A field study of turbulent flows in shallow gravel-bed rivers

The study of turbulent flows has always been a challenge for scientists. Turbulent flows are common in nature and have an important role in several geophysical processes related to a variety of phenomena such as river morphology, landscape modeling, atmospheric dynamics and ocean currents. At present, new measurement and observation techniques suitable for fieldwork can be combined with laboratory and theoretical work in order to advance in the understanding of river processes. In this Ph.D. dissertation, an Acoustic Doppler Velocity Profiler (ADVP) suspended from a deployable structure allowed the investigation of turbulent gravel-bed river flows. The ADVP, which was developed by the Laboratoire d'Hydraulique Environnementale (LHE), permits to obtain over the entire water depth, three-dimensional quasi-instantaneous information on the fluctuating velocity flow field in the production and inertial subranges of the spectral space. Improvements on the ADVP data quality were made with the implementation of a correction methodology for errors due to data aliasing. This reduced the range-velocity ambiguity in Doppler-based instruments. The results presented in this dissertation contribute to the understanding of transport and mixing processes in river flows. They are based on three sets of field measurements made in gravel-bed rivers with blockage ratios of h/D84≈3.0 and aspect ratios B/h between 23 and 32. The measurements were made during low-water periods. The fieldwork provided results on the mean and instantaneous velocity field. The flow was divided into three inviscid vertical layers with different mean field and Reynolds stress characteristics: the roughness layer, the blending or intermediate layer and the surface layer. In the lower layers of the flow three types of mean velocity profile were found: mono-logarithmic, s-shaped due to bed perturbations and double-logarithmic downstream bed perturbations. The determination of the shear stress distribution for each of these profile types is studied. In double-log profiles, the friction velocity and roughness length determined for the outer logarithmic layer are required for the velocity profile parameterization. The s-shaped profiles are described by a tangent-hyperbolic function in the lower layers compatible with an external log layer. Limitations of 2D open-channel theories to parameterize the velocity distribution and to characterize the bottom drag are discussed. Bottom drag occurs in the predominant momentum direction. The direction changes as a function of the local bed forms. To estimate bottom drag one has to consider the actual momentum transport direction which varies with the flow depth. The wall effect of the riverbank is visible until y/h≈5. Bottom topography produces important secondary mean motion in the flow. A permanent structure of the flow was described in the upper layers, near the surface (z/h

An experimental study of coherent structures, secondary currents and surface boils and their interrelation in open-channel flow

