Showing papers on "Pipe flow published in 1992"

Transition to shear-driven turbulence in Couette-Taylor flow.

Abstract: Turbulent flow between concentric cylinders is studied in experiments for Reynolds numbers 800R1.23\ifmmode\times\else\texttimes\fi{}${10}^{6}$ for a system with radius ratio \ensuremath{\eta}=0.7246. Despite predictions for the torque scaling as a power law of the Reynolds number, high-precision torque measurements reveal no Reynolds-number range with a fixed power law. A well-defined nonhysteretic transition at R=1.3\ifmmode\times\else\texttimes\fi{}${10}^{4}$ is marked by a change in the Reynolds-number dependence of the torque. Flow quantities such as the axial turbulent diffusivity, the time scales asociated with the fluctuations of the wall shear stress, and the root-mean-square fluctuations of the wall shear stress and its time derivative are all shown to be simply related to the global torque measurements. Above the transition, the torque measurements and observed time scales indicate a close correspondence between this closed-flow system and open-wall--bounded-shear flows such as pipe flow, duct flow, and flow over a flat plate.

Dense, vertical gas‐solid flow in a pipe

Abstract: The hydrodynamics of gas-solid flow, usually referred to as circulating fluidizedbed flow, was studied in a 7.5-cm clear acrylic riser with 75-μm FCC catalyst particles. Data were obtained for three central sections as a function of gas and solids flow rates. Fluxes were measured by means of an extraction probe. Particle concentrations were measured with an X-ray densitometer. In agreement with previous investigators, these data showed the flow to be in the core-annular regime, with a dilute rising core and a dense descending annular region. However, unlike the previous studies conducted worldwide, the data obtained in this investigation allowed us to determine the viscosity of the suspension. The viscosity was a linear function of the volume fraction of solids. It extrapolates to the high bubbling-bed viscosities.

Low-Reynolds-number effects in a fully developed turbulent channel flow

Abstract: Low-Reynolds-number effects are observed in the inner region of a fully developed turbulent channel flow, using data obtained either from experiments or by direct numerical simulations. The Reynolds-number influence is observed on the turbulence intensities and to a lesser degree on the average production and dissipation of the turbulent energy. In the near-wall region, the data confirm Wei and Willmarth's (1989) conclusion that the Reynolds stresses do not scale on wall variables. One of the reasons proposed to account for this behavior, namely, the 'geometry' effect or direct interaction between inner regions on opposite walls, was investigated in some detail by introducing temperature at one of the walls, both in experiment and simulation. Although the extent of penetration of thermal excursions into the opposite side of the channel can be significant at low Reynolds numbers, the contribution these excursions make to the Reynolds shear stress and the spanwise vorticity in the opposite wall region is negligible. In the inner region, spectra and cospectra of the velocity fluctuations u and v change rapidly with the Reynolds number, the variations being mainly confined to low wavenumbers in the u spectrum.

The role of non-uniqueness in the development of vortex breakdown in tubes

Abstract: Numerical solutions of viscous, swirling flows through circular pipes of constant radius and circular pipes with throats have been obtained. Solutions were computed for several values of vortex circulation, Reynolds number and throat/inlet area ratio, under the assumptions of steady flow, rotational symmetry and frictionless flow at the pipe wall. When the Reynolds number is sufficiently large, vortex breakdown occurs abruptly with increased circulation as a result of the existence of non-unique solutions. Solution paths for Reynolds numbers exceeding approximately 1000 are characterized by an ensemble of three inviscid flow types: columnar (for pipes of constant radius), soliton and wavetrain. Flows that are quasi-cylindrical and which do not exhibit vortex breakdown exist below a critical circulation, dependent on the Reynolds number and the throat/inlet area ratio. Wavetrain solutions are observed over a small range of circulation below the critical circulation, while above the critical value, wave solutions with large regions of reversed flow are found that are primarily solitary in nature. The quasi-cylindrical (QC) equations first fail near the critical value, in support of Hall's theory of vortex breakdown (1967). However, the QC equations are not found to be effective in predicting the spatial position of the breakdown structure.

