Effects Of Gravitational Forces Research Articles

The formation of water droplets in an air-breathing PEMFC

Air-Breathing Proton Exchange Membrane Fuel Cells (AB-PEMFC) have the potential to supersede lithium-ion batteries in portable electronics. However, their water management issue has yet to be resolved to ensure optimum cell performance and safe system operation. In this paper, the formation of water droplets and their aggregation in the cathode flow channels of an operating AB-PEMFC is investigated by direct visualisation under various operating conditions. The developed optical set-up enables observation of droplet formation on the surface of the membrane from the top and side view of the channels simultaneously. The two orthogonal views reveal that during formation the receding and advancing droplet contact angles are almost identical with values that increase, in a similar trend to the droplet height, with increasing droplet diameter. Water films were able to develop and maintain direct contact with the side wall of the channels even under the effect of gravitational force. The aggregation of water droplets in the channels was strongly influenced by the change in the air and hydrogen stoichiometry conditions. However, these operating parameters appear to have no significant effect on the water extraction from the channels contrary to load and temperature, where temperature has proved to be the most effective water removal mechanism with minimum reduction in the current density of AB-PEMFC.

キャピラリージェット形状を利用した動的表面張力の測定法 : 第1報,噴流形状の理論的解析(流体工学,流体機械)

A theoretical and experimental consideration is conducted to propose a novel method to measure the dynamic surface tension of surfactant solution. The trajectory of laminar jet injected from a horizontal capillary tube is considered from an approximate theory. The momentum balance is discussed between surface tension, gravitational force, pressure and viscous force. Since the effect of gravitational force is relatively small compared with the surface tension, the cross section of jet is assumed as a circle and the pressure inside the jet is approximately obtained from the Laplace equation. The ordinary differential equation of motion is solved numerically by the simple Runge Kutta's method to obtain the profile of jet trajectory. The trajectory profiles are measured experimentally for water and two kinds of ethanol solutions with smaller surface tension than that of water. The theoretical profiles agree well with experimental ones for each condition and the trajectories are strongly dependent on the surface tension. It is demonstrated that the change of surface tension of surfactant solution could be measured by the method proposed in this study.

A Brownian Dynamics Simulation to Study the Influence of Gravity on the Process of Particle Coagulation in Suspension

The Brownian dynamics method was used to simulate the influence of gravity on the process of particle coagulation in a dilute solution. By including or excluding the gravitational influence in a computer simulation, the relative change in the number of aggregates with time for these two gravitational conditions could be compared. Time-dependent variations of the number and size of all aggregates under the gravitational conditions were obtained. We conclude that the influence of gravity is negligible during the earlier stage of the coagulation process and that the rate of aggregation is accelerated by the gravitational force in the latter stage. The effect of gravitational force on the process of particle aggregation in suspension is discussed by using a dynamic analysis method.

202 風車ブレードの非線形振動解析 : 弾性ブレードに生じる面外振動

The use of the wind turbine generator has rapidly spread as the one of the clean energy sources. Recently, as the size of the wind turbine generator becomes larger, its maintenance becomes more difficult. However, there are few studies on the vibration analysis and its suppression in the conventional researches. The wind turbine is a special type of rotating machinery which has a long heavy blade rotating in the vertical plane under the action of the gravitational force. The wind power acting on the turbine blade changes periodically because of the height-dependent characteristic of the wind power. Therefore, the wind turbine blade requires the original point of view in the dynamical analysis. This paper investigates the fundamental vibration characteristic of the elastic wind turbine blade. The nonlinear analysis of the super-harmonic resonance is performed, and its characteristics are explained. Furthermore, the interaction effect of gravitational force and wind force on the super-harmonic resonance is clarified.

