Laboratoire de Recherche Hydrodynamique, Energétique et Environnement Atmosphérique

At neighbourhood scale, the urban climate is governed by multi-scale exchanges of momentum, heat, and water vapour occurring at the canopy top and between districts through turbulent diffusion and advection. At the scale of several neighbourhoods, individual buildings cannot be explicitly resolved using for example an immersed boundary method in atmospheric models, and their parameterisation through a simple roughness length is insufficient to accurately represent turbulent exchanges within the roughness sublayer. An alternative consists in modelling the urban canopy as a porous medium, following the drag–porosity approach commonly used for vegetation canopies, in order to improve the representation of wind dynamics inside the roughness sublayer. However, unlike vegetation canopies, urban canopies are characterised by solid volumes, sharp edges and strong spatial heterogeneity, which strongly modulate the dominant turbulent motions responsible for turbulent exchanges. Instead of assuming a horizontally homogeneous porosity field as in vegetation canopies, we investigate an adaptation of the drag–porosity approach for urban canopies by concentrating the porosity at building locations, explicitly accounting for the three-dimensional urban morphology at the metre scale. This so-called object-based porosity approach is evaluated using large-eddy simulations of the flow over a staggered array of buildings in neutral thermal stratification. We analyse the differences in wind dynamics obtained by representing the urban canopy through a drag–porosity approach and the object-based porosity method, in comparison with explicit building-resolving configurations from literature. Flow statistics and conditional averaging show that, compared to the homogeneous drag–porosity approach, the object-based formulation yields a more realistic representation of turbulent momentum exchanges at canopy top and better captures the dominant coherent structures within the roughness sublayer.

Abstract. As the number of floating offshore wind farm projects grows across the world, several questions remain unanswered in the domain of floating wind turbine wakes. Previous studies observed that the motions impact the wake under laminar inflow conditions, and that the modifications are observable even under high turbulent inflow conditions, that are more realistic. However, these spatiotemporal behavior modifications still require precise characterization, especially under turbulent conditions. Thus, the present article aims to extend the previous work by studying the wake dynamics induced by surge motion under turbulent inflow, and to find the frequency and the spatiotemporal characteristics related to this motion. For this purpose, a porous disk is placed in a wind tunnel with a high level of turbulent inflow, and some experiments are performed at 8.125D (D the turbine diameter) downwind of the disk. First, hot-wire measurements are used to identify a 'critical' frequency at which wake dynamics are the greatest, and, then, stereo-PIV measurements are conducted for this frequency configuration to characterize the spatiotemporal wake dynamics. The results indicate a critical Strouhal number of approximately St = 0.24, for which the surge motion induces periodic contraction and expansion of the wake at the same frequency as the imposed motion. A quasi-steady-state analysis reveals that these modifications are larger than the predictions of basic wake models, highlighting the dynamic nature of the wake response. Moreover, inclined surge-generated periodic coherent structures are observed during this work, which are supposed to be related to shear effects. Additionally, an unexpected subharmonic frequency peak appears in the far wake velocity spectra. POD and coherence analyses suggest that this is related to a far wake meandering phenomenon, locked onto this subharmonic frequency.

Abstract From a coastal observatory located on Belle-Île-en-Mer Island off the west coast of France, Doppler scanning wind lidar (S-LIDAR) measurements were continuously performed during a 7-month deployment. The site was selected for its exposure to dominant wind and swell. Range–height indicator scans were routinely achieved over a wide range of wind and wave conditions, allowing to compute 30-min-averaged wind speed profiles over the 1–120-m height range and to monitor water level. Using the gradient flux method, near-surface friction velocities compare well with the COARE 3.5 algorithm, but differences are observed when computed higher above the surface. From the profiles, Monin–Obukhov boundary layer theory is solved at heights of 80–120 m, assumed sufficiently far from surface-induced perturbations. Comparison between theory and near-surface wind speeds reveals that departure is common. Departure is found to be highest near the sea surface and to decay exponentially as a function of height. We propose an analytic solution to this departure and attempt to scale the amplitude of the perturbation decay as a function of wind speed and wave age. A novelty is the wave age computed from wind speeds measured at 100-m height, assumed independent from the surface-induced perturbations.

