Changes In Velocity Distribution Research Articles

Comparison of macroscopic particle‐in‐cell and semikinetic models of the polar wind

The outflow of the polar wind along diverging geomagnetic field lines has been the subject of many modeling studies for the past 25 years. As the plasma drifts up and out of the topside ionosphere, it undergoes several transitions; for instance, its velocity changes from subsonic to supersonic and its velocity distribution changes from Maxwellian to non‐Maxwellian. The complexity of the flow led to the development of several modeling approaches, such as the generalized moment, the kinetic, and the semikinetic models. Recently, a “macroscopic” particle‐in‐cell (PIC) model was adopted to study the polar wind. However, because one is always restricted to a finite number of particles, the validity of the approach must be established when it is applied to macroscopic flows. In this study the polar wind predictions obtained from a macroscopic PIC simulation were compared to those obtained from the more rigorous semikinetic model for steady state conditions. The study also shows the sensitivity of the PIC simulation to the adopted modeling parameters for both time‐dependent and steady state conditions, including the number of simulation particles, the time step, the spatial bin size, etc. The study indicates that (1) the PIC model can be a powerful simulation tool if special attention is given to its potential pitfalls; (2) because of the finite number of particles, the PIC technique is subject to a considerable amount of noise; (3) the noise level is higher for the higher‐order moments, such as heat flow, and for the velocity distribution function; (4) the use of a time step that is too large leads to a modulation of the results; (5) an insufficient number of spatial bins yields a poor spatial resolution, while too many spatial bins leads to more noise; (6) the noise level can be reduced by averaging over time and/or space, but this affects the spatial and/or temporal resolution; (7) the bin size in velocity space must be carefully chosen to balance numerical noise and velocity space resolution; (8) in the steady state the PIC technique can achieve the same accuracy as the semikinetic model if all of the PIC modeling parameters are optimized; and (9) a few hundred thousand simulation particles, as used in some previous studies, are not adequate to resolve the tail of the velocity distribution, even in the steady state when time averaging is possible. For 106 particles the noise in the tail is still appreciable; and (10) when poor spatial resolution is used, important features can be missed, as was the case in some previous studies.

Flow shear across solar wind discontinuities: WIND observations

We examine the tangential magnetic field and velocity shears across directional discontinuities (DDs) with significant change in magnetic field intensity observed by WIND in slow and fast solar wind streams. The magnetic field rotation sense in fast wind DDs is that predicted by theory for outward propagating rotational discontinuities (RDs), but flow shear magnitude and orientation do not always satisfy RD theory. Alternatively, DDs with small normal magnetic field can be regarded as tangential discontinuities (TDs); the observed shears imply that the length scale over which the proton velocity distribution changes at the discontinuity can be both smaller or larger than that of the electron distribution. The slow wind includes a larger fraction of DDs that disagree with RD theory. It is shown that the flow shear orientations allowed in a TD provide a continuous transition between the opposite orientations for RDs propagating along or against the magnetic field direction.

Spray jets in a cross-flow

When droplets are expelled at a high velocity by a spray, a strong vertical air jet is induced throughout which the smallest droplets are dispersed (their Reynolds numbers associated with their relative motion being small). In our analysis we focus on the interaction between an external cross-flow and this spray jet. This interaction and the distances by which the spray jet and, over a longer distance, the large droplets are deflected are found to depend largely on the ratio of the cross-wind speed to the induced air speed U0/Uj. Using a multi-zone analysis we show that with a weak cross-flow (U0/Uj[les ]0.1), in the region immediately below the nozzle the spray entrains the external cross-flow and acts like a line sink; the streamlines close to the spray curve inwards to the centre, while further away the sink flow is weak and the streamlines follow the cross-wind. The external flow stagnates at a certain distance from the spray centreline which depends on U0/Uj. When U0/Uj[ges ]0.1 the cross-section of the spray jet and its velocity distribution change in the same way as a fluid jet in a cross-flow, whose inertia causes the deflection of the external flow around it and whose surface vorticity causes a pair of axial vortices on the downwind side of the spray. These vortices have a significant effect on the spray because they induce a back flow which reduces the tendency of the small droplets to leave the spray. When the cross-wind is strong (U0/Uj>0.3; U0[ges ]10 m s−1) the flow is too strong to be entrained; in this limit the main effect of the larger spray droplets is simply to resist the cross-flow which causes the cross-flow to slow down as it passes through the spray and to divert some of the cross-flow around the spray jet. Since the cross-flow now passes through the spray it carries the smallest droplets downwind.In this paper analytical models have been developed for all the practical ranges of the ratio of the jet speed to the cross-wind speed. This enables spray drift to be calculated. These models require very little computer time and can be run interactively. Spray droplet trajectories can be plotted straightforwardly for both axisymmetric and flat-fan sprays.

