Absolute Value Of Velocity Research Articles

Flow Structures in a U-Shaped Fuel Cell Flow Channel: Quantitative Visualization Using Particle Image Velocimetry

Flow through an experimental model of a U-shaped fuel cell channel is used to investigate the fluid dynamic phenomena that occur within serpentine reactant transport channels of fuel cells. Achieving effective mixing within these channels can significantly improve the performance of the fuel cell and proper understanding and characterization of the underlying fluid dynamics is required. Classes of vortex formation within a U-shaped channel of square cross section are characterized using high-image-density particle image velocimetry. A range of Reynolds numbers, 109⩽Re⩽872, corresponding to flow rates encountered in a fuel cell operating at low to medium current densities is investigated. The flow fields corresponding to two perpendicular cross sections of the channel are characterized in terms of the instantaneous and time-averaged representations of the velocity, streamline topology, and vorticity contours. The critical Reynolds number necessary for the onset of instability is determined, and the two perpendicular flow planes are compared in terms of absolute and averaged velocity values as well as Reynolds stress correlations. Generally, the flow undergoes a transition to a different regime when two recirculation zones, which originally develop in the U-bend region, merge into one separation region. This transition corresponds to generation of additional vortices in the secondary flow plane.

Investigation of the projectile ion velocity inside the interaction media by the x-ray spectromicroscopy method

A method for space resolved measurements of the fast heavy ion velocity during the interaction with matter are presented. The main idea is based upon the fact that the characteristic radiation from ions traveling in the stopping media undergoes the Doppler shift, which varies along the ion beam trajectory due to the ion deceleration. High spectrally (λ/Δλ=1000–3000) and spatially (up to 30–100 μm) resolved x-ray K-shell spectra of Ca projectile ions as well as of the ionized stopping media have been obtained using focusing spectrometers with spatial resolution (FSSR). Spherically bent crystals of quartz and mica with small curvature radii R=150 mm and large apertures (15×50 mm) have been used as dispersive elements. Fast Ca+6 ions with energies of 5.9 and 11.4 MeV/u were stopped in quartz, SiO2 aerogels and CaF2 targets. High spectral and spatial resolution of the spectra allowed measuring the velocity of heavy projectile ions at different points along the beam trajectory. A method based on the utilization of two equal spectrometers at different angles of observation (along and downstream the ion beam propagation direction) is proposed to measure the absolute values of ion velocity inside the stopping media. The use of aerogel targets with extremely low volume density (0.15 g/cc) increases the ion stopping range up to 50 times in comparison to solid quartz. This provides the resolution of the stopping process dynamics. Experimentally it was found that Ca+6 ions with the initial energies of 5.9 or 11.4 MeV/u propagate in solid dense matter as Ca+17–Ca+19 down to the energies of 2.0–2.2 MeV/u.

Force–velocity characteristics of upper limb extension during maximal wheelchair sprinting performed by healthy able-bodied females

The aim of this study was to determine the relationship between force and velocity parameters during a specific multi-articular upper limb movement – namely, hand rim propulsion on a wheelchair ergometer. Seventeen healthy able-bodied females performed nine maximal sprints of 8 s duration with friction torques varying from 0 to 4 N · m. The wheelchair ergometer system allows measurement of forces exerted on the wheels and linear velocity of the wheel at 100 Hz. These data were averaged for the duration of each arm cycle. Peak force and the corresponding maximal velocity were determined during three consecutive arm cycles for each sprint condition. Individual force–velocity relationships were established for peak force and velocity using data for the nine sprints. In line with the results of previous studies on leg cycling or arm cranking, the force–velocity relationship was linear in all participants (r = −0.798 to −0.983, P <0.01). The maximal power output (mean 1.28 W · kg−1) and the corresponding optimal velocity (1.49 m · s−1) and optimal force (52.3 N) calculated from the individual force–velocity regression were comparable with values reported in the literature during 20 or 30 s wheelchair sprints, but lower than those obtained during maximal arm cranking. A positive linear relationship (r = 0.678, P <0.01) was found between maximal power and optimal velocity. Our findings suggest that although absolute values of force, velocity and power depend on the type of movement, the force–velocity relationship obtained in multi-articular limb action is similar to that obtained in wheelchair locomotion, cycling and arm cranking.

