DAUC-Dynamics of Urban and Coastal Atmosphere

Investigating the dynamic and thermodynamic processes that take place in the lower atmosphere in urban and coastal environment, and their interaction with surfaces is the main research interest of the group Dynamics for urban and coastal Atmosphere (DAUC).
The research efforts of the DAUC group have the common objective of providing decision support for sustainable urban planning to face the issues of climate change and the evolution of coastal and continental cities. They aim to address societal and environmental challenges related to urban micro-climate, air quality or optimizing wind energy production.
 
The group employs complementary approaches for understanding physical processes driving the lower atmosphere, and for taking them into account to control aerodynamic surfaces:
 
  • Physical modeling in atmospheric wind tunnel for the study of turbulent flows
  • Field observation of the micro-climatology and micro-meteorology
  • Multi-scale numerical modeling of the coastal and urban atmosphere
  • Development of Input/Output dynamics models for flow control
The group maintains its own experimental facilities, but also benefits from the off-shore test site SEM-REV and the WindMorbihan network. Numerical simulations developed by the group are based on the concept of Large-Eddy Simulation of the turbulence and the derivation of dedicated models to account for the urban environment and its interactions with the atmospheric boundary layer. Analysis methods for data from both experiments and numerical simulations are developed in order to investigate and improve the understanding of the canopy-atmosphere turbulent transfers or to characterize the micro-meteorology of complex coastal sites. Experimental methods for the input/ouput modeling of complex systems are also derived in the framework of flow control.

Research areas

The DAUC research group organizes its research in 4 axes.
Urban Micro-climatology

The main objective is to understand, assess and predict the role of urban morphology over micro-climate. Transfer of heat and water and aerodynamic exchanges between surfaces and the atmosphere play a crucial role in modifying micro-climate condition in urban environments, one of the most well-known effects being the formation of the urban heat-island. A correct estimation of these transfers is therefore necessary for the adequate modeling of the micro-climate of the urban atmosphere, at different scales (street, neighborhood and city).

Understanding the behavior of urban environments while accounting for the different contribution from all the surfaces to the exchanges with the atmosphere or explaining the micro-climate variability within the city rely on :

conducting in situ field campaigns (ONEVU or shorter-term campaigns) and the development of data analysis methods specific to heterogeneous terrains ; developing and employing models for the simulation of urban micro-climate, at urban conglomeration-scale . These models also have to both be compatible with data from Geographic Information Systems and allow for the use of a fine enough spatial resolution to account for the heterogeneities of the city neighborhoods (from the point of view of morphology and physical properties of the urban fabric). Last-years research efforts have been dedicated to the development of the ARPS-Canopy model. This research is conducted within the framework of collaborations with IRSTV members.


Dynamics of turbulent transfers between the urban canopy and the lower atmosphere

Physical processes driving the exchanges of mass and momentum between the urban canopy and the atmospheric surface layer are the results of complex interactions involving turbulent structures both from the boundary layer and induced by the roughness obstacles. Investigating their unsteady dynamics and their interaction within the roughness sub-layer is crucial for increasing our understanding of the transfer processes, improving their modeling and how they are accounted for in urban-scale numerical models, and also to derive relationships between the flow dynamics and pollutant transport, in order to be able to design modeling strategies for near-field dispersion.

Research efforts addressing this topic are conducted via three complementary approaches: physical modeling in an atmospheric wind-tunnel, mainly using Particle Image Velocimetry, Large-Eddy Simulation with or without explicit obstacle resolving techniques and on-site or field observations with scaled-model or real sites.

Targeted applications are atmospheric pollutant dispersion and urban micro-climate (in link with the previous research theme).



 

Atmospheric processes in complex coastal sites

Atmosphere over coastal areas is characterized by atmospheric phenomena such as sea-breezes (due to the diurnal variation of the land-sea temperature difference) but is also influenced by larger-scale meteorological conditions. In coastal sites with complex geography (bays, islands, estuaries), wind and turbulence are also the results of site effects at very local scale and of interactions between the marine surface and the atmosphere. Given the rapid development of off-shore wind farms and the issues related to the dynamics of marine aerosols, the group's objective is to gain a better understanding and to characterize in more details the processes at play, which can turn out to be of very complexity when topographic or urban effects are involved.