• Both the RAO and the QTF are employed to determine response-conditioned waves. • Extreme resonant responses are predicted using short-duration design wave episodes. • The method is validated for the surge motions of a moored container ship. • The results are insensitive to the choice of the added damping in the modelling. Design waves have been used in the past for the probabilistic assessment of wave-induced loads and responses of offshore structures. Various response-conditioning techniques have been employed to determine suitable wave episodes, typically based on linear response transfer functions. Nevertheless, extreme events are not always driven by linear phenomena but can be triggered by near-resonant effects, as in the case of the slow-drift motions of moored floating bodies. Limited research has been devoted to addressing this class of responses using response-conditioned waves (RCW). This paper presents a new approach for deriving RCWs that accounts for combined wave- and low-frequency responses. Both the response amplitude operator (RAO) and the quadratic transfer function (QTF) are employed in an iterative response-conditioning procedure. That permits the identification of appropriate short-duration wave episodes that excite resonant slow-drift motions. These wave episodes are then used in a two-step multi-fidelity design wave methodology for the probabilistic evaluation of the fully nonlinear extreme responses. The proposed approach is validated experimentally for predicting the surge excursions of a moored container ship, and good agreement is found against Monte Carlo results in irregular waves.

<strong class="journal-contentHeaderColor">Abstract.</strong> A comprehensive meteorological dataset from an operational wind farm, consisting of six 2 MW turbines, has been made available. A meteorological mast, equipped with sonic anemometers at four different heights, was installed at the center of the farm and has collected data over three years. The dataset is further supplemented with radiometer measurements for atmospheric stability analysis. Simultaneously, supervisory control and data acquisition (SCADA) data were acquired to provide operational information about the wind turbines, including inter alia power production and wind direction. Additionally, the turbine blades were scanned to support aerodynamic simulations. This unique and comprehensive database has been made accessible to the research community through the AERIS platform.

A realistic three-dimensionnal large-eddy simulation is performed for the study of turbulent transport of sea spray aerosol in the coastal region of Le Croisic, France. A new transport model for the aerosol is implemented in the ARPS code. Simulation results are compared with field measurements, both for the mean wind field and aerosol concentration. The numerical results fit well with the observations. The mean vertical concentration profile takes an exponential shape when the data is averaged over sufficiently long timeframe, whereas the 15-min averaged profiles vary and deviate from the theory. The transport of the aerosols is analyzed in relation to the sea-land transition and the changes in thermal stability of the atmosphere during the diurnal cycle. Turbulence is found to play an important role in the mixing of aerosols in the unstable surface layer. The turbulent vertical transport of aerosols is enhanced through convective cells over the land during the day, whereas, at night, aerosols remain trapped near the surface and are transported over appreciable horizontal distances under quasi-neutral or stable thermal conditions.

The objective of this study is to understand how each variable impacts the optimal configuration of a marine diesel engine equipped with an electric hybrid air-charging system that allows energy assistance and recovery. The aim is to minimize CO2 emissions by reducing fuel consumption. The hybrid system offers flexibility in adjusting parameters from both the engine and air-charging system. It is compared with the baseline engine, which uses a free-floating turbocharger. The results show a significant improvement at low engine loads, where the baseline engine struggles to provide sufficient air. While turbine speed has little influence, compressor power reduces fuel consumption at low loads. However, at mid loads, resizing the turbomachine is necessary for further improvements. At high loads, full optimization of all variables is required to reduce fuel consumption. The electric hybrid system is particularly effective in tugboat-like conditions, where low loads dominate, but less impactful for ro-pax ferries. Despite the potential of the hybrid system, a fully optimized turbocharger could provide greater benefits due to reduced losses. Future studies could explore combining the adaptability of the hybrid system with a highly efficient turbocharger to reduce emissions across all load conditions.

In the present study, a multi-fidelity methodology based on response-conditioned waves (RCW) is demonstrated for the probabilistic assessment of wave-induced loads and responses of offshore structures. The methodology consists of two steps: (i) the RCW are determined using a surrogate response model and (ii) they are reproduced in an experimental or CFD-based numerical wave tank (NWT) to obtain the fully nonlinear response, as a high-fidelity evaluation. The main novelty of the proposed response-conditioning technique is that it uses a fully nonlinear wave solver that calculates the wave propagation inside an NWT. In this work, the extreme Vertical Bending Moment (VBM) of a zero-speed containership is investigated. It is found that the proposed approach provides nonlinear wave sequences that can be explicitly and exactly reproduced experimentally, up to very extreme sea states (Hs=17 m, Tp=15.5 s). Moreover, through comparison of the obtained results with an experimental Monte Carlo approach, it is shown that the overall framework can predict the short-term distribution of the VBM accurately and efficiently. Finally, given that model tests are costly and not widely accessible, the implementation of the high-fidelity response evaluation within a CFD-based NWT is also demonstrated and validated against the available experimental results.

Babarit et al., (2024). CN-AeroModels: A C++ implementation of aerodynamic models for wind propulsion systems of cargo ships. Journal of Open Source Software, 9(102), 6940, https://doi.org/10.21105/joss.06940