Various regimes of motion of a spherical cavity at a negative external pressure

1. Study of the behavior of defects in the internal structure of a solid is of fundamental importance for simulation of its behavior under external action. The problem of the behavior of a spherical cavity is a classical problem for point defects. As early as in 1917, Rayleigh [1] showed that the absolute value of the velocity of the cavity boundary in an ideal incompressible fluid increases as R–3/2 with a decease in the cavity radius R to zero. The compression of the cavity in a viscous medium with surface tension can lead to several collapse regimes [2–4]. The basic properties of cavitycollapse regimes under such conditions were studied by methods of the qualitative theory of differential equations in [5].

Vein graft function improvement after percutaneous intervention: evaluation with MR flow mapping.

To provide functional reference values in single and sequential vein grafts by using magnetic resonance (MR) flow mapping and to examine the effect of percutaneous intervention (PCI) on coronary artery bypass graft function. Fast MR flow mapping at baseline and during adenosine-induced stress was performed in 39 nonstenotic single vein grafts and 20 nonstenotic sequential vein grafts, as well as in 15 stenotic vein grafts before and 7.3 weeks +/- 1.5 after successful PCI. We evaluated the following parameters (in terms of mean values +/- SDs): average peak velocity (APV) at baseline, stress APV, and velocity reserve. Parameters in nonstenotic single and sequential vein grafts were compared by means of unpaired two-tailed Student t testing. To evaluate changes in velocities before and after PCI, a paired two-tailed Student t test was used. P <.05 was considered to indicate a statistically significant difference. Reference values in single vein grafts for baseline APV, stress APV, and velocity reserve were 8.6 cm/sec +/- 3.4, 20.2 cm/sec +/- 9.5, and 2.4 +/- 0.8, respectively. In sequential vein grafts, significantly higher values for baseline APV (12.2 cm/sec +/- 5.0) and stress APV (27.2 cm/sec +/- 10.6) but a similar velocity reserve (2.3 +/- 0.7) were found. Significant improvements were observed after PCI in baseline APV (before PCI: 9.2 cm/sec +/- 6.6; after PCI: 12.9 cm/sec +/- 7.9; P =.008) and stress APV (before PCI: 12.9 cm/sec +/- 6.3; after PCI: 27.1 cm/sec +/- 13.9; P <.001). No improvement in velocity reserve was observed. Significantly higher absolute velocity and flow values were observed in sequential versus single vein grafts, underscoring the need for separate functional reference values for different graft types. Graft function showed significant improvement after PCI to the point that it was restored or nearly restored to reference values.

A new benthic foraminiferal proxy for near-bottom current velocities in the Gulf of Cadiz, northeastern Atlantic Ocean

Recent benthic foraminiferal assemblages were analyzed in the Gulf of Cadiz, northeastern Atlantic, to study the impact of the Mediterranean Outflow Water (MOW) undercurrent on the benthic environment. Foraminiferal counts and the analysis of specimens attached to hard substrates from 26 surface samples reveal a relationship of epibenthic assemblages with sedimentary and hydrodynamic environment. Epibenthic species make up as much as 60% of the living assemblage at proximal sites with high current velocities and 3–18% in distal areas or near the margins of the MOW flow paths at low velocities. These foraminifers inhabit elevated substrates only within the MOW, which evidently provides an ecological niche for opportunistic suspension feeders. They adapt their settling elevation dynamically and occur at greater heights above the ambient sediment surface under stronger currents. Mobility, fixation strength, suspension feeding, and reproduction efficiency emerge as individual capabilities promoting the occupation of elevated substrates by certain epibenthic species. The active microhabitat selection is pursued as basic strategy of these foraminifers to optimize their food acquisition. A better access to food sources stimulates reproduction and leads to a greater contribution of foraminiferal tests to the surface sediments. Elevated epibenthos percentages from the dead assemblage and current velocities prevailing at the sampling sites are closely correlated. A compilation including other data from southern Portugal, Florida Straits, and the English Channel infers an exponential relationship between epibenthic abundances and flow strength implying that endobenthic species prevail even under high current velocities. A linear model provides a significantly better fit for the Gulf of Cadiz data however. This relation is used for a case study in order to estimate near-bottom current strengths for the late Holocene Peak III contourite in core M39008-3. Trends and absolute values of current velocities, inferred from the benthic foraminiferal proxy, are the same scale as estimates obtained from sediment grain-size distribution and hydrodynamic models. Epibenthic foraminifera thus bear a high potential as proxy for palaeocurrent studies that even may overcome objections by predetermined grain-size distributions in deep-sea cores.