Primary breakup in gas/liquid mixing layers for turbulent liquids

Abstract: An experimental study of primary breakup of turbulent liquids in gadliquid mixing layers is described. The experiments involved mixing layers along large liquid jets (3.6.6.4 and 9.5 mm dia.) injected at various velocities into still air at atmospheric pressure with fully-developed turbulent pipe flow at the jet exit. Liquids studied included water, glycerol (42% glycerin by mass) and n-heptane. Pulsed shadowgraph photography and holography were used to find conditions where turbulent primary breakup was initiated and drop sizes and velocities after primary breakup. Drop sizes after primary breakup satisfied Simmons' universal root normal distribution and can be characterized solely by their SMD. Mass weighted mean streamwise and crosstream drop velocities after primary breakup were comparable to mean streamwise and crossweam rms fluctuating velocities in the liquid, respectively, with effects of mean velocity distributions in the jet passage reflected by somewhat lower streamwise drop velocities near the jet exit. Conditions for the initiation of turbulent primary breakup and the variation of SMD with distance from the jet exit were correlated reasonably well by a phenomenological analyses considering effects of surface tension and liquid turbulence properties alone. However, limited data in the literature indicates that aerodynamic effects begin to influence drop sizes after primary breakup at ambient pressures greater than atmospheric pressure.

Modifying turbulent structure with drag-reducing polymer additives in turbulent channel flows

Abstract: New power spectra computed from LDA measurements of the fluctuating u- and v-velocity signals in a turbulent channel flow with and without drag-reducing polymer (polyethylene oxide) injection are presented. LDA data rates were sufficiently high to reconstruct the simultaneous time-dependent u- and v-velocity signals along with the time-dependent Reynolds stress signal. Time-averaged statistics of the turbulent flow are presented in conjunction with the power spectral measurements which show a dramatic reduction in both the v-velocity fluctuations and Reynolds stress fluctuations throughout the channel over all frequencies. There is also a redistribution of energy in the u-velocity fluctuations from high frequencies to low frequencies throughout the channel. Different injection conditions were examined: different polymer concentrations were injected at different flow rates such that the total amount of polymer in the channel remained constant. For certain polymer concentrations, ‘large’ negative Reynolds stress, -〈uv〉/uτ2 ≈ − 0.2, was measured in the near-wall region. In addition, there is a marked difference in the u-velocity spectra and the Reynolds stress spectra close to the wall for the different injection conditions.

The late stages of transition to turbulence in channel flow

Abstract: The late stages of transition, from the ?-vortex stage up to turbulence, are investigated by postprocessing data from a direct numerical simulation of the complete K-type transition process in plane channel flow at a Reynolds number of 5000 (based on channel half-width and laminar centreline velocity). The deterministic roll-up of the high-shear layer that forms around the ?-vortices is examined in detail. The new vortices arising from this process are visualized by plotting three-dimensional surfaces of constant pressure. Five vortices are observed, with one of these developing into a strong hairpin-shaped vortex. Interactions between the different vortices, and between the two channel halves, are found to be important. In the very last stage of transition second-generation shear layers are observed to form and roll up into new vortices. It is postulated that at this stage a sustainable mechanism of wall-bounded turbulence exists in an elementary form. The features which are locally present include high wall shear, sublayer streaks, ejections and sweeps. Large-scale energetic vortices are found to be an important part of the mechanism by which the turbulence spreads to other spanwise positions. The generality of the findings are discussed with reference to data from simulations of H-type and mixed-type transition

Efficient calculation of transient flow in simple pipe networks

Abstract: Extensions to the conventional method of characteristics allow transient conditions in simple pipe networks to be efficiently calculated. In particular, treating both boundary conditions and network topology in a general and comprehensive fashion simplifies the solution of many combinations of hydraulic devices. The algebraic framework presented includes a flexible integration of the friction loss term that reduces to previous linear approximations as special cases. In addition, an explicit algorithm is derived for a general hydraulic element called an external energy dissipator. This boundary condition conveniently represents surge tanks, relief valves, storage reservoirs, valves discharging to the atmosphere, and many other common devices. Significantly, the solution remains explicit even if friction losses and inertia effects are present in both the storage element and a connecting pipe. This comprehensive approach to transient analysis simplifies control logic, encourages accurate reporting of field data, and improves execution times. The procedure is illustrated by analyzing transient conditions in a small network containing a variety of devices.

Experiments on convective heat transfer in corrugated channels

Abstract: Experiments have been performed to study convective heat transfer in the entrance region of corrugated channels. With water as the working fluid, two channel spacings were examined for a single corrugation angle of 20°. The flow rate was varied over the range I50 ≤ Re ≤ 4000. Flow visualization under low-Reynolds-number flow conditions suggested the presence of longitudinal vortices, while at somewhat higher Reynolds numbers, the existence of spanwise vortices was clearly revealed. For Re > 1500, Nusselt numbers in the corrugated channels exceeded those in the parallel-plate channel by approximately 140% and 240% for the two channel spacings, the corresponding increases in friction factor being 130% and 280%. Performance evaluations under the criteria of equal mass flow rate, equal pumping power, and equal pressure drop per unit length established both the corrugated channels as superior to the parallel-plate channel in intensifying heat transfer.

