Papers by Frederic Moulin
Journal of Geophysical Research: Earth Surface, 2019
In steep rivers, sediment is often transported over immobile cobbles and boulders. Previous studi... more In steep rivers, sediment is often transported over immobile cobbles and boulders. Previous studies of such conditions have observed that the entrainment rate of the mobile sediment strongly depends on the level of protrusion of the immobile grains. Here experiments are conducted in a laboratory flume in order to quantify how different levels of protrusion of large aggregates above a fixed fine-sediment bed, modeled as a patch of hemispheres, modify the local hydrodynamics near the fine-sediment bed. Five protrusion levels defined by P = k∕R = {0%, 20%, 40%, 60%, 80%} were investigated, where k is the protrusion height and R the radius. For small protrusion (P = 20%), enhanced shear stress and turbulence intensity on the fine-sediment bed is observed as the mixing layer generated at the hemisphere top impacts the fine-sediment bed. Moreover, sweep events generated near the top of the hemispheres reach the fine-sediment bed. For large protrusions (P ≥ 60%), the mixing layer generated near the top of the hemispheres does not reach the fine grains with the consequence that the shear stress drops. The remaining turbulence near the fine-sediment bed, although enhanced by the wakes generated by the hemispherical caps, is quasi-isotropic. The transition between these two distinct near-bed flow regimes is found to be around P = 40%, corresponding to the protrusion levels observed by Grams and Wilcock (2014,
E3S Web of Conferences, 2018
River beds frequently exhibit a lateral variation of roughness. For example, in the case of an ov... more River beds frequently exhibit a lateral variation of roughness. For example, in the case of an overflowing river, the main channel has a smoother topography compared to the adjacent floodplains where vegetation and land occupation yield an important hydraulic roughness. The lateral difference in roughness can induce a high lateral velocity gradient within the river cross- section that gives birth to a mixing layer. This mixing layer leads to fluid and momentum transfers between the two adjacent beds. To understand such mix- ing processes in rivers is important for predicting stage-discharge relationships and the velocity distribution within the cross-section. In order to address these issues in the context of a shallow water flow with a water depth h of the same order as the roughness elements of the bed, experiments were undertaken in a 26 m long and 1.1 m wide glass-walled open-channel flume. One half-side of the bed was covered with an array of cubes of height k arranged in a squ...
Physical Modeling of Water, Fauna and Flora: A Foresight Study for Ecohydraulics
E3S Web of Conferences, 2018
Multi-plane PIV measurements were performed in an open-channel flume filled with elongated prisms... more Multi-plane PIV measurements were performed in an open-channel flume filled with elongated prisms of height k and width l to investigate the effect of the deepening of the canopy on the flow structure. Velocity measurements were performed both inside the canopy and above it. Analysis of the spatial convergence for the double-averaged quantities shows that for canopy flow investigations (z < k), at least 5 measurement planes are required to obtain a relative spatial convergence error below 3% for the dispersive shear stress, the quantity the most sensible to spatial sampling. With only three measurement planes, the spatial convergence is below 1% only in the flow region above the canopy (z > k). Three canopy aspect ratios, k/l = [1, 3, 6] were investigated for a fixed modified-submergence ratio β = (h - k)=l = 3 where h is the water depth. As the canopy deepens, the hydraulic roughness decreases and the velocity near the bottom of the canopy becomes gradually constant, as expec...
