RESEARCH TOPICS

HERCULES: PHYSICAL MODEL OF THE GIBRALTAR STRAIT: REALISTIC IMPLEMENTATION IN THE CORIOLIS PLATFORM

(ANR ASTRID HERCULES, 2023-26

NEGRETTI ME, WIRTH A., SOMMERIA J., TASSIGNY A., VIBOUD S., VALRAN T (LEGI), BORDOIS L., DUMAS F. (SHOM), CARTON X., ROUSTAN J.-B. (LOPS), GOSTIAUX L. (LMFA), LAFUENTE JM, SANCHEZ GARRIDO JC (UNIVERSITY OF MALAGA))

Predicting Earth’s climate depends on our understanding of the ocean dynamics on a wide variety of interacting scales in time and space. Gravity currents represent one of the key sub-mesoscale processes that drive energy transfer, impact the thermohaline structure and vertical exchange of water masses in the ocean, but their representation remains challenging for numerical models.

The targeted study area is the Strait of Gibraltar (columns of Hercules) which connects the Mediterranean to the Atlantic Ocean, located between southern Spain and northern Morocco. This narrow passage is today one of the most frequented maritime routes in the world.

The choice of the Strait of Gibraltar and its adjacent areas (Gulf of Cadiz and the Alboran Sea) as targeted study area for this proposal is relevant regarding many aspects . It is particularly suitable for a first exploration of unknown sub-mesoscale regimes since the mechanisms that generate them are easily localized: "intense" turbulent structures can be more easily reproduced in the laboratory and observed within the water column. It also represents a place of formation of large-scale eddy structures. The Strait of Gibraltar is also a good model to better understand the impact (feedback) that turbulent dissipation has on the general circulation in the Mediterranean and North Atlantic basins and the way it may shape the ocean dynamics at a much larger scale.

The main objective is to understand the processes underlying the energy transfer between the sub-mesoscale and dissipative scales, and the feedback of small-scale processes on the mesoscale; besides, it will contribute to quantify the impact of small-scale processes on the modulation of non-hydrostatic dynamics, the dynamics of localized exchange fluxes, and its feedback on the larger-scale synoptic dynamics.

These objectives will be achieved through high spatial and temporal resolution measurements in the first realistic physical model ever built of the Strait of Gibraltar ; it will encompass the Gulf of Cadiz and Alboran Sea. It will represent a region of 250km x 150km in the world's largest infrastructure dedicated to the study of ocean flows, the Coriolis Platform (LEGI) and will include baroclinic, barotropic (tidal), rotational, and surface wind forcing. It will be a real engineering challenge. These data will be exploited in synergy with high resolution in situ observational data (SHOM) over several campaigns (2014, 2020, 2022) and numerical data (LES) mimicking the experimental physical model. Diagnostic tools will be developed to describe the small-scale turbulent processes - turbulent mixing, potential vorticity evolution, turbulence statistics - necessary for the evaluation of the non-hydrostatic dynamics and its feedback on the mesoscale.

The data and tools (experimental, numerical and theoretical) produced in the framework of this project will also provide the necessary data to (i) test the parameterizations of the unresolved processes in the CROCO code related to gravity currents (entrainment, mixing, turbulent processes), (ii) calibrate, test and validate the CROCO code under non-hydrostatic conditions as produced by gravity currents.

Although the project is focused on the Gibraltar Strait and the nearby environment, the methodology and approaches validated can be subsequently applied and extended to other locations with similar hydrological and dynamical characteristics and will thus offer a new way of modeling sub-grid processes and their integration into more global models, essential for a correct representation and forecasting of the ocean circulation.

TUBE : THE TURBULENT LIFE OF DOWNSLOPE OCEANIC DENSE CURRENTS

(M.E. NEGRETTI, A. WIRTH, F AUCLAIR (LA TOULOUSE), F. DUMAS (SHOM))

Understanding Earth’s climate dynamics is one of today’s greatest challenges. It depends on our understanding of the ocean dynamics over a large variety of interacting scales in time and space. Downslope currents induced by density differences represent one of the key submesoscale processes that lead to energy transfers to larger/smaller scales, impact the thermohaline structure and the vertical exchange of water masses in the ocean, and play a crucial role for the overall ocean circulation and climate.

The aim of this project is to study the submesoscale turbulence (km scale) generation induced at boundary layers of rotating downslope currents, its advection to the interior ocean and the resulting interaction and evolution between the different scales.

Laboratory experiments on the Coriolis platform are performed using an axysimmetric conical slope descending toward the center of the tank and injecting salt solutions from 32 injectors equally spaced on the outer edge of the tank at a given flow rate within a discrete or continuously stratified ambient fluid. Measurements are based on PIV and Fluorescent dye visualizations to capture the velocity and scalar fields, respectively, that are performed in the full basin domain. An Acoustic doppler Profiler is used to measure the three velocity components on the along slope section in order to resolve the boundary layers produced by the rotating gravity current.