Equilibrium after violent relaxation in numerical dynamical models of open clusters

Models of open clusters in a state of equilibrium in the space of the three parameters of the stellar motion and simultaneously far from virial equilibrium are analyzed. A formula for the phase-space stellar number density in such cluster models is derived, as well as formulas for the integrated and differential energy distributions of the phase-space volume occupied by cluster stars per unit stellar mass. These three quantities are computed for several times exceeding the time for violent relaxation of the cluster model. The phase-space density function obtained is used to compute the distribution of the absolute values of stellar velocities for the cluster model considered.

Dorsal aortic flow velocity in chick embryos of stage 16 to 28

The objective of this study was to evaluate two Doppler frequency-detection methods to measure blood flow velocity in the developing chick embryo. We compared the commonly used directional zero-crossing counter and a customized digital bidirectional spectrum analyzer. At development stages 16 up to 28 (2.5 to 6 days incubation), a reversed flow component in the dorsal aorta was demonstrated using the bidirectional spectrum analyzer. Dorsal aortic velocities obtained with the directional zero-crossing counter were significantly lower than with the bidirectional spectrum analyzer in stages 16, 20 and 28. In addition to the differences in the absolute velocity values, there was also a remarkable discrepancy in the velocity waveform shape using the two Doppler frequency processors. The calculated heart rate using the two Doppler frequency processors was identical. It is concluded that a Doppler velocity detector based on spectral analysis is superior to the hitherto used zero-crossing counter in the chick embryo. With the customized digital bidirectional spectrum analyzer, we can accurately measure the hemodynamics of the developing chick embryo. (E-mail: ursem@gyna.azr.nl)

Assessment of cerebral hemodynamics during roll over test in healthy pregnant women and those with pre-eclampsia

Objective To compare cerebral autoregulatory responses obtained during roll over tests in healthy pregnant women and those with pre-eclampsia in order to assess the middle cerebral artery velocity changes in relation to the roll over test in normotensive and pre-eclamptic women. Participants Twenty-two healthy pregnant women and 26 with pre-eclampsia underwent transcranial Doppler measurements of the middle cerebral artery. Methods Systolic, mean and diastolic blood flow velocities and mean arterial blood pressures were recorded in the left lateral position and five minutes after turning to the supine position. Absolute values of mean blood flow velocities, mean arterial blood pressure values and calculated cerebral blood flow indices as well as cerebrovascular resistance area products were compared at different positions among the groups. Results Mean arterial blood pressure increased in both groups while turning from the left lateral to the supine position. In women with pre-eclampsia both mean arterial blood pressure and absolute values of mean blood flow velocity values were higher in both positions, compared with healthy pregnant women. In both groups, changing the position resulted in a decrease of absolute values of mean blood flow velocities. Calculated cerebral blood flow indices did not change, while cerebrovascular resistance area products increased significantly in the groups during roll over testing. In women with pre-eclampsia, the increase of cerebrovascular resistance area products was more pronounced as compared with healthy pregnant women. Conclusions In women with pre-eclampsia roll over test results in an increase of the mean arterial blood pressure, which is accompanied by a decreased mean blood flow velocity in the middle cerebral artery. Further studies are needed to clarify the pathophysiological background of cerebral haemodynamic changes in pre-eclampsia.

Influence of the magnitude of the two initial velocities on the flow field of a coaxial turbulent jet

The present investigation is a continuation of a program to study the effects of various parameters on the flow field of a coaxial turbulent jet. In the present study the effect of varying the absolute value of the two initial axial mean velocities of the jet is investigated experimentally, while keeping their ratio constant. The effect of absolute value of each stream velocity, on the flow field, was investigated by means of achieving the velocity ratio, λ= U i/ U o=2, three times with two different velocities each time. Moreover, The similarity of the axial mean velocity profiles was investigated, for two other values of λ=3.3 and 4.5, and compared with the results of the single jet. The results show that the reduction in the absolute values of the velocities of both streams while keeping the same velocity ratio constant made the coaxial jet decay faster along the centerline. In addition, similarity of the radial profiles of the axial mean velocity was obtained in the fully merged region.

