Alexander N. Sukhodolov

Field investigation of three-dimensional flow structure at stream confluences: 1. Thermal mixing and time-averaged velocities

Stream confluences are characterized by complex patterns of three-dimensional fluid motion. This paper examines the three-dimensional time-averaged flow structure at three concordant-bed confluences in east central Illinois. Two of the junctions have symmetrical planforms, whereas the other has an asymmetrical planform. Similarities among the sites include (1) pronounced convergence of flow at the upstream end of the confluences, (2) a region of stagnated fluid near the upstream junction corner, (3) a well-defined thermal mixing interface between the converging flows that persists downstream of each confluence, (4) a downstream velocity field characterized by two zones of maximum velocity separated by an intervening region of low velocity centered on the mixing interface, (5) convective acceleration of flow within the mixing interface leading to increasing uniformity of the downstream velocity field in the downstream direction, and (6) lateral deflection of flow by the dominant tributary. Prominent helical motion occurs at the asymmetrical confluence, whereas weak helicity is detectable only at one of the two symmetrical confluences. The downstream persistence of a well-defined mixing interface at the two symmetrical confluences and the disruption of this interface at the asymmetrical confluence suggest that helical motion enhances patterns of mixing at confluences.

Turbulence structure in a river reach with sand bed

Measurements and analysis of the three-dimensional turbulence structure in a straight lowland river reach are presented. Accurate measurements of velocity profiles were taken with an acoustic Doppier velocimeter (ADV) and a multichannel micropropeller-based system. It is shown that analytical expressions derived from investigations of laboratory open-channel flows agree well with the measured data only in the central part of the river where flow can be considered weakly three-dimensional. At the same time, clear differences of empirical parameters between river and laboratory flows are detected. It was established that river turbulence is isotropic for spatial scales smaller than the river depth. The data also allowed the detection and analysis of the turbulence anisotropy impact on the formation of secondary currents. Special attention is paid to the investigation of coherent structures. Their spatial scales were evaluated applying the conditional-average procedure based on uw quadrant analysis. On average the scales of ejection and sweep events are as large as 1.5 times the flow depth.

Field investigation of three-dimensional flow structure at stream confluences 2. Turbulence

Stream confluences are among the most highly turbulent locations in fluvial systems. This paper examines the three-dimensional structure of turbulence at three stream confluences in east central Illinois. The analysis focuses on the characteristics of turbulence both within the shear layer and in the ambient flow. Results show that at the upstream end of each confluence the shear layer occupies a limited portion of the flow cross-sectional area, but turbulence kinetic energy within this layer is 2–3 times greater than the turbulence kinetic energy of the ambient flow, which has turbulence characteristics similar to those for flow in straight channels. Turbulence within the shear layer can be characterized as quasi-two-dimensional in the sense that large-scale turbulence generated by transverse shear is predominantly two dimensional, whereas small-scale turbulence associated with bed friction is three dimensional. Spectral analysis suggests that the structure of fluid motion within the shear layer differs for confluences with symmetrical versus asymmetrical planforms. The shear layer dissipates rapidly as flow enters the downstream channel, even though a well-defined mixing interface persists at downstream locations.

