Tubes Of Different Radii Research Articles

The relationship between threshold power of wave mode transition and tube radius in argon helicon discharge

We report in this Letter a geometric similarity of threshold radio frequency (RF) power absorbed by the helicon wave coupled mode in argon helicon discharge. Helicon plasma was excited by a half-helical antenna in tubes of different radii at a pressure of 0.3 Pa. Plasma density was measured by using a Langmuir probe, and wave structure was measured by a B-dot probe. The results show that the threshold powers for triggering helicon wave mode are proportional to the tube cross sections, and the plasma densities at the transition point are the same, indicating a geometric scaling law between the RF power and the tube size. This similarity of threshold power might provide a comparison of results in different systems.

Electric Double Layer and Orientational Ordering of Water Dipoles in Narrow Channels within a Modified Langevin Poisson-Boltzmann Model.

The electric double layer (EDL) is an important phenomenon that arises in systems where a charged surface comes into contact with an electrolyte solution. In this work we describe the generalization of classic Poisson-Boltzmann (PB) theory for point-like ions by taking into account orientational ordering of water molecules. The modified Langevin Poisson-Boltzmann (LPB) model of EDL is derived by minimizing the corresponding Helmholtz free energy functional, which includes also orientational entropy contribution of water dipoles. The formation of EDL is important in many artificial and biological systems bound by a cylindrical geometry. We therefore numerically solve the modified LPB equation in cylindrical coordinates, determining the spatial dependencies of electric potential, relative permittivity and average orientations of water dipoles within charged tubes of different radii. Results show that for tubes of a large radius, macroscopic (net) volume charge density of coions and counterions is zero at the geometrical axis. This is attributed to effective electrolyte charge screening in the vicinity of the inner charged surface of the tube. For tubes of small radii, the screening region extends into the whole inner space of the tube, leading to non-zero net volume charge density and non-zero orientational ordering of water dipoles near the axis.

Dynamic permeability functions for partially saturated porous media

SUMMARY While the frequency-dependence of permeability under fully saturated conditions has been studied for decades, the corresponding characteristics of partially saturated porous media remain unexplored. Notably, it is not clear whether the use of effective pore fluid approaches under such conditions is valid. To address this issue, we propose a method that allows us to obtain dynamic permeability functions for partially saturated porous media. To this end, we conceptualize the considered pore space as a bundle of capillary tubes of different radii saturated by two immiscible fluid phases. We then solve the Navier–Stokes equations within the pore space and define a capillary pressure–saturation relationship, which permits to obtain saturation- and frequency-dependent effective permeability estimates. The application of this method to a realistic model of an unconsolidated granular sediment demonstrates that dynamic effective permeability functions for wetting and non-wetting fluid phases exhibit distinct characteristics, thus rendering effective pore fluid approaches inadequate. Finally, we explore the capability of the seminal dynamic permeability model developed by Johnson et al.[J. Fluid Mech. 176, 379 (1987)] to account for the effects of partial saturation. We find that the frequency scaling proposed by Johnson et al. prevails in partially saturated scenarios. However, the parameters associated with this model need to be redefined to account for saturation-dependent effects.

Paperboard tubes failure due to lateral compression-experimental and numerical study

The work provides an analysis of paperboard tubes subjected to transversal compression force. The investigations were performed for 8 tubes of different radii and different wall thicknesses. The number of layers for detailed pipe ranged from 4 up to 13. The each layer of the paper was modelled with orthotropic material properties. To compare the obtained results of experimental tests, numerical simulations of considered tubes in Ansys 16.2® software were conducted. The calculations were carried out for total failure of pipes using the anisotropic Hill potential theory to implement progressive phase of damage. The scores of investigations were compared with each other and discussed.

Analysis and experimental study on the effect of a resonant tube on the performance of acoustic levitation devices