Structural and performance studies of poly(vinyl chloride) hollow fiber membranes prepared at different air gap lengths

Poly(vinyl chloride) hollow fiber membranes were prepared by the dry/wet and wet/wet spinning technique at different air gap lengths keeping all other spinning parameters constants. Mean pore size, pore size distribution and mean roughness of both the internal and external surfaces of the hollow fibers were determined by atomic force microscopy. Cross-sectional structure was studied by scanning electron microscopy. Ultrafiltration experiments of pure water and aqueous solutions of different solutes having different molecular weights (bovine serum albumin, polyethylene glycol and polyvinyl pyrrolidone) were carried out. It was found that the inner and outer diameters of the PVC fiber membranes decreased with the increase of the air gap distance due to the gravitational force effect. The hollow fiber membranes prepared without and with air gap distances up to 7 cm exhibited a quite symmetric cross-structure consisting of four layers, two small finger-like structure layers at both edges of the hollow fibers and two larger finger-like voids mixed with macrovoids layers in the middle of the cross-section. The outer-middle layer thickness decreased when the air gap distance was increased to 10 cm and disappeared from the cross-section of the hollow membranes prepared with higher air gap lengths than 15 cm. For all dry/wet spun PVC hollow fibers, the outer pore size and the pure water permeation flux both increased with the increase of the air gap distance. In contrast, the solute separation factor decreased with the air gap distance. This was related to the pore size of the external surface of the PVC hollow fibers.

Thermal Energy Storage in the Ground of a Greenhouse by the Polypropylene Capillary Heat Exchanger

Nonlinearities and parametric uncertainties are unavoidable problems faced in controlling robot manipulator. A single link manipulator driven by a permanent magnet brushed dc motor is a nonlinear dynamics due to effects of gravitational force, mass of the payload, posture of the manipulator and viscous friction coefficient. Furthermore, uncertainties arise because of changes of the rotor resistance with temperature and random variation of friction while operating. Due to this fact, classical PID controller can not be used effectively since it is developed based on linear system theory. In order to overcome this problem, in this research, a neural network control scheme, NARMA-L2 Control is adopted and implemented in real time for controlling a DC motor driven single link manipulator with unknown dynamics. However, the real time experimentation showed that the proposed system results in chattering of the control signal. Hence, the system also chatters within the desired trajectory. As a solution, real time Smoothed NARMA-L2 Control scheme is implemented. Physical results showed that the improved control scheme has not only reduced the chattering but has successfully controlled the single link manipulator for both point-to-point and continuous path motion control.

The effects of gravity on human walking: a new test of the dynamic similarity hypothesis using a predictive model

The dynamic similarity hypothesis (DSH) suggests that differences in animal locomotor biomechanics are due mostly to differences in size. According to the DSH, when the ratios of inertial to gravitational forces are equal between two animals that differ in size [e.g. at equal Froude numbers, where Froude = velocity2/(gravity x hip height)], their movements can be made similar by multiplying all time durations by one constant, all forces by a second constant and all linear distances by a third constant. The DSH has been generally supported by numerous comparative studies showing that as inertial forces differ (i.e. differences in the centripetal force acting on the animal due to variation in hip heights), animals walk with dynamic similarity. However, humans walking in simulated reduced gravity do not walk with dynamically similar kinematics. The simulated gravity experiments did not completely account for the effects of gravity on all body segments, and the importance of gravity in the DSH requires further examination. This study uses a kinematic model to predict the effects of gravity on human locomotion, taking into account both the effects of gravitational forces on the upper body and on the limbs. Results show that dynamic similarity is maintained in altered gravitational environments. Thus, the DSH does account for differences in the inertial forces governing locomotion (e.g. differences in hip height) as well as differences in the gravitational forces governing locomotion.

Following tracheal intubation, mucus flow is reversed in the semirecumbent position: Possible role in the pathogenesis of ventilator–associated pneumonia