Statistical sand wave dynamics in one-directional water flows

Moving sand waves and the overlying tubulent flow were measured on the Wilga River in Poland, and the Tirnava Mica and Buzau Rivers in Romania. Bottom elevations and flow velocities were measured at six points simultaneously by multi-channel measuring systems. From these data, the linear and two-dimensional sections of the three-dimensional correlation and structure functions and various projections of sand wave three-dimensional spectra were investigated. It was found that the longitudinal wavenumber spectra of the sand waves in the region of large wavenumbers followed Hinos −3 law ( S ( K x ) ∝ K −3 x ) quite satisfactorily, confirming the theoretical predictions of Hino (1968) and Jain & Kennedy (1974). However, in contrast to Hino (1968), the sand wave frequency spectrum in the high-frequency region was approximated by a power function with the exponent −2, while in the lower-frequency region this exponent is close to −3. A dispersion relation for sand waves has been investigated from analysis of structure functions, frequency spectra and the cross-correlation functions method. For wavelengths less than 0.15–0.25 of the flow depth, their propagation velocity C is inversely proportional to the wavelength λ. When the wavelengths of spectral components are as large as 3–4 times the flow depth, no dispersion occurs. These results proved to be in good qualitative agreement with the theoretical dispersion relation derived from the potential-flow-based analytical models (Kennedy 1969; Jain & Kennedy 1974). We also present another, physically-based, explanation of this phenomenon, introducing two types of sand movement in the form of sand waves. The first type (I) is for the region of large wavenumbers (small wavelengths) and the second one (II) is for the region of small wavenumbers (large wavelengths). The small sand waves move due to the motion of individual sand particles (type I, C ∝λ −1 ) while larger sand waves propagate as a result of the motion of smaller waves on their upstream slopes (type II, C ∝λ 0 ). Like the sand particles in the first type, these smaller waves redistribute sand from upstream slopes to downstream ones. Both types result in sand wave movement downstream but with a different propagation velocity. The main characteristics of turbulence, as well as the quantitative values characterizing the modulation of turbulence by sand waves, are also presented.

Aquatic interfaces: a hydrodynamic and ecological perspective

ABSTRACT Ecologically-appropriate management of natural and constructed surface water bodies has become increasingly important given the growing anthropogenic pressures, statutory regulations, and climate-change impacts on environmental quality. The development of management strategies requires that a number of knowledge gaps be addressed through interdisciplinary research efforts particularly focusing on the water-biota and water-sediment interfaces where most critical biophysical processes occur. This paper discusses the current state of affairs in this field and highlights potential paths to resolve critical issues, such as hydrodynamically-driven mass transport processes at interfaces and associated responses of organisms through the development of traits. The roles of experimental methods, theoretical modelling, statistical tools, and conceptual upscaling methods in future research are discussed from both engineering and ecological perspectives. The aim is to attract the attention of experienced and emerging hydraulic and environmental researchers to this research area, which is likely to bring new and exciting discoveries at the discipline borders.

Case Study: Effect of Submerged Aquatic Plants on Turbulence Structure in a Lowland River

Interactions of aquatic plants with turbulent flows in fluvial systems have attracted considerable interest. While there have been recent advances in theories that describe vegetation-flow interactions in idealized laboratory flows, their practical application is still problematic due to limited knowledge of effects caused by heterogeneously (patchy) distributed plants in naturally formed vegetative mosaics in rivers. This paper reports on a study in a lowland river, aimed at quantification and parameterization of vegetation effects on redistribution of mean and turbulent characteristics of the flow and their consequences for hydraulic resistance. The measurements were carried out in summer on a river reach with a patchy mosaic dominated by submerged flexible aquatic plants and repeated at the same water level in early spring before the plants start growing. This design of the study allowed for quantitative evaluation of the effects caused by flow-plants interactions on bulk flow parameters at comparable submergences of riverbed roughness elements (sediment grains and sand bars). The study indicates that symmetrical quasi-two-dimensional open-channel flow structure in unvegetated riverbed was transformed into highly fragmented complex flow pattern spatially arranged by patches and free paths in the mosaic. Despite complexities and three dimensionality of the flow, normalized mean velocity profiles in the patches were satisfactory described by hyperbolic tangent function while flow in the free paths, similarly to unvegetated channel, was in a reasonable agreement with the conventional logarithmic law.

Experimental and numerical validation of the dead-zone model for longitudinal dispersion in rivers

Experimental results of longitudinal dispersion in rivers are discussed in the light of new findings with respect to the “dead-zone” theory. Both the experiments made by authors as well as some literature results are taken into consideration. The dead-zone model parameters, namely the relative dead-zone volume, penetration time of the tracer into dead-zones, constant mean flow velocity and dispersion coefficient, are obtained with the use of the nonlinear least square technique applied to the image functions of concentration time-distributions. The statistical moments of the concentration time distributions as functions of distance are also analyzed. Expressions for statistical moments, recently obtained by authors, are found to agree well with both experimental and computational results.

