Velocity In Turbulent Flow Research Articles

An Eulerian approach for large eddy simulation of particle transport in turbulent flows

The objective of this paper is to advance an Eulerian continuum approach for modeling the transport of inertial particles in the framework of large eddy simulation. This approach is based on a kinetic equation for the filtered probability density function of particle velocity in turbulent flow. Some models developed for the subgrid-scale turbulent stresses of the particulate phase are presented. The effect of filter width on the averaged subgrid particle kinetic energy is explored. It is revealed that, in the limit of zero filter width, the subgrid kinetic stress and energy of inertial particles can have nonzero values.

0213 平行平板間クエット乱流中における大規模縦渦維持機構の解明(OS2-3 乱流の計測・解析・制御,オーガナイズドセッション)

In order to investigate the sustaining mechanism of the large-scale streamwise vortical structures in a turbulent Couette flow, a vorticity-probe consisted of 4-X type hotwire has been developed. The probe performance was estimated by measuring the fluctuating velocity and vorticity components in turbulent Poiseuille flow and comparing them with DNS and/or LDV measurements. Contributions to Dω^^-_1/Dt from various terms of ω^^-_1 transport equation in Poiseuille flow and Couette flow are estimated using data obtained from vorticity-probe. In Poiseuille flow the total of these terms weaken the streamwise vortex, while the one in Couette flow has no such tendency. These results are consistent with the decaying or sustaining properties of the streamwise vortex in Poiseuille or Couette flow.

On chaotic flow regimes in a rotating spherical layer

We present results of an experimental investigation and numerical simulations of chaotic flow regimes in a layer of a viscous incompressible fluid confined between counter-rotating spherical boundaries. The turbulent flow velocity pulsation profiles obtained by straightforward calculations are qualitatively similar to the analogous profiles known for a flat mixing layer. At the boundaries of their formation, chaotic flow regimes are characterized by a continuous spectrum of velocity pulsations and a high correlation dimension (D > 8), in contrast to the chaotic regimes observed in an analogous layer with one (inner) rotating boundary.

Late Diagnosis of Multiple Fibroelastomas in a 41-Year-Old Woman with Hypertrophic Cardiomyopathy and Atrial Fibrillation

A 39 year old woman with hypertrophic cardiomyopathy with severe latent obstruction and mitral regurgitation, presented with symptoms of dyspnea and atrial fibrillation, and developed an acute embolic stroke. After thrombolysis and complete neurologic recovery, a transesophageal echo revealed six mobile densities, on her mitral and aortic valves, and in the outflow tract. Surgical resection of multiple fibroelastomas, with septal myectomy, aortic and mitral valve replacement, and pulmonary vein ablation, led to clinical improvement. The etiology and pathogenesis of fibroelastomas are unknown; we speculate that their formation may be promoted by endocardial injury from surgery, radiation therapy, or the high velocity turbulent flow of valve dysfunction or outflow tract obstruction.

Estimation of the Friction Velocity in Stably Stratified Boundary-Layer Flows Over Hills

A method is suggested for the calculation of the friction velocity for stable turbulent boundary-layer flow over hills. The method is tested using a continuous upstream mean velocity profile compatible with the propagation of gravity waves, and is incorporated into the linear model of Hunt, Leibovich and Richards with the modification proposed by Hunt, Richards and Brighton to include the effects of stability, and the reformulated solution of Weng for the near-surface region. Those theoretical results are compared with results from simulations using a non-hydrostatic microscale-mesoscale two-dimensional numerical model, and with field observations for different values of stability. These comparisons show a considerable improvement in the behaviour of the theoretical model when the friction velocity is calculated using the method proposed here, leading to a consistent variation of the boundary-layer structure with stability, and better agreement with observational and numerical data.