Decrease in nerve temperature: a model for increased temporal dispersion

A decrease in nerve temperature causes a proportional decrease in conduction velocity which, in percentage terms, is equal for all nerve fibers. The absolute decrease in conduction velocity is larger for faster conducting nerve fibers. This results in a compression and a shift to lower values of the conduction velocity distribution and an increase in temporal dispersion. The purpose of this study was to determine if these effects could be detected by a combination of two collision techniques designed to obtain the motor conduction velocity distribution and refractory period distribution. In 12 healthy volunteers we measured the conduction velocity distribution in the median nerve at nerve temperatures of 25 and 40°C. The results showed that our method could detect the predicted changes in conduction velocity distribution and temporal dispersion. We conclude that temperature change is a model that can be used to study temporal dispersion. This may be a first step towards distinguishing between the effects of conduction block and (abnormal) temporal dispersion in demyelinated nerve fibers.

Initial observations of fine plasma structures at the flank magnetopause with the complex plasma analyzer SCA-1 onboard the Interball Tail Probe

Abstract. The fast plasma analyzer EU-1 of the SCA-1 complex plasma spectrometer is installed onboard the Interball Tail Probe (Interball-1). It provides fast three-dimensional measurements of the ion distribution function on the low-spin-rate Prognoz satellite (about 2min). The EU-1 ion spectrometer with virtual aperture consists of two detectors with 16 E/Q narrow-angle analyzers and electrostatic scanners. This configuration allows one to measure the ion distribution function in three dimensions (over 15 energy steps in 50 eV/Q–5.0 keV/Q energy range in 64 directions) in 7.5 s, which makes it independent of the slow rotation speed of the satellite. A description of the instrument and its capabilities is given. We present here the preliminary results of measurements of ions for two cases of the dawn low- and mid-latitude magnetopause crossings. The properties of observed ion structures and their tentative explanation are presented. The 12 September 1995 pass at low latitude at about 90° solar-zenith angle on the dawn side of the magnetosphere is considered in more detail. Dispersive ions are seen at the edge of the magnetopause and at the edges of subsequently observed plasma structures. Changes in ion velocity distribution in plasma structures observed after the first magnetopause crossing suggest that what resembles multiple magnetopause crossings may be plasma blobs penetrating the magnetosphere. Observed variations of plasma parameters near magnetopause structures suggest nonstationary reconnection as the most probable mechanism for observed structures.

Initial observations of fine plasma structures at the flank magnetopause with the complex plasma analyzer SCA-1 onboard the Interball Tail Probe

The fast plasma analyzer EU-1 of the SCA-1 complex plasma spectrometer is installed onboard the Interball Tail Probe (Interball-1). It provides fast three-dimensional measurements of the ion distribution function on the low-spin-rate Prognoz satellite (about 2min). The EU-1 ion spectrometer with virtual aperture consists of two detectors with 16 E/Q narrow-angle analyzers and electrostatic scanners. This configuration allows one to measure the ion distribution function in three dimensions (over 15 energy steps in 50 eV/Q–5.0 keV/Q energy range in 64 directions) in 7.5 s, which makes it independent of the slow rotation speed of the satellite. A description of the instrument and its capabilities is given. We present here the preliminary results of measurements of ions for two cases of the dawn low- and mid-latitude magnetopause crossings. The properties of observed ion structures and their tentative explanation are presented. The 12 September 1995 pass at low latitude at about 90° solar-zenith angle on the dawn side of the magnetosphere is considered in more detail. Dispersive ions are seen at the edge of the magnetopause and at the edges of subsequently observed plasma structures. Changes in ion velocity distribution in plasma structures observed after the first magnetopause crossing suggest that what resembles multiple magnetopause crossings may be plasma blobs penetrating the magnetosphere. Observed variations of plasma parameters near magnetopause structures suggest nonstationary reconnection as the most probable mechanism for observed structures.

