Increase Of Critical Velocity Research Articles

Effect of Unbalanced Magnetic Pull of Generator Rotor on the Dynamic Characteristics of a Pump—Turbine Rotor System

In pumped storage units, the rotor-bearing electromagnetic system is under the joint influence of hydraulics, mechanics, and electromagnetics, and the mechanism of unit vibration problems is very complex to investigate. ANSYS software is used to establish a three-dimensional model of a pumped storage power plant’s rotor-bearing electromagnetic system, and the stiffness coefficient of the unbalanced magnetic traction forces is calculated using the Fourier series of the magnetic conductivity of the air gap. This shows that the nonequilibrium magnetic attraction increases non-linearly with increasing excitation current and eccentricity of the rotor. At each order, the critical velocity of the rotor system increases as the stiffness factor of the bearing increases, with the greatest increase in critical velocity at the third and fourth orders. In the first-order mode-oscillation pattern, the unbalanced magnetic attraction has an effect on the intrinsic frequency of the transverse oscillation, with a reduction in the amplitude of the intrinsic frequency by 34.65%. Axial and transverse modal vibrations manifest themselves as upward and downward motions and transverse oscillations in different portions of the rotor system, respectively, whereas torsional modal vibrations manifest as a radial broadening or reduction in the generator rotor, runner, and coupling portions of the rotor system. The results of the study provide a theoretical foundation and a computational method for the dynamic analysis and design of the rotor system of pumped storage power stations.

Numerical investigation of critical velocity in curved tunnels: Parametric study and establishment of new model

Curved tunnels are more likely to have an accident and a fire source due to their physical characteristics. Using a 3D computational fluid dynamics tool with body-fitted grids, three different fire locations were investigated numerically in a series of curved tunnels with a turning radius of 50–1000 m and a heat release rate of 5–15 MW. Results showed that the critical velocity increased with increasing tunnel turning radius, and the straight tunnel had the highest critical velocity in all scenarios. For a tunnel with a 10 MW heat release rate, the critical velocity increased by 19 % from R = 50 to 1000 m. Furthermore, it was shown that the critical velocity was proportional to one-third power of the heat release rate. A gradual increase in critical velocity was also observed as the fire source was displaced from the tunnel's center to the walls. Furthermore, this increment increased as the fire source approached the internal curvature. During the displacement of the fire to the inner and outer walls of a tunnel with R = 100 m and a 10 MW heat release rate, the critical velocity increased by 7.7 % and 5.6 %, respectively. Therefore, the worst fire scenario occurs when a fire is close to a tunnel's internal curvature and has the largest turning radius and the maximum heat release rate. Tunnel curvature and fire location are not considered in normal dimensionless analysis (commonly used for tunnels). Thus, the numerical results were unified in a non-dimensional form, and a modified formula was developed to calculate critical velocities in curved tunnels. The proposed formula includes tunnel curvature, heat release rate, and fire location with a coefficient of determination of 0.98. Numerical simulations confirmed the predictions of the proposed formula. Compared to other models in straight and curved tunnels, the predicted correlation developed in the present study can accurately predict critical ventilation velocity in curved tunnels.

Dynamics of axially functionally graded conical pipes conveying fluid

Abstract In this paper, the dynamics of axially functionally graded (AFG) conical pipes conveying fluid are analyzed. The materials are distributed along the conical pipe axis as a volume fraction function. Either the elastic modulus or the density of the AFG conical pipe is assumed to vary from the inlet to the outlet. The governing equation of the AFG conical pipe is derived using the Hamiltonian principle and solved by the differential quadrature method. The effects of the volume fraction index, volume fraction function type and reduction factor on the natural frequency and critical velocity are analyzed. It is found that for a power function volume fraction type, the natural frequency and critical velocity increase with increasing volume fraction index and clearly increase when the volume fraction index is within the range (0, 10). For an exponential function volume fraction type, the natural frequency and critical velocity change rapidly within the range (−10, 10), besides the above range the relationship between the natural frequency, critical velocity and volume fraction index is approximate of little change. The natural frequency and critical velocity decrease linearly with increasing reduction factor.

