Search

Background The relationship between the visual sensory and center-of-mass characteristics during the sit-to-stand task is not well documented. Do the center of mass strategies change with restricted vision during the sit-to-stand task? Methods The participants in this study were fifteen girls with congenital blindness (age (94.6 (5.6) months), mass (25.6 (2.0) kg), and height (1.3 (0.0) m)) and 30 healthy girls with no visual impairment who were divided randomly into two groups. The two experimental condition groups consisted of one in which the participants were assigned to keep their eyes open (age (95.8 (5.4) months), by mass (26.1 (5.1) kg), and by height (1.3 (0.0) m)) and the other in which the participants were assigned to keep their eyes closed (age (93.8 (0.0) months), by mass (24.2 (0.0) kg), and by height (1.1 (0.0) m)) for 20 min before the test. The center-of-mass displacement of the foot, leg, and thigh were calculated for dominance and non-dominance during the sit-to-stand performance at initiation, seat-off, and end events. Findings Congenital blindness was associated with the non-dominant center-of-mass displacement of the foot increasing upwards during all events (initiation, p = 0.025; seat-off, p = 0.036; end, p = 0.034), as well the non-dominant center of mass of the foot moving anteriorly during the initiation point (p = 0.016). In addition, the center-of-mass displacements for the dominant and non-dominant feet (initiation, p = 0.006, and p = 0.016; seat-off, p = 0.006, and p = 0.014; end, p = 0.006, and p = 0.013, respectively) and legs (16.98%, p = 0.024, d = 0.95; and 17.88%, p = 0.032, d = 0.99, respectively) in the eyes-closed group shifted to a forward direction, and the center of mass of the non-dominant foot (initiation, p = 0.024; seat-off, p = 0.021; end, p = 0.012) moved significantly upwards. Furthermore, the whole body center of the mass velocity (anterior-posterior axis (initiation, p = 0.042; seat-off, p = 0.006; end, p = 0.005)) in the eyes-closed group was smaller than that in the eyes-open group. Interpretation The findings show that the sit-to-stand center of the mass components in blind children may have clinical importance for the rehabilitation of these subjects.

An experimental research has been carried out for vertically upward two-phase flow of air-water across inverted half U-tube bundle with or without anti-vibration bars. Pressure drop characteristics in inverted half U-tube bundle and anti-vibration bars were obtained and analyzed. New correlations for total pressure drop of two-phase flow in inverted half U-tube bundle and local pressure drop of two-phase flow in anti-vibration bars were developed. The measured two-phase pressure drops in inverted half U-tube bundle of three walls in the test sections were unequal. The pressure drop in the side wall dominated the total pressure drop of two-phase flow in inverted half U-tube bundle for 57 ≤ G −2 ·s −1 , while the pressure drop in the front wall dominated for 114 G ≤ 143 kg·m −2 ·s −1 . An obvious mass velocity effect was observed for the total pressure drop of two-phase flow in inverted half U-tube bundle. For a given Martinelli parameter, the total pressure drop of two-phase flow in inverted half U-tube bundle decreased with an increasing mass velocity. The two-phase local pressure drop data of anti-vibration bars could be fitted well in terms of the Martinelli parameter and mass velocity.

In this study, an experimental investigation was carried out to determine the increase of pressure drop during convective condensation of R-404A vapor inside coiled wire inserted tubes. Experiments were performed for a plain tube and five coiled wire inserted tubes. Data were collected for four mass velocities. For each mass velocity, six different vapor qualities were considered. From the results, it is found that the coiled wire inserts increased the pressure drop up to 1200% compared to plain tube values. An empirical correlation is also developed to predict the pressure drop of coiled wire inserted tubes during R404-A condensation with accuracy of ±20%.

This article presents dryout results of propane in single vertical circular minichannels made of stainless steel with internal diameters of 1.70 mm and 1.224 mm. The heat flux is increased in steps up to occurrence of dryout. The effects of different parameters such as mass flux, saturation temperature, vapour quality and internal diameter on the dryout are investigated. The results show that the dryout heat flux increases with the increase in mass velocity, with the decrease of vapour quality and with the increase of internal diameter. Almost no effect of saturation temperature on dryout heat flux is observed. Generalised CHF correlations developed for macro and micro scale from the literature are also compared with the experimental results. QC 20120210

In this paper, 325 pressure drop data points measured in return bends have been collected from the literature. The database includes 3 different fluids (R-12, R-134a and R-410A) from two laboratories. Based on this database, a new method is proposed for predicting the pressure gradient in return bends, which is the sum of the frictional pressure gradient that would be obtained in straight tubes (predicted by Muller-Steinhagen and Heck, 1986 correlation) and the singular pressure gradient (proposed equation). The proposed correlation includes only two empirical constants and exhibits the correct physical limits. Using the proposed equation, simulations have been performed to predict the effect of fluid, mass velocity and saturation temperature on the singular pressure gradient.