Trailing-edge jet control of leading-edge vortices of a delta wing

The effect of using a trailing-edge jet to control the leading-edge vortices of a delta wing is investigated experimentally in a water towing tank facility. The Reynolds number, based on the freestream velocity and the root chord, is 9.8 X 103. Both static and dynamic (pitching-up) conditions are tested. For the dynamic cases, the wing is pitched from 10- to 45-deg angle of attack with pitch rates varied from 0.043 to 0.26. From the dye flow visualization, it is shown that a downward vectored trailing-edge jet can significantly delay the vortex breakdown on a delta wing. Strong asymmetric breakdown of the leading-edge vortices can be induced by arranging the vectored jet in an asymmetric configuration. Transient pitching motion delays the onset of the vortex breakdown. The initial delay is independent of the pitch rate. Also, the use of jet control is found to be effective for the dynamic cases. During the initial pitching-up period, the use of jet control has a dominant influence on the propagation of the vortex breakdown. In general, with jet control, the propagation of the vortex breakdown slows down. From instantaneous particle image velocimetry measurements, a quasiperiodic variation of the leading-edge vorticity field is detected before the vortex breakdown. This variation appears to relate to the strong interaction between the separating shear layer, the secondary vortex, and the primary vortex. Along the vortex axis, the velocity distribution changes from a jet-type profile to a wake-type profile, signifying the onset of vortex breakdown.

Transmission Line Model Equivalent Circuit for Piezoelectric Transducers Including the Effect of Electrical Terminal Impedance

A new transmission line model equivalent circuit for a thickness-mode piezoelectric vibrator is derived. The equivalent circuit includes an electrical impedance connected to the terminals of a piezoelectric vibrator. A bolt-clamped frequency-controllable piezoelectric transducer is analyzed by using the equivalent circuit. The changes in particle velocity distribution by varying the electrical impedance in the frequency-controllable ultrasonic transducer are measured by a noncontact method based on laser Doppler velocimetry. The changes in particle velocity distribution agree well with results calculated using the new equivalent circuit.

Experimental Investigation of the Flow in Diffusers Behind an Axial Flow Compressor

The flow fields of four diffusers situated at the rear of a one-stage axial flow compressor were experimentally investigated. Through modification of the compressor operating point, a wide range of variations of the side wall boundary layers and the radial velocity distribution outside of the boundary layers at diffuser inlet could be achieved. The three-dimensional flow field at both diffuser inlet and outlet is analyzed. Changes of inlet blockage and radial velocity distribution and their resulting effects on pressure recovery are thoroughly presented. Compared with the results of measurements at diffusers, typically with ducted flow inlet conditions, higher values of pressure recovery are observed. Established design rules, based on investigations of diffusers with carefully developed inlet flow, are checked regarding their applicability for diffusers in turbomachine environments.

Numerical Analysis of the Helium Flow in Regenerator. Performance Calculation of a Stirling Cryocooler.

Computer simulation of flow in packed wire screens of a regenerator is successfully attempted using a technique similar to the boundary element method. One-dimensional potential flow is assumed and the local helium volume increase by heating and pressure change is replaced by source or sink. Finally, fictious sources are attached to both ends, satisfying the given pressure conditions. Darcy's law is applied to obtain flow velocity from potential. This method requires no iterative processes. This fairly efficient calculation is installed in overall crycooler performance calculations, and the change of velocity distribution in the regenerator is successfully investigated in the course of the refrigeration cycle.

Size and settling velocity distributions of flocs in the Tamar estuary during a tidal cycle

Data extracted from video recordings of individual estuarine flocs near the estuary bed during the advance and retreat of the salt intrusion show changes in size and settling velocity distributions. The recordings were taken using INSSEV —IN Situ SEttling Velocity instrument. Size coupled with effective density variations due to both changes in floc structure and ambient salinity result in changes in the settling velocity during the tidal cycle. In particular, just after high water slack, the appearance of high settling velocity medium size flocs and individual particles suggest that the lower density flocs have been broken up by the intense vertical shear in the currents caused by the salt wedge intrusion. Current shear is shown to have a significant influence on floc effective density.