Ismail Albayrak

River and open-channel flows are free surface boundary layer flows with complex 3D, large-scale, turbulent structures. The study of 2D and 3D large-scale turbulent flow structures is a great challenge for physicists, mathematicians and engineers from such different domains as civil, environmental and mechanic engineering. Different processes can generate 3D, large-scale, turbulent structures which occur at the same time. On the one hand, large scale vortical structures such as secondary currents of Prandtl's second kind play an important role in the understanding of 3D turbulent structures in straight channels and rivers. Secondary currents affect bottom shear stress and longitudinal mean velocity, and contribute to sediment transport and air-water gas exchange by creating upwelling and downwelling motion in the water column. At the free surface, such upwelling and downwelling motion is an important mechanism for the air-water gas exchange and is considered to be responsible for surface boils. On the other hand, experimental work in turbulent boundary layers revealed the existence of bursting resulting in hairpin shaped structures which are responsible for the link between the inner and the outer layer. The interaction between these two layers in turbulent boundary layers is considered in terms of the dynamics of momentum, energy, and Reynolds shear stress transport. In order to advance in the understanding of this fundamental problem in turbulent open-channel flow, recently developed measurement and observation techniques are used in this Ph.D study. A non-intrusive Acoustic Doppler Velocity Profiler (ADVP), Surface Large Scale Particle Image Velocimetry (LSPIV) and a hot-film probe were combined in the investigation of coherent structures, secondary currents, surface boils and their interaction in turbulent rough-bed open-channel flow. The ADVP permits to measure 3D quasi-instantaneous velocity profiles in the entire water depth and to investigate the mean field and the fluctuating field of all three velocity components. The LSPIV system, developed at the LHE, allows visualizing the water surface and obtaining the surface velocity information in relation to instantaneous surface vortical structures. Bottom shear stress was measured with a sensor based on the hot film principle. The instruments provided the mean and instantaneous velocity field in the entire water depth and at the free surface. Six sets of experiments were carried out in turbulent rough bed open-channel for three different width-to-depth ratios (12.25, 15 and 20) at high, moderate and low Reynolds numbers. The results of the ADVP measurements show mean longitudinal velocity patterns undulating across the channel which indicate patterns of secondary currents in the mean flow structure. Upwelling regions can be identified by lower relative mean longitudinal velocities close to the free surface, and downwelling regions can be identified by higher relative mean longitudinal velocities. It is observed that the existence of secondary currents affects the distribution of bed shear stress and Reynolds stress across the channel. Bed shear stress show a cross-channel undulation pattern with bed shear stress in downwelling areas being higher than in upwelling areas. The Reynolds shear stress distribution in the water column has revealed the same undulating pattern. The number of secondary flow cells is determined by the aspect ratio and relative roughness. It is found that the bottom roughness elements of the channel bed make these longitudinal cells stable. The Reynolds number does not affect the spanwise position of the upwelling and downwelling regions of the secondary cells, but it does affect and slightly increase the normalized Reynolds shear stress. Secondary currents with cells whose dimensions are equal to the flow depth are the most stable and dominant pattern. Changes in the vorticity pattern causes changes in turbulence characteristics in upwelling and downwelling regions. Our study and existing investigations demonstrated that one of the most probable mechanisms for the initiation of multi cellular secondary currents is the mutual interaction between the rough bed and the pre-existing secondary currents near the side wall. The occurrence of small and large scale coherent structures, such as hairpin packets, and their relation to secondary currents are investigated through a quantitative analysis of instantaneous flow fields over the entire turbulent boundary layer across the channel. Uniform momentum zones are clearly detected in the instantaneous velocity fields in the longitudinal direction. In the logarithmic layer, the coherent vortex packets originating from the wall layer frequently occur within larger moving zones of uniform momentum, and extend up to the middle of the boundary layer. Good results in terms of dimension and position of large coherent structures relating to zones of uniform streamwise momentum support the concept of a dynamic link of hairpin packets and zonal organization in the outer layer. Secondary currents are large-scale streamwise vortical structures that affect the organization of coherent structures in the outer layer. More hairpin vortex packets could be carried by upwelling, with positive vertical velocity increasing the height of Zone 2. In the downwelling region, the height of Zone 2 and the growth angle of the hairpin packets decrease compared to the upwelling region, because of the negative vertical mean velocity of the secondary currents. This may prevent hairpin vortex packets from reaching the free surface due to the higher gradient of the longitudinal velocity. A quadrant analysis in the upwelling and downwelling regions revealed that in the wall region of downwelling areas, sweep events are dominant, and in the region close to the free surface, ejection events dominate over sweep events. The dominance of ejection events at the free surface explains the occurrence of a large number of surface boils observed in the upwelling regions. The measurements have shown that secondary currents and coherent structures are correlated, thus producing 3D flow structures. The results from LSPIV show a mean multi-cellular pattern of faster and slower primary longitudinal surface velocities. Streaks of faster longitudinal velocity are found in downwelling areas. Upwelling areas are identified with lower velocities. In addition, we observed mean transversal surface currents between upwelling and downwelling zones. We have shown that near the surface, ejections which are part of the large scale burst cycle are more common in upwelling zones between secondary current cells. Measurements reveal that these vortex boils mainly occur in upwelling areas with high vorticity, whereas downwelling areas show lower vorticity. Up- and downwelling zones, as well as surface boils are observed at all Reynolds numbers and aspect ratios. Therefore, they can be considered important processes in river dynamics and affect transport between the surface and the pelagic zone. Based on the combined results from LSPIV, ADVP and hot-film data, this study experimentally demonstrated that secondary currents, surface boils and coherent structures are correlated and produce 3D flow structures. The effect of secondary currents on tracer distribution in open-channel and river flow is to disperse and mix tracers in three dimensions more rapidly than would be the case if turbulent diffusion were acting alone. This has important consequences for pollutant spreading. Together with surface boil vortices, these currents contribute to surface renewal and gas transfer. In downwelling zones, the water masses moved along the surface by the transversal currents are transported downwards faster than by turbulent mixing. Again, the dispersion and mixing due to secondary currents discussed above will then provide for rapid 3D distribution in the entire water column. This thesis is a contribution to understanding of the transport and mixing dynamics in open-channel flow with an emphasis on the effects of coherent structures, secondary currents and surface boils, as well as the interaction between them. The information which was obtained advances the understanding of fine and large scale dynamics in open-channel flow. At the same time it contributes to the improvement of algorithms in numerical predictive water quality models, which in turn improve effective water resources management.