Investigation methods for characteristics of movement along the tubes, combustion dynamics and gasification of separate drops were developed for the coal–water mixtures (CWM). The following parameters were determined on the basis of laser heating: thermometric, pyrometric and concentration dynamics of single-drop combustion, complete combustion times, duration of temperature phases of combustion, as well as the moment and temperature of ignition. Information on the combustion mass velocity and gasification products was also obtained using laser heating.

In this paper, dynamic response of Timoshenko beams under moving mass is analyzed using a numerical method called discrete element technique (DET). In DET, continuous flexible beam elements are replaced by a system of rigid bars and flexible joints. We present a DET model of Timoshenko beams under moving mass. The results of our DET model are compared with the solutions obtained by PAFEC (programs for automatic finite element calculations) for Euler–Bernoulli beams and finite difference method for Timoshenko beams. The effects of beam thickness and moving mass velocity on dynamic response of beams under moving mass are numerically studied.

Experimental results are presented that show the effect of fin geometry on condensation of refrigerant HFC134a in a staggered bundle of horizontal finned tubes. Two types of conventional low-fin tubes and three types of three-dimensional fin tubes were tested. The refrigerant mass velocity ranged from 8 to 23 kg/m2s and the condensation temperature difference from 1.5 to 12 K. The effect of condensate inundation was more significant for the three-dimensional fin tubes than for the low-fin tubes. In most cases, the highest performance was obtained by the tube with a three-dimensional structure at the tip of low fins. In the case of high mass velocity and high condensate inundation rate, however, the highest performance was obtained by one of the low-fin tubes. The results were compared with previous results for bundles of smooth tubes and low-fin tubes.

Linear acoustic theory is still the most widely used tool for exhaust system design in I. C. engines. In this paper, the pressure and mass velocity distributions along the exhaust system are used to evaluate the results from linear acoustic theory. These results are compared with those obtained from a full non-linear calculation validated against engine test experiments. The analysis performed allows the influence of two relevant aspects to be separated: the linear representation of the source (engine and manifold) and the linear description of the exhaust system itself. The results indicate that good qualitative agreement may be attained in general, but some differences associated with non-linear effects can appear.

Relativistic ab initio calculations of the 183 W magnetic shielding constant and the chemical shift of WX 6 (X = F and Cl) and WO 4 2− are presented. The computational method is a combination of the relativistic spin-free no-pair theory and the spin-orbit unrestricted Hartree-Fock method, which has been applied previously to 199 Hg magnetic shielding constants. The spin-tree relativistic (SFR) terms, involving the mass velocity and Darwin terms, are shown to be important for 183 W shielding constants. The spin-orbit interaction, which is smaller than the SFR term, works differently on WCl 6 and WO 4 2− . The effects of relaxing the inner s and p orbitals of W are investigated.

Experimental results on void fraction and friction pressure drop in vapour-potassium flows in the high-vapour-quality region up to unity are presented. The experimental data obtained and the pertinent results of other authors are generalized, and empirical relationship are suggested to calculate void fraction and pressure drop in two-phase liquid metal flows for channels of various configurations and orientations. The relationships are valid within the range of vapour qualities from almost zero to unit. The experimental data prove the mass velocity to have no influence on the hydrodynamic characteristics within the range of the parameters investigated. It is found in the experiments with heat supply that friction pressure losses are smaller than those for adiabatic conditions. It is shown that this result is in good correspondence with the model of the effect of injection in a boundary layer on the value of shear stresses between cases.

An experimental investigation of flow transient critical heat flux (CHF) for water at pressures 6.9 and 15.5 MPa is conducted under the simulation of both Boiling Water Reactor (BWR) and Pressurized Water Reactor (PWR) conditions. The influence of flow decay rates on local mass velocity and quality at CHF is explored to ensure the adequacy of local-condition approach in the CHF predictions. The assesment of transient upstream correction factor to modify the predictions of steady-state CHF correlations during the BWR flow transients are presented. A mechanistic CHF model based on the dryout of a sublayer and CHF correlations are evaluated in the predictions of PWR flow transient CHF.