Grain Boundary Motion during Ag and Cu Grain Boundary Diffusion in Cu Polycrystals

Grain boundary (GB) motion in high-purity Cu material (5N8 and 5N Cu) is investigated using the results of radiotracer GB diffusion measurements with tracers exhibiting fundamental differences in the solute-matrix atom interactions. The results on GB solute diffusion of Ag (revealing a miscibility gap in the Ag-Cu phase diagram) and Au (forming intermetallic compounds with Cu) in Cu and on Cu self-diffusion are analyzed. The initial parts of the Ag and Cu penetration profiles turned out to be substantially curved. The profile curvature is explained via the effect of GB motion during 110m Ag and 64Cu GB penetration. The activation enthalpies of GB motion in these two independent measurements occurred to be very close, 95 and 103 kJ/mol, respectively. Moreover, these values turn out to be close, but still somewhat larger than the activation enthalpy of Cu GB self-diffusion in Cu material of the same very high purity, Q Cu gb = 72 kJ/mol. Although tracer diffusion measurements of Au GB diffusion in Cu yielded only limited information on GB motion, the absolute values of GB velocities are consistent with those calculated from the Ag and Cu GB diffusion data.

Two-dimensional analysis of the relativistic parapotential electron flow in a magnetically insulated transmission line oscillator (MILO)

This paper presents a two-dimensional model for studying the relativistic parapotential electron flow in a magnetically insulated transmission line oscillator (MILO). The distribution expressions of the velocity, energy, density, and self-electric and self-magnetic fields of electron flow are derived and then analyzed numerically. Results of the model show that the self-electric and self-magnetic fields and density of the electron flow are quite high near the surface of the slow-wave structure of a MILO where they may reach their peak values. In addition, the formation of the insulated electron flow requires a large current flowing through the inner conductor (cathode) of the MILO, which is identical with the previous works. It is also found that considerable increases in the absolute values of axial and radial velocities of the electron flow occur when electrons approach the surface of the slow-wave structure. The electron flow is mainly along the axial direction in between the surfaces of cathode and slow-wave structure except the regions near the two surfaces. More interestingly, the radial velocity of electron flow ran still be increased but the axial velocity decreased when the electrons go into the region in between the inner and outer radii of the slow-wave structure, where the electron flow is not always dominated by axial flow. The results of the present paper are more realistic than those of the one-dimensional model in describing the parapotential electron flow in a MILO.

An experimental investigation of fluid flow and combustion characteristics of dual-fuel in a cylindrical combustion chamber

Combustion and fluid flow characteristics of coflowing LPG and kerosene have been investigated experimentally. A cylindrical water cooled combustion chamber was used to investigate the effect of changing the injection location and percentage of liquid fuel during gaseous fuel combustion. It was found that the injection of liquid fuel leads to an increase in the absolute value of maximum positive axial velocity and reduces the absolute value of the maximum negative axial velocity compared to the case of LPG alone. Also, a stable temperature distribution is noticed at axial distance of X / D approximately equal to 2.15 (where X is the axial distance measured from the inlet of diffuser, and D is chamber diameter). This is less than that of gaseous fuel combustion (approximately equal to 2.91). The change of injection location leads to a reduction in values of gas temperatures at X inj / D =0.15 then it increased to reach maximum values at X inj / D =0.35 which is approximately the same value for combustion of LPG fuel only. Any further increase in the injection location leads to a reduction in gas temperature, especially at the upstream sections of the combustion chamber. Also, it was found that values of temperature along the combustion chamber were decreased with increasing the percentage of the injected liquid fuel due to incomplete combustion of liquid fuel. Values of combustion chamber efficiency ( η ) for all percentage of liquid fuel at X inj / D =0.35 are higher than those for combustion of LPG alone.