This research topic relies on the multi-scale modeling of the atmospheric boundary layer of real site allowing to model phenomena ranging from a regional scale down to the local scale with a LES approach fine enough to account for the both the shore complexity and the spatial and temporal heterogeneities of meteorological quantities (wind, turbulence, thermal stratification, etc). The group is also strongly involved in the analysis of meteorological data from the SEM-REV site and in the improvement of the experimental aspect of its deployment in order to refine the characterization of the atmospheric processes.

These research efforts are conducted in collaboration with the group EMO and SEM-REV from the LHEEA.

Interaction between aerodynamical boundary layer/control/atmospheric disturbances

In spite of the large improvement of the understanding of the intrinsic spatio-temporal organization of aerodynamic turbulent boundary layers over the past few years, flow control of this type of flow remains a challenge as well as choosing and designing the most adequate control strategy depending on the final objective (drag reduction, lift modification, mixing enhancement, etc). In many cases, local flow disturbance due to the inhomogeneous and unsteady character of the atmospheric boundary layer also play a key role. For instance, variations in aerodynamic load over a wind turbine blade due to both the mean vertical gradient of wind speed in the atmospheric boundary layer and the strong wind variability in the case of site of complex topography induce a non-negligible structural fatigue of the blade.

The main goal of this research theme is therefore to develop a better understanding of the dynamical interactions of the disturbed aerodynamic boundary layer (via flow control and/or the atmospheric boundary layer) and to provide control loop models for the flow (input/output models from automation and control science). In the framework of the ANR SMARTEOLE research project, two prototypes of « smart » wind turbine blades with integrated sensors and actuators have been developed to allow for the integration of distributed control jets in order to conduct studies on optimization of aerodynamic surfaces. The perturbation imposed at the aerodynamic surface is for the time being simplified, consisting only in the simulation of the mean direction of the incident wind, but the long-term goal is to be able to simulate accurately the atmospheric perturbations.

This research activity relies on experiments performed in the aerodynamic wind tunnel of the DAUC group but also on numerical simulations performed in collaboration with the MEHTRIC group from the LHEEA lab. The objective is to develop operational flow control strategies to reduce the structural fatigue for wind turbine blades but also concerns the other transportation means such as aircraft, trucks or UAV for which the dynamics of the atmosphere can play a key role.                     

Main test facilities of the research group

Atmospheric Wind Tunnel

This open-circuit wind tunnel is dedicated to the modeling and investigation of atmospheric boundary layer flows interacting with dense canopies. The test section is 26-meters-long, with a cross-section of 2m x 2m, with a maximum wind speed of 10 m/s. Besides the use of more classical techniques, measurements are mainly performed using Stereoscopic Particle Image Velocimetry (SPIV).

 
Aerodynamic Wind Tunnel

This closed-circuit wind tunnel is dedicated to the development and design of smart wind turbine blades. The test section is 0.8-meters-long, with a cross-section of 0.5m x 0.5 m, with a maximum wind speed of 40 m/s. This facility is equipped with a real-time acquisition system.
 

LiDAR-Light Detection And Ranging

The LHEEA acquired in 2019 a scanning LiDAR, an innovative measuring device to characterize wind and atmosphere. It will be used for marine renewable energy (MRE), urban atmosphere and air quality applications.
 

Nantes Observatory for Urban Environments (ONEVU)

This observatory is dedicated to the long-term study of hydorology, micro-meteorology, and quality of air, water and soils in urban environments. Within the framework of Research Institute for Sciences and Techniques in Cities (IRSTV, FR CNRS 2488), the DAUC group develops the instrumentation required to analyse the relationship between the urban fabric and the local micro-climate (measurement of flux, wind temperature and moisture) (Read more) and to derive energy budget.
 

 
Published on March 26, 2017 Updated on January 25, 2021