The influence of a resonant tube on the performance of acoustic standing wave-based levitation device (acoustic levitation device hereinafter) is studied by analyzing the acoustic pressure and levitation force of four types of acoustic levitation devices without a resonance tube and with resonance tubes of different radii R using ANSYS and MATLAB. Introducing a resonance tube either enhances or weakens the levitation strength of acoustic levitation device, depending on the resonance tube radii. Specifically, the levitation force is improved to a maximum degree when the resonance tube radius is slightly larger than the size of the reflector end face. Furthermore, the stability of acoustic levitation device is improved to a maximum degree by introducing a resonance tube of R=1.023λ. The experimental platform and levitation force measurement system of the acoustic levitation device with concave-end-face-type emitter and reflector are developed, and the test of suspended matters and liquid drops is conducted. Results show that the Φ6.5-mm steel ball is suspended easily when the resonance tube radius is 1.023λ, and the Φ5.5-mm steel ball cannot be suspended when the resonance tube radius is 1.251λ. The levitation capability of the original acoustic levitation device without a resonance tube is weakened when a resonance tube of R=1.251λ is applied. These results are consistent with the ANSYS simulation results. The levitation time of the liquid droplet with a resonance tube of R=1.023λ is longer than without a resonance tube. This result is also supported by the MATLAB simulation results. Therefore, the performance of acoustic levitation device can be improved by introducing a resonant tube with an appropriate radius.

Numerical simulation of air permeability in covercrete assuming molecular flow in circular tubes

Torrent’s testing method is extensively used for measuring the surface air permeability of concrete. However, the equation for the coefficient of surface air permeability adopted in the test has not been well validated. We numerically simulated the surface air permeability test assuming molecular flow in different number of circular tubes of different radii. Our numerically obtained coefficients agreed well with the theoretical one and also with Torrent’s coefficients of surface air permeability when the correct porosity was input. The relationships between the surface air permeability, pore (tube) radius, and porosity (effective area) were summarised.

Flexible-bending of profiles with asymmetric cross-section and elimination of side bending defect

Flexible-bending is a relatively novel bending process, particularly suitable for profile and tube bending. Advantageous characteristics, such as manufacturing profiles or tubes of different radii without die change and continuous forming of bent profiles or tubes, make flexible-bending highly applicable and efficient especially in small batch production. In this paper, flexible-bending process of angle iron which is a typical asymmetric-section profile was investigated with finite element analysis method. The effect of die offset on radius of bent angle iron in outward and inward bending process was investigated. The cause and elimination method of side bending defect in bent angle irons were analyzed and compared with the experimental results. The simulation results agree well with the experimental results, which verify the feasibility of using simulation to guide the experiment. The results of both simulations and experiments indicate that the curvature of bending angle iron and side bending increases linearly with the die offset. The curvature of side bending outward is larger than the curvature of side bending inward of same die offset. The side bending defect can be eliminated by shifting the die in reverse direction of side bending.

Intercomparison of gamma ray scattering and transmission techniques for gas volume fraction measurements in two phase pipe flow

Void fraction simulating stratified air–water flow in cylindrical tubes of different radii was measured using transmission and scattering of gamma rays. A simple experimental set-up using 137Cs γ-ray point source of 10μCi and NaI(Tl) detector was used. The void fractions determined from Compton–Compton scattering and transmission peaks were found in good agreement with the real void fractions. However, deviations were noticed between the results obtained from traditional Compton scattered gamma rays and real void fraction. It was shown that sensitivity of gamma ray scattering is better than the transmission measurements. The set-up used in the present work is simpler than those existing in literature and radiation safety and shielding requirements are minimized.

Numerical simulation of the upward propagation of a flame in a vertical tube filled with a very lean mixture

Upward propagation of a premixed flame in a vertical tube filled with a very lean mixture is simulated numerically using a single irreversible Arrhenius reaction model with infinitely high activation energy. In the absence of heat losses and preferential diffusion effects, a curved flame with stationary shape and velocity close to those of an open bubble ascending in the same tube is found for values of the fuel mass fraction above a certain minimum that increases with the radius of the tube, while the numerical computations cease to converge to a stationary solution below this minimum mass fraction. The vortical flow of the gas behind the flame and in its transport region is described for tubes of different radii. It is argued that this flow may become unstable when the fuel mass fraction is decreased, and that this instability, together with the flame stretch due to the strong curvature of the flame tip in narrow tubes, may be responsible for the minimum fuel mass fraction. Radiation losses and a Lewis number of the fuel slightly above unity decrease the final combustion temperature at the flame tip and increase the minimum fuel mass fraction, while a Lewis number slightly below unity has the opposite effect.

Elastic moduli of a boron nitride nanotube

The elastic properties of boron nitride nanotubes have been calculated using theTersoff–Brenner potential which is a bond order potential used successfully previously forcarbon nanotubes. In the present calculation, the same form of potential is used withadjusted parameters for hexagonal boron nitride. The Young’s modulus and shear modulusfor single-walled armchair and zigzag tubes of different radii have been calculated. Theeffects of tube diameter are investigated. The computational results show the variation ofYoung’s modulus and shear modulus of boron nitride nanotubes with nanotube diameter.The results have been compared with available data, experimental as well as calculated.