Stoichiometry-dependent scattering of trimethylgallium from GaAs(100) surface

We examined the scattering of pulsed trimethylgallium (TMG) beams from GaAs surfaces with c(4 × 4), (2 × 4) and (1 × 6) reconstructions in order to obtain dynamical information concerning surface chemical reactions in metalorganic molecular beam epitaxy (MOMBE). The time-of-flight (TOF) spectra for scattered TMG were analyzed by taking into account the change in velocity distribution and the finite surface residence. We also investigated the sticking of TMG on these surfaces as a function of the TMG dosage. For the (2 × 4)-reconstructed surface, we observed no permanent sticking of TMG, although incident TMG molecules were dominantly desorbed after a long surface residence (900 micro s at 546 K). This result indicates that the (2 × 4)-reconstructed surface is highly stabilized, although it has a deep trapping potential, which is consistent with the strong stoichiometry dependence of the TMG decomposition rate. On the other hand, clean c(4 × 4)- and (1 × 6)-reconstructed surfaces were considered to be rather reactive because of the high sticking coefficients. However, these surfaces were inactivated by TMG exposure. After about one monolayer of TMG exposure, most of the incident TMG molecules were scattered without any observable surface residence.

Diamagnetic Temperature Control in the Solar Wind

AbstractWe have developed a model to account for changes in velocity distributions of protons in the solar wind. The essential operative mechanism is the diamagnetic effect on charged particles. Through numerical simulation, we explore the (distance 0.3 AU to 1 AU) evolution of thermal anisotropy. We find that as the initial ratio T+/T∥is increased, there is a larger growth in bulk speed, as is observed. Since any form of wave-particle interactions is exluded from our simulations but they fit well the observations, we conclude that the diamagnetic effect is most relevant in controlling both thermal anisotropy and bulk speed evolution.

Oscillatory Flow in Curved Pipes. 4th Report. Velocity Distribution in Entrance Region.

Oscillatory laminar flow in the inlet region of a circular curved pipe, with a curvature ratio of 11.7, has been investigated by the flow visualization method. The experiments were carried out under the conditions of the Womersley numbers α=5.5∼18 and the Dean number D=300. The velocity was measured using a tracer technique and the instantaneous velocity distributions were obtained at four selected phases in one cycle. The flow mechanism has been made clear in the region from the upper stream tangent to the curved pipe. The flow in the curved pipe is closely related to the flow in the upper stream tangent, and the velocity distribution changes drastically with the phase at the inlet region. The inlet length of oscillatory flow is less than that of steady flow and becomes shorter as α increases.

Turbulence Structure in Plane Turbulent Couette Flow.

Turbulence structure in a fully developed turbulent plane Couette flow is studied experimentally at Reb=2×104. Turbulence structure in the wall region is similar to that in the two-dimensional channel flow. In the core region, however, turbulence structure is very different from that in the channel flow case because of uniform shear existing there. Since energy is produced only on the streamwise turbulence quantity u according to the term - uvdU/dy, the anisotropy of turbulence is greater than that of channel flow. Distributions of velocity-fluctuating components are Gaussian, and triple velocity correlation is almost zero. Viscous diffusion and turbulence diffusion are negligibly small. Consequently, the production and dissipation of turbulence energy are balanced throughout the core region. Energy transport among u, v and w is due to the pressure-strain term. Sinuisoidal change of mean velocity distribution has been measured in the spanwise direction, so that large-scale motions having spanwise expansion of eddies are assumed to exist.

Velocity Distribution Inside and Above Branched Flexible Roughness

The purpose of this study is to develop function relationships for the velocity distribution of water flow in and above a model of branched grass. A trapezoidal cross‐section flume is constructed for this study, and branched plastic strips are used to simulate the grass in nature. A correct determination of flow characteristics for channels suffering from submerged weeds is desirable. Furthermore, knowledge of the changes of velocity distribution as a result of the presence of weeds and the changes in the shear stress at the bed will improve understanding of the different problems associated with this type of channel. Power‐ and logarithmic‐function relationships are developed to describe the velocity distribution of water inside and above branched flexible‐roughness elements (which represent the submerged weeds) in trapezoidal open channels. An experimental study was carried out in the hydraulic lab at Cairo University, Cairo, Egypt, to check the validity of the prediction equations that relate the velocity distribution to the depth of water in the channel cross section. More than 170 velocity profiles have been measured at different locations along the channel length and its cross sections.