Application of higher order finite element methods to viscoelastic flow in porous media

Abstract: A higher order Galerkin finite element scheme for simulation of two‐dimensional viscoelastic fluid flows has been developed. The numerical scheme used in this study gives rise to a stable discretization of the continuum problem as well as providing an exponential convergence rate toward the exact solution. Hence, with this method, spurious oscillatory modes are effectively eliminated by increasing the order of the interpolant within each subdomain. In our calculations, an upper limit for the Weissenberg number due to numerical instability was not encountered. However, the memory requirements of the discretization grow quadratically with the polynomial order. Consequently, the maximum attainable We is determined by the availability of computational resources. The algorithm was tested for flow of upper convected Maxwell and Oldroyd‐B fluids in the undulating tube problem and it was subsequently applied to viscoelastic flow past square cylindrical arrangements. The results obtained show no increase in the fl...

Swirling, particle-laden flows through a pipe expansion

Abstract: The present study concerns a particle-laden, swirling flow through a pipe expansion. A gas-particle flow enters the test section through a center tube, and a swirling air stream enters through a coaxial annulus. The swirl number based on the total inflow is 0.47. Numerical predictions of the gas flow were performed using a finite-volume approach for solving the time-averaged Navier-Stokes equations. The predicted mean velocity profiles showed good agreement with experimental results when using the standard k-e turbulence model. The turbulent kinetic energy of the gas phase, however, is considerably underpredicted by this turbulence model, especially in the initial mixing region of the two jets. The particle dispersion characteristics in this complex flow were studied by using the Lagrangian method for particle tracking and considering the particle size distribution. The influence of the particle phase onto the fluid flow was neglected in the present stage, since only low particle loadings were considered. The particle mean velocities were again predicted reasonably well and differences between experiment and simulation were only found in the velocity fluctuations, which is partly the result of the underpredicted turbulent kinetic energy of the gas phase. The most sensitive parameter for validating the quality of numerical simulations for particle dispersion is the development of the particle mean number diameter which showed reasonable agreement with the experiments, except for the core region of the central recirculation bubble. This, however, is attributed again to the predicted low turbulent kinetic energy of the gas phase.

Upward Vertical Two-Phase Flow Through an Annulus—Part II: Modeling Bubble, Slug, and Annular Flow

Abstract: This paper reports that mechanistic models have been developed for each of the existing two-phase flow patterns in an annulus, namely bubble flow, dispersed bubble flow, slug flow, and annular flow. These models are based on two-phase flow physical phenomena and incorporate annulus characteristics such as casing and tubing diameters and degree of eccentricity. The models also apply to the new predictive means for friction factor and Taylor bubble rise velocity. Given a set of flow conditions, the existing flow pattern in the system can be predicted. The developed models are applied next for predicting the flow behavior, including the average volumetric liquid holdup and the average total pressure gradient for the existing flow pattern. In general, good aggrement was observed between the experimental data and model predictions.

Molecular dynamics study of flow at a fluid-wall interface

Abstract: A novel wall model, including inelastic fluid-wall collisions, molecular structure of the wall, and a long-ranged fluid-wall interaction, is used in molecular dynamics simulations of flow in two dimensions. For strong fluid-wall attraction the fluid velocity goes smoothly to zero at the wall; there is no velocity slip. In less dense fluids the velocity drops to zero in an ever-narrower region. Our results differ from standard kinetic theory but agree with Eu's theory [Phys. Rev. A 36, 400 (1987)]. The nature of the fluid-wall interaction is important; repulsive walls give velocity slip.

On transition due to three-dimensional disturbances in plane Poiseuille flow

Abstract: The purpose of the present study is to characterize the process of laminar–turbulent transition at Reynolds numbers which are subcritical from the two-dimensional linear point of view. The development of a point-like disturbance was studied in an air flow channel with hot-wire anemometry at a Reynolds number of 1600. Localized disturbances were triggered at one of the walls and their development followed downstream by traversing the hot-wire probe in the streamwise direction over a distance of 90 half channel heights, as well as in the normal and spanwise directions. The disturbance evolved into elongated streaky structures with strong spanwise shear (i.e. normal vorticity) which grew in amplitude and streamwise extension and thereafter either decayed or gave rise to a turbulent spot. The results indicate that the mechanism underlying the initial growth is a linear one resulting from the coupling between the normal velocity and the normal vorticity, as described by the three-dimensional linear equations. The nonlinear development of the structure leads to the formation of intense normal shear layers and the appearance of oscillations and ‘spikes’, which multiply and form the rear or a turbulent spot.

