Showing papers on "Open-channel flow published in 1989"

Reynolds-number effects on the structure of a turbulent channel flow

Abstract: A high resolution, two component laser-Doppler anemometer has been used for turbulence measurements at a high data rate in a channel flow of water. Measurements of the velocity components in the stream direction and in a direction normal to the wall are reported over the Reynolds number range of 3000–40000. The combination of high spatial resolution and high data rates enabled accurate reconstruction of time dependent velocity traces. Long-time statistical averages of these signals clearly show that profiles of the dimensionless turbulence quantities such as turbulence intensities and Reynolds stress are strongly Reynolds-number dependent over a large part of the channel flow. For instance, in the Reynolds-number range of this investigation, it is shown that the fluctuating turbulence quantities do not scale with wall variables even as close as 15 viscous lengths from the wall. The velocity traces and associated power spectra exposed two phenomena which may explain the Reynolds number dependencies.

Characteristic-eddy decomposition of turbulence in a channel

Abstract: The proper orthogonal decomposition technique (Lumley's decomposition) is applied to the turbulent flow in a channel to extract coherent structures by decomposing the velocity field into characteristic eddies with random coefficients. In the homogeneous spatial directions, a generaliztion of the shot-noise expansion is used to determine the characteristic eddies. In this expansion, the Fourier coefficients of the characteristic eddy cannot be obtained from the second-order statistics. Three different techniques are used to determine the phases of these coefficients. They are based on: (1) the bispectrum, (2) a spatial compactness requirement, and (3) a functional continuity argument. Results from these three techniques are found to be similar in most respects. The implications of these techniques and the shot-noise expansion are discussed. The dominant eddy is found to contribute as much as 76 percent to the turbulent kinetic energy. In both 2D and 3D, the characteristic eddies consist of an ejection region straddled by streamwise vortices that leave the wall in the very short streamwise distance of about 100 wall units.

Transport of Passive Scalars in a Turbulent Channel Flow

Abstract: A direct numerical simulation of a turbulent channel flow with three passive scalars at different molecular Prandtl numbers is performed. Computed statistics including the turbulent Prandtl numbers are compared with existing experimental data. The computed fields are also examined to investigate the spatial structure of the scalar fields. The scalar fields are highly correlated with the streamwise velocity; the correlation coefficient between the temperature and the streamwise velocity is as high as 0.95 in the wall region. The joint probability distributions between the temperature and velocity fluctuations are also examined; they suggest that it might be possible to model the scalar fluxes in the wall region in a manner similar to the Reynolds stresses.

Aerosol particle deposition in numerically simulated channel flow

Abstract: The trajectories of rigid spherical particles in a turbulent channel flow are computed using a pseudospectral computer program to simulate the three‐dimensional, time‐dependent flow field. It is assumed that the channel is vertical so that gravity cannot directly cause the deposition of particles on the walls. The particles are assumed to be sufficiently small and widely separated so that their influence on the fluid velocity field can be ignored. It is found that when the particles are assigned random initial locations with initial velocities that are equal to the local fluid velocity, the particles tend to accumulate in the viscous sublayer. At the edge of the viscous sublayer, the particles that deposit on the wall typically possess normal components of velocity that are comparable in magnitude to the intensity of the normal component of the velocity in the core of the channel (i.e., of the order of magnitude of the friction velocity). A shear‐induced lift force having the form derived by Saffman for laminar flow is found to have virtually no effect on particle trajectories, except within the viscous sublayer where it plays a significant role both in the inertial deposition of particles and in the accumulation of trapped particles. The Reynolds number of the particles that deposit does not remain small compared with unity.

New approximate boundary conditions for large eddy simulations of wall-bounded flows

Abstract: Two new approximate boundary conditions have been applied to the large eddy simulation of channel flow with and without transpiration. These new boundary conditions give more accurate results than those previously in use, and allow significant reduction of the required CPU time over simulations in which no‐slip conditions are applied. Mean velocity profiles and turbulence intensities compare well both with experimental data and with the results of resolved simulations. The influence of the approximate boundary conditions remains confined near the point of application and does not affect the turbulence statistics in the core of the flow.

On the structure of pressure fluctuations in simulated turbulent channel flow

Abstract: Pressure fluctuations in a turbulent channel flow are investigated by analyzing a database obtained from a direct numerical simulation. Detailed statistics associated with the pressure fluctuations are presented. Characteristics associated with the rapid (linear) and slow (nonlinear) pressure are discussed. It is found that the slow pressure fluctuations are larger than the rapid pressure fluctuations throughout the channel except very near the wall, where they are about the same magnitude. This is contrary to the common belief that the nonlinear source terms are negligible compared to the linear source terms. Probability density distributions, power spectra, and two-point correlations are examined to reveal the characteristics of the pressure fluctuations. The global dependence of the pressure fluctuations and pressure-strain correlations are also examined by evaluating the integral associated with Green's function representations of them. In the wall region where the pressure-strain terms are large, most contributions to the pressure-strain terms are from the wall region (i.e., local), whereas away from the wall where the pressure-strain terms are small, contributions are global. Structures of instantaneous pressure and pressure gradients at the wall and the corresponding vorticity field are examined.