Phonon and thermal properties of achiral single wall carbon nanotubes

A detailed theoretical study of the phonon and thermal properties of achiral single wall carbon nanotubes has been carried out using force constant model considering up to third nearest-neighbor interactions. We have calculated the phonon dispersions, density of states, radial breathing modes (RBM) and the specific heats for various zigzag and armchair nanotubes, with radii ranging from 2.8 A to 11.0 A. A comparative study of phonon spectrum with measured Raman data reveals that the number of Raman active modes for a tube does not depend on the number of atoms present in the unit cell but on its chirality. Calculated phonon modes at the zone center more or less accurately predicted the Raman active modes. The radial breathing mode is of particular interest as for a specific radius of a nanotube it is found to be independent of its chirality. We have also calculated the variation of RBM and G-band modes for tubes of different radii. RBM shows an inverse dependence on the radius of the tube. Finally, the values of specific heat are calculated for various nanotubes at room temperature and it was found that the specific heat shows an exponential dependence on the diameter of the tube.

A laboratory experiment on inferring Poiseuille's law for undergraduate students

In this paper a laboratory experiment is proposed to infer Poiseuille's law. A simple set-up based on two flasks joined by a detachable tube allows one to measure using tubes of different radii and different lengths. One of the flasks is connected to a vacuum pump to control the pressure differential between the tube extremes. The influence on the flow of different radii, lengths, pressures and viscosities can be studied in a didactic way by measuring the flow rate for each of these variables. The experiment can be performed getting together the students in groups, so that each group concentrates on the effect on the flow of a specific variable, leaving the rest fixed. After putting together all these results the law of Poiseuille can be obtained as a very good approach.

Size and temperature dependence of the specific heat capacity of carbon nanotubes

The specific heat capacity ( C v ) of carbon nanotubes for different temperatures and radii is calculated and presented. These calculations are based upon the continuum model of Zhang et al. The change in dimensionality behaviour (from quasi-1D to quasi-2D) as a function of temperature for tubes of different radii is investigated. It is found that for a range of radii the changeover occurs between 180 K and 400 K. Thicker nanotubes have intrinsically stronger quasi-2D behaviour beyond this temperature range than thinner nanotubes. The present numerical results confirm a qualitative discussion of this behaviour presented earlier by Benedict et al. For a given temperature, the variation of C v as a function of radius is also discussed. An expression has been obtained to show the linear variation of C v in the appropriate low temperature region.

Elastic moduli of single-walled carbon nanotubes and their ropes

We report the results of a model calculation for studying the effects of pressure on a bunch of carbon nanotubes as well as individual nanotubes. Carbon nanotubes are extremely rigid in the axial direction. At pressures that we work with, the deformation in the axial direction comes out to be negligibly small in comparison to that in the transverse direction. We use the six-exponential and Brenner potentials to account for inter- and intratube interactions, respectively. Using second derivatives of potential, Young's modulus for single-walled armchair, zigzag and chiral tubes of different radii have been calculated. The values found by us in this simple model turn out to be in good agreement with other theoretical and experimental values. The strain dependence of Young's modulus has also been studied. We have also calculated the Poisson ratio and shear modulus of various single-walled nanotubes. We find that hydrostatic pressure is an ideal probe to study the radial deformations of the nanotubes. The nanotubes are considered to be flexible, identified by a flattening of cylinders under pressure through a parameter $f$. We calculate the total energy of the bunches having faceted tubes. The free energy thus calculated enables us to calculate phase changes at various pressures. From our calculations, we find the phase transformation to occur at about $5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. Young's modulus of nanoropes has also been calculated at various pressures and at the phase transition we obtain a discontinuity in the curve. A much simplified form of the Brenner potential has been suggested and results are compared with those obtained from original form of Brenner potential.

Helium molecules within carbon nanotubes

The ground-state properties of 3 He and 4 He dimers and trimers in infinite carbon nanotubes are studied. The ground state eigen-functions of a helium atom in tubes of different radii are obtained numerically and then fitted by analytic expressions. Total wave function is assumed to be a product of Jastrow–Feenberg pair correlation and one particle functions. After extensive Monte Carlo (MC) calculation in two successive stages, simple variational MC and diffusion MC it is found that binding depends on the tube radius and it has the maximum for a certain tube width. This radius is between 5 and 7 A ̊ , and the average distance between atoms in such tubes is ranges from 5 to 10 A ̊ . The difference in binding between bosons and fermions disappears for R→0.