The detailed measurements and simulations are expected to deliver the necessary data to improve present representation of downslope currents in circulation and climate models and to validate non-hydrostatic codes (CROCO).

Member :

Eletta NEGRETTI - LEGI, France
Joël SOMMERIA - LEGI, France
Achim WIRTH - LEGI, France

HARP: HYPERPYCNAL SEDIMENT-LADEN RIVER PLUMES IN LAKES: FLOW-SEDIMENT-BATHYMETRY INTERACTIONS

(ANR AAPG PRCI 2023-27, NEGRETTI ME, CHAUCHAT J (LEGI), BLANCKAERT K (TU VIENNA)

Wider research context

The focus is on the near-field flow dynamics and the flow-sediment-bathymetry interactions of hyperpycnal plumes, i.e., plumes induced by sediment-laden river inflows with a density higher than that of the receiving water body (lake, reservoir, ocean). The near-field hydro-sedimentary processes control the fate of substances (e.g., sediments, nutrients, oxygen, contaminants) introduced by the river inflow, which is of critical importance in practical applications, e.g., water-quality modelling in lakes or sediment management in reservoirs.

Research questions and objectives

The current understanding of plume dynamics is limited to simple confined geometries, and flow-sediment-bathymetry interactions are not well understood because of a lack of field and laboratory data. This project will address three research questions for a broad range of confined and unconfined geometries: (i) What are the characteristics and dominant control parameters of the flow processes? (ii) What is the influence of sediment on these processes? (iii) What are the dominant flow-sediment-bathymetry interactions?

Approach and methods

In a preparatory research phase, the partners have: (i) developed a field set-up at the Rhône inflow into Lake Geneva that combines boat-towed ADCP measurements of velocity and sediment concentration with continuous remote-sensing imagery of near-field plume surface patterns, and collected data covering a broad range of conditions; (ii) validated a large-scale laboratory infrastructure for investigating unconfined saline river plumes; (iii) established the two-phase numerical model SedFoam for sediment-laden flows.

This project will build on the preparatory phase and integrate field investigations, laboratory experiments and numerical modelling. The available field data will be analysed and additional data will be acquired if needed. The large-scale laboratory infrastructure will be adapted for sediment-laden plumes and experiments will be performed. Results of both field and laboratory investigations will be generalized by numerical experiments with the SedFoam model.

Level of originality and innovation

The integration of the unique field and laboratory set-ups and the innovative two-phase numerical model will lead to transformative advances. Existing conceptualizations will be extended to geometrically unconfined configurations and flow-sediment-bathymetry interactions during intermittent episodes of sedimentation and pick-up.

High-quality field and laboratory data acquired under a broad range of conditions will be made available to the research community.

Primary researchers

A Postdoc will perform the field investigation, a PhD the laboratory experiments, and a Postdoc the numerical modelling. They will be supervised by Koen Blanckaert (TU Wien, PI), Eletta Negretti (CNRS-LEGI, co-PI) and Julien Chauchat (UGA/GINP-LEGI, co-PI).

ill be supervised by Koen Blanckaert (TU Wien, PI), Eletta Negretti (CNRS-LEGI, co-PI) and Julien Chauchat (UGA/GINP-LEGI, co-PI).

GRAVITY CURRENTS OVER COMPLEX TERRAIN

(M.E. NEGRETTI, C. ADDUCE (UNIV ROMA 3, ITALY))

Gravity currents are abundant in the geophysical context. They can develop over short/long distances before reaching continental slopes. Most laboratory experiments considered gravity currents on horizontal or constant slope, smooth boundaries, using finite volume releases. Studies of the layer flow of constant supply gravity currents on slopes have focused on the equilbrium state of the current. This state is considered to be reached at a distance from the supply of about 10 times the initial depth of the current. The flow up to this equilibrium state and the dependency on initial conditions and slope angle, which includes spatially developing currents due to topography changes, has received so far only little attentions.