Stress-driven nonlocal elasticity for the instability analysis of fluid-conveying C-BN hybrid-nanotube in a magneto-thermal environment

The Eringen’s strain-driven nonlocal differential model is well-established to exhibit inconsistencies when applied to bounded continua of applicative interest. The stress-driven nonlocal theory leads instead to well-posed nonlocal elastic formulations demonstrating stiffening structural responses. In the present article, using the stress-driven nonlocal model, a comprehensive analysis is conducted to explore the vibrational characteristics and critical divergence velocity of a hybrid-nanotube constructed by carbon (C) and boron nitride (BN) nanotubes conveying magnetic fluid. The impact of size-dependence, magnetic field and thermal medium on the dynamic behavior of the systems is included in the proposed model. The obtained governing equations of two-segment nanotubes are then examined using the finite element method. It is interestingly showed that the threshold of the divergence/flutter instability of the system would be enhanced by employing a hetero-nanotube instead of a nanotube composed of a uniform material. Furthermore, the results demonstrate that the configuration of the mode shapes may be dramatically changed for a nanotube conveying fluid. Therefore, the classical modes do no longer exist, and should not be considered in the dynamics of the system. It is also shown that by assuming a low temperature medium, the critical velocity increases by increasing the temperature and decreases in the case of high temperature.

Effects of fluid rheology and wetting properties on the droplet motion by a shear flow

AbstractThe paper presents an experimental investigation of the detaching mechanisms of adhering droplets exposed to a turbulent air flow, focusing on the specific case of water, glycerine and ethanol droplets on a PMMA and coated silicon wafer surface. A rectangular Plexiglas channel (22 x 22 mm) with a replaceable substrate is used. A controlled air flow is established and increased during the experiment, until the droplet deforms and starts to move from its initial position along the surface. Shadowgraph technique is applied to measure the droplet behavior. The images are automatically analyzed by an edge‐detection algorithm to gain information on the droplet contour, the contact angles and the contact line movement. It was found that the critical velocity required for the onset of the droplet movement decreases with the droplet volume. As the viscosity of the droplet increases, the critical velocity increases. For a particular liquid it is easier to detach the droplets on coated silicon wafers compared to PMMA due to the increase of the hydrophobicity of the substrate. Glycerine droplets move at a slow rate over the surface due to the high viscous damping and reach a certain position only at higher air velocities. The results show that contact angle hysteresis seems not to be the unique factor influencing the critical velocity, since several parameters superpose and affect the droplet detachment process.

Lubrication performance analysis of sealing structures in rodless open cylinders

As the lubrication performance of sealing structures in rodless open cylinders varies with sliding velocity, the concept of a critical velocity is proposed based on the aforementioned characteristics and the lubrication performance of sealing structure is analysed. A finite-element model of sealing structure is established and the oil film load under dynamic lubrication condition is obtained. A two-dimensional theoretical model of lubrication performance in sealing structure is established based on the proposed critical velocity. The influence on critical velocity of parameters including pre-compression, geometry size and microscopic morphology is analysed. Considering surface microscopic morphology, generation of heat by friction, heat flux distribution and other factors, a three-dimensional numerical model of lubrication performance of sealing structure is established to reveal the influence of texture mechanism on lubrication performance. As concluded, with the increase of pre-compression, the lip angle and the temperature of lubrication oil, the critical velocity increases. The three-dimensional microscopic morphology of seal ring is found to pose obvious influence on critical velocity, and the isotropic microscopic morphology tends to form total lubrication under the same conditions.

Numerical investigation of nonlinear vibration analysis for triple-walled carbon nanotubes conveying viscous fluid

PurposeThe purpose of this paper is to investigate nonlinear vibrations of triple-walled carbon nanotubes buried within Pasternak foundation carrying viscous fluids.Design/methodology/approachConsidering the geometry of nanotubes, the governing equations were initially derived using Timoshenko and modified couple stress theories and by taking into account Von-Karman expressions. Then, by determining boundary conditions, type of fluid motion, Knudsen number and, ultimately, fluid viscosity, the principal equation was solved using differential quadrature method, and linear and nonlinear nanotube frequencies were calculated.FindingsThe results indicated that natural frequency is decreased as the fluid velocity and aspect ratio increase. Moreover, as the aspect ratio is increased, the results converge for simple and fixed support boundary conditions, and the ratio of nonlinear to linear frequencies approaches. Natural frequency of vibrations and critical velocity increase as Pasternak coefficient and characteristic length increase. As indicated by the results, by assuming a non-uniform velocity for the fluid and a slip boundary condition at Kn = 0.05, reductions of 10.714 and 28.714% were observed in the critical velocity, respectively. Moreover, the ratio of nonlinear to linear base frequencies decreases as the Winkler and Pasternak coefficients, maximum deflection of the first wall and characteristic length are increased in couple stress theory.Originality/valueThis paper is a numerical investigation of nonlinear vibration analysis for triple-walled carbon nanotubes conveying viscous fluid.

