The role of coherent structures in the production and dissipation of turbulence in a boundary layer is characterized, summarizing the results of recent investigations. Coherent motion is defined as a three-dimensional region of flow where at least one fundamental variable exhibits significant correlation with itself or with another variable over a space or time range significantly larger than the smallest local scales of the flow. Sections are then devoted to flow-visualization experiments, statistical analyses, numerical simulation techniques, the history of coherent-structure studies, vortices and vortical structures, conceptual models, and predictive models. Diagrams and graphs are provided.

Coherent Motions in the Turbulent Boundary Layer

Aquatic plants convert mean kinetic energy into turbulent kinetic energy at the scale of the plant stems and branches. This energy transfer, linked to wake generation, affects vegetative drag and turbulence intensity. Drawing on this physical link, a model is developed to describe the drag, turbulence and diffusion for flow through emergent vegetation which for the first time captures the relevant underlying physics, and covers the natural range of vegetation density and stem Reynolds' numbers. The model is supported by laboratory and field observations. In addition, this work extends the cylinder-based model for vegetative resistance by including the dependence of the drag coefficient, CD, on the stem population density, and introduces the importance of mechanical diffusion in vegetated flows.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1998WR900069

Drag, turbulence, and diffusion in flow through emergent vegetation

/pdf/evidence-of-very-long-meandering-features-in-the-logarithmic-2m2f4f0daq.pdf

Evidence of very long meandering features in the logarithmic region of turbulent boundary layers

A simple raster-based model for flood inundation simulation

This paper argues that the active turbulence and coherent motions near the top of a vegetation canopy are patterned on a plane mixing layer, because of instabilities associated with the characteristic strong inflection in the mean velocity profile. Mixing-layer turbulence, formed around the inflectional mean velocity profile which develops between two coflowing streams of different velocities, differs in several ways from turbulence in a surface layer. Through these differences, the mixing-layer analogy provides an explanation for many of the observed distinctive features of canopy turbulence. These include: (a) ratios between components of the Reynolds stress tensor; (b) the ratio K H /K M of the eddy diffusivities for heat and momentum; (c) the relative roles of ejections and sweeps; (d) the behaviour of the turbulent energy balance, particularly the major role of turbulent transport; and (e) the behaviour of the turbulent length scales of the active coherent motions (the dominant eddies responsible for vertical transfer near the top of the canopy). It is predicted that these length scales are controlled by the shear length scale L s = U(h)/U′(h) (where h is canopy height, U(z) is mean velocity as a function of height z, and U′ = dU/dz). In particular, the streamwise spacing of the dominant canopy eddies Λ x = mL s , with m = 8.1. These predictions are tested against many sets of field and wind-tunnel data. We propose a picture of canopy turbulence in which eddies associated with inflectional instabilities are modulated by larger-scale, inactive turbulence, which is quasi-horizontal on the scale of the canopy.

Coherent eddies and turbulence in vegetation canopies: The mixing-layer analogy

A review of open channel turbulence, focusing especially on certain features stemming from the presence of the free surface and the bed of a river. Part one presents the statistical theory of turbulence; Part two addresses the coherent structures in open-channel flows and boundary layers.

Turbulence in open-channel flows

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...

Open‐channel Flow Measurements with a Laser Doppler Anemometer

In this paper we intend to predict the magnitude of the contribution to the Reynolds stress of bursting events: ‘ejections’, ‘sweeps’, ‘inward interactions’ and ‘outward interactions’. We shall do this by making use of the conditional probability distribution of the Reynolds stress − uv, which can be derived by applying the cumulant-discard method to the Gram-Charlier probability distribution of the two variables u and v. The Reynolds-stress fluctuations in openchannel flows over smooth and rough beds are measured by dual-sensor hot-film anemometers, whose signals are conditionally sampled and sorted into the four quadrants of the u, v plane by using a high-speed digital data processing system.We shall verify that even the third-order conditional probability distribution of the Reynolds stress shows fairly good agreement with the experimental results and that the sequence of events in the bursting process, i.e. ejections, sweeps and interactions, is directly related to the turbulent energy budget in the form of turbulent diffusion. Also, we shall show that the roughness effect is marked in the area from the wall to the middle of the equilibrium region, and that sweeps appear to be more important than ejections as the roughness increases and as the distance from the wall decreases.

Prediction of the contributions to the Reynolds stress from bursting events in open-channel flows

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...

Turbulent Structure in Compound Open‐Channel Flows

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.

Iehisa Nezu

Papers

Turbulence in open-channel flows

Open‐channel Flow Measurements with a Laser Doppler Anemometer

Prediction of the contributions to the Reynolds stress from bursting events in open-channel flows

Turbulent Structure in Compound Open‐Channel Flows

Open-Channel Flow Turbulence and Its Research Prospect in the 21st Century