Appel à candidatures, Recrutement | Recherche, Emploi

PhD "Towards a unified modeling of wave-current-turbulence interactions in three dimensions: application to Alderney Race (France) and Gregory Sound (Ireland), Universités de Caen and Paris-Saclay

Du 1 septembre 2022 au 31 août 2025

PhD thesis beginning : 2022
PhD duration : 36 months 
Site actualite

Location: University of Caen Normandy, Coastal and Continental Morphodynamics lab. (M2C), Caen, France. Research stays in Paris at University Paris-Saclay and ENS Paris-Saclay

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PhD subject: Towards a unified modeling of wave-current-turbulence interactions in three dimensions: application to Alderney Race (France) and Gregory Sound (Ireland)

Supervising team: A.-C. Bennis (dir., Univ. Caen), F. Dias (co-director, ENS-Paris Saclay/Univ. College of Dublin)

Location: University of Caen Normandy, Coastal and Continental Morphodynamics lab. (M2C), Caen, France. Research stays in Paris at University Paris-Saclay and ENS Paris-Saclay

Salary: 1975€ -2300€

Summary. The main objective of this thesis is to move towards a unified modeling of wave- current-turbulence interactions in three dimensions (hereafter named 3D), with two application sites in coastal seas, the Alderney Race (France) and the Gregory Sound (Ireland). These sites are key for the development of marine renewable energies. This fundamental research work will improve the knowledge of hydrodynamic interaction processes and help in the design and deployment of energy recovery devices.

To address this unified modeling problem, we propose to integrate the original LES turbulence models, Navier-Stokes-α (LANS-α) and Leray-α (e.g. Geurts and Holm, 2006; Hecht et al, 2008) to the set of equations modeling the 3D phase-averaged wave-current interactions (McWilliams et al., 2004; Ardhuin et al., 2008; Bennis et al., 2011) for non-hydrostatic flow.

The α-models have been successfully implemented and tested for the first time in a 3D coastal oceanographic model for application to the Alderney Race (Bennis et al., 2021; Adong and Bennis, 2019) without consideration of wave effects. In these approaches, turbulence is accounted for via nonlinear advection terms, rather than by adding a dissipative effect modeled by sub-grid scale viscosity and introducing a smooth velocity. This smooth velocity is obtained from the velocity containing all scales of the turbulence (ie. rough velocity) to which a differential filter is applied. This modeling was first tested on the test case of Hecht et al, (2008), aiming to reproduce the wind-induced circulation, and then applied to Alderney Race (Bennis et al, 2021; Adong and Bennis, 2019). The results obtained are: (i) the LANS-α modeling re-energizes the flow by recovering the turbulence statistics obtained at high resolution in lower resolution simulations, saving 30% of computational time, ii) LANS-α reproduces both types of inertial regime for barotropic turbulence, with k-5/3 and k-3 turbulent energy decays (k being the wavenumber), and iii) LANS-α strongly impacts the deformation-induced turbulence. The Leray-α modeling shows similar behaviors to the LANS-α modeling, with also a re-energization of the flow.

Through this thesis project, we want to answer the following scientific questions:

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1/ What are the contributions of Leray-α and LANS-α modeling to the representation of three- dimensional marine turbulence in calm weather and for the 2 sites?

2/ Which set of equations is the most relevant for a unified modeling of wave-current- turbulence interactions in 3D in coastal oceanographic models?

3/ What is the relevance of this new set of equations in realistic situations according to the meteorological conditions and with respect to existing models?

Study sites. Two study sites were chosen for their complementarity: the Alderney Race (France) and Gregory Sound (Ireland).

The French site is located between Cap de la Hague and Alderney island. Alderney Race is the most energetic tidal site in Western Europe, with a maximum tidal potential of 5.1GW (Coles et al., 2017). Due to the influence of the semi-diurnal tide, the current speed can reach 5 m/s during spring tides for a water depth of 20-80m. This strong current causes numerous wave breakers that lead to a permanent foam at the surface.

Former studies have shown high variability and turbulence in the water column and near the bottom due to interactions between hydrodynamics and bottom morphology (e.g. Furgerot et al. 2020; Thiebaut et al. 2020). However, despite recent efforts to measure this turbulent activity, difficulties remain in assessing 3D turbulent structures, especially near the bottom, as reported by Mercier et al. (2021). In addition, innovative numerical models based on LES turbulent modeling (e.g. Bennis et al. 2021; Mercier et al. 2020; Bourgoin et al., 2020) have been developed to help in the understanding of turbulent motions. These works do not consider the effects of wave-current interactions on marine turbulence. However, these interactions have a significant influence in Alderney Race as shown by Bennis et al., 2020, 2022 and Furgerot et al., 2020. In-situ and High-Frequency radar data have revealed that wave effects were absent only 6% of the time from June 2017 to July 2018 and that strong current shears were observed in the upper half of the water column. During the Eleanor storm (2018), wave effects have increased the ebb/flood asymmetry of 3% to 13%, leading to a reduction in the tidal yield by up to 30% (Bennis et al., 2022).