Open Archive Toulouse Archive Ouverte (OATAO)
To cite this version: Lionard, Marie and Azémar, Frédéric and Boulêtreau, Stéphanie and Muylaert,... more To cite this version: Lionard, Marie and Azémar, Frédéric and Boulêtreau, Stéphanie and Muylaert, Koenraad and Tackx, Micky and Vyverman, Wim Grazing by meso- and microzooplankton on phytoplankton in the upper reaches of the Schelde estuary
Stereoscopic PIV measurements in turbid waters on natural beds
ABSTRACT
Vortex dipole evolution in shallow water
We present here results of a study of three-dimensional aspects of nominally two-dimensional vort... more We present here results of a study of three-dimensional aspects of nominally two-dimensional vortex dipoles in a shallow-water configuration. Such structures have been shown to result from transient rip currents for example (see Johnson et al (2006)). A recent study by Sous et al (2005) has shown that impulsive turbulent jets in a shallow-water configuration generate quasi-two dimensional vortex dipoles through an inverse energy-cascade process and observed via a flow visualization method a spanwise vortex at the front of the vortex dipole. The vertical motions associated with such spanwise vortices are expected to modify the sediment transport and put in suspension characteristics induced by the dipole structure. The origin and characteristics of this secondary vortex are however still unknown. Thus, aiming ultimately for a better understanding of the transport properties associated with these structures, our objective is to investigate the three-dimensional structure and its tempo...
Boundary layers and instabilities of a suction vortex
Velocity field measurements and visualization is applied to a sink vortex of water (with a free s... more Velocity field measurements and visualization is applied to a sink vortex of water (with a free surface on top) produced in a cylindrical tank rotating about the vertical axis. The controlling parameters are the rotating rate of the tank (0.2 and 0.4 rad s−1) and the volume flux of a water withdrawn from a hole at the center of the bottom (50 and 150 cm3 s−1). It is found that a Rankine-like vortex is produced in the steady state when the volume flux of a water withdrawn from the hole is large. When the volume flux is small, however, the conservation of the angular momentum around the central axis of the vortex does not established. A simple theoretical model that assumes Ekman boundary layer on bottom plate turns out to reproduce successfully the nonconservation of angular momentum. The injection of fluorescence dye at the periphery of the cylindrical tank reveals that the water introduced at the periphery of the tank descends to the bottom along the side wall and flows to the cent...
Experimental Observations of Wave-vortex Interactions In A Stratified Rotating Fluid
We present the results of an experimental investigation on the interactions between inertia-gravi... more We present the results of an experimental investigation on the interactions between inertia-gravity waves with a single cyclonic vortex. The experiments were conducted in a 1meter size tank containing a rotating stratified fluid, of which the ratio buoyancy frequency to rotation frequency, N/f, was varied between 2 and 9. The vortices ranged from baroclinic to close to barotropic and were generated using different ways of forcing. The vortex was perturbed by planar internal waves generated by the vertical oscillation of a horizontally placed circular cylinder. The wave propagation in the vortex velocity field was well reproduced by a numerical simulation based on WKB approximation. For baroclinic vortices, the waves propagating in the same direction as the azimuthal velocity were trapped near the radius of maximum velocity without breaking due to viscous damping. In some cases, the waves triggered oscillating modes of the vortex. For barotropic vortices, waves opposing the the veloc...
Experimental study on wave-vortex interaction in a rotating stratified fluid
Experimental Study On The Mixing Induced By Inertia-gravity Wave Breaking On The Dynamical Barrier of A Vortex
We present the results of an experimental investigation on the material transport across the shea... more We present the results of an experimental investigation on the material transport across the shear zone of a cyclonic vortex induced by the breaking of inertia-gravity waves. As has been suggested by McIntyre (1995) this could be a possible mechamism of ozone transport across the dynamical barrier of the polar vortex. The experiments were conducted in a 1 meter size tank containing a rotating stratified fluid. Barotropic vortices were generated by siphoning off fluid with a long perforated tube and per- turbed by planar internal waves generated by the vertical oscillation of a horizontal circular cylinder. As predicted by theorical results based on the WKB approximation, the waves opposing the vortex velocity field were trapped in the outer edge of the vor- tex. In some cases, the increase of wave energy in this region was strong enough to lead to 3-dimensional breaking of the wave pattern. Experimental visualization tech- niques were used to determine the class of instability respo...