Over a wide range of length-to-diameter ratios l / d extending from 50 to 800, the systematic nature of the critical heat flux (CHF) in tubes is explored by analyzing CHF data upto 1051, which have recently been obtained in experiments of R-12 at 1.47 MPa for a tube diameter d = 3, 5 and 8 mm, and a mass velocity G = 510–6055 kg m −2 s −1 . First, concrete evidence is obtained to show the unfitness of the expedient method substituting CHF data in tubes of low l / d values with subcooling enthalpy ΔH i l / d values with ΔH i > 0. Next, the CHF data with ΔH i > 0 are analyzed on the basis of two typical kinds of existing generalized correlations of CHF data, showing a fact that the value of CHF for tubes of diameter as small as 3 or 5 mm tends to deviate from standard values predicted by the foregoing generalized correlations based on the CHF data for l / d less than, say, 370. A tentative means to predict the above-mentioned deviation peculiar to long and narrow tubes is also obtained.

The pressure drops through contractions in horizontal single and two-phase flow were investigated. A total of 167 measurements were made for four different entrance shapes to study the effects of the entrance shape on the pressure drop through a contraction in horizontal single and two-phase flow. From this data, pressure drops were calculated and compared with the pressure drops predicted by analytical models for single and two-phase flow. For single phase flow the agreement between the data and predictions is within ±25% , whereas for two-phase flow, Hoopes model which gives a better agreement than the homogeneous model, underpredicts the data as much as 45%. In addition, the effects of void fraction and liquid phase mass velocity on the pressure drop through the sudden flow channel contraction were investigated for two-phase flow.

Eighteen two-phase friction pressure drop models and correlations were tested against about 2220 experimental steam-water pressure drop measurements under adiabatic conditions and about 1230 in diabatic flow conditions. The data represented several geometries and had the following property ranges: Pressure 1.7–10.3 MN/m2 (250–1500 psia); Mass velocity 270–4340 kg/m2sec (0.2–3.2 Mlb/ft2hr); Quality Subcooled to 100%; Equivalent diameters 2.3–33.0 mm (0.09–1.3 in.). The four models and correlations which were found to have the best performance were the Baroczy correlation, the Thom correlation and the homogeneous model two-phase friction multipliers, φ 2 fo = 1+X V fg V f and φ 2 fo = 1+X V fg V f 1+X μ g μ f −1 0.25 The correlations were also evaluated with the data being sub-divided into sets which were based on properties and flow conditions.

The application of a conical fluidized bed as a remedial measure for certain inherent drawbacks of gas-solid systems is suggested. Investigations were carried out in Perspex cones with apex angles of 10°, 30°, 45° and 60°, with spherical glass beads of different sizes as the bed material, and air as the fluidizing medium. With the help of available correlations and packed bed pressure drop data, minimum fluidization velocities for the above cases were computed and compared with experimental data obtained from bed pressure drop versus fluid mass velocity plots, and the mean and standard deviations were calculated.

All loss-of-coolant-accident analysis codes solve the one-dimensional hydraulic conservation equations for a two-phase compressible fluid. Some codes allow slip between the phases, this slip being specified by a slip model. In this paper the characteristics of these conservation equations with well-known slip models (Jones, Moody, Fauske, Smith and CISE) are evaluated as a function of pressure, quality and mass velocity. Regions in which two of the characteristics are complex are shown on contour maps. The significance of these regions is discussed.

The impulsive and gradual phases of solar flares are investigated, comparing simultaneous SMM UVSP and HXRBS observations. Data for several 1980 flares are presented graphically and characterized in detail, with reference to the results of numerical simulations based on theoretical models. Consideration is given to the density and mass velocity in the transition region, the temporal and spatial relationships between the impulsive UV and X-ray bursts, and the relationships between the impulsive and gradual phases.

A gas turbine counterflow plate-finned regenerator is sized for (a) a minimum volume at a given cycle efficiency degrade (η∞−η) which is the same magnitude as entropy generation, NST, and (b) the volume resulting in minimum generation, NST, along a path of constant mass velocity, G, as suggested by Bejan (A S M E J. Heat Transfer 99, 374 (1977)). This latter procedure results in much larger regenerators for any (η∞−η) or NST.