In addition to Alderney Race, the Gregory Sound site will be used to validate and assess the contribution of 3D wave-current-turbulence modeling. This site, located between the Aran Islands (Inis Mor and Inis Meain) on the Irish Atlantic coast at the exit of Galway Bay, is also influenced by the semi-diurnal tide and its depth reaches 35m. The maximum annual barotropic currents are about 2.3 m/s (McCullagh et al., 2020), with strong asymmetries in intensity and direction. Tidal ranges average 4/5 m and wind also plays an important role. Regarding the sea states, in case of storm there is mainly incoming swell (typical case inside the gully: 3m Hs, 6s in entrance - 4s in exit for Tm02, swell directed north-west) and crossed sea at the exit of the gully on the Galway Bay side with swell entering from the North Sound. The site is characterized by strong turbulence that has not been studied to date. The bottom is rough (rocks, gravel and gravelly sand). The breaking of the waves is almost omnipresent. The cliffs bordering Gregory Sound allow the deployment of various sensors (radar, stereo- cameras, seismometer and upstream a wave buoy as well as a current profiler).

Profile required. The candidate is currently in a 2nd year of Msc or final year of engineering school in the following fields: i) physical oceanography, ii) fluid mechanics for the environment, iii) mathematics applied to the environment and any other related field.

The successful candidate must have:
- a strong knowledge in physical oceanography, fluid mechanics and turbulence,
- a strong background in numerical modeling for oceanography and/or fluid mechanics, - a good knowledge of scientific programming (e.g. Fortran, python, matlab),
- a good knowledge of French and English (written and oral).

Application procedure. Please send by March 31, 2022 a detailed CV, a motivation letter, two letters of recommendation and a transcript of higher education records (at Master level) to Anne-Claire Bennis (anne-claire.bennis@unicaen.fr) and Frédéric Dias (frederic.dias@ucd.ie).

Références.

Adong, F. and A.-C. Bennis, (2019). Lans-alpha and leray-alpha turbulence models for coastal simulations: application to alderney race. Proceedings of EWTEC 2019, Sustainable energy research Group.
Aiki, H. and R. J. Greatbatch, (2012). Thickness-weighted mean theory for the effect of surface gravity waves on mean flows in the upper ocean. J. Phys. Oceanogr.

Ardhuin, F., et al., (2008). Explicit wave-averaged primitive equations using a generalized Lagrangian mean.

Ocean Modelling.

Bennis A.-C., et al., (2011). On the coupling of wave and three-dimensional circulation models: Choice of theoretical framework, practical implementation and adiabatic tests. Ocean Modelling.
Bennis A.-C., et al., (2020). Numerical modelling of three-dimensional interactions in a complex environment: application to Alderney Race. Applied Ocean Research.

Bennis, A.-C., et al., (2021). LANS-alpha turbulence modeling for coastal sea : An application to Alderney Race. J. Comp. Phys.
Bennis, A.-C., et al., (2022). A winter storm in Alderney Race: impacts of 3D wave-current interactions on the hydrodynamic and tidal stream energy. Applied Ocean Research.

Bourgoin, A., et al. (2020). Turbulence characterization at a tidal energy site using large-eddy simulations: case of the Alderney Race. Phil. Trans. R. Soc. A.
Coles, D. S., et al., (2017). Assessment of the energy extraction potential at tidal sites around the channel islands. Energy.

Craik, A. D. D. and S. Leibovich, (1976). A rational model for Langmuir circulations. J. Fluid Mech.
Smith, J. A., (2006). Wave-current interactions in finite-depth. J. Phys. Oceanogr.
Furgerot, L., et al., (2020). One year measurement in Alderney Race: what did we learn ? Phil. Trans. R. Soc. A. Geurts, B. J., et D. D. Holm, (2006). Leray and LANS-alpha modeling of turbulent mixing. Journal of turbulence. Groeneweg, J. and G. Klopman (1998). Changes in the mean velocity pro les in the combined wave-current motion described in GLM formulation. J. Fluid Mech.
Hecht, M. W., et al., (2008). Implementation of the lans-alpha turbulence model in a primitive equations ocean model. J. Comp. Phys.
Longuet-Higgins, M. S. and R. W. Stewart, (1962). Radiation stresses and mass transport in surface gravity waves with application to `surf beats'. J. Fluid Mech.
Marchesiello, P., et al., (2021). Tridimensional nonhydrostatic transient rip currents in a wave-resolving model. Ocean Modelling.
Marchesiello P., et al., (2015). On tridimensional rip current modeling. 
Ocean Modelling.
McCullagh, D., et al. (2020). Geomorphology and substrate of Galway Bay, Western Ireland. 
J. Maps. McWilliams, J. C., et al., (2004). An asymptotic theory for the interaction of waves and currents in coastal waters. J. Fluid Mech.
Mercier, P., et al., (2020). Numerical study of the turbulent eddies generated by the seabed roughness. case study at a tidal power site. Applied Ocean Research.
Mercier, P., et al., (2021). Turbulence measurements: An assessment of acoustic doppler current profiler accuracy in rough environment. Ocean Engineering.
Phillips, O. M., (1977). The dynamics of the upper ocean. Cambridge University Press, London