Misleading Turbulent Flow Through Pulmonary Venous Anastomoses During Lung Transplantation

Misleading Turbulent Flow Through Pulmonary Venous Anastomoses During Lung Transplantation

The nature of near-wall convection velocity in turbulent channel flow

A novel notion of turbulent structure—the local cascade structure—is introduced to study the convection phenomenon in a turbulent channel flow. A space–time cross-correlation method is used to calculate the convection velocity. It is found that there are two characteristic convection speeds near the wall, one associated with small-scale streaks of a lower speed and another with streamwise vortices and hairpin vortices of a higher speed. The new concept of turbulent structure is powerful to illustrate the dominant role of coherent structures in the near-wall convection, and to reveal also the nature of the convection—the propagation of patterns of velocity fluctuations—which is scale-dependent.

Settling velocities of particulate systems 15: Velocities in turbulent Newtonian flows

The success in previous papers of this series in obtaining closed form solutions to the sedimentation of individual particles and to the suspensions of particles in fluids motivated the application of the same procedure to the turbulent flow of Newtonian fluids in conduits. The purpose of this paper is to present explicit equations for the Fanning friction factor and of the average velocity of a Newtonian fluid in a cylindrical tube, for any value of the Reynolds number and dimensionless roughness. Previously published equations for the friction coefficient are either implicit or explicit in the friction coefficient but dependent on the Reynolds number. Since the Reynolds number depends on the pipe diameter and fluid velocity, in both cases iterative method must be used to calculate the flowrate. By defining a dimensionless pipe diameter and a dimensionless average fluid velocity, and using conduit-flow material properties to define characteristic size and velocity parameters, explicit equations were obtained for the friction coefficient and for the average flow velocity valid for different roughness and flow regimes.

Velocity and turbulence measurements by ultrasound pulse Doppler velocimetry

This paper describes a velocimeter, based on back-scattering of ultrasonic waves by particles, designed for velocity measurement of suspended sediments within the flow of sewer systems. The ultrasound pulse Doppler velocimeter (UPDV) needs no calibration and is therefore a potentially useful tool for measuring velocities in laboratory experiments or in sewer systems. The potential of the developed system to determine the velocity in turbulent pipe flow was investigated. Two different approaches were used to estimate velocity: the well-known temporal method, i.e., the pulse–pair technique and the spectral identification based method, developed by our research group. Measurements have demonstrated the ability of the apparatus, on the one hand, to measure unsteady turbulent velocities, and on the other hand, to investigate experimentally the statistical properties of homogeneous and isotropic turbulence. A good agreement between experimental results and theory was observed. The validation of these laboratory measurements permits to extrapolate them to sewer systems.

Numerical simulation of flow velocity distribution in cyclone with impulse electrostatic excitation

A single-phase of gas, three-dimensional turbulent flow velocity distribution model – k– ɛ model, which considers the effects of impulse high-voltage electrostatic fields, was founded. The k– ɛ model was dispersed by control-volume method and solved by Semi-Implicit Method for Pressure Linked Equations (SIMPLE) algorithm. The axial-, the tangential-, and the radial-velocity distributions of flow field inside cyclone with impulse electrostatic excitation on the various operating parameters were obtained by computer numerical simulation, and verified partly by experiment. It was concluded that shape of curves of velocity distributions of flow field inside cyclone impulse electrostatic precipitation (CIESP) was similar to that of cyclone with no impulse electrostatic field while the values of the axial-, the tangential-, and the radial-velocity of airflow had distinctly changed in impulse electrostatic field, in particular, the radial-velocity of airflow. Compared to cyclone with no impulse electrostatic field, the radial-velocity of airflow in CIESP was influenced strongly by the electric field at same inlet gas rate.

Influence of initial conditions at the nozzle exit on the structure of round jet

Experimental data concerning the influence of initial conditions at the nozzle exit on the structure and development characteristics of round jets are reported. Features in the development of laminar and turbulent round jets emanating from variously elongated nozzles at identical Reynolds numbers are revealed. Smoke visualization pictures obtained for jets formed under different initial conditions (with different distributions of mean and pulsating flow velocities at the nozzle exit) are discussed. It is shown possible to make the zone of laminar flow in the jet stream more extended, and to delay the jet turbulization process in space, by making the flow-velocity profile more parabolic at the exit of elongated nozzle. Features in the development of vortical structures in a jet under an acoustic action are identified. It is shown that, for a turbulent round jet to be produced right at the nozzle exit, the nozzle length must be increased in excess of a certain value so that to provide for spatial growth of turbulent boundary layer thickness, finally ending in the formation of a fully turbulent flow velocity profile across the channel.