Analytical Studies of Two-Dimensional Channel Turbulent Flow Subjected to Coriolis Force.

Coriolis effects on fully developed turbulent flow in a two-dimensional channel rotating about an axis perpendicular to its axis are considered. The Coriolis force has stabilizing/destabilizing effects on turbulence, and the mean velocity distribution changes accordingly. Experimental and numerical studies on the velocity characteristics have already been conducted by other researchers for various conditions. However, we cannot assemble the overall picture of the Coriolis effect on the velocity distributions. In this paper, analytical studies have been performed using dimensional analysis, which indicates that the three lenght scales and their ratios constitute the dominant parameters governing the flow. The flow regime space has been shown to account for the overall Coriolis effect on the flow. Furthermore the velocity laws for various flow regions have been derived.

Experimental study of a three-dimensional large-scale structure in a reciprocating oscillatory flow

An experimental investigation has been conducted on the large-scale structure of the reciprocating oscillatory turbulent flow in a rectangular duct. Using hot-wire anemometers and conditional sampling technique, we found that the Reynolds stress generated explosively in the decelerating phase, a characteristic feature of the flow, is caused by the three-dimensional large-scale structure. These structures are revealed to be a pair of counter-rotating fluid motions which change their form gradually as the phase-averaged velocity distribution changes. Phase differences of the turbulence intensity distribution in an oscillatory flow are found to take place in accordance with the generation-growth-decay process of the large-scale structure.

Ionospheric modification ‐ An outdoor laboratory for plasma and atmospheric science

With the ability to induce large‐scale changes in the ionosphere by high‐power ground‐based HF transmitters, laboratory‐like experiments can be performed repetitively in space plasmas. Using satellites and rockets, as well as ground‐based magnetometers, optical and radar diagnostics, this large‐scale plasma has been investigated. The absence of walls and the long confinement time make this plasma distinctively different from conventional laboratory plasmas. Particle and energy transports can be studied in this large‐scale well‐confined plasma. Significant changes in density profiles and velocity distributions can be brought about by modest incident EM wave intensity because the wave fields are resonantly enhanced and the perturbations are cumulative in the same plasma over a long time. Free energy sources can further enhance the excitation of waves. Experimental methods are reviewed together with major experiments performed at the Platteville, Arecibo, Max Planck, and HIPAS observatories. The ionospheric plasma with a rich source of free energy is highly variable especially in the auroral zone, and fast temporal resolution techniques used in laboratory experiments have been introduced to these outdoor experiments to observe caviton collapse and other fast processes. High‐frequency and low‐frequency wave phenomena are observed to be excitable simultaneously and coupled by non‐linear processes. Laboratory experiments, computer simulation, and analytic theories are used to interpret and to guide field experiments. Future experiments include the studies of the formation of potential double layers and ion acoustic shocks from density cavities and their coupling to free energy sources in the ionosphere. Active experiments are desirable in the atmosphere whose response to a known perturbation such as the controlled heating of electrons and neutrals and the formation of negative ions can be quantified. With higher focusing of the available power, there is the exciting possibility of generating a steady state ionospheric plasma and an opening to a new era in controlled atmospheric experiments and long‐range communications.

Quantitative color flow imaging to measure the two-dimensional distribution of blood flow velocity and the flow rate.

A quantitative Doppler color flow imaging was employed to measure the two-dimensional distribution of blood flow velocity and flow rate in a large vessel. Regional blood flow velocity was determined by converting the color intensity at the respective pixel into corresponding flow velocity and correcting the flow velocity for spatial ultrasound beam incident angle. Instantaneous flow rate was determined precisely from the image of velocity distribution on the cross-section of the flow tract in a steady flow model circuit. In clinical application, the differences in phasic changes in two-dimensional velocity distribution in the ascending aorta, between normal subjects and the patient with aortic regurgitation, were clearly depicted. The quantitative color flow imaging may have great potential to determine noninvasively and real-timely the two-dimensional distribution of flow velocity as well as flow rate in a large vessel.