Critically ill intubated patients are positioned in the semirecumbent position to prevent pneumonia. In tracheally intubated sheep, we investigated the effects of gravitational force on tracheal mucus transport and on bacterial colonization of the respiratory system. Prospective randomized animal study. Animal research facility at the National Institutes of Health. Sixteen healthy sheep. Spontaneously breathing or mechanically ventilated sheep were randomized to be positioned with the orientation of the trachea above (40 degrees, trachea-up) or below (5 degrees, trachea-down) horizontal. Tracheal mucus velocity was measured through radiographic tracking of radiopaque tantalum disks, insufflated into the trachea. After 24 hrs, sheep were euthanized, and samples from the airways and lungs were taken for microbiological analysis. The proximal trachea was colonized in all sheep. In trachea-down sheep, all mucus moved toward the glottis at a mean velocity of 2.1 +/- 1.1 mm/min. When mucus reached the endotracheal tube, it either entered the endotracheal tube or was lodged at the inflated endotracheal tube cuff. In all trachea-up sheep, abnormal tracheal mucus clearance was found. Mucus, mostly on the nondependent part of the trachea, moved toward the glottis at an average velocity of 2.2 +/- 2.0 mm/min and constantly accumulated at the inflated endotracheal tube cuff. From the proximal trachea, mucus eventually moved toward the lungs on the dependent part of the trachea, leading to an "intratracheal route" of colonization of the lungs. Pneumonia was found in 6/8 of trachea-up sheep and the same microorganisms were isolated from the lungs and the proximal trachea. No pneumonia was found in trachea-down sheep (p = .007). The study indicates that following tracheal intubation gravitational force influences tracheal mucus clearance. When the trachea is oriented above horizontal, a flow of mucus from the proximal trachea toward the lungs is highly associated with bacterial colonization of the airways and pneumonia.

Variable-density miscible displacements in a vertical Hele-Shaw cell: linear stability

A computational study based on the Stokes equations is conducted to investigate the effects of gravitational forces on miscible displacements in vertical Hele-Shaw cells. Nonlinear simulations provide the quasi-steady displacement fronts in the gap of the cell, whose stability to spanwise perturbations is subsequently examined by means of a linear stability analysis. The two-dimensional simulations indicate a marked thickening (thinning) and slowing down (speeding up) of the displacement front for flows stabilized (destabilized) by gravity. For the range investigated, the tip velocity is found to vary linearly with the gravity parameter. Strongly stable density stratifications lead to the emergence of flow patterns with spreading fronts, and to the emergence of a secondary needle-shaped finger, similar to earlier observations for capillary tube flows. In order to investigate the transition between viscously driven and purely gravitational instabilities, a comparison is presented between displacement flows and gravity-driven flows without net displacements.The linear stability analysis shows that both the growth rate and the dominant wavenumber depend only weakly on the Péclet number. The growth rate varies strongly with the gravity parameter, so that even a moderately stable density stratification can stabilize the displacement. Both the growth rate and the dominant wavelength increase with the viscosity ratio. For unstable density stratifications, the dominant wavelength is nearly independent of the gravity parameter, while it increases strongly for stable density stratifications. Finally, the kinematic wave theory of Lajeunesse et al. (J. Fluid Mech. vol. 398, 1999, p. 299) is seen to capture the stability limit quite accurately, while the Darcy analysis misses important aspects of the instability.

543 風車ブレードの振動解析 : 剛性1枚翼単体モデルを用いた理論解析と実験

Recently, the use of the wind turbine generator has rapidly spread as the one of the clean energy sources. Its size is becoming larger, and simultaneously, the maintenance of the turbine generator becomes more difficult. However, the conventional reserches aimed at the fluid engineering in order to gain higher efficiency, and only few investigation have been reported in the area of the vibration analysis and its suppression. The wind turbine is a special type of rotating machinery, and it requires the original point of view in the dynamical analysis. It has a long heavy blade which rotates in the vertical plane under the action of the gravitational force. The wind power acting on the turbine blade changes periodically because, for example, the wind power increase as the function of the height. This paper investigates the fundamental vibration characteristic of the wind turbine blade by using rigid blade model. The interaction effect of gravitational force and wind force on the vibration of the turbine blade is clarified. The super-harmonic resonances occur due to the effect of parametric excitation effect which is caused by gravitational force. Also, the occurrences of the unstable vibration ranges are indicated.