The Value of Doppler Ultrasound in Cirrhosis and Portal Hypertension

Background: Cirrhosis and portal hypertension affect the flow profile of the liver vasculature. In these conditions Doppler ultrasound can provide important information on the hemodynamics of the portal venous system, the hepatic artery and the hepatic veins. Methods: The value of Doppler ultrasound in the assessment of the patient with cirrhosis and portal hypertension was determined by reviewing the literature. Results: Portal venous blood flow becomes reversed with advanced portal hypertension. Reversed flow is also demonstrated in patients with veno-occlusive disease and portosystemic shunts. Despite general agreement that portal flow velocity is decreased in cirrhotic patients, the absolute values of portal flow velocity in both healthy subjects and cirrhotic patients vary considerably. Errors in Doppler measurements, observer variability and collateral pathways contribute to these variations. Furthermore, portal blood flow is influenced by numerous factors such as changes in the body position, phase of respiration, timing of meals, exercise and cardiac output. Finally, portal flow may be unaltered due to a combination of high inflow from the splanchnic organs and increased resistance within the liver. High resistive index of the hepatic artery is seen in patients with end-stage liver disease, particularly in children with severe cirrhosis secondary to biliary atresia. However, hepatic artery flow remains normal in most patients. Abnormal hepatic vein flow profiles are seen in patients with cirrhosis, but dampening or flattening of the flow profile has a multifactorial origin (Budd-Chiari, metastases, ascites) and can be observed in healthy subjects. Conclusions: Although many factors may affect the accuracy of volume flow and velocity measurements and the flow profile of the liver vasculature may change in different situations, Doppler ultrasound is useful in the assessment of the patient with cirrhosis and portal hypertension.

Observation and calculation of marangoni convection induced thermally in a molten salt

Marangoni convection has a large effect on materials processing since it remarkably enhances heat and mass transport near a free interface. To have a better understanding of the behavior of Marangoni convection in molten salts, observation and calculation of the thermal convection in a column held between a pair of Pt disks (2 mm in diameter) fixed to hot thermocouples, were carried out. Calculated flow patterns and absolute values of fluid velocity agreed well with the observed results. When the temperatures of both disks were set equal and the height of liquid column was 2.4 mm, four vortices were generated on the longitudinal plane with a typical velocity of 15 mm⧹s. From the calculated isothermal lines, the behavior of the enhanced heat transport was obvious ; fluid heated at the disks was conveyed to the free surface and the cooled fluid at the surface was conveyed to the inner region.

On the velocities of localized vibration modes in immersed solid wedges

The approximate theory of localized elastic waves in immersed solid wedges earlier developed for wedges with small values of the apex angle [V. V. Krylov, Proc. IEEE Ultrason. Symp., Cat. #94, CHO 793–796 (1994)] has predicted that the effect of water loading results in velocity decrease for wedge modes travelling in the subsonic regime of wave propagation. The results of this theory, in particular the absolute values of wedge wave velocity calculated for slender Plexiglas wedges, agree well with the corresponding experiments. The present study demonstrates that for relative values of wedge wave velocity, as compared with those for wedges in vacuum, this theory provides good quantitative agreement with the experiments on Plexiglas samples also for large values of the apex angle. In addition to this, a generalization of the theory is undertaken to describe the effect of heavier wedge material and a supersonic regime of wave propagation. The corresponding results show good agreement with the existing velocity measurements in immersed brass wedges.

Relationships between cerebral regional blood flow velocities and volumetric blood flows and their respective reactivities to acetazolamide.

The technique of transcranial Doppler ultrasonography (TCD) is widely used for assessment of cerebral blood flow velocity. Whether measurement of changes in TCD velocity can be used for studying volumetric cerebral blood flow variations remains a matter of debate. We therefore investigated the relationship between flow velocity and volumetric cerebral blood flow before and during acetazolamide-induced vasodilation. The middle cerebral artery mean blood flow velocity (MV) measured by TCD and the corresponding regional and hemispheric cerebral blood flows assessed with 133Xe single-photon emission CT were measured in 52 unselected patients. Absolute values of flow and velocity before and after stimulation and their reactivity to acetazolamide were compared. When the correlation was statistically significant, the linearity of the relationship was tested. Absolute values of hemispheric cerebral blood flow were correlated with MV both before (r = .315, P = .02) and after acetazolamide (r = .436, P = .001), whereas regional cerebral blood flow was correlated with MV only after acetazolamide (before, r = .262, P = .06; after, r = .446, P = .001). All these relationships fitted a linear model. In contrast, there was no correlation between acetazolamide-induced relative increments of flow and velocity. Our results support a linear model describing the relationship between absolute values of flow and velocity when arterial section is the slope and anastomotic blood flow is the intercept. In contrast, relative increments in volumetric flow and velocity may be proportional only if anastomotic flow is negligible, ie, in subjects without cerebrovascular disease. We conclude that, for patients with cerebrovascular disease, TCD does not satisfactorily model cerebral vasoreactivity in terms of volumetric cerebral blood flow.