Chaotic advection by laminar flow in a twisted pipe

Abstract: The appearance of chaotic particle trajectories in steady, laminar, incompressible flow through a twisted pipe of circular cross-section is demonstrated using standard dynamical systems diagnostics and a model flow based on Dean's perturbation solutions. A study is performed to determine the parameters that control fluid stirring in this mixing device that has no moving parts. Insight into the chaotic dynamics are provided by a simple one-dimensional map of the pipe boundary onto itself. The results of numerical experiments illustrating the stretching of material lines, stirring of blobs of material, and the three-dimensional trajectories of fluid particles are presented. Finally, enhanced longitudinal particle dispersal due to the coupling between chaos in the transverse direction and the non-uniform longitudinal transport of particles is shown.

The turbulent flow field around a circular cylinder

Abstract: The flow field around a circular cylinder mounted vertically on a flat bottom has been investigated experimentally. This type of flow occurs in several technical applications, e.g. local scouring around bridge piers. Hydrogen bubble flow visualization was carried out for Reynolds numbers ranging from 6,600 to 65,000. The main flow characteristic upstream of the cylinder is a system of horse-shoe vortices which are shed quasi-periodically. The number of vortices depends on Reynolds number. The vortex system was found to be independent of the vortices that are shed in the wake of the cylinder. The topology of the separated flow contains several separation and attachment lines which are Reynolds number dependent. In the wake region different flow patterns exist for each constant Reynolds number.

Velocity Distribution in Open Channel Flow

Abstract: Velocity distribution equations for open channel flows are derived and compared. These equations are derived by a combined application of: (1) A probabilistic formulation of the velocity distribution problem; (2) the entropy concept in the selection of the probability distribution function of velocity; (3) a geometrical technique in modeling a curvilinear coordinate and the coordinate transformation between this and the Cartesian coordinates; and (4) the basic hydrodynamics concerning the rates of transport of mass, momentum, and kinetic energy by the flow through an open channel cross section. A technique to estimate the parameters of these velocity distribution equations is also developed. The equations are capable of modeling and simulating the velocity distribution from the channel bed to the water surface, which may have the maximum velocity occurring on or below the water surface.

Three-dimensional turbulent structure in straight open channel flows

Abstract: Secondary currents are generated and modified as a result of the anisotropy of turbulence, which is caused by the boundary conditions of the bed, the side-wall and the free surface, as well as the aspect ratio of the channel and the channel geometry. Secondary currents affect the primary mean flow, producing threedimensional structures. Such a generation mechanism for secondary currents in closed and open channel flows can be explained well by using the longitudinal vorticity equation. The secondary motions in open channel flows are quite different from those in closed channel flows.

Calculation of unsteady flows in turbomachinery using the linearized Euler equations

Abstract: A method for calculating unsteady flows in cascades is presented. The model, which is based on the linearized unsteady Euler equations, accounts for blade loading shock motion, wake motion, and blade geometry. The mean flow through the cascade is determined by solving the full nonlinear Euler equations. Assuming the unsteadiness in the flow is small, then the Euler equations are linearized about the mean flow to obtain a set of linear variable coefficient equations which describe the small amplitude, harmonic motion of the flow. These equations are discretized on a computational grid via a finite volume operator and solved directly subject to an appropriate set of linearized boundary conditions. The steady flow, which is calculated prior to the unsteady flow, is found via a Newton iteration procedure. An important feature of the analysis is the use of shock fitting to model steady and unsteady shocks. Use of the Euler equations with the unsteady Rankine-Hugoniot shock jump conditions correctly models the generation of steady and unsteady entropy and vorticity at shocks. In particular, the low frequency shock displacement is correctly predicted. Results of this method are presented for a variety of test cases. Predicted unsteady transonic flows in channels are compared to full nonlinear Euler solutions obtained using time-accurate, time-marching methods. The agreement between the two methods is excellent for small to moderate levels of flow unsteadiness. The method is also used to predict unsteady flows in cascades due to blade motion (flutter problem) and incoming disturbances (gust response problem).

Flow Resistance : A Design Guide for Engineers

Abstract: Flow in straight tubes and conduits flow at the entrance into tubes and conduits flow through orifices with sudden change in velocity and flow area flow with a smooth change in velocity flow with changes of the stream direction merging of flow streams and division into flow streams flow through barriers uniformly distributed over the channel cross section flow through pipe fittings and labryinth seals flow past obstructions in a tube flow at the exit from tubes and channels.