Dynamics of shallow lateral shear layers: Experimental study in a river with a sandy bed

Shallow lateral shear layers forming between flows with different velocities, though essential for mixing processes in natural streams, have been examined only in laboratory settings using smooth, fixed?bed channels. This paper reports the results of an experimental study of a shear layer in a straight reach of a natural river where the layer, in contrast to the two?dimensional structure observed in the laboratory, is highly three?dimensional. The study included pronounced transverse pressure gradients, which influenced shear layer structure compared to flume experiments. It also introduces an analysis that complements conventional theory on mixing layers. The lateral velocity gradient between the flows downstream from a splitter plate placed in the river, the principal controlling factor, was adjusted for three experimental runs to determine the influence of different gradients on shear?layer dynamics. In each run, detailed three?dimensional measurements of mean and turbulent characteristics were obtained at five cross sections downstream from the splitter plate. Although experimental results agreed with conventional mixing?layer theories with respect to turbulence, the dynamics of the shear layers were dominated by the mean lateral fluxes of momentum. After re?examining the governing equations, we developed a parabolic equation describing the shear layer evolution and several scaling relations for essential terms of the energy budget: mean and turbulent lateral fluxes of momentum, turbulent kinetic energy, and dissipation rates. The study also provides insight into the spectral dynamics of turbulence in the shear layer and clarifies previous observations reported for confluences in natural streams.

Numerical evaluation of the effects of planform geometry and inflow conditions on flow, turbulence structure, and bed shear velocity at a stream confluence with a concordant bed

This study numerically investigates the effects of variations in inflow conditions and planform geometry on large-scale coherent flow structures and bed friction velocities at a stream confluence with natural bathymetry and concordant bed morphology. Several numerical experiments are conducted in which either the Kelvin-Helmholtz mode or the wake mode dominates within the mixing interface (MI) between the two confluent streams as the junction angle and alignments of the tributaries are altered. In the Kelvin-Helmholtz mode, the MI contains mostly corotating vortices driven by the mean transverse shear across the MI, while in the wake mode the MI contains counterrotating vortices forming by the interaction of the separated shear layers on the two sides of a zone of stagnant fluid near the junction corner. A large angle between the two incoming streams is not necessary for the development of strongly coherent streamwise-oriented vortical (SOV) cells in the immediate vicinity of the MI. Results show that such SOV cells can develop and produce high bed friction velocities even for cases with a low angle between the two tributaries and for cases where the downstream channel is approximately aligned with the axes of the two tributaries (low-curvature cases). SOV cells tend not to develop only when the incoming streams are parallel and aligned with the downstream channel (junction angle of zero), and the incoming flows produce a strong Kelvin-Helmholtz mode. Under such conditions, quasi 2-D MI vortices play the primary role in mixing and the production of high bed shear velocities. Simulations with and without natural bed morphology/local bank line irregularities indicate that planform geometry and inflow conditions primarily govern the development of coherent flow structures, but that bathymetric and bank line effects can locally modify details of these structures.

Assessment of a River Reach for Environmental Fluid Dynamics Studies

Turbulence is the fundamental mechanism governing energy transfer in river flows that was conventionally examined in laboratory flumes. Recently, a trend has been observed for constructing larger scale and outdoor facilities that tend to avoid the problems of upscaling of experimental results. This paper presents the results of an experimental study performed on a river reach used as an environmental field laboratory. The study is focused on the understanding of the spatial arrangement of the flow structure and its dependency on the temporal variability of the flow. Detailed measurements were taken using acoustic Doppler velocimeters and their analysis was completed applying the theory of open-channel flows. The obtained results reveal that the flow structure on the river reach resembles characteristics of a typical three-dimensional open-channel flow. Away from the riverbanks, the flow behaves as a quasi-two-dimensional fully developed turbulent open-channel flow thus providing conditions favorable for field experimental studies of shallow mixing layers and flows over patches of submerged aquatic plants. An interesting observation in the seasonal dynamics of turbulent shear stress patterns was that the height of the roughness layer was reduced in the central part of the flow, though the overall roughness coefficient was increased. At the same time, the structure of the secondary flow near the banks was also substantially altered as the secondary circulations observed at low water levels were replaced by flow separation and internal boundary layers at medium water levels.