Ventilation through building walls — wind tunnel studies

Abstract The usual way of extracting the waste air in a multi-storey residential building is to conduct the air above the roof. The purpose of this research program was to examine whether exhaust air from each apartment could be discharged outdoors via exhaust vents mounted on the outer wall in such a way that no odours or other harmful effects are caused. This would facilitate the use of apartment-based mechanical ventilation using wall ventilators which would be of great help specially in renovating old multi-storey apartment houses. A wind tunnel investigation was carried out using tracer gas measurements with models of three multi-storey buildings to determine how much waste air from a wall outlet is carried into the intakes on the same wall. Also full-scale experiments were performed and for comparison some of full-scale test wind conditions were reproduced in wind tunnel tests. Several parameters were varied in wind tunnel investigations, for example: exhaust jet-to-wind velocity ratio, the distance between outlet and intake openings and the position of the wall outlet. The results showed that the exhaust jet-to-wind velocity ratio is a suitable parameter for determining dilution rates and the tests can be carried with different absolute velocity values. Owing to the turbulence caused by the building itself and near-by buildings the trajectories of the exhaust plumes are much more complicated with wall ventilators than with roof ventilators. Near exhaust vents on the open wall, the measured dilution was usually much more higher than that predicted by the minimum dilution theory for roof exhaust. Thus, the theory could be useful when dimensioning exhaust vents on an open wall. In niches and corners the theory does not function reliably.

A kinematic analysis of contrastive stress.

The present investigation examined the shape and variation of movement patterns associated with specific stress contexts. Movements of the lower lip were recorded using the Optotrak, a noninvasive system in which light emitting diodes are attached to the moving structure. Eight adult subjects produced versions of the utterance ‘‘Buy Bobby a puppy’’ that varied in stress. Initally, this utterance was produced with normal stress patterns. Then, a scenario was presented that elicited a contrastive stressed form (e.g., to stress ‘‘Bobby,’’ the subject responded to the utterances ‘‘Don’t buy Kathy a puppy. Buy Bobby a puppy.’’). A standard technique in the analysis of kinematic data is to measure absolute values of peak velocity, displacement, and duration. Analyses revealed that, as expected, increased duration and amplitude occurred in stressed contexts. A second set of analysis procedures involved the evaluation of underlying movement patterns through time and amplitude normalization (i.e., collapsing absolute differences in duration and amplitude). Pattern recognition techniques were then applied, revealing that underlying patterning differed as a function of contrastive stress. [Work supported by NIDCD.]

Experimental study of Lamb waves having a negative energy velocity in a thin plane plate immersed in liquid.

Lamb waves having a negative energy velocity have been theoretically predicted; they are called negative waves for short. This word is used if the phase velocity is in the opposite direction to the energy velocity. When a plate is immersed in a liquid, this phenomenon still occurs. These waves become leaky as their energy is being reradiated into the liquid. Any negative wave has the same limit frequency as a Lamb wave, and the same label with a prime (′) sign (i.e., S2−S2′). Some common solids were studied: duralumin, stainless steel, brass, and glass. The negative waves were generated (as Lamb waves are) by a tone burst bulk wave and observed after propagation over the plate by translating one of the transducers in order to receive the negative waves. The existence of several negative waves was proved; especially the A3′ glass wave which can be observed quite easily. When the frequency × thickness product decreases, several properties were observed in accordance with the theory: (i) the absolute value of energy velocity increased and reached a maximum near the cutoff frequency, (ii) the attenuation increased. A time‐frequency study follows which enables the calculation of the energy velocity.