Showing papers on "Open-channel flow published in 1983"

Experimental and Theoretical Investigation of Backward-Facing Step Flow

Abstract: Laser-Doppler measurements of velocity distribution and reattachment length are reported downstream of a single backward-facing step mounted in a two-dimensional channel. Results are presented for laminar, transitional and turbulent flow of air in a Reynolds-number range of 70 < Re < 8000. The experimental results show that the various flow regimes are characterized by typical variations of the separation length with Reynolds number. The reported laser-Doppler measurements do not only yield the expected primary zone of recirculating flow attached to the backward-facing step but also show additional regions of flow separation downstream of the step and on both sides of the channel test section. These additional separation regions have not been previously reported in the literature.Although the high aspect ratio of the test section (1:36) ensured that the oncoming flow was fully developed and two-dimensional, the experiments showed that the flow downstream of the step only remained two-dimensional at low and high Reynolds numbers.The present study also included numerical predictions of backward-facing step flow. The two-dimensional steady differential equations for conservation of mass and momentum were solved. Results are reported and are compared with experiments for those Reynolds numbers for which the flow maintained its two-dimensionality in the experiments. Under these circumstances, good agreement between experimental and numerical results is obtained.

Flood Plain and Main Channel Flow Interaction

Abstract: Experimental results are presented concerning the discharge characteristics, the boundary shear stress and boundary shear force distributions in a compound section comprising of one rectangular main channel and two symmetrically disposed flood plains. Equations are presented giving the shear force on the flood plains as a percentage of the total shear force in terms of two dimensionless parameters. The experimental shear force results are used to derive ancillary equations for the lateral and vertical transfer of momentum within the cross section. The apparent shear force acting on the vertical interface between one flood plain and the main channel is shown to increase rapidly for low relative depths and high flood plain widths. Equations are also presented giving the proportion of the total flow which occurs in the various sub areas. The division of flow based on linear proportion of the areas is shown to be inadequate on account of the interaction between the flood plain and main channel flows.

Flow transformations in sediment gravity flows

Abstract: Sediment gravity flows (subaerial or subaqueous) are those in which movement is driven by gravity and the sediment motion moves the interstitial fluid (gas or liquid). Such flows exhibit flow transformations , a term introduced here, referring to changes between laminar and turbulent flow, in turn related chiefly to particle concentration, thickness of the flow, and flow velocity (slope).

Diffusion in oscillatory pipe flow

Abstract: The rate of mass transfer of a diffusing substance along a pipe is augmented by an oscillatory motion of the ambient fluid in the pipe. The increase of the flux is evaluated for the cases of a circular pipe and of a two-dimensional channel. Results are given for a general cross-section in the limiting cases of slow and fast oscillations of the flow.

Flow pattern in horizontal and vertical two phase flow in small diameter pipes

Abstract: New data on flow pattern transition for gas liquid flow in small diameter tubes (4 to 12 mm) is presented. The experimental results are compared with previously published models for horizontal and vertical flows considered to be valid for medium and large diameter pipes. The effect of surface tension which might be expected to be important in small diameter pipe flow has been found to affect only the stratified-slug transition in horizontal flow. A modification to the model to include this effect is proposed.

Flow Near Groin‐Like Structures

Abstract: This paper presents the results of an experimental study on the structure of turbulent flow near groin‐like structures. Based on experimental observations, the deflected flow has been analyzed using the model of the three‐dimensional turbulent boundary layer. The flow bounded by the separating stream line, the groin, and the adjacent bank has been analyzed by treating it as a shear layer in which the velocity profiles have been found to be similar. The amplified bed shear stress near the groin has also been analyzed using the similarity idea. Even though the majority of the present experiments were done with the thin‐plate groin, some observations were also made on a groin with a semicylindrical hose.