Dispersion of fine settling particles from an elevated source in an oscillatory turbulent flow

The present paper examines the stream-wise dispersion of suspended fine particles with settling velocities in an oscillatory turbulent shear flow with or without a non-zero mean over a rough-bed surface when the particles are being released from an elevated continuous source. A finite-difference implicit method is employed to solve the unsteady turbulent convective-diffusion equation. A combined scheme of central and four-point upwind differences is used to solve the steady state equation and the Alternating Direction Implicit (ADI) method is adopted for unsteady equation. It is shown how the mixing of settling particles is influenced by the tidal oscillatory current and the corresponding eddy diffusivity when the initial distribution of concentration regarded as a line-source. The vertical concentration profiles of suspended fine particles with settling velocities are presented for different downstream stations for various values of settling velocity and the frequency of the oscillation in tidal flow. For two-dimensional unsteady dispersion equation, the behaviour of iso-concentration lines for different values of settling velocity, frequency of the oscillation, dispersion time and releasing height is studied in terms of the relative importance of convection and eddy diffusion.

Coloured noise for dispersion of contaminants in shallow waters

In this article, we explore the application of a set of stochastic differential equations called particle model in simulating the advection and diffusion of pollutants in shallow waters. The Fokker–Planck equation associated with this set of stochastic differential equations is interpreted as an advection–diffusion equation. This enables us to derive an underlying particle model that is exactly consistent with the advection–diffusion equation. Still, neither the advection–diffusion equation nor the related traditional particle model accurately takes into account the short-term spreading behaviour of particles. To improve the behaviour of the model shortly after the deployment of contaminants, a particle model forced by a coloured noise process is developed in this article. The use of coloured noise as a driving force unlike Brownian motion, enables to us to take into account the short-term correlated turbulent fluid flow velocity of the particles. Furthermore, it is shown that for long-term simulations of the dispersion of particles, both the particle due to Brownian motion and the particle model due to coloured noise are consistent with the advection–diffusion equation.

Flow and Heat Transfer Characteristics of Cone Orifice Free and Impinging Jets (Effects of Cone Angle)

A jet from an orifice nozzle having a saddle-backed-shape velocity profile and a contracted flow at the nozzle exit may improve the heat transfer characteristics on an impingement plate because of a large centerline velocity, but it requires more driving power than a common nozzle due to the higher flow resistance. Thus, we considered newly, a cone orifice nozzle with a wish of better heat transfer performance and a small flow resistance. In this study, the effects of the cone angle α on the cone orifice free jet flow and heat transfer characteristics of the impinging jet are examined. A comparison with a pipe nozzle, which provides a fully developed turbulent pipe flow velocity profile, and a quadrant nozzle, which has a uniform velocity profile at the nozzle exit, are performed. The considerable heat transfer enhancement of the cone orifice jets are observed at Re=1.5×104.All of the cone orifice impinging jets has enhanced the heat transfer rates under the same operation power, compared to the quadrant jet. For instance, a maximum enhancement up to approximately 22% atr/do≅0.5 is observed for α=15°In addition, an increase of approximately 7% is attained compared to the pipe jet.

Carotid bruitを聴取する虚血性脳血管障害における頚部血管超音波検査所見の検討

We investigated the clinical significance of carotid bruits in 238 patients with ischemic stroke using carotid ultrasonography. We recognized carotid bruits in 4 1 (17.2%)of the patients on auscultation of the neck. Patients with carotid bruits tended to have risk factors of ischemic heart disease and to be male. and symptomatic cerebral infarction with significantly more frequent than in patients without carotid bruits (p<0.05). Internal carotid artery stenosis (area of stenosis ≥50%), turbulence flow, and peak systolic velocity ≥150 cm/sec were observed significantly more frequently in patients with carotid bruits (p<0.05). Age, diabetes mellitus, hypertension. cigarette smoking. and dyslipidemia were not proved to have a significant correlation with the presence of carotid bruits. These findings indicate that carotid bruits are associated with advanced atherosclerosis in many organs.