The consistent streamline-upwind/Petrov-Galerkin method for viscoelastic flow revisited

Abstract: In an earlier paper, Marchal and Crochet introduced two mixed finite-element methods for calculating viscoelastic flow. The first one, based on a consistent streamline-upwind/Petrov-Galerkin integration of the constitutive equations, was disregarded because it produced wiggles in the numerical calculation of the stick-slip flow of a Maxwell fluid. The second method, based on a non-consistent streamline-upwind integration, was found to be stable at high values of the Weissenberg number; however, it introduces artificial diffusivity of the order of the element size, h, which decreases with mesh refinement. In the present paper, we re-examine the first method. We show that it is both stable and accurate for solving flows of a Maxwell fluid in smooth geometries. The test problems are the flow around a sphere in a tube and the flow through a corrugated tube. The results coincide with those of other accurate methods for solving the same problems. Finally, we show that the results obtained with the second method converge towards the most accurate results when the element size decreases. In particular, we show that the velocity field is little affected by numerical diffusion in the stress constitutive equations.

Velocity matching and Poiseuille pipe flow of superfluid helium.

Abstract: We show that in laminar pipe flow of helium II with the average normal-fluid and superfluid velocities in the same direction, a single superfluid vortex filament can form a series of large vortex rings oriented with the flow. These rings interact to form an array of vortex rings that gives the superfluid a parabolic velocity profile matching the normal-fluid velocity profile. We conclude with some generalizations of this behavior to other flow geometries.

Experimental investigation of turbulent flow through a circular-to-rectangular transition duct

Abstract: Steady, incompressible, turbulent, swirl-free flow through a circular-to-rectangular transition duck was studied experimentally. The cross-sectional area remains the same at the exit as at the inlet, but varies through the transition section to a maximum value approximately 15 percent above the inlet value. The cross-sectional geometry everywhere along the duct is defined by the equation of a superellipse. Mean and turbulence data were accumulated utilizing pressure and hot-wire instrumentation at five stations along the test section. Data are presented for operating bulk Reynolds numbers of 88,000 and 390,000. Measured quantities include total and static pressure, the three components of the mean velocity vector, and the six components of the Reynolds stress tensor. In addition to the transition duct measurements, a hot-wire technique which relies on the sequential use of single rotatable normal and slant-wire probes was proposed. The technique is applicable for measurement of the total mean velocity vector and the complete Reynolds stress tensor when the primary flow is arbitrarily skewed relative to a plane which lies normal to the probe axis of rotation.

Developing Heat Transfer and Friction in a Ribbed Rectangular Duct With Flow Separation at Inlet

Abstract: The local heat transfer and pressure drop characteristics of developing turbulent flows in a rectangular duct with an abrupt-contraction entrance and repeated square-rib pairs on the two opposite walls have been investigated experimentally. Both entrance-region and periodic-fully-developed-region results were obtained. Laser holographic interferometry was employed in the local and average heat transfer measurements. The Reynolds number was varied from 5.0x10[sup 3] to 5.0x10[sup 4]; the rib pitch-to-height ratios were 10, 15, and 20; and the rib height-to-duct height ratio was kept at a value of 0.13. The results allowed the entry length to be determined and the regions susceptible to hot spots to be located. Semi-empirical heat transfer and friction correlations for the periodic fully developed region were developed. Moreover, performance comparisons between the ribbed and smooth ducts were made under two types of constraint, namely equal mass flow rate and equal pumping power. Finally, the effect of thermal entry length on the length mean Nusselt number was also investigated. The results showed that the length mean Nusselt number ratio was a function of only the duct length and independent of PR and Re, and could be further correlated by an equation of the form [bar N]u[sub m]/[bar N]u[sub p]=1more » + 1.844/(X/De).« less

Slug flow mitigtion for production well fluid gathering system

Abstract: Oil and gas well production flow is gathered in field common line manifolds and conducted through a separator to make a coarse separation of gas from liquid and minimize slug flow in the conduits leading from the manifolds to further separation, treatment and pumping facilities. A liquid level signal transmitter provides a signal to a valve controller which controls separate liquid and gas discharge flow control valves or a variable speed pump to maintain a set point level of liquid in the separator. Liquid level control is accomplished automatically by varying the flow from the fluid discharge conduit which is connected to the lower pressure flow line, which is usually the gas flow line. Manifold pressures may be sensed to prevent exceeding a predetermined pressure in the manifolds and the well flow lines.