Material transport during wave-vortex interactions in rotating stratified fluids
Many geophysical flows are dominated by the presence of large-scale coherent vortices surrounded ... more Many geophysical flows are dominated by the presence of large-scale coherent vortices surrounded by an intense wave field (e.g the polar vortex and meso-scale vortices in the ocean). We investigate whether these waves alter the transport along and across potential-vorticity barriers of one or multiple vortices. We carried out experiments and numerical simulations for flows in rotating-stratified fluids. First we consider the interaction of inertia-gravity wave with a single baroclinic cyclonic vortex. When waves propagate in the same direction as the vortex velocity field, the wave energy is trapped in the outer edge of the vortex while waves opposing the velocity field are refracted. These effects were well predicted by a WKB simulation of wave propagation in an axisymetric velocity field which results in amplification of wave energy in the inviscid case, and due to viscous effects may lead to mixing. As a next step to model geophysical turbulent flows, we investigated the mixing p...
Experimental study of the impact of biogenic macrorugosities on the benthic boundary layer
The turbulent boundary layer over a bed of Crepidula Fornicata shells, representative of typical ... more The turbulent boundary layer over a bed of Crepidula Fornicata shells, representative of typical biogenic macrorugosites, is investigated experimentally in an hydraulic flume by PIV and combined PLIF-PIV techniques for different frontal densities lf. Double-averaged profiles of longitudinal velocity and turbulent stress tensor share similarities with profiles over urban canopies. Values of the parameters describing such profiles (roughness length z0, displacement height d, friction velocity u* and attenuation coefficient of the canopy flow a) are deduced from our experiments for flows above beds covered with shells. Measurements of the turbulent Schmidt number are performed that show that values close to unity are found near the canopy. This result justifies the use of an exchange velocity to describe the exchanges between the canopy and the flow above. Typical values of this exchange velocity are given for the flows over shells investigated here.
Computational Methods in Multiphase Flow VI, 2011
The transport of solid bodies (e.g. algae, debris or sediment grains) along a coastline is a nece... more The transport of solid bodies (e.g. algae, debris or sediment grains) along a coastline is a necessary consideration for the sustainable management of beaches and coasts, including any structure built along the shoreline. The use of a stochastic transport model allows to take into consideration a wide scale of physical processes, such as the current around a coastal structure, the turbulence generated by the flow and the effects of inertia and drag of each body. In order to validate the developed model, in view of industrial applications, a set of experiments was performed. The objective of the experiments was to validate to numerical model in isotropic turbulence. The experiment consisted of oscillating grid generated turbulence, inside which spherical particles were released. Measurements were done using PIV to quantify the turbulence and video particle tracking to measure displacement. The experimental result were then compared to various numerical simulations.
Experimental study of a river biofilm growth on artificial cobbles in contrasted flow conditions
Proceedings of the International Workshop on Environmental Hydraulics, IWEH09, 29 & 30 October 2009, Valencia, Spain, 2009
Peer reviewe
Journal of Hydraulic Research, 2014
Physical modelling is a key tool for generating understanding of the complex interactions between... more Physical modelling is a key tool for generating understanding of the complex interactions between aquatic organisms and hydraulics, which is important for management of aquatic environments under environmental change and our ability to exploit ecosystem services. Many aspects of this field remain poorly understood and the use of physical models within eco-hydraulics requires advancement in methodological application and substantive understanding. This paper presents a review of the emergent themes from a workshop tasked with identifying the future infrastructure requirements of the next generation of eco-hydraulics researchers. The identified themes are: abiotic factors, adaptation, complexity and feedback, variation, and scale and scaling. The paper examines these themes and identifies how progress on each of them is key to existing and future efforts to progress our knowledge of eco-hydraulic interactions. Examples are drawn from studies on biofilms, plants, and sessile and mobile fauna in shallow water fluvial and marine environments. Examples of research gaps and directions for educational, infrastructural and technological advance are also presented.