Spacecraft stability, dynamics and control near the triangular Lagrange points influenced by multiple Trojan asteroids

In view of the importance of Lagrange points to the exploration and development of space, the dynamics and stability of a satellite were studied under multiple Trojan asteroids influence. Through the use of a numerical simulator developed in MATLAB, consideration was given to the effects of gravitational forces exerted by the asteroids themselves, simulating the resulting insignificant influence of the Trojan asteroids on a satellite placed at the triangular Lagrange points. The study of optimized satellite transfers between triangular Lagrange points allowed the enforcement of multiple, specific, non-linear constraints on critical mission parameters of maximum thrust, mission duration, propellant consumption and accelerations. The optimized transfer trajectory between the two triangular Lagrange points was direction sensitive. That is, the minimum thrust optimized transfer trajectory for a satellite from L 4 to L 5 was unique and vastly different to that from L 5 to L 4 . A further exciting discovery highlighted that superposition of the latter trajectories formed a perfectly smooth, uninterrupted kidney-shaped loop, fused at the two relevant points of connection. Implications for this phenomenon extend directly to future mission planning.

Effect of gravitational force on type-J counter-current chromatography by mathematical analysis

The gravitational force exerts an important effect on the counter-current chromatography (CCC) and has been used to retain the stationary phase and improve the efficiency in the various CCC separations. This paper deals with the effect of gravitational force on type-J CCC, one of the most popular CCC methods by the mathematical analysis for the first time. The theoretical analyses reveal a close relationship between the effect of gravitational force, the placed orientation of type-J CCC apparatus, and the revolutional speed. Two placed orientations are more suitable for preparative CCC separation: one is common horizontal, and the other upright. Thus, there are three optimum conditions for type-J CCC separation, the first using horizontal apparatus at very low revolutional speed around 10 rpm in the gravitational filed, the second by use of upright apparatus at moderate revolutional speed around 100 rpm in the three-dimensional complex force field composed of centrifugal and orthogonal gravitational force, and the third employing the apparatus placed at any orientation including horizontal and upright CCC apparatus at high revolutional speed above 300 rpm in the centrifugal force field. Therefore, we should consider the effect of gravitation force on CCC at the selection of proper operational conditions according to properties of two immiscible phases, the desired revolutional speed and the parameters of apparatus in order to utilize efficiently the effect of gravitational force. These analyses and results will benefit to improve the efficiency of CCC separation, especially large-scale industrial preparation.

Pendulum Mode Control of Free-Free Launcher Structural Models in Gravity Fields

For structural models that contain tanks filled with liquid propellants subjected to gravitational forces, so-called pendulum modes exist. The same holds true for structures with prestressed elements due to gravitational forces. Depending on the sign of the eigenvalue, the pendulum mode can be classified as either stable or unstable. For launchers, the thrust point is positioned below the launcher center of gravity. Hence, unstable pendulum modes with negative eigenvalues, both due to prestress and liquid propellants, do exist. In reality, the unstable pendulum modes are controlled by thrust vectoring. In launcher‐payload coupled dynamic analysis, a simple control system could be employed using control moments. The control moments can be introduced by means of a preconditioned stiffness matrix. For this purpose, the rigid-body forces or the rigid-body strain energy of the free‐free stiffness matrix will be considered. A global sensing system is needed to minimize the effect of the control method on the flexible modes. The low-frequency modes are affected most by the control method. In addition to the prestress effect of gravitational forces (dead loads), the prestress effect of pressure forces (follower loads) can also be significant. The latter type of forces do not need to be controlled because they do not cause a resulting moment under rigid-body rotation.

