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

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.

Turbulent Flows over Rough Permeable Beds in Mountain Rivers: Experimental Insights and Modeling

Gauthier Paul Daniel Marie Rousseau

Steep mountain streams exhibit shallow waters with roughness elements such as stones and pebbles that are comparable in size to flow depth. Owing to the difficulty in measuring fluid velocities at the interface, i.e., from the rough permeable bed to the free surface, experimental results are rare although they are essential to improve models. Using a novel experimental procedure, this thesis attempts to improve predictions of the vertical structure of turbulent flows over rough permeable beds. To explore flows at the bed interface, I devised an experimental set-up where a fluid flowed over glass spheres (8 mm < dp < 14 mm) in a narrow flume (W = 6 cm) with slopes varying from 0.5 % to 8 %. The Refractive Index Matching (RIM) technique has been employed. This involves matching the refractive index of the fluid with that of the glass spheres, thereby allowing the interior of the medium to be examined and velocities to be measured by Particle Image Velocimetry (PIV). Vertical profiles are retrieved by employing the spatiotemporal double averaging method. In the course of this manuscript, flow processes are studied at the mesoscopic scale, i.e., by averaging quantities over distances ranging from 5 to 10 grain diameters. For open-channel flows over rough permeable beds, the spatial averaging procedure yields a continuous porosity profile. When applied to the Navier-Stokes equations, it produces a momentum equation with several terms including drag forces and three stresses: the turbulent, dispersive, and viscous stresses. The momentum equation was employed to devise a one dimensional (1D) model describing the vertical structure of unidirectional turbulent flow. A turbulent boundary layer over the rough bed was observed while experiments were performed at intermediate Reynolds numbers, i.e., Re = O (1000). In such conditions, viscosity plays a critical role through the van Driest damping effect. To model vertical profiles, the Darcy-Ergün equation is well suited to the prediction of friction forces in the permeable bed, i.e., in roughness and subsurface layers. Based on the \textit{Prandtl mixing length theory}, turbulent stress is predicted from a mixing length distribution that considers dispersive effects and assumes a continuous porosity profile. This alternative contrasts with most existing boundary layer models which postulate a discontinuous porosity profile for permeable or impermeable walls. Finally, hydraulic conditions collected by an Unmanned Aerial Vehicle (UAV) and classical flow resistance equations (Chézy, Keulegan, ...) were compared with profile simulations and demonstrate a good agreement between predictions and observations. It reveals the crucial role of fluid depth definition in equations in small submergence conditions. Furthermore, incipient sediment motion conditions have been estimated and compared to empirical results showing the importance of turbulence and lift force for grain entrainment. With regard to fluid dynamics, mountain streams are a case study of the larger scientific family of turbulent flows interacting with porous structures. Insights and developments acquired in the course of this thesis are likely to be transferable to other domains working with these phenomena such as flows over buildings, vegetal canopies or rough wings.

EPFL2019

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

Part load flow in radial centrifugal pumps

Olivier Braun

Centrifugal pumps are required to sustain a stable operation of the system they support under all operating conditions. Minor modifications of the surfaces defining the pump's water passage can influence the tendency to unstable system operation significantly. The action of such modifications on the flow are yet not fully understood, leading to costly trial and error approaches in the solution of instability problems. The part-load flow in centrifugal pumps is inherently time-dependent due to the interaction of the rotating impeller with the stationary diffuser (Rotor-Stator Interaction, RSI). Furthermore, adverse pressure gradients in the pump diffuser may cause flow separation, potentially inducing symmetry-breaking non-uniformities, either spatially stationary or rotating and either steady or intermittent. Rotating stall, characterized by the presence of distinct cells of flow separation on the circumference, rotating at a fraction of the impeller revolution rate, has been observed in thermal and hydraulic turbomachines. Due to its complexity, the part-load flow in radial centrifugal pumps is a major challenge for numerical flow simulation methods. The present study investigates the part-load flow in radial centrifugal pumps and pump-turbines by experimental and numerical methods, the latter using a finite volume discretization of the Reynolds-averaged Navier-Stokes (RANS) equation. Physical phenomena of part load flow are evidenced based on three case studies, and the ability of numerical simulation methods to reproduce part-load flow in radial centrifugal pumps qualitatively and quantitatively is assessed. A numerical study of the flow in a high specific speed radial pump-turbine using steady approaches and the hypothesis of angular periodicity between neighboring blade channels evidences the relation of sudden flow topology changes with an increase of viscous losses, impacting on the energy-discharge characteristic, and thus increasing the risk of unstable operation. When the flow rate drops below a critical threshold, the straight through-flow with flow separation zones attached to the guide vanes changes to an asymmetrical flow. Energy is drawn off the mean flow and dissipated in a large vortex-like structure. Besides flow separation in some diffuser channels, time-dependent numerical simulations of the flow in a double suction pump evidence a flow rate imbalance between both impeller sides interacting with asymmetric flow separation in the diffuser. Viscous losses increase substantially as this imbalance occurs, the resulting segment of positive slope in the energy-discharge characteristic is found for a flow rate sensibly different from measurements. Different modes of rotating stall are identified by transient pressure measurements in a low-specific-speed pump-turbine, showing 3 to 5 zones of separated flow, rotating at 0.016 to 0.028 times impeller rotation rate, depending on discharge. For operating conditions where stall with 4 cells is most pronounced, velocity is measured by Laser-Doppler methods at locations of interest. The velocity field is reconstructed with respect to the passage of stall cells by definition of a stall phase obtained from simultaneous transient pressure measurements. Time-dependent numerical simulation predicting rotating stall with 4 cells shows velocity fields that are in reasonable agreement with the measured velocity fields, but occurring at a sensibly higher flow rate than found from experiments. In consideration of the quantitative shortcomings of the numerical simulation, a novel modelling approach is proposed: Replacing the costly 3-dimensional simulation of the major part of the impeller channels by a 1-dimensional model allows a significant economy in computational resources, allowing an improved modeling for the remainder of the domain at constant computational cost. The model is validated with the challenging cases of rotating stall and impeller side flow rate imbalance. The satisfying coherence of the results with the simulation including the entire impeller channels qualifies this approach for numerous turbomachinery applications. It could also provide improved, time-dependent boundary conditions for draft tube vortex rope simulations at reasonable computational cost. Parameter studies modifying deliberately some quantities of mean flow and turbulence at the modeled boundary surfaces can be implemented in the framework of the method.

EPFL2009