Journal of Fluid Mechanics, 2006
In this paper we present ray-tracing results on the interaction of inertia-gravity waves with the... more In this paper we present ray-tracing results on the interaction of inertia-gravity waves with the velocity field of a vortex in a rotating stratified fluid. We consider rays that interact with a Rankine-type vortex with a Gaussian vertical distribution of vertical vorticity. The rays are traced, solving the WKB equations in cylindrical coordinates for vortices with different aspect ratios. The interactions are governed by the value of FrR/λ where Fr is the vortex Froude number, R its radius, and λ the incident wavelength. The Froude number is defined as Fr = U max /(NR) with U max the maximum azimuthal velocity and N the buoyancy frequency. When FrR/λ > 1, part of the incident wave field strongly decreases in wavelength while its energy is trapped. The vortex aspect ratio, H/R, determines which part of this incident wave field is trapped, and where its energy accumulates in the vortex. Increasing values of FrR/λ are shown to be associated with a narrowing of the trapping region and an increase of the energy amplification of trapped rays. In the inviscid approximation, the infinite energy amplification predicted for unidirectional flows is retrieved in the limit FrR/λ → ∞. When viscous damping is taken into account, the maximal amplification of the wave energy becomes a function of FrR/λ and a Reynolds number, Re wave = U 2 L + U 2 H /νk 2 , with U L and U H typical values of the shear in, respectively, the radial and vertical directions; the kinematic viscosity is ν, and the wavenumber k, for the incident waves. In a sequel paper, we compare WKB simulations with experimental results.
Journal of Fluid Mechanics, 2004
We investigate the response of a rotating stratified fluid to the local spin-up by a small rotati... more We investigate the response of a rotating stratified fluid to the local spin-up by a small rotating disk of radius R, with Rossby number Ro = ω d /2Ω around unity where ω d is the rotating-disk vorticity and Ω the background rotation frequency. During an initial stage τ su = O(E k −1/2 N −1) with Ekman number, E k = ν/ΩR 2 (ν the kinematic viscosity and N the buoyancy frequency), fluid ejected by the Ekman boundary layer mixes with ambient fluid, and forms an intermediate-density intrusion the radial spreading of which is arrested by background rotation. This flow resembles a concentric source-sink configuration with the sink represented by the Ekman layer above the disk and the source by the ejected fluid, which, by conservation of potential vorticity, leads to the formation of a cyclonic vortex embedded in an anti-cyclonic ring. In the next stage, the radial and axial diffusion of momentum dominate the flow evolution, and the flow is characterized by a balance between viscous dissipation of momentum and the amount of momentum applied by the rotating disk. Vorticity diffusion dominates the flow and smooths out the flow history when E −1/2 k (f/N) < 3, whereas the initial stage can be recognized as a separate flow stage when E −1/2 k (f/N) > 3. The stability of the density front is discussed.