Redistribution of an inlet temperature distortion in an axial flow turbine stage

Abstract: The results of an experimental program aimed at determining the extent of the redistribution of an inlet temperature distortion in an axial flow turbine stage are presented. The program was conducted in a large-scale, low-speed, single-stage turbine where air, seeded with CO2, was introduced at one circumferential location upstream of the inlet guide vane. The migration of the seeded air through the turbine was determined by sensing CO2 concentration inside the stage. A temperature distortion was introduced by heating the seeded air. The CO2 concentration contours measured downstream of the vane showed h'ttle change with heating, indicating that the vane flowfield was relatively unaffected by the introduction of the temperature distortion. However, the CO2 contours observed on the rotor airfoil surfaces for the case with inlet heating indicated segregation of hot and cold gas with the higher temperature gas migrating to the pressure side and the lower temperature gas migrating to the suction side. Significant increases in rotor secondary flow were also observed. Two separate physical mechanisms are postulated to explain the observed experimental trends. Calculations performed by using a three-dimensional Euler solver show qualitative agreement with the experimental data.

Uniform flow in a smooth open channel

Abstract: Laboratory experiments on the structure of uniform flow in a smooth open channel are reported. Velocity profiles (Figs. 2 and 3) and turbulence intensity profiles (Fig. 6) as well as friction velocity (Table 2) were measured. It was found that: the universal law-of-the-wall describes rather well the data over the entire channel depth (Fig. 2); in the core of the outer region (0.2 < y/d < 0.7), a wake of limited strength (Π ≈ 0.08) exists; in the near-surface zone (0.7< y/d < 1.0), a retarding effect, possibly due to weak secondary currents, tends to compensate the wake divergence (Fig. 4), resulting in a modified distribution of the mixing length (Fig. 5).

The relationship between surface-renewal and bursting motions in an open-channel flow

Abstract: Surface-renewal motions in the interfacial region below a gas-liquid interface were experimentally investigated in relation to bursting motions in the wall region. To estimate the frequency of the appearance of surface-renewal eddies, mass-transport experiments with methylene-blue solution, together with velocity measurements, were done in an open-channel flow. The instantaneous concentration of methylene-blue tracer emitted from a point source positioned in the buffer layer was measured at the free surface downstream from the source by an optical probe. Instantaneous streamwise velocity was measured using a laser-Doppler velocimeter at a position in the buffer region. Frequencies of both surface-renewal and bursting events were computed from these concentration and velocity signals using a conditional-averaging method. In order to clarify whether the surface-renewal eddies actually dominate mass transfer across the gas-liquid interface, gas-absorption experiments were added. Carbon dioxide was absorbed into the water flow across the calm free surface and its mass-transfer coefficient on the liquid side was measured under the same flow conditions as used in the above mass-transport experiments. The results show that the surface-renewal motions originate in the bursting motions which vigorously occur in the buffer region. That is, the decelerated fluid which is strongly lifted towards the outer layer by bursting almost always arrives at the free surface and renews the free surface. The frequency of the surface renewal, as well as the bursting frequency, is uniquely determined by the wall variables or the outer-flow variables and the Reynolds number. Mass transfer across the gas-liquid interface is dominated by the large-scale surface-renewal eddies, and the mass-transfer coefficient on the liquid side is proportional to the square-root of the surface-renewal frequency.

River-Meander Model: I. Development

Abstract: Meander flow and meanderplanform development are described. The basis is a steady, twodimensional model of flow and bed topography in an alluvial channel with variable curvature. The model is devel...

Secondary Flow in Mildly Sinuous Channel

Abstract: Most analytical models of the flow and bed topography in sinuous channels include the assumption that the secondary flow is locally adapted to channel curvature. Recently, Kitanidis and Kennedy (1984) and Ikeda and Nishimura (1986) have, however, established the existence of a phase lag between the two, induced by inertia, i.e., downstream convective acceleration of the secondary flow. The analysis of Ikeda and Nishimura (1986), nevertheless, includes an unverified assumption. Herein a more rigorous derivation of the effect is obtained with a minimum of assumptions. The predicted lag appears to be sufficiently small to warrant neglect in natural channels. The theory predicts much higher values in many experimental channels, in agreement with data. The reason for this is that a scaled dimensionless meander wave number r, found to be order‐one in natural channels, is found to be an order of magnitude higher in many laboratory meandering flumes.

Turbulent Velocity Profiles for Smooth and Rough Open Channel Flow

Abstract: Velocity measurements, using a laserdoppler anemometer, are carried out in a fully developed, rectangular, subcritical open channel flow on smooth and rough beds. The average roughness heights of t...

