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Unsteady depth-varying open-channel flows are really observed in flood rivers. Owing to highly accurate laser Doppler anemometers (LDA), some valuable experimental databases of depth-varying unsteady open-channel flows are now available. However, these LDA measurements are more difficult to conduct in open-channel flows at higher unsteadiness, in comparison with unsteady wall-bounded flows such as oscillatory boundary layers and duct flows. Therefore, in this study, a low-Reynolds-number k–e model involved with a function of unsteadiness effect was developed and some numerical calculations were conducted using the volume of fluid method as a free-surface condition. The present calculated values were in good agreement with the existing LDA data in the whole flow depth from the wall to the time-dependent free surface. These values were also compared with those of unsteady wall-bounded flows. The present calculations were able to predict the distributions of turbulence generation and its dissipation, and con...

In 1883, Osborne Reynolds reported experiments in real viscous pipe flows and made the seminal distinction between laminar or turbulent flow regimes, each with quite distinct flow characteristics. Early pioneering research on turbulence emanated from the Gottingen group of Ludwig Prandtl, who developed the concept of the boundary layer and proposed a mixing-length turbulence closure, and from the Cambridge group of G. I. Taylor, who provided a theoretical basis for a statistical description of turbulence, with specific results for isotropic turbulence. Important flows, such as those in open channels, boundary layers, and pipes are, however, turbulent shear flows, so the theory of isotropic turbulence cannot be directly applied to such shear flows. The concept of local isotropy, introduced by the Russian group of Kolmogoroff has proved useful in understanding these flows. These theoretical concepts will be briefly reviewed. Experimental research in turbulent boundary-layer and pipe flows has been conducted in air flows since the 1950s using hotwire anemometry. In contrast, most basic research on openchannel turbulence has only been conducted since the 1970s, because of difficulties in applying thermal anemometry to typical water flows in hydraulic laboratories. Since the 1980s, laser anemometry has made experimental studies in open-channel turbulence much less arduous ~though still rather expensive!, permitting detailed investigations of not only basic two-dimensional ~2D! uniform flows, but also, more recently, unsteady and ~3D! channel flows. The following highlights contributions to openchannel turbulence research, particularly those by our group at Kyoto University.

Mechanism of sediment transport is composed of complicated interactions between turbulent flow, particle motion, and bed configurations. Of particular significance is the interaction between turbulence and particle motion, although turbulence measurements of particle-laden two phase flow have been a problem for a long time, especially in the near-wall region. In this study, simultaneous measurements of both the particles and fluid (water) were conducted in particle-laden two phase open channel flows by means of a discriminator particle-tracking velocimetry. The mean velocity and turbulence characteristics for fluid and particles each were examined in comparison with those in clear-water (particle-free) flow, together with previous existing data measured by laser Doppler anemometer and phase Doppler anemometer. The relative velocity and the turbulence modulation, which are the most important topics in two phase-flow approach, were revealed by varying the particle diameter and specific density. The fluid-sweeps are more contributory to the motion of particles than the fluid ejections in the near-wall region. In turn, the particle-sweeps transport the high momentum to the carrier fluid and enhance the turbulence intensities of fluid.

Turbulence measurements over a smooth wall in unsteady depth-varying open-channel flows were conducted by the simultaneous use of a two-component laser Doppler anemometer (LDA) and a water-wave gauge. In the present study, velocity measurements in the viscous sublayer of highly unsteady flow have been first conducted to evaluate the friction velocity independently of the log-law. The von Karman constant κ was calculated using this friction velocity. It was proven then that the value of κ was really a constant of 0.41 even in unsteady open-channel flows. Next, statistical structures of mean velocity and turbulence characteristics were investigated across the whole flow from the near-wall region up to the free surface. In particular, turbulence measurements near the depth-varying water-surface zone of flood flow were conducted successfully for the first time with the LDA; such measurements have been impossible with conventional velocity instruments such as hot-film anemometers. Mean velocity profiles and turbulence characteristics were revealed in both the rising and the falling stages of the flood period.

The hydrodynamic response of turbulent flow in a compound wide rectangular open channel with a vegetated domain at the channel bank, in a channel corner and in a floodplain, to the vegetation density and diameter is discussed. To this end a phenomenological model was imbedded in an algebraic stress model with the vegetation modeled as an internal resistance that exerts drag force, produces energy of turbulence, and interferes with its anisotropy and length scale. Simple suggestions for some modeling problems associated with the presence of wide vegetated domains are used to yield the three-dimensional flow. Vorticity sources relaxation was implemented to smooth periodic behavior, and dissipation correction was introduced as a coordinate-invariant damping. With these, the results represent fully developed flow.

Steps leading to the numerical study of the hydrodynamic behavior of a compound open channel are described. The method uses an algebraic stress model imbedded, together with an energy‐dissipation model, in a parabolic pressure‐correction algorithm. The steps include the distinction between a compound channel and a channel with small longitudinal step in bed level, the applicability of the algebraic stress model to flow along a 270°corner formed between the flood‐plain bed and the main‐channel bank, justification of the use of the flux boundary condition for the dissipation at the free surface of the compound channel, and other topics.

