imaging studies. As a result of this survey, 13 stars were determined to be youthful with ages less than 600 Myr (Hyades age) and eight of these were found to have projected rotational velocities in excess of 15 km s−1. In addition, five spectroscopic binary systems were identified. Of those stars observed for this survey, HD 106506 is the most outstanding target for mapping active regions. It is an apparently young and single star with rapid rotation (v sin i ∼ 80 km s−1), strong Hα chromospheric activity (log R′Hα ∼ −4.2), and deformation of the spectral line profiles indicating the presence of large starspots.

Development of a naval railgun

The U.S. Navy is considering the development of electromagnetic railguns for future ships for naval surface-fire support (NSFS) and other missions. To reach long ranges, muzzle velocities in excess of 2000 m/s with projectile flight mass of 16 kg and above are needed. Relatively high firing rates are desired; typically 6-12 rounds per minute, so substantial power demands will be made on the ship. For an electrically driven ship, as in the DD(X) concepts presently being explored by the Navy, an existing electrical infrastructure would be in place that could be modified to recharge the pulsed power supplies for the railgun. This study focused on the railgun, the pulsed power system needed to drive the gun, the power system needed to restore the energy required to match the firing rate, and special ship interfaces. Railgun concepts and sizing for the NSFS mission were undertaken, and research and development issues of importance for the success of a future system were identified. Recommendations for analytical and experimental studies that should be undertaken to further address these issues are provided.

Origin of pressure fluctuations in fluidized beds

The present paper shows a novel approach in interpreting the standard deviation of pressure fluctuations in a fluidized bed with respect to the spectral analysis. Based on several realistic assumptions for a freely bubbling bed, the physical model was proposed for the standard deviation of incoherent part of pressure fluctuations. Using the concept of energy dissipated in a fluidized bed the plausible explanation of linear dependence of standard deviation calculated from the total pressure signal on the excess gas velocity was given and verified experimentally.

Internal Rotation of Subdwarf B Stars: Limiting Cases and Asteroseismological Consequences

Observations of the rotation rates of horizontal branch (HB) stars show puzzling systematics. In particular, cooler HB stars often show rapid rotation (with velocities in excess of 10 km s-1), while hotter HB stars (those with Teff in excess of 11,000 K) typically show much smaller rotation velocities of 8 km s-1 or less. Simple models of angular momentum evolution of stars from the main sequence through the red giant branch fail to explain these effects. Assuming solid body rotation throughout produces models that rotate much too slowly. On the other hand, assuming local conservation of angular momentum, but with solid body rotation in the convective regions, produces HB models that also rotate too slowly at all Teff but preserve a rapidly rotating core. In these cases, the observed angular velocities of HB stars require that some of the angular momentum stored in the core reaches the surface. Models that assume constant specific angular momentum in the surface convection zone have faster rotation in the envelope; while the predictions of these models match the observed rotation rates of the cooler HB stars, hot HB stars rotate more slowly than the models. Thus, while there is not yet a coherent explanation of the trends of rotation on the HB, evolutionary models in all cases preserve a rapidly rotating core. To test the idea that HB stars contain such a core, one can appeal to detailed computations of trace element abundances and rotational mixing. However, a more direct probe is available to test these limiting cases of angular momentum evolution. Some of the hottest horizontal branch stars are members of the pulsating sdB class. They frequently show rich pulsation spectra characteristic of nonradially pulsating stars. Thus, their pulsations probe the internal rotation of these stars and should show the effects of rapid rotation in their cores. Using models of sdB stars that include angular momentum evolution, we explore this possibility and show that some of the sdB pulsators may indeed have rapidly rotating cores.