Turbulence characteristics of sediment-laden flow

Abstract: Experiments are reported which were carried out in open channel flow in a laboratory flume; and turbulent structures of both clear water flow and sediment-laden flow, which were kept under exactly the same conditions, were measured, compared and analyzed. It was found that the various statistical parameters of turbulence measured in clear water flow are essentially consistent with those obtained by other researchers. The turbulent intensity decreases with increase in concentration. In sediment-laden flow, the probability density distribution and the autocorrelation coefficient are similar to those of the clear water flow. Turbulent frequency decreases and turbulent energy is concentrated to large-size eddies with low frequency. The longitudinal sizes of macroscale and microscale eddies increase. It was found that for a Newtonian flow with noncohesive particles, the fundamental turbulent structure has no essential change - only the turbulent intensity and frequency have some changes in magnitude. These and other findings are discsussed.

Cavity Heat Transfer on a Transverse Grooved Wall in a Narrow Flow Channel

Abstract: Etude du transfert de chaleur par convection dans une cavite rectangulaire dans la paroi d'un canal etroit

An Experimental Investigation of Heat Transfer Coefficients in a Spanwise Rotating Channel With Two Opposite Rib-Roughened Walls

Abstract: Liquid crystals are used in this experimental investigation to measure the heat transfer coefficient in a spanwise rotating channel with two opposite rib-roughened walls. The ribs (also called turbulence promoters or turbulators) are configured in a staggered arrangement with an angle of attack to the mainstream flow, α, of 90° for all cases. Results are presented for three values of turbulator blockage ratio, e/Dh (0.1333, 0.25, 0.333) and for a range of Reynolds numbers from 15,000 to 50,000 while the test section is rotated at different speeds to give Rotational Reynolds numbers between 450 and 1800. The Rossby number range is 10 to 100 (Rotation number of 0.1 to 0.01). The effect of turbulator blockage ratios on heat transfer enhancement is also investigated. Comparisons are made between the results of geometrically identical stationary and rotating passages of otherwise similar operating conditions. The results indicate that a significant enhancement in heat transfer is achieved in both the stationary and rotating cases, when the surfaces are roughened with turbulators. For the rotating case, a maximum increase over that of the stationary case of about 45% in the heat transfer coefficient is seen for a blockage ratio of 0.133 on the trailing surface in the direction of rotation and the minimum is a decrease of about 6% for a blockage ratio of 0.333 on the leading surface, for the range of rotation numbers tested. The technique of using liquid crystals to determine heat transfer coefficients in this investigation proved to be an effective and accurate method especially for nonstationary test sections.Copyright © 1989 by ASME

Instabilities in channel flow with system rotation

Abstract: A flow visualization study of instabilities caused by Coriolis effects in plane rotating Poiseuille flow has been carried out. The primary instability takes the form of regularly spaced roll cells aligned in the flow direction. They may occur at Reynolds numbers as low as 100, i.e. almost two orders of magnitude lower than the critical Reynolds number for Tollmien-Schlichting waves in channel flow without rotation. The development of such roll cells was studied as a function of both the Reynolds number and the rotation rate and their properties compared with results from linear spatial stability theory. The theoretically obtained most unstable wavenumber agrees fairly well with the experimentally observed value. At high Reynolds number a secondary instability sets in, which is seen as a twisting of the roll cells. A wavytype disturbance is also seen at this stage which, if the rotational speed is increased, develops into large-scale ‘turbulence’ containing imbedded roll cells.

Open channel flows with submerged obstructions

Abstract: Free-surface flows past a submerged triangular obstacle at the bottom of a channel are considered. The flow is assumed to be steady, two-dimensional and irrotational; the fluid is treated as inviseid and incompressible and gravity is taken into account. The problem is solved numerically by series truncation. It is shown that there are solutions for which the flow is suberitical upstream and supercritical downstream and other flows for which the flow is supercritical both upstream and downstream. The latter flows have limiting configurations with a stagnation point on the free surface with a 120° angle at it. It is found that solutions exist for triangular obstacles of arbitrary size. Local solutions are constructed to describe the flow near the apex when the height of the triangular obstacle is infinite.

On the shape and dynamics of wall structures in turbulent channel flow

Abstract: Turbulence‐producing events in turbulent channel flow were found to be predominantly associated with asymmetric vortical structures rather than pairs of counter‐rotating structures. An asymmetry‐preserving averaging scheme was devised, allowing a picture of the ‘‘average’’ structure that more closely resembles the instantaneous one to be obtained. In addition, these structures were found to persist for long distances with little change while convecting downstream.

Turbulent free shear layer mixing

Abstract: Some experimental data on turbulent shear layer growth, mixing and chemical reactions in the limit of fast kinetics are reviewed. The dependence of these phenomena on such fluid and flow parameters as Schmidt number, Reynolds number, and Mach number are discussed with the aid of some recent models and consequences deducible from the large scale organization of the flow.