Experimental study of turbulent jet induced by steam jet condensation through a hole in a water tank

A turbulent jet induced by steam jet condensation in a water pool was investigated experimentally. An experimental apparatus equipped with a steam boiler, a single-hole steam sparger, and a water pool, etc. was used. For the measurements, a pitot tube and thermocouples were used for turbulent flow velocity and temperatures, respectively. Overall flow shapes of the turbulent jet by the steam jet condensation are similar to those of axially symmetric turbulent jet flows. The angular coefficients of turbulent rays are quantitatively comparable between the traditional turbulent jet flows and the turbulent jet flows induced by the steam jet condensation in this work. Although the turbulent flows were induced by the steam jet condensation, general theory of turbulent jets was found to be applicable to the turbulent flows of this work.

Differential and algebraic models for the second moments of particle velocity and temperature fluctuations in turbulent flows

Differential and algebraic models are constructed for the dispersed-phase turbulent stresses and heat fluxes and for the mixed moments of the velocity and temperature fluctuations in the continuous and dispersed phases. The models are based on a kinetic equation for the joint probability density of the particle velocity and temperature in an anisotropic turbulent flow. The results are compared with the available direct numerical simulation (DNS) data.

330 渦度プローブによる平行平板間クエット乱流中における大規模縦渦維持機構の解明(2)(OS3-7 噴流,後流,せん断流の構造・普遍的特性(せん断流1),OS3噴流,後流,せん断流の構造・普遍的特性)

In order to investigate the sustaining mechanism of the large-scale streamwise vortical structures in a turbulent Couette flow, a vorticity-probe consisted of 4-X type hotwire has been developed. The effects of the probe-size and mean-shear on the probe-performance as well as effectiveness of the temperature compensation scheme developed are discussed. The final probe performance was estimated by measuring the fluctuating velocity and vorticity components in turbulent Poiseuille flow and comparing them with DNS and/or LDV measurements. Contributions to Dω^^-_1/Dt from various terms of ω^^-_1 (streamwise vorticity) transport equation in Poiseuille flow and Couette flow are estimated using data obtained from vorticity-probe experiment. In Poiseuille flow the total of these terms weaken the streamwise vortex, while these in Couette flow have no such tendency. These results are consistent with the decaying or sustaining properties of the streamwise vortex in Poiseuille or Couette flow.

Connection between two statistical approaches for the modelling of particle velocity and concentration distributions in turbulent flow: The mesoscopic Eulerian formalism and the two-point probability density function method

The objective of the paper is to elucidate a connection between two approaches that have been separately proposed for modelling the statistical spatial properties of inertial particles in turbulent fluid flows. One of the approaches proposed recently by Février, Simonin, and Squires [J. Fluid Mech. 533, 1 (2005)] is based on the partitioning of particle turbulent velocity field into spatially correlated (mesoscopic Eulerian) and random-uncorrelated (quasi-Brownian) components. The other approach stems from a kinetic equation for the two-point probability density function of the velocity distributions of two particles [Zaichik and Alipchenkov, Phys. Fluids 15, 1776 (2003)]. Comparisons between these approaches are performed for isotropic homogeneous turbulence and demonstrate encouraging agreement.

Eulerian Monte Carlo method for the joint velocity and mass-fraction probability density function in turbulent reactive gas flows

A new Eulerian (field) Monte Carlo method for solving an equation that describes the one-time one-point probability density of species mass fractions in turbulent reactive gas flows has been proposed in a previous article. In the present paper, this method is extended to an equation for the joint velocity and mass-fraction probability density function. The method is based on passing from Lagrangian variables used in Lagrangian Monte Carlo methods to Eulerian variables. In this manner, stochastic ordinary differential equations for the Lagrangian trajectories of fluid particles are transformed to partial stochastic equations. As compared to the classical hydrodynamics, the stochastic velocity field satisfies only the mean continuity constraint and not the instantaneous one. As a consequence, one has to introduce a stochastic density, which differs from the physical density but has the same mean value. The case of the mass-fraction probability density is revised. The equations differ from those derived previously: they can be written in divergent form. Both formulations, however, are statistically equivalent.