Video Games and the Future of Learning

Video Games and the Future of Learning

An Integrated Experimental Study of Foamy Oil Flow During Solution Gas Drive

Abstract One of the important mechanisms in the primary production of heavy oil reservoirs under solution gas drive is the foamy oil flow effect. This form of flow occurs only under certain combinations of capillary, viscous, and gravity forces. Several studies have been carried out to examine this effect on the recovery process, but it remains poorly understood and difficult to model. The objective of this study was to examine the roles of capillary, viscous, and gravity forces in foamy oil flow. Primary depletion experiments were conducted in a 2 m long sand pack holder to measure the oil and gas production at different depletion rates. The effect of gravitational forces was investigated by comparing the experiments conducted in horizontal and vertical orientations of the 2 m long sand pack. In the vertical orientation, the solution gas drive performance was evaluated with production from the bottom end of the sand pack, as well as with production from the top end. The experimental results are presented in terms of the oil and gas production behaviour affected by the capillary number and gravity forces. The results indicate that the gravity forces do not significantly influence the ultimate oil recovery when the oil is produced under foamy oil flow. Even at the lowest depletion rate used in these tests, when the oil flow and gravity are in the same direction, gravity segregation of gas did not improve the oil recovery significantly. Recovery efficiency and the critical gas saturation at which free gas flow starts are primarily dependent on the capillary number. Introduction Some of the heavy oil reservoirs in the world show anomalous primary production under solution gas drive. Smith(1) was one of the first researchers who presented a detailed analysis of such unusual production behaviour. Since then, it has been the subject of many studies, due to its importance in making reliable projections of future production and optimization of field operations. These studies can be divided into three categories. The first category focuses mainly on the geomechanical aspects(2, 3). It is believed that the production of sand with oil improves the inflow performance by creating wormholes (high permeability channels). Experimental and theoretical studies(3, 4) have shown that the high pressure gradients present during cold production can overcome the cohesive strength of the sand matrix and this leads to the creation of wormholes. However, the mechanics of wormhole network development are not yet clear. The second category focuses on multiphase flow behaviour, and can be further divided into two subcategories. First, there are the conventional two-phase flow models(5, 6), which suggest that solution gas drive, whether in light oils or in heavy oils, is describable by the critical gas saturation and oil-gas relative permeability. In these, the high oil recovery is attributed mainly to low gas mobility. However, a good history match of the field performance is not obtained by using low gas mobility; it also requires enhanced permeability far from the wellbore due to sand production(7).

Analysis of the effects of gravity and wall thickness in a model of blood flow through axisymmetric vessels

The effects of gravitational forces and wall thickness on the behaviour of a model of blood flow through axisymmetric vessels were studied. The governing fluid dynamic equations were derived from the Navier-Stokes equations for an incompressible fluid and linked to a simple model of the vessel wall. A closed form of the hyperbolic partial differential equations was found, including a significant source term from the gravitational forces. The inclination of the vessel is modelled using a slope parameter that varied between -1 and 1. The wave speed was shown to be related to the wall thickness, and the time to first shock formation was shown to be directly proportional to this thickness. Two non-dimensional parameters were derived for the ratio of gravitational forces to viscous and momentum forces, respectively, and their values were calculated for the different types of vessel found in the human vasculature, showing that gravitational forces were significant in comparison with either viscous or momentum forces for every type of vessel. The steady-state solution of the governing equations showed that gravitational forces cause an increase in area of approximately 5% per metre per unit slope. Numerical simulations of the flow field in the aorta showed that a positive slope causes a velocity pulse to change in amplitude approximately linearly with distance: -4% per metre and +5% per metre for vessels inclined vertically upwards and downwards, respectively, in comparison with only +0.5% for a horizontal vessel. These simulations also showed that the change relative to the zero slope condition in the maximum rate of change of area with distance, which was taken to be a measure of the rate of shock formation, is proportional to both the slope and the wall thickness-to-inner radius ratio, with a constant of proportionality of 1.2. At a ratio of 0.25, typical of that found in human arteries, the distance to shock formation is thus decreased and increased by 30% for vessels inclined vertically downwards and upwards, respectively. Gravity and wall thickness thus have a significant impact on a number of aspects of the fluid and wall behaviour, despite conventionally being neglected.

In vivo evaluation of the effects of gravitational force (+Gz) on over-the-wire stainless steel Greenfield inferior vena cava filter in swine.