Parallel path Coriolis mass flow rate meter

Abstract: A flow meter apparatus for measuring the mass flow rates of fluids passed through two essentially parallel, cantilever mounted U-shaped flow tubes where, instead of the fluid serially passing through one of the U-shaped flow tubes and then through the second U-shaped flow tube, the fluid is essentially evenly divided and half of the fluid is passed through one U-shaped flow tube and the other half of the fluid is passed through the other U-shaped flow tube. The two U-shaped flow tubes are sinusoidally driven as the tines of a tuning fork, and when a fluid is passed through the oscillating U-shaped flow tubes Coriolis forces are generated which torsionally deflect the U-shaped flow tubes. Such torsional deflections are a function of the mass flow rate of the fluid passing through the two U-shaped flow tubes.

Turbulence structure in stably stratified open-channel flow

Abstract: The effects of stable stratification on turbulence structure have been experimentally investigated in stratified open-channel flow and a theoretical spectral-equation model has been applied to the stably stratified flow. The measurements were made in the outer layer of open-channel flow with strongly stable density gradient, where the wall effect was small. Velocity and temperature fluctuations were simultaneously measured by a laser-Doppler velocimeter and a cold-film probe. Measurements include turbulent intensities, correlation coefficients of turbulent fluxes and coherence–phase relationships. These turbulent quantities were correlated with the local gradient Richardson number and compared with the values calculated using a spectral-equation model and with other laboratory measurements. In stable conditions, turbulent motions approach wavelike motions, and negative heat and momentum transfer against the mean temperature and velocity gradient occurs in strongly stable stratification.

Flow Depth in Sand‐Bed Channels

Abstract: Six methods for predicting friction factor are evaluated and a new method is presented which better predicts flow depth (and therefore friction factor) when discharge and slope values are known. The goal of the investigation is to define a relationship between depth, discharge, and slope that can be used in a wide range of situations where uniform flow conditions are present, and which can also be adapted for numerical modeling applications. None of the six methods completely satisfy the writer's criteria; therefore a new relationship is presented. The proposed technique is based on dimensional analysis, a statistical analysis of a large body of laboratory and field data, and basic principles of hydraulics. Separate equations are developed for the lower (dune and ripple) and upper (flat bed and antidune) flow regimes. The technique also includes a simple method for flow regime determination. Finally, the proposed method is verified through comparison with data sets not used in the derivation of the technique.

Effects of imperfect spatial resolution on measurements of wall-bounded turbulentbx shear flows

Abstract: The effects of imperfect spatial resolution on hot-film and hot-wire measurements of wall-bounded turbulent shear flows were studied. Two hot-film probes of different length were used for measurements of fully developed turbulent channel flow in a water tunnel. In the near-wall region significant effects of spanwise spatial averaging due to finite probe size were found for a probe 32 viscous units long. The maximum turbulence intensity attained a 10% lower value than that for a probe about half as long, and the zero-crossing of the skewness factor was shifted away from the wall. This could be attributed to spatial averaging of narrow low-speed regions. Results for different Reynolds numbers, but with the same sensor length in viscous units, showed that Reynolds-number effects are small, and that much of the reported discrepancies for turbulence measurements in the near-wall region can be ascribed to effects of imperfect spatial resolution. Also the number of events detected with the variable-interval time-averaging (VITA) technique was found to depend strongly on the sensor length, especially for events with short duration.

Meander Bends of High Amplitude

Abstract: Meander bends of high amplitude in alluvial rivers often display a high degree of coherency. The only analytical equation heretofore available to describe such bends is the sine‐generated curve, which has no rigorous derivation in the context of rivers. Herein a nonlinear “bend equation,” based on a dynamical description of flow in bends and a kinematical description of bank erosion, is used to describe channel migration. This equation admits solutions of constant amplitude that migrate downstream with constant speed. The solution at high amplitude displays a prominent skewing that reveals the direction of flow. At low amplitude, the solution reduces to the sine‐generated curve.

On the structure of wall‐bounded turbulent flows

Abstract: The variable-interval time-averaging (VITA) technique developed by Blackwelder and Kaplan is applied to data obtained from large-eddy simulation of turbulent channel flow in an investigation of the organized structures associated with the bursting phenomenon in the near-wall region. Conditionally averaged velocities, shear stress, pressure, and vorticity are discussed in conjunction with the bursting phenomenon detected by the VITA technique. The conditionally averaged pressure reveals that the ejection process is associated with a localized adverse pressure gradient. In the plane perpendicular to the flow direction, the conditionally averaged vorticity field indicates that a pair of counterrotating streamwise vorticity is being lifted through the ejection process.