Blast Effects of High Explosive Charges Detonating in Cylindrical Steel Tubes

Summary Described are tests of cylindrical high explosive charges with an L=D ratio of 2, centrally detonating in steel tubes of different radii and wall thicknesses to measure the bulge respectively the damaging effects of these steel tubes. The results can be described with analytical equations. For a protecting container with minimum weight the radius should be as large as possible, because the necessary tube wall thickness is quadratically decreasing and therefore the weight of the tube is linearly decreasing with increasing tube radius.

The motion of long bubbles in a network of tubes

The motion and interaction of discrete bubbles in porous materials is studied numerically using a network model. The goal is to extend analytical results for the motion of bubbles through a single straight tube to a more ‘realistic’ geometry for porous materials, modeled here as a planar network of straight tubes of different radii. The problem is characterized by two dimensionless parameters, the capillary number (Ca) and the volume fraction of bubbles (φ); results are characterized by determining the effective permeability of the network and the mean residence time of bubbles in the material. The simulations indicate that at low volume fraction most of the bubbles follow a limited number of high-flow pathways through the network. In this case the predictions of our simulations can be approximated by a simple analytical model. Bubbles interact with each other because their presence changes the local resistance to flow in individual tubes. As φ increases, interactions between individual bubbles become important resulting in a wider range of residence times in the porous material.

Calibration of Jointed Tube Model for the Gas Diffusion Coefficient in Soils

AbstractModeling of gas and vapor transport in soils requires knowledge of the relative gas diffusion coefficient. The relative diffusion coefficient was determined as a function of air‐filled porosity, using a pore model based on two tortuous tubes of different radii joined in series. This model was fitted to measured relative diffusion coefficients on undisturbed samples of nine soil types at a wide range of water contents. It was found that model parameters vary with the soil types studied. Combined measurements of the water retention curve, air‐filled porosity, and relative diffusion coefficient were made to evaluate the effects of water distribution in the soil pores on the relative diffusion coefficient. After the samples were saturated with water, the residual air‐filled porosity ranged from 0.051 to 0.167 m3 m−3 and the relative diffusion coefficient was practically zero, which is due to pore blocking by soil water. At pressure heads corresponding to the air‐entry value in the water retention function, air‐filled porosity ranged from 0.095 to 0.256 m3 m−3 and the relative diffusion coefficient ranged from 0.002 to 0.013. From this point, with increasing air‐filled porosity, the relative diffusion coefficient increased rapidly to values ranging from 0.165 to 0.383 at zero water content. It was concluded that the air‐entry value is an important parameter, which indicates a priori the water content at which pore blocking becomes relevant.

Energetics of Large Fullerenes: Balls, Tubes, and Capsules

First-principles calculations were performed to compare the energies of 29 different fullerene structures, with mass number from 60 to 240, and of eight nonhelical graphite tubes of different radii. A quantity called the planarity, which indicates the completeness of the pi-bonding, is the single most important parameter determining the energetics of these structures. Empirical equations were constructed for the energies of nonhelical tubes and for those fullerene structures that may be described as balls or capsules. For a given mass number, bail-shaped fullerenes are energetically favored over capsular (tube-like) fullerenes.

Effects of surface tension and viscosity on airway reopening

We studied airway opening in a benchtop model intended to mimic bronchial walls held in apposition by airway lining fluid. We measured the relationship between the airway opening velocity (U) and the applied airway opening pressure in thin-walled polyethylene tubes of different radii (R) using lining fluids of different surface tensions (gamma) and viscosities (mu). Axial wall tension (T) was applied to modify the apparent wall compliance characteristics, and the lining film thickness (H) was varied. Increasing mu or gamma or decreasing R or T led to an increase in the airway opening pressures. The effect of H depended on T: when T was small, opening pressures increased slightly as H was decreased; when T was large, opening pressure was independent of H. Using dimensional analysis, we found that the relative importance of viscous and surface tension forces depends on the capillary number (Ca = microU/gamma). When Ca is small, the opening pressure is approximately 8 gamma/R and acts as an apparent "yield pressure" that must be exceeded before airway opening can begin. When Ca is large (Ca greater than 0.5), viscous forces add appreciably to the overall opening pressures. Based on these results, predictions of airway opening times suggest that airway closure can persist through a considerable portion of inspiration when lining fluid viscosity or surface tension are elevated.