The calcium exosphere of Mercury

A tenuous calcium atmosphere at Mercury, principally seen in the polar regions, was first observed in July, 1998, using the High Resolution Echelle Spectrograph (HIRES) at the W.M. Keck I telescope (Bida et al., Nature 404, 159, 2000). We report four years of observations of the calcium exosphere of Mercury, confirming the initial findings of a very tenuous atmosphere. These observations show a persistent but spatially variable blue shift, indicating an excess velocity toward the observer of up to 3 km s −1, with an average excess velocity of 2.2 km s −1 above the south pole. In addition, the line profiles reveal a hot corona at the equivalent of 12,000–20,000 K in a thermalized atmosphere, indicating a large range of motion with respect to the observer. The calcium is not confined to the polar-regions: rare and low Ca abundance is seen in the equatorial regions. Strong emission was seen anti-sunward on 3 May 2002. Apparent weak emission on the sunward hemisphere may be due to scattered light from the surface, or may indicate a high latitude source. We show that the likely source of the calcium is either impact vaporization in the form of CaO and clusters, which are subsequently photo-dissociated, or ion-sputtering of atoms, molecules and ions. The column abundance is somewhat, but not strongly, correlated with solar activity. We predict a very hot (probably escaping) oxygen component to the hermean exosphere.

An Optical Survey of Supernova Remnants in M83

Observations of the face-on spiral galaxy M83 (NGC 5236) performed at the Cerro Tololo Inter-American Observatory in Chile have yielded a catalog of optical supernova remnant (SNR) candidates. These observations were performed with the 4 m Blanco telescope and a prime focus CCD imaging system using narrowband interference filters centered on the light of [S II], Hα, [O III], and red and blue continuum bands. Based on strong relative [S II] : Hα emission, 71 emission nebulae have been identified as SNR candidates. Positions and Hα fluxes of the candidates are presented. Follow-up spectra of 25 of the SNR candidates, also performed at CTIO, have confirmed many of the SNR identifications, although the spectra of a few objects are discrepant, perhaps because of inaccurate aperture placement. In addition, the low mean excitation of M83 H II regions has allowed a separate search for young oxygen-dominated (core collapse) SNRs similar to Cas A in our Galaxy, using [O III] : Hα. This search found a number of the same objects as the [S II] : Hα search, indicating that many of these SNRs have shock velocities in excess of 100 km s-1. However, no bona fide young core-collapse SNRs were detected with this technique, with the possible exception of the independent recovery of SN 1957D, which had been seen previously. We have also attempted to identify optical counterparts for the six historical supernovae that have occurred in M83. Except for SN 1957D, none of the historical supernovae have been detected by this survey. We compare our SNR candidate list against the Chandra X-ray source list of Soria and Wu and identify 15 X-ray sources as likely SNRs, based on positional coincidence within 1''. The sources identified have hardness ratios that are soft compared to the general X-ray source population in M83.

Shape-Based Algorithm for the Automated Design of Low-Thrust, Gravity Assist Trajectories

Given the benefits of coupling low-thrust propulsion with gravity assists, techniques for easily identifying candidate trajectories would be extremely useful to mission designers. The computational implementation of an analytic, shape-based method for the design of low-thrust, gravity-assist trajectories is described. Two-body motion (central body and spacecraft) is assumed between the flybys, and the gravity-assists are modeled as discontinuities in velocity arising from an instantaneous turning of the spacecraft’s hyperbolic excess velocity vector with respect to the flyby body. The method is augmented by allowing coast arcs to be patched with thrust arcs on the transfers between bodies. The shape-based approach permits not only rapid, broad searches over the design space, but also provides initial estimates for use in trajectory optimization. Numerical examples computed with the shape-based method, using an exponential sinusoid shape, are presented for an Earth‐Mars‐Ceres rendezvous trajectory and an Earth‐Venus‐Earth‐Mars‐Jupiter flyby trajectory. Selected trajectories from the shape-based method are successfully used as initial estimates in an optimization program employing direct methods.

Electrokinetic micropump and micromixer design based on ac faradaic polarization

A microfluidic pump and mixer design based on ac faradaic polarization is proposed. Unlike ac electrokinetic devices based on capacitive charging of the electrodes, the design yields a net electro-osmotic flow for high-conductivity electrolytes at high voltages and frequencies without producing gas bubbles or generating pH gradients. The average velocity, which can be more than an order of magnitude higher than that generated by the capacitive mechanism, has an exponential dependence on the voltage and increases monotonically at low frequencies. Vortices and net flows with linear velocities in excess of 1mm∕s are generated with orthogonal microfabricated planar electrodes based on the unique flow and polarization features of this new ac charging mechanism.