Basic knowledge of turbulent structures in open‐channel flows are now available from accurate experimental data that were obtained using a laser Doppler anemometer in laboratory flumes. Many of these experiments have, however, been conducted at moderate Reynolds numbers. Therefore, it is not yet clear whether such laboratory data of open‐channel turbulence can be applied to river turbulence at high Reynolds numbers. In this study, velocity measurements of the Biwako‐Sosui River in Kyoto and the Aichi Irrigation Channel in Nagoya have been conducted by making use of three‐component electromagnetic flow meters. Multicellular secondary currents were evident in a wide river, whereas large‐scale free‐surface secondary currents associated with velocity‐dip phenomena were generated in a narrow river. The features of such secondary currents in rivers could be explained by theory of turbulence, and also they coincided well with the laboratory experiments and numerical calculations.

The hydrodynamic response of the turbulent flow in a compound rectangular open channel to the flood-plain depth, roughness, and symmetry, and to the channel Reynolds number is demonstrated. The flow in the four basic asymmetric channels measured by Tominaga and Nezu in 1991 is numerically simulated, as well as other channel configurations, including symmetric channels. Turbulence is modeled by the energy-dissipation model, combined with the algebraic stress model suggested by Naot and Rodi in 1982. The three-dimensional flow is solved using the parabolic pressure correction algorithm of Patankar and Spalding. The examples suggest estimations for the friction factors, and the mass exchange rate between the flood plain interior and its threshold.

Velocity measurements with a fiberoptic laserDoppler anemometer were conducted in steep openchannel flows over smooth and incompletely rough beds. The channel slopes were changed between 1500 and 1...

An investigation of three‐dimensional (3‐D) turbulent structure, including turbulence‐driven secondary currents in compound open‐channel flows, is a very important topic in hydraulic and river engineering, as well as in fluid mechanics. In this study, accurate measurements in fully developed compound open‐channel flows are conducted by means of a fiber‐optic laser Doppler anemometer (FLDA). Secondary velocities can be measured very accurately with the present 3‐D measurement system. The characteristics of compound open‐channel flows are recognized in the junction region between the main channel and flood plain, whereas the characteristics of rectangular open‐channel flows are observed in a region near the sidewall of the main channel. Strong, inclined secondary currents, which are associated with a pair of longitudinal vortices, are generated in the junction region between the main channel and the flood plain. The primary mean velocity field is directly influenced by these secondary currents. Turbulence i...

Unstable patterns are identified in the results of numerical simulations of two cases of partly vegetated open channels, at the free unobstructed flow side of the shear layer that separates the unobstructed channel from the vegetated zones. A time-dependent pattern was identified in a rectangular open channel compound with a vegetated bank, and cellular structure was found in a wide ractangular open channel spanning over a vegetated thin layer at the channel bed. Distinction between three modes of algebraic stress models: a prompt algebraic stress model, a delayed algebraic stress model, and an algebraic stress model with longitudinal vorticity sources relaxation, and the modeling of time-dependent turbulence is made, and discussed in the present context.

A powerful two‐color Laser Doppler Anemometer (LDA) system, with direct digital signal processing has been used to measure accurately the longitudinal and vertical velocity components in two‐dimensional, fully‐developed open‐channel flow over smooth beds. The law of the wall and the velocity defect law were re‐examined because the log‐law has been often applied to open channels without detailed verification. It was found that the log‐law can be applied strictly only to the near‐wall region. In this region, the von K´rm´n constant κ and the integral constant A are truly universal, having values of κ=0.412 and A=5.29 irrespective of the Reynolds and Froude number. As the Reynolds number becomes larger, the deviation from the log‐law cannot be neglected in the outer region. This deviation can be expressed well by Coles' wake function which involves a Reynolds‐number dependent parameter Π. The distributions of eddy viscosity and mixing length were evaluated and found to depend on Π. All the data including the...

An investigation of cellular secondary currents, i.e., three-dimensional flow patterns, is very important in hydraulic engineering because these currents might cause three-dimensional sediment distributions and bed configurations such as sand ribbons in straight rivers. It is, however, fairly difficult to measure secondary currents of water flows, even by making use of hot-film anemometers, because the velocity of secondary currents is within 5% of the mainstream velocity. Thus, on the basis of the hypothesis that the existence of free surface may not be an essential cause of secondary currents, the present study has investigated experimentally the turbulent structure of their currents in air conduit. Considering an essential interaction between secondary currents and bedform, longitudinal ridge elements were attached onto both lower and upper bottoms of the conduit, which simulated the longitudinal ridges and troughs of river bedform, i.e., sand ribbons. All three components of the velocity were measured accurately by hotwire anemometers. The structure of secondary currents was examined through the equations of mean flow vorticity and mean flow energy.