Focus of this research is on the spatial development due to rapid topography changes of a continuously supplied dense current. We considered well developed currents on a horizontal boundary, having a large interfacial Richardson number before reaching concave or linear slopes. The resulting down-slope current was found to have a completely different behavior. Three distinct development regions were identified characterized by strong variations of velocity, entrainment and bottom friction coefficient (Negretti Flor and Hopfinger JFM 2017)

Based on these results, further experiments have been conducted to understand the influence of the initial conditions (Richardson number Ji) on the development of a steady-state gravity current over sloping boundaries. The initial development of the current before reaching the slope, expressed in terms of the initial Richardson number Ji, is crucial in determining its further development so that the current do not always reach the commonly assumed self-similar regime even within long distances.
The drag terms (entrainment and bottom friction) cannot be predicted using the proposed relations available in the literature. The different reported current behaviours can be expressed in terms of an overall acceleration parameter which scales with the initial Richardson number Ji and the slope angle. By solving numerically the governing equations, the gravity flow velocity, depth and buoyant acceleration in the flow direction can be well predicted for all the performed experiments over the full measured domain. The numerical solution for the experiments with Ji>0.3 predicts that the current requires a distance 50 times larger than the current initial depth to reach an equilibrium state of constant velocity, which is much larger than the distance required in the case of a current with critical interface already at slope begin (Ji<0.3). These results are of interest since natural overflows may be far from this idealized condition and already well developed before reaching a descending topography (see in situ measurements in the Romanche fracture channel of van haren and Gostiaux 2014). Martin, Negretti and Hopfinger JFM 2019

TURBULENT MIXING PROCESSES IN STRATIFIED BOUNDARY LAYERS OVER CURVED BOUNDARIES

(M.E. NEGRETTI, C. BRUN, AND G. BALARAC)

This research is jointly conducted with my colleagues C Brun and G Balarac to put togheter physical and numerical experiments.

The effects of curved boundaries on the development of a the gravity current has been investigated experimentally Negretti et al 2017. Through comparison of currents on concave and straight slopes, the downhill deceleration on concave slopes reveals to have no quantitative influence, without formation of centrifugal instabilities: the dynamics is entirely dominated by the initial acceleration and ensuing shear instabilities induced by the sudden slope change. Specific experiments need to be designed to explore the onset of such instabilities in stratified flows.

Instead, the problem of the effects of a curved boundary on the development of a (non) stratified boundary layer has been tackled using numerical simulations which was the core of the PhD thesis of J Dagaut that I co-supervised, with C Brun and G Balarac (LEGI). This work is motivated by previous observations and realistic numerical simulations of the atmospheric boundary layer, that the so-called Görtler instability can arise due to a local unbalance between the centrifugal force and the radial pressure gradient.
Prior of investigating the effects of a background stratification on the Görtler instability, highly resolved Large-Eddy Simulations of a boundary layer flow over a concave wall has been performed using a Blasius inflow profile and both with and without turbulence and wavelength forcing. The figure below gives a 3D view of of iso-contours of the normalized Q-criterion, computed from the instantaneous flow velocity field obtained from a simulation without initial turbulence and wavelength forcing, with high initial inlet Reynolds and Görtler numbers. Transition to turbulence is then induced by the natural development of the Görtler instability and a developed turbulent region was reached at the end of the computational domain.

In this case, the developed flow over a concave wall exhibits steady large scale vortical structures that induce a spanwise heterogeneity of the mean flow properties even in the developed turbulent region, with a first clear wavelength in the initial development region L1 and a second clear wavelength L2 in the turbulent region.

The predictions of both the most amplified wavelength along with the associated spatial modes obtained by extending the linear stability analysis of Floryan 1982 to a wider parameter domain of G and the non-dimensional wavelength well compare to the computed wavelength and the spanwise averaged spatial modes in the linear region for L1. The dominant wavelength L2 in the turbulent domain is well predicted with the LSA if a turbulent Görtler number is considered, in which the kinematic viscosity is replaced by the turbulent viscosity, as proposed by Tani 1956. Also, the spatial modes associated with L2 converge with a low scatter in the turbulent region.

The skin friction coefficient Cf increases locally up to a factor 3.5 in the non-linear region of dominance of L1 and up to a factor of 1.4 in the developed turbulent region for the downwash location of the L2 Görtler instability, very different from the homogeneous distribution of Cf reported previously in the literature, in which no Görtler vorticies develop in the turbulent region because the estimated turbulent Görtler number is close or even below the neutral stability curve. The high value of initial Reynolds number induces non-linear effects that act primarily on the first appearing Görtler wavelength L1 damping its growth, so that the skin friction coefficient Cf slightly exceeds the flat plate turbulent prediction for the second dominant wavelength L2 at the end of the computational domain, and no overshoot of Cf is reported in the region of dominance of L1. Additional simulations we performed with a smaller initial Reynolds number are in agreement with previous studies with an overshoot of Cf for the first dominant Görtler wavelength with respect to the turbulent plate predictions. These results are given in the journal Dagaut et al 2020.

The next step of this research topic is to explore the influence of a background stratification on the onset and development of the Görtler instability and is part of the PhD Thesis of J Dagaut. Together with my colleagues C Brun and G Balarac, we will pursuit on this research topic.

My research: Recherche