Stochastic differential equations and turbulent dispersion

Abstract: Aspects of the theory of continuous stochastic processes that seem to contribute to an understanding of turbulent dispersion are introduced and the theory and philosophy of modelling turbulent transport is emphasized. Examples of eddy diffusion examined include shear dispersion, the surface layer, and channel flow. Modeling dispersion with finite-time scale is considered including the Langevin model for homogeneous turbulence, dispersion in nonhomogeneous turbulence, and the asymptotic behavior of the Langevin model for nonhomogeneous turbulence.

Stabilization effects in flow through helically coiled pipes

Abstract: When a flow through a straight pipe is passed through a coiled section, two stabilizing effects come into play. First, in a certain Reynolds number range, the flow that is turbulent in the straight pipe becomes completely laminar in the coiled section. Second, the stabilization effect of the coil persists to a certain degree even after the flow downstream of the coil has been allowed to develop in a long straight section. In this paper, we report briefly on aspects related to these two effects.

Flow between rotating disks. Part 1. Basic flow

Abstract: Laser-Doppler velocity measurements were obtained in water between finite rotating disks, with and without throughflow, in four cases: ω1 = ω2 = 0; ω2/ω1 = −1; ω2/ω1 = 0; ω2/ω1 = 1. The equilibrium flows are unique, and at mid-radius they show a high degree of independence from boundary conditions in r. With one disk rotating and the other stationary, this mid-radius ‘limiting flow’ is recognized as the Batchelor profile of infinite-disk theory. Other profiles, predicted by this theory to coexist with the Batchelor profile, were neither observed experimentally nor were they calculated numerically by the finite-disk solutions, obtained here via a Galerkin, B-spline formulation. Agreement on velocity between numerical results and experimental data is good at large values of the ratio RQ/Re, where RQ = Q/2πνs is the throughflow Reynolds number and Re = R22ω/ν is the rotational Reynolds number.

Flow in an Axially Rotating Pipe : A calculation of flow in the saturated region

Abstract: If a flow enters an axially rotating pipe, it receives a tangential component of velocity from the moving wall, and the flow pattern and hydraulic loss suffer a change according to the ratio of the rotational speed to the through flow velocity. A flow laminarization is set up by an increase in the rotational speed of the pipe if the flow in the pipe is initially turbulent, and a flow destabilization is brought conversely about if the flow is initially laminar. Velocity distributions and friction coefficient in the fully developed region of the pipe were calculated by using a modified mixing length theory, and the results were compared with those by the experiments.

Modeling solid-fluid turbulent flows with application to predicting erosive wear

Abstract: Abstract Numerical modeling of fluid-particulate turbulent flow transport processes, including erosion, cannot substitute (at least for the present) good experimentation. Notwithstanding, the calculation approach offers a viable, relatively inexpensive and complementary alternative for gleaning useful information on the relative dependence of particle transport and erosion on the flow system parameters of relevance. In the present study these parameters include: the particle phase response time, Reynolds number and concentration; the turbulent characteristics of the fluid phase; the channel aspect ratio and curvature. This communication summarizes the main results predicted by means of a numerical procedure for the motion of a dilute suspension of solid particles driven by turbulent flow in curved and straight two-dimensional channels. An exposition of the theoretical development, the numerical model, its testing and application to these configurations of special engineering interest is provided. A simple model is used to illustrate the prediction of erosive wear.

Numerical study of secondary flows and roll-cell instabilities in rotating channel flow

Abstract: A numerical study is conducted on the pressure-driven laminar flow of an incompressible viscous fluid through a rectangular channel subjected to a spanwise rotation. The full nonlinear time-dependent Navier–Stokes equations are solved by a finite-difference technique for various rotation rates and Reynolds numbers in the laminar regime. At weak rotation rates, a double-vortex secondary flow appears in the transverse planes of the channel. For more rapid rotation rates, an instability occurs in the form of longitudinal roll cells in the interior of the channel. Further increases in the rotation rate leads to a restabilization of the flow to a Taylor–Proudman regime. It is found that the roll-cell and Taylor–Proudman regimes lead to a substantial distortion of the axial-velocity profiles. The specific numerical results obtained are shown to be in excellent agreement with previously obtained experimental measurements and theoretical predictions.