Rebound characteristics for the impact of SiO2 particle onto a flat surface at different temperatures

A dynamic model has been developed to simulate the normal impact of micro-sized SiO2 particle onto a flat surface at different experimental temperatures. The model describes the impact process through the combination of the static contact theory proposed by Brach et al. and energy dissipation considering both adhesion effect and viscoelastic effect. The present paper focuses on the effects of particle size, incident velocity, temperature, and damping coefficient on the particle/surface collisions. The model is verified by comparing the predicted critical velocities with the experimental results. Specifically, temperature and damping coefficient were treated as variables except for particle size and incident velocity. The influences of the contact force and the contact displacement on the contact time were also analyzed. It is found that both adhesion effect and viscoelastic effect increase with increasing surface temperature, which leads to the increase of critical velocity.

The influence of lateral shock absorber valve parameters on vehicle dynamic performance

The shock absorber is important for improving train dynamics characteristics, and its valve parameters determine its performance and also directly affect the train dynamics characteristics. Hydraulic control model of lateral shock absorber is established by using parametric modeling method in Easy5, and CRH2 trailer model is constructed in VI-Rail. Calculation result of shock absorber model’s speed characteristic is embedded into the vehicle model. The accuracy of the vehicle model is verified by a real track-test, and numerical analysis is done about the influence of various parameters on vehicle dynamics performance. The research results show that critical velocity and stability index increase with the increase of damping aperture and through-flow aperture. One-way valve aperture of bottom valve and one-way valve aperture of piston have little influence on those performance. As spring stiffness increases in the range of 800–2000 N/cm, critical velocity increases and stability index decreases, and its influence is little if spring stiffness is beyond this scope. As preload force of spring increases in the range of 100–250 N, critical velocity increases and stability index decreases, and its influence is also little if the preload force of spring is beyond this scope.

Structural and properties characterization of stellite coatings obtained by cold gas spraying

The work was aimed at obtaining cold gas spray deposits of stellite-6 minimizing the porosity content with further characterization of such coatings in terms of structure as well as wear and high temperature behaviour. The high strength and melting point of this alloy make the critical velocity increase; for that reason spraying stellite means a challenge. Then, although the deposition efficiencies are relatively low, the spraying parameters have been optimized leading to pore-free deposits where the slightly deformed particles clearly exhibit the dendritic structure of the feedstock.The structural changes investigated at the nanoscale level by transmission electron microscopy revealed a high deformation by twinning and the shear stresses produced by the wear conditions led to the occurrence of the hexagonal phase coexisting with the original cubic phase.Concerning the oxidation resistance, stellite coatings were capable to withstand up to 900°C and the wear performance was improved by reinforcing the ultimate layers with alumina particles.

Investigating the use of column inserts to achieve better chromatographic bed support

Chromatography plays an important role in the downstream processing of proteins. Over the past years, there has been a steady move toward the adoption of more rigid, porous particles to combine ease of manufacture with increased levels of productivity. The latter is still constrained by the onset of compression where the level of wall support becomes incapable of withstanding flow-induced particle drag. In this study, we investigate how, by the installation of cylindrical column inserts, it is possible to enhance the level of wall support. Experiments were conducted to examine the effect of the position of the insert in the column, and also of the insert dimensions on the critical velocity at which the onset of compression occurs. It was found that when installed at the bottom of the column, inserts can provide up to a 20% increase in critical velocity without significantly affecting column hydrodynamics, as measured by the level of axial dispersion.