This study was done to determine the effect of exposure to gravitational force (acceleration stress) on in vivo over-the-wire stainless steel Greenfield inferior vena cava filters. Fifteen pigs underwent venous cut down and placement of a stainless steel Greenfield filter. A 4-week observation period simulated realistic convalescence and allowed sufficient time for epithelialization. Ten pigs were exposed to acceleration stress in a centrifuge (3G run for 15 sec followed by rest until return to baseline heart rate, then a 9G run for 15 sec), with inertial loading in a head-to-tail direction (+Gz). Fluoroscopy during acceleration stress allowed assessment for filter migration. Five pigs were not exposed to acceleration stress. AP and lateral abdominal radiographs were obtained at post-filter placement, convalescence, and centrifuge exposure to determine the position and integrity of the filter. All 15 IVCs were resected and evaluated for gross or histological injury to the vessel wall. IVC filter placement was technically successful in all 15 pigs. Radiographic measurements were limited secondary to differences in pig positioning. Fluoroscopy showed no filter migration. All filters were securely attached to the vena cava by the hooks without gross evidence of perforation or hemorrhage. There were varying degrees of fibroplasia involving the hooks and tip of the filters in both the control and experimental groups. Histologically, there was evidence of prior hemorrhage at the level of the hooks, which was similar between the control and experimental groups. It is concluded that Greenfield filter position and vena caval integrity at the implantation site is unaffected by high acceleration stress.

Numerical and experimental analysis of periodic patterns and sedimentation of lysozyme

This paper deals with experimental investigation, mathematical modelling and numerical simulation of the crystallization processes induced by counter diffusion method of a precipitant agent in a lysozyme protein solution. Novel mathematical strategies are introduced to simulate the experiments and in particular to take into account the kinetics of the growth process and the motion of the crystals due to the combined effect of gravitational force and viscous drag if the sedimenting process is allowed (protein chamber free of gel). Comparison between experimental observations and numerical simulations in the presence of convection and sedimentation and without them provides a validation of the model. The crystal formation in gel results modulated in space. If the gel matrix is not present, convective cells arise in the protein chamber due to local inversions in the density distribution associated to nucleation phenomena. As time passes, these vortex cells migrate towards the top of the protein chamber exhibiting a different wave number according to the distance from the gel interface. The sedimentating particles produce a wake due to depletion of protein from the surrounding liquid. The models and the experiments may represent a useful methodology for the determination of the parameters and conditions that may lead to protein crystallization.

COMPLEX DYNAMICS OF RHYTHMIC PATTERNS AND SEDIMENTATION OF ORGANIC CRYSTALS: A NEW NUMERICAL APPROACH

This article deals with the mathematical modeling and numerical simulation of the complex phenomena related to the crystallization of an organic macromolecular substance due to solubility modulation (lysozyme). Novel mathematical models and numerical strategies are introduced to simulate the protein nucleation and further precipitation (or resolution) according to the concentration distribution and in particular to take into account the motion of the crystals due to the combined effect of gravitational force and viscous drag if the sedimenting process is allowed (protein reactor free of gel). The numerical simulations show that gellified lysozyme reacts to produce "spaced deposits." The phenomenon is characterized by a certain degree of periodicity in time and in space (Liesegang patterns). If the gel matrix is not present, crystals settle in the growth chamber. In this case, due to solutal buoyancy forces, a very complex convection pattern arises whose dynamics depend on the nucleation phenomena occurring in the bulk of the protein chamber.

Prediction of Agglomerate Size for Fine Particles in a Vibro-fluidized Bed.

A model for estimating the agglomerate size of Geldart group C powder in a vibro-fluidized bed was proposed. The agglomerate size was estimated by using the experimental value of the lower limit of gas velocity for channel breakage, ucha, and the void fraction, echa, at ucha. The force balance between cohesive force (van der Waals force) and separation force (gravity, vibration and shear force by a gas flow) was considered. The vibrational force was considered to act on the agglomerates to increase the effect of gravitational force. The calculated agglomerate size decreased with increasing vibration strength.The value of ucha decreased with increasing vibration strength for all tested group C powders in this study. The effects of particle properties such as particle diameter, density and material on ucha were discussed. It was found that a higher value of ucha was obtained when the cohesive force was large in the cohesive-separation force balance. This indicates the particle properties affect the cohesive–separation force balance.