One of the primary functions of the space station is related to the propellant resupply of orbital transfer vehicles, orbital maneuvering vehicles, and satellites. Difficulties arise in the case of an acquisition of cryogenic propellants by means of a use of zero-gravity techniques. The use of the 'tethered propellant resupply technique' is, therefore, considered. A study is being conducted to determine the feasibility, design requirements, and operational limitations of this technique. Attention is given to aspects of gravity feed, transfer method selection, requirements related to the orbital transfer vehicle, hazard clearance, attitude control, depot operations, end mass velocity, the microgravity laboratory, and concept evaluation activities.

A best-estimate prediction of transient critical heat flux (CHF) is based on instantaneous local-conditions. Eight correlations were tested against most recent blowdown heat-transfer data obtained in the U.S. The prediction results are summarized in a table in which both CISE and Biasi round-tube correlations are found to be capable of predicting the early CHF that occurs after flow reversal in the order of 1 s. The Griffith-Zuber correlation is credited for its prediction of the delay CHF that occurs in a quiescent state with slowly decaying mass velocity. A mass-velocity criterion is also suggested for its application. In many experiments, the early CHF can be closely correlated by the X  1.0 criterion; this is certainly indicative of a dryout mechanism. As for the delay CHF, a pool-boiling-type hydrodynamic crisis is suggested.

Diffusiophoretic separation of 0·79 μm latex spheres from nitrogen—ammonia mixtures was studied under turbulent conditions. The experiments were conducted with a small wetted wall column in which the gas mixture flowed countercurrently to a descending water film. The measured particle removal efficiencies are compared with theoretical predictions based on the hypotheses that the radial particle velocity is given by the mean molar velocity of the gas mixture, the mean mass velocity of the mixture, or Schmitt and Waldmann's (1960) expression. It is shown that the theoretical efficiencies are only functions of the molecular weights and terminal concentrations of the gaseous species and that the second hypothesis is in best agreement with experimental data. Diffusiophoresis under turbulent conditions requires large changes in gas composition to achieve even moderate particle removal efficiencies.

Axial dispersion or backmixing appears to be responsible for adverse mass velocity effects observed in trickle-flow laboratory reactors. At low Reynolds numbers typical of bench-scale units, the dispersion problem can be at least an order-of-magnitude more severe in trickle-flow than vaporphase operation. A simple perturbation criterion is derived for the minimum reactor length required for freedom from significant axial dispersion effects. It shows that the minimum length increases with both conversion and reaction order, and is inversely proportional to the Bodenstein number.

Results of observations on the initiation process in trotyl (TNT) charges shocked by air blast waves are presented. The evolution of the pre-detonation waves was investigated by the electromagnetic method. In the experiments, the shock wave velocity, the mass velocity and the reaction time were measured for the pre-detonation waves. Based on these measurements, the Rankin-Hugoniot relation for unreacted trotyl and the chemical kinetics law were derived. A simplified picture of the process is proposed.

The hydrodynamics of a fluid consisting of several components can be formulated in two different ways by applying the methods of the thermodynamics of irreversible processes. The first formulation is obtained if Gibbs' fundamental formula for the entropy per unit of mass of the system is assumed to be still valid when applied to the movement of the center of gravity of the system; statistically this corresponds to an approximate equilibrium velocity distribution function around the mean mass velocity, that is, to a normal transfer of impulse between the various components. This first formulation contains the classical Stokes-Navier equation for the hydrodynamics of a fluid. A second formulation is obtained if we assume the transfer of impulse between the various components to be a small perturbation or even negligible. This corresponds statistically to approximate equilibrium velocity distribution function around the macroscopical velocity of each component or “fluid” as we say in this case. Assuming therefore Gibbs' formula for the entropy s α per unit of mass of each fluid to be still valid when applied to the macroscopical movement of each fluid, we can derive a new set of hydrothermodynamical relations. When specialising, these equations to the two fluid model of liquid Helium II, Gorter's equations of motion for the normal and the superfluid are obtained.

The paper presents new measurements of the effectiveness of a two-dimensional wall jet and a calculation procedure which is shown to closely predict the measured values for a restricted, but practically important, class of injection conditions. The measurements were performed in the range of slot to free stream mass velocity from 0·467 to 2·26, using helium as a tracer gas and gas chromatographic equipment to detect the relevant concentrations. The prediction method uses empirical correlations to relate the flow properties at the slot to those downstream of the potential core. Thereafter, an explicit method of solving the boundary-layer equations is employed. The advantages and disadvantages of the method are discussed.

A canonical form of generalised two-fluid equations is derived. The Landau equations are contained as a special case. It turns out that superfluid vortices are transported with the mass velocity.