EPFL2008

Experimental study of suspension flow in open channels

The study of the sediment transport in open-channel as well as in river flow is of great importance for fluvial hydraulics. While the transport of sedimentary particles at the bed, as the bed-load, has been the subject of much research, less attention has been paid to the transport of sediments in suspension. This thesis is a contribution to our knowledge of the transport of sediments in suspension. The most important theory on the vertical distribution of the mean sediment concentration in suspension flows is the diffusion-convection theory, given by the Rouse equation. This equation is rather simple and contains few parameters. About one of these parameters, the β-value, there is little known, despite its importance and physical meaning. One of the main aims of this thesis is to experimentally measure the β-values for different suspension flow configurations. The major objective was to investigate suspension flows over moveable beds without bed forms and this in capacity condition. Since flows are at times not in capacity, i.e. the flows are not saturated with sediments, an additional study (see Appendix C) was also performed. Furthermore, two special runs were done (see Appendix D), where the bed was covered with artificial bed forms. In order to evaluate the β-values, the vertical profiles of the mean velocity and its fluctuations as well as the mean concentration and its fluctuations had to be measured. This could only be done by using the new ultrasonic instrument (Acoustic Particle Flux Profiler, APFP), developed and assembled at the Laboratoire de Recherches Hydrauliques (LRH). Suspension flows were investigated focusing on the possible modification of the clear-water turbulence by suspended sediments. The concentration was carefully measured and interpreted by the Rouse equation. In particular, the ratio of the sediment, εs(y) , and momentum, εm(y) , diffusion coefficients, being the β(y) -value, was for the first time, directly measured. The strongest effect of particles on the flow was noticed on the vertical component of the turbulence intensity, , which was considerably suppressed, when compared to clear-water flow. The longitudinal velocity, u(y) , its turbulence intensity component, , and the Reynolds stress, – ρu'v' , were only slightly affected. By using the APFP instrument, the instantaneous sediment concentrations, cs, were, for the first time, directly measured. The calibration of the APFP instrument was achieved by measuring the mean concentration profiles, cs, with the suction method. The largest measured fluctuating concentrations, , and sediment flux, profiles were observed close to the bed, where the mean concentration is also very large. The sediment, εs, and the momentum, εm, diffusion coefficients, which represent the "ability" of sediment and fluid particles to be diffused in the flow by turbulence, were computed from the APFP measurements. For suspension flows over plane-bed, the sediment diffusion-coefficient profiles are always smaller than the momentum ones, εs < εm. This indicates that sediment particles undergo less diffusion than fluid particles; consequently the β-values are less than unity, β < 1. Thus, the usual assumption of εs = εm, that leads to β = 1, is not justified for the present measurements. The Rouse equation gives a better agreement to the concentration distributions measured with the suction method, by using the experimental obtained β-values, , rather than by using β = 1. For practical use, the experimental values and the ones obtained by a best-fit (least-squares method) of the Rouse equation to some concentration distributions taken from the literature are summarized in a plot. It seems that the β-values increase with either scaling parameters, vss/u*, and (vss/u*) · (Cs/csa). The effects of suspended particles on the clear-water turbulence were also investigated in non-capacity suspension flows having increasing concentrations. The measurements show that, the suppression (damping) of the vertical component of the turbulence intensity – caused by suspended particles – increases with the depth-averaged concentration. The tendency of the - values to decrease approaching the capacity condition, i.e. increasing the concentration, was also found. Suspension flows over bed forms were also investigated; in this study special attention was paid to the evolution of the flow structure behind the bed-form crest as well as the effect of bed forms on the β-values. The bed-form crest seems to generate a high-turbulence region with consequent peaks in both the longitudinal and vertical turbulence-intensity profiles as well as in the Reynolds-stress profiles. This high-turbulence region enhances the sediment diffusion coefficient but suppresses the momentum one. As a consequence, we found that for suspension flows over beds with bed forms, εs > εm, leading to β > 1. The most important result of this thesis is a recommendation that the Rouse equation with an improved β-value – itself to be estimated from the experimental plots obtained in this study – can be used to establish the dimensionless concentration profile of suspension flows. For beds without bed forms the β-values are β < l, while for beds with bed forms the β-values are β > 1. The correlation between the velocity fluctuations, associated with coherent structures (burst cycle), and the concentration was also investigated. From this study it becomes evident that the ejection event, being the most important phase of a burst cycle, is responsible of the erosion of sediment on the bed, behaving as an "injector" of sediments into the main flow.