Molecular-dynamics simulation of compressible fluid flow in two-dimensional channels.

Abstract: We study compressible fluid flow in narrow two-dimensional channels using a molecular-dynamics simulation method. In the simulation area, an upstream source is maintained at constant density and temperature while a downstream reservoir is kept at vacuum. The channel is sufficiently long in the direction of the flow that the finite length has little effect on the properties of the fluid in the central region. The simulated system is represented by an efficient data structure, whose internal elements are created and manipulated dynamically in a layered fashion. Consequently the computer code is highly efficient and manifests completely linear performance in simulations of large systems. We obtain the steady-state velocity, temperature, and density distributions in the system. The velocity distribution across the channel is very nearly a quadratic function of the distance from the center of the channel and reveals velocity slip at the boundaries; the temperature distribution is only approximately a quartic function of this distance from the center to the channel. The density distribution across the channel is nonuniform. We attribute this nonuniformity to the relatively high Mach number, approximately 0.5, in the fluid flow. An equation for the density distribution based on simple compressibility arguments is proposed; its predictions agree well with the simulation results. The validity of the concept of local dynamic temperature and the variation of the temperature along the channel are discussed.

Effect of nozzle configuration on transport in the stagnation zone of axisymmetric, impinging free-surface liquid jets: Part 1-turbulent flow structure

Abstract: This study characterized the mean and fluctuating parts of the radial component of the local velocity in the stagnation region of an impinging, free-surface liquid jet striking a smooth flat plate. Four different nozzle exit conditions were studied, including fully developed pipe flow, a contoured nozzle, and turbulence-damped and undamped sharp-edged orifices. Liquid jet Reynolds numbers in the range 30,000 to 55,000 were investigated. Velocities were measured using laser-Doppler velocimetry. Mean velocities were found to vary nearly linearly with radial location, with the slope of the line being a function of distance from the impingement plate. Dimensionless mean velocity gradients, of relevance to the heat transfer, were found to be a strong function of nozzle type, but roughly independent of jet Reynolds number for a given nozzle type. Turbulence levels were also found to be strongly influenced by the nozzle exit-condition. Local heat transfer data corresponding to the flow structure measurements presented here are reported in Part 2 of this study. 22 refs., 9 figs.

A Power-Law Formulation of Laminar Flow in Short Pipes

Abstract: In the quantification of air flow through penetrations in buildings, it is necessary to be able to characterize the flow without detailed knowledge of the geometry of the paths. At the conditions typical of buildings, the flow regime is partially developed laminar flow. This report develops a theoreticfal description of the hydrodynamic relationship based an a power-law representation between the air flow and applied pressure for laminar flow in short pipes

Large-eddy simulation of turbulent flow and heat transfer in plane and rib-roughened channels

Abstract: Large-eddy simulation results are presented and discussed for turbulent flow and heat transfer in a plane channel with and without transverse square ribs on one of the walls. They were obtained with the finite-difference code Harwell-FLOW3D, Release 2, by using the PISOC pressure-velocity coupling algorithm, central differencing in space, and Crank-Nicolson time stepping. A simple Smagorinsky model, with van Driest damping near the walls, was implemented to model subgrid scale effects. Periodic boundary conditions were imposed in the streamwise and spanwise directions. The Reynolds number based on hydraulic diameter (twice the channel height) ranged from 10 000 to 40 000. Results are compared with experimental data, k-ϵ predictions, and previous large-eddy simulations.

Applications of Domain Decomposition Spectral Collocation Methods in Viscoelastic Flows Through Model Porous Media

Abstract: A domain decomposition spectral collocation method is developed for steady-state, viscoelastic flow simulations through model porous media. The method is first tested on the one dimensional linear equation set resulting from the first order domain perturbation analysis of the flow of an Upper Convected Maxwell (UCM) fluid in an undulating channel. The splitting of the domainin the direction normal to the flow allows for an efficient local mesh refinement in the region of steep stress gradients and results in more accurate solutions than the fully pseudospectral implementation for the same degrees of freedom. The method was applied to the full 2-d, non-linear, viscoelastic equations with the same success. In addition to the undulating channel, solutions convergent with mesh refinement have been obtained for the flow of an UCM fluid through a square array of cylinders up to We = 1.5. No increase in the flow resistance is found, for increasing elasticity. This observation is consistent with the findings in the undulating channel flow.