Effect of 12 weeks of training on critical velocity and maximal lactate steady state in swimmers

The aim of this study was to determine the effect of 12 weeks of training on the critical velocity and maximal lactate steady state of elite swimmers. The tests to determine critical velocity and maximal lactate steady state were performed before and after 12 weeks of training. Critical velocity after 12 weeks of training was significantly higher than before training (1.45±0.10 m · s−1 vs. 1.41±0.11 m · s−1). In contrast, no significant differences in the velocity at maximal lactate steady state were observed before and after training (1.41±0.10 m · s−1 vs. 1.43±0.10 m · s−1). There was also a decrease in mean lactate concentration after 12 weeks of training. Before training, the velocity at maximal lactate steady state occurred at 100% of critical velocity, with a mean lactate concentration of 4.34 mmol · l−1. After training, the velocity at maximal lactate steady state occurred at 98% of critical velocity, with a reduced mean lactate concentration of 3.69 mmol · l−1. Based on these results, it would appear that 12 weeks of training was enough to promote an increase in critical velocity. Although no significant differences in the velocity at maximal lactate steady state were observed before and after training, the decrease in mean lactate concentration after training demonstrated greater efficiency of the aerobic system, leading to less wear during the tests.

A Single-phase Fluid Flow Pattern in a Kind of Fractured-vuggy Media

The oil reserves of a fractured-vuggy carbonate reservoir are abundant. It is important to study the flow pattern in fractured-vuggy media for developing fractured-vuggy carbonate reservoirs. In this article, the curves of velocity vs. pressure gradient under different vug density have been drawn by completing the experiment of single-phase fluid flow. Using experimental data, the critical velocity at which the linear flow period changes to a non-linear flow period has been calculated by researching the relationship of the Reynolds number and the resistance coefficient. Based on the Darcy formula and the Forchheimer formula, the expressions of permeability and coefficients of the Forchheimer formula, which are function of vug density, have been calculated by fitting experimental data. A single-phase fluid flow pattern in fractured-vuggy media with different vug density has been built. Conclusions are as follows: With an increase of vug density, the pressure gradient at the same velocity decreases and the critical velocity increases. The flow pattern can be described by the Darcy formula when the velocity is smaller than the critical velocity and by the Forchheimer formula when the velocity is bigger than the critical velocity.

Contact line dynamics of a superhydrophobic surface: application for immersion lithography

The dynamic contact line behavior of water on nanotextured rough hydrophobic and superhydrophobic surfaces is studied and contrasted to smooth hydrophobic surfaces for application in immersion lithography. Liquid loss occurs at the receding meniscus when the smooth substrate is accelerated beyond a critical velocity of approximately 1 m/s. Nanotexturing the surface with average roughness values even below 10 nm results in critical velocity larger than 2.5 m/s, the upper limit of the apparatus. This unexpected increase in critical velocity is observed for both sticky hydrophobic and slippery superhydrophobic surfaces. The authors attribute this large increase in critical velocity both in increased receding contact angle and in increased slip length for such nanotextured surfaces.

Stability of suspension bridge catwalks under a wind load

A nonlinear numerical method was developed to assess the stability of suspension bridge catwalks under a wind load. A section model wind tunnel test was used to obtain a catwalk\'s aerostatic coefficients, from which the displacement-dependent wind loads were subsequently derived. The stability of a suspension bridge catwalk was analyzed on the basis of the geometric nonlinear behavior of the structure. In addition, a full model test was conducted on the catwalk, which spanned 960 m. A comparison of the displacement values between the test and the numerical simulation shows that a numerical method based on a section model test can be used to effectively and accurately evaluate the stability of a catwalk. A case study features the stability of the catwalk of the Runyang Yangtze suspension bridge, the main span of which is 1490 m. Wind can generally attack the structure from any direction. Whenever the wind comes at a yaw angle, there are six wind load components that act on the catwalk. If the yaw angle is equal to zero, the wind is normal to the catwalk (called normal wind) and the six load components are reduced to three components. Three aerostatic coefficients of the catwalk can be obtained through a section model test with traditional test equipment. However, six aerostatic coefficients of the catwalk must be acquired with the aid of special section model test equipment. A nonlinear numerical method was used study the stability of a catwalk under a yaw wind, while taking into account the six components of the displacement-dependent wind load and the geometric nonlinearity of the catwalk. The results show that when wind attacks with a slight yaw angle, the critical velocity that induces static instability of the catwalk may be lower than the critical velocity of normal wind. However, as the yaw angle of the wind becomes larger, the critical velocity increases. In the atmospheric boundary layer, the wind is turbulent and the velocity history is a random time history. The effects of turbulent wind on the stability of a catwalk are also assessed. The wind velocity fields are regarded as stationary Gaussian stochastic processes, which can be simulated by a spectral representation method. A nonlinear finite-element model set forepart and the Newmark integration method was used to calculate the wind-induced buffeting responses. The results confirm that the turbulent character of wind has little influence on the stability of the catwalk.