EPFL1998

High velocity aerated flow on stepped chutes with macro-roughness elements

Stéphanie Joëlle André

Uncontrolled overtopping during flood events can endanger embankment dams. Erosion of the downstream slope and scouring of its base caused by the high velocity and energy of the overflow can indeed lead to breach formation until complete failure. In this context and faced with the important number of overtopped embankment dams to be rehabilitated, since the early eighties, researchers have investigated surface protection solutions for downstream slope. Overlays against erosion such as seeded goetextile or cable-tied cellular concrete blocks, are not sufficient. In fact, they can resist only short events with low discharge and velocity. Solution to overcome more severe overflow lies in overlays which dissipate flow energy along the downstream embankment slope. Conventional steps resulting from Roller Compacted Concrete (RCC) techniques fulfill efficiently this challenge. However, flows over steep stepped chutes are quite complex, characterizing by great aeration, high turbulence and confused wavy free surface. Then, most of hydraulic studies of such flows are performed on physical model. Yet, understanding and definition of flow behaviour and accurate approach to estimate energy dissipation are still lacking. General guidelines of hydraulics of aerated flows over stepped macro-roughness chutes and for optimal design of protection overlay remain confusing. To contribute to reduce these uncertainties, experimental study of flow over stepped chutes equipped with macro-roughness elements is performed in a laboratory gated flume for mild (~ 1:7H : 1V ) and weak (~ 3H : 1V ) chutes. Thus, they are representative of the range of embankment dams and spillways slopes. Three types of stepped macro-roughness overlays are assessed, namely rectangular conventional steps, steps equipped with endsills fixed on their nose over all the flume width and steps equipped with rectangular spaced blocks. Endsills overlays were characterized with different longitudinal distributions whereas blocks overlays consisted in different transverse patterns. Tests were conducted for the three nappe, transition and skimming flow regimes. Results can be extrapolated to 1/5 to 1/15 scaled prototypes using the Froude similarity with negligible scale effects. Flow depth, local air concentration and longitudinal velocities are measured with a double fiber-optical probe. Pressures at macro-roughness faces are taken with piezo-resistive sensors. Sequent depths of the hydraulic jump forced in the stilling basin at the flume base are measured with ultrasound sensors. Thus, this experimental phase of the thesis has allowed: to define flow parameters (regimes, depths, velocity and air concentration distributions, hydrodynamic forces) for tested overlays, to highlight that air-water flow depth is divided into: a rough boundary layer influenced by shear stress and by drag form (macro-turbulence) caused by macro-roughness, a homogeneous aerated layer which represents the main portion of flow involved in energy dissipation mechanism, a free surface layer which must be considered in the side walls design, to stress that energy dissipation is mainly a question of drag losses, to validate indirect method of hydraulic jump for energy dissipation estimation, to estimate relative energy loss for several stepped macro-roughness overlays. Tests finally show that an optimal alternative to dissipate the overflow energy during an overtopping event consists in spaced blocks, with transverse space larger than the width of block and fixed alternately on conventional steps. However, experimental results remain related and limited to their tested domains. Then, in order to provide more general governing equations of aerated flows over macro-roughness stepped chutes, a numerical modeling of two phase flows over conventional stepped flume was performed in collaboration with the Laboratory of Applied Hydrodynamics and Hydraulic Constructions at University of Liège. A quasi-2D numerical model based on the finite volume method was developed. It consists in applying the classical depth-averaged simplified Navier-Stokes equations (viscosity and Coriolis terms neglected) to a 1D incompressible air-water mixture flow over mild and steep slopes with a stepped topography. Self-aeration process is modeled by a transport equation of depth-averaged air concentration whereas turbulent structures are indirectly implemented through the Boussinesq coefficient. This first 1D-approach of semi-theoretical description of aerated flow over steps is tested for a 30o gated stepped flume and a 52° crested spillway laboratory model. This numerical model leads to realistic results regarding mixture depth, mean flow velocity, air concentration and wave amplitudes of the flow free surface. Finally, on the basis of existing protections of embankment dams and previous studies, the present experimental and numerical results contribute to extend the knowledge of high velocity aerated flows over macro-roughness and to provide elements of guidelines to optimize stepped macro-roughness overlays for embankment dams safety.

EPFL2004