Two-Equation Turbulence Model for Flow in Trenches

Abstract: Steady recirculating flow is described by means of a mathematical model based on the full unsteady Reynolds equations and the k-ϵ model for turbulence closure. The model is applied to the flow in a steep-sided trench perpendicular to the main flow direction. Inlet profiles are taken with reference to developed channel flow. For the wall boundary a local equilibrium is assumed, yielding among others a logarithmic behaviour for the mean flow velocity. The constants of the k-ϵ model are related to the roughness conditions. A sensitivity study is reported to identify the relative importance of the various constants and inlet conditions. Numerical results are compared with laboratory experiments.

Experimental Studies of Damping and Hydrodynamic Mass of a Cylinder in Confined Two-Phase Flow

Abstract: This paper presents the results of experiments done to determine the effects of cylinder mass and flow regime on the damping and hydrodynamic mass characteristics of a cylinder vibrating in simulated two-phase air-water flows. It was found that two-phase damping varied in inverse proportion to the combined cylinder and two-phase hydrodynamic masses. This is in agreement with a postulated detuning process for damping in two-phase flow. As in earlier studies, the two-phase hydrodynamic mass was found to be less than that calculated from the mixture density, for all flow conditions. 4 refs.

Development of Three-Dimensional Turbulent Boundary Layer in an Axially Rotating Pipe

Abstract: The time-mean velocities and turbulent fluctuations inside the turbulent boundary layers which developed in an axially rotating pipe were measured in the case where an undeveloped flow with a rectangular axial velocity distribution was introduced in the pipe. The pipe rotation gives two counter effects on the flow: one is a destabilizing effect due to a large shear caused by the rotating pipe wall and the other is a stabilizing effect due to the centrifugal force of the swirling velocity component of the flow. The destabilizing effect prevails in the inlet region, but the stabilizing effect becomes dominant in the downstream sections. The intensity of turbulence in the rotating pipe decreases ultimately below that in a stationary state of the pipe. Using the experimental results, the relationship between the mixing length and Richardson number proposed by Bradshaw was examined for the turbulent boundary layer that develops in the rotating pipe.

Hydraulie Control and Flow Separation in a Multi-Layered Fluid with Applications to the Vema Channel

Abstract: Observations from a recent field experiment in the Vema Channel are briefly described. These show a remarkable change in the configuration of isopycnal surfaces within the channel and the development of thick, nearly homogeneous regions near the bottom which are capped by sharp vertical gradients. Contrary to previous speculation that these “bottom boundary layer” result from enhanced vertical mixing, a dynamical mechanism is explored. This involves the hydraulic adjustment of an inertial, semi-geostrophic flow to the channel geometry. First, an active two-layer flow in a rectangular geometry is studied to show that internal flow separation can occur when the flow is accelerated sufficiently by a narrowing channel. Almost always this separation accompanies hydraulic control: the slowest upstream moving Kelvin wave is stopped and upstream and downstream states are not symmetric with respect to the channel width. An active three-layer flow with a variable bottom profile is then presented as a more ...

On the Theory of the Long-Term Variability of the Atmosphere

Abstract: Much of the atmosphere's long-term variability is contained in the planetary modes with zonal wavenumber m≤5. It is proposed that a considerable fraction of this variability is induced by the nonlinear interaction of synoptic-scale modes (m>5) with the planetary-scale modes. TO test this hypothesis, the synoptic-scale forcing of the planetary-scale 500 mb streamfunction is determined from data. It is found that this forcing can be fitted approximately to a Markov process of first order, and that it strongly depends on locality. The response of the planetary motions to this forcing is calculated within the framework of the linear barotropic vorticity equation on the sphere and for β-plane flow. The power spectra and the low-frequency variance of the induced planetary-scale motions are presented. It is shown that there is a good correspondence between the observed low-frequency variance and the result of the computations on the sphere. The results for the β-plane channel flow are less satisfactory.