Influence of free-stream turbulence on the movement-induced vibrations of an elongated rectangular cylinder in cross-flow

We present an experimental study of the galloping oscillations of an elongated cylinder. The shape ratio of the rectangular section is 1:2 and we study the influence of free-stream turbulence on the vibrations. The main studied parameter is the turbulence intensity. In smooth flow, the structure is also submitted to vortex shedding lock-in, which disappears in turbulent flow. At low intensity (4–5%), the critical velocity decreases by comparison with smooth flow. A jump of the Strouhal number is reported and can explain this decrease. At 6% intensity, the critical velocity increases, then the system becomes stable at 7.5%.

Melt convection effects on the critical velocity of particle engulfment

Liquid convection ahead of the solidifying interface alters particle behavior in the vicinity of the interface. This effect has not been quantified to date. Relevant directional solidification experiments were conducted using samples of varying thicknesses, as well as normal and low-gravity experiments. A mixture of transparent biphenyl matrix and spherical glass particles, as well as one of succinonitrile matrix with polystyrene particles were used. Two experimental setups were used: a horizontal gradient heating facility (HGF) for horizontal solidification, and a Bridgman-type furnace (BF) for vertical solidification. The convection level during solidification in the HGF was varied by changing the distance between the glass slides containing the composite sample. The BF was used on ground and during parabolic flights, and thus the convection level was changed by alternating low-gravity and high-gravity solidified regions. It was found that the convection level and/or particle buoyancy significantly influences the critical velocity for particle engulfment. At higher natural convection during solidification the critical velocity increases by up to 40%. At very high convection levels engulfment may become impossible because particles fail to interact with the interface. A systematic analysis of some theoretical models was performed in an attempt to evaluate the present level of theoretical understanding of the problem. Methods of evaluating the surface energies required for model validation are also presented.

Downstream migration and critical water velocities in stream channels for fry of four salmonid species

Fry of brown trout, Atlantic salmon, brook trout and lake trout were tested for downstream migration and critical velocities with a method of stepwise increasing water velocities. Each velocity was tested for 15 min before increase to the next step. Critical velocities for fry entering the free‐feeding stage, defined as the stage when the fry has resorbed its yolk sac and will have to ascend from the bottom gravel to catch food, were between 0.10 and 0.25 m s−1, varying among individuals and depending on species and water temperature. Downstream displacement started at lower velocities. Lake trout had the lowest critical velocity. Temperature influenced swimming performance considerably. On average, a 7°C increase in temperature resulted in a 0.05 m s−1 increase in critical velocity. The fry actively searched out the low‐velocity niches in the channels. Flow‐sensivity gradually decreases with fry development; when the fry had reached a length of 40–50 mm they were able to tolerate water velocities higher than 0.50 m s−1.

Shock initiation of an explosion. II. Critical energy

1) The critical energy for initiating an explosion in TNT, introduced by the impact of a plate, has been determined. Critical velocities and the corresponding critical energies have been obtained for sample densities of 1.43, 1.54, and 1.60 g/cm3. 2) Critical plate velocities have been, determined for various explosives: TNT, RDX, tetryl, picric acid, and TNT-RDX mixtures (50∶50). The accuracy of determination is defined by deviation of the plate velocity from the critical value by ±0.5%. An increase in critical velocity by 0.5% gives 100% explosions and a decrease by 0.5% gives 100% failures (no explosion). 3) The shock velocity has been measured and the principal dynamic characteristics at the shock front determined for TNT with the densities given above (p, V, u, E−Eo). The relationship between the velocity of the material and the critical plate velocity has been obtained $$\begin{gathered} u = \frac{a}{{a + \rho _0 D}} \cdot \mathcal{W},E - E_0 = \frac{{a^2 }}{{(a + \rho _0 D)^2 }} \cdot \frac{{w^2 }}{2}, \hfill \\ a = 0.4 \cdot 10^7 [g \cdot cm^{ - 2} \cdot \sec ^{ - 1} ], \hfill \\ \end{gathered}$$ which enables one to make a quantitative analysis of the effect on the critical energy (or the velocity of the material u) of the velocity and mass of the impact plate, and also of the density of the explosive investigated. It follows from Eq. (4) that the energy of compression (E−Eo) is determined only by the plate velocity (w) and is independent of its mass, which does not enter into the equation.