Experimental study of the interaction between a turbulent flow and a river biofilm growing on macrorugosities
ABSTRACT In fluvial ecosystems dominated by fixed biomass at the bottom, the epilithic biofilm im... more ABSTRACT In fluvial ecosystems dominated by fixed biomass at the bottom, the epilithic biofilm imust be included in numerical modellings of biogeochemical fluxes. Among the external factors that control the growth of this biofilm, the hydrodynamics is essential since involved in many relevant processes (colonization, nutrients fluxes, sloughing). Retroactively, its growth modifies the bottom boundary condition for the turbulent flow, yielding a complex interaction that motivated many studies in the last decades. At the river scale, an equation of evolution for the epilithic biomass derived formerly by Uehlinger (1996) was applied successfully to the Garonne river (Boulêtreau et al 2006) : an optimal submodel was used that includes an exponential growth term counterbalanced by an erosion term proportional to the water discharge. However, Godillot et al. (2001) showed that horizontally averaged characteristics of the bottom turbulent boundary layer (shear velocity u * , roughness length z 0 and displacement height d) are more relevant to predict the epilithic biomass dynamics than vertically integrated quantities like the water discharge. Then, the retroaction of the biofilm growth on the flow appears through an evolution of z 0 and d with the amount of biofilm biomass. Recently, some bottlenecks for these approaches have been put in evidence : firstly, a description of the biofilm evolution through its sole biomass is not always appropriate (like for the autogenic sloughing that requires a separate description of the bacterial biomass, see Boulêtreau et al. 2006) ; secondly, the horizontally averaged parameters u * , z 0 and d do not represent necessarily the local hydrodynamical conditions felt by the biofilm during its growth on natural macrorugosities like cobbles. The flow around the macrorugosities and the exchanges between this region and the free stream above are clearly related to horizontally averaged quantities u * , z 0 and d, but through non-trivial relationships as shown e.g. in Moulin et al 2007. In the present study, we investigated how the growth of epilithic biofilm on macrorugosities and its impact on turbulence characte-ristics differ from growth on beds covered with small substrates. We also studied the selection by the sole hydrodynamics of different biofilms and its consequences on the evolution of the turbulent boundary layer. Finally, we quantified how different hydrodynamical conditions during the growth of the biofilm yielded different behaviors during exposition to higher flow velocities. To achieve these objectives, experiments of epilithic biofilm -turbulent flow interactions were performed in an 11-m long by 50-cm wide by 20-cm deep laboratory flume adapted from the one described in Godillot et al. (2001), covered with artificial cobble macrorugosities (20 mm radius hemispheres, see figure 1). In a first experiment, stable hydrodynamical conditions were maintained during 4 months and monitored by Laser Doppler Velocimetry (LDV) measurements along parallel vertical profiles. The biological and chemical parameters were monito-red by regular samplings of the biofilm and water every week (see Boulêtreau et al., 2008). In a second experiment, three different flow conditions in the flume were created and Particle Image Velocimetry (PIV) measurements were performed every week in two vertical planes in the middle of the tank (see figure 2). After 45 days of growth in the three different flow regions, the resistance to sloughing of the three different cultivated biofilms was investigated by exposing samples to increasing flow velocities in a separate flume. In the scope of this conference, we will focus our presentation on results directly related to the hydrodynamical properties (u * , z 0 , d and other turbulence descriptors inferred from the PIV measurements). From comparisons of the evolution of these properties during the biofilm growth with results obtained previously for flat bottoms or bottoms covered with small substrates, we will discuss the impact of macrorugosities on biofilm dynamics. We will show how this evolution is also correlated with the morphology of the biofilm (inferred from a daily photographic monitoring) that depends on the flow conditions to which the biofilm is exposed during its growth. This selection also leads to different behaviors in sloughing resistance tests : flow conditions during the growth control the biomass repartition between two components of the biofilm, a part strongly attached to the macrorugosities and a part composed of streamers and filaments easily sloughed by an increase of flow velocity. Boulêtreau S., Garabétian F., Sauvage S. &amp; Sánchez-Pérez J-M., 2006. Assessing the importance of a self-generated detachment process in river biofilm models. Freshwater Biology, 51, 901-912. Boulêtreau et al., 2008. Long-term dynamics of epilithic biomass in controlled flows : structural and functional changes. in preparation.., 2007. Impact of an…
Nous présentons des résultats expérimentaux préliminaires obtenus lors de l'étude d'un dipôle tou... more Nous présentons des résultats expérimentaux préliminaires obtenus lors de l'étude d'un dipôle tourbillonnaire en eau peu profonde. En particulier, la formation d'un tourbillon transverse secondaire se positionnant à l'avant de la structure dipolaire est mise en évidence. Enfin, nous identifions le processus de formation de ce tourbillon secondaire lié à l'étirement de la vorticité contenue dans la couche limite développée lors du déplacement du dipôle.
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Papers by Frederic Moulin