Self-nested large-eddy simulations in PALM model system v21.10 for offshore wind prediction under different atmospheric stability conditions

Large-eddy simulation (LES) resolves large-scale turbulence directly and parametrizes small-scale turbulence. Resolving micro-scale turbulence, e.g., in wind turbine wakes, requires both a sufficiently small grid spacing and a domain large enough to develop turbulent flow. Refining a grid locally via a nesting interface effectively decreases the required computational time compared to the global grid refinement. However, interpolating the flow between nested grid boundaries introduces another source of uncertainty. Previous studies reviewed nesting effects for a buoyancy-driven flow and observed a secondary circulation in the two-way nested area. Using a nesting interface with a shear-driven flow in LES, therefore, requires additional verification. We use PALM model system 21.10 to simulate a boundary layer in a cascading self-nested domain under neutral, convective, and stable conditions and verify the results based on the wind speed measurements taken at the FINO1 platform in the North Sea. We show that the feedback between parent and child domains in a two-way nested simulation of a non-neutral boundary layer alters the circulation in the nested area, despite spectral characteristics following the reference measurements. Unlike the pure buoyancy-driven flow, a non-neutral shear-driven flow slows down in a two-way nested area and accelerates after exiting the child domain. We also briefly review the nesting effect on the velocity profiles and turbulence anisotropy.</p

Wind Farm Inflow Wind Simulation based on Mesoscale and Microscale Coupling

Inflow wind simulation is a critical issue that dominates the wind farm design regarding the actual wind environment. Different from traditional real measurement based wind simulation, this paper proposed a mesoscale and microscale coupling strategy, which applies the forecasting information from WRF model to the LES model based SOWFA. Firstly, an offline coupling strategy is implemented with the modular software interface between WRF and SOWFA. The wind speed, potential temperature and pressure data are converted from Geographic Coordinate to Cartesian coordinate that is a readable format to SOWFA. Then, the simulation domain is selected in daytime for neutral ABL condition at a 1km*1km region where the wind information from WRF is interpolated and averaged at center point with 100m height. Time-series ABL conditions are extracted from center point and force the SOWFA internal solver to simulate the same environment with predictive data. The mesoscale lacked information, turbulence, is generated by periodically running precursor with the surface roughness and boundary conditions. Finally, the comparison between WRF exacted data and SOWFA output verifies the coupling strategy. The result shows that the mesoscale and microscale coupling has high fidelity and accuracy simulation at stable ABL conditions and slow-changing wind environments. This work provides a low cost and reliable data source which allows the inflow wind simulation to have the predictive ability for actual wind

Interaction Between Mesoscale Eddies and the Gyre Circulation in the Lofoten Basin

The interaction between the mesoscale eddies and the cyclonic gyre circulation of the Lofoten Basin is studied using a suite of satellite altimeters, a regional coupled ocean‐sea‐ice data assimilation system (the TOPAZ reanalysis) and Argo float data. An automated method identified 5,373/5,589 individual anticyclonic/cyclonic eddies in the Lofoten Basin from more than 65,000 altimeter‐based eddy observations, of which 70–85% are found to be nonlinear. The nonlinearity of eddies is estimated from its translational and rotational velocities. The study found clustering of highly intense nonlinear eddies on either side of the Lofoten Basin. Further, we show the distinct cyclonic drift of the anticyclonic and cyclonic eddies, both confined to the western side of the basin, and its similarity to the middepth gyre circulation also confined to the same region. A well‐defined cyclonic drift pattern of eddies is found during the time period when the gyre circulation of the basin is strengthened, while a clear cyclonic drift of eddies is absent during a weakened gyre. Analysis of barotropic energy conversion in the reanalysis data shows maximum transfer of energy from the eddy field to the mean flow in the Lofoten Vortex region. Even though comparatively smaller (roughly 9 times) there is also notable transfer of energy from the mean flow to the eddies in the region located outside the Lofoten Vortex. Our study shows that the gyre circulation when strengthened, receives more energy from the Lofoten Vortex and loses less energy to those eddies circulating around the Lofoten Vortex. Plain Language Summary Lofoten Basin situated in the path of Atlantic Water flow from the North Atlantic to the Arctic is the largest heat reservoir in the Nordic Seas. The mesoscale eddies and the gyre circulation of the basin can impact the heat transported into the basin interior and the heat lost to the atmosphere. In this paper, we use a suite of satellite altimeters, Argo floats, and an ocean reanalysis data set to study the interaction between the mesoscale eddies and the gyre circulation of the Lofoten Basin. Our study shows that the energy transfer associated with the mesoscale eddies influence the gyre circulation of the basin

Turbulence structure in the upper ocean: a comparative study of observations and modeling

Observations of turbulent dissipation rates measured by two independent instruments are compared with numerical model runs to investigate the injection of turbulence generated by sea surface gravity waves. The nearsurface observations are made by a moored autonomous instrument, fixed at approximately 8 m below the sea surface. The instrument is equipped with shear probes, a highresolution pressure sensor, and an inertial motion package to measure time series of dissipation rate and nondirectional surface wave energy spectrum. A free-falling profiler is used additionally to collect vertical microstructure profiles in the upper ocean. For the model simulations, we use a one-dimensional mixed layer model based on a k–ε type second moment turbulence closure, which is modified to include the effects of wave breaking and Langmuir cells. The dissipation rates obtained using the modified k–ε model are elevated near the sea surface and in the upper water column, consistent with the measurements, mainly as a result of wave breaking at the surface, and energy drawn from wave field to the mean flow by Stokes drift. The agreement between observed and simulated turbulent quantities is fairly good, especially when the Stokes production is taken into account