Gerald Steinfeld

On the Offshore Advection of Boundary-Layer Structures and the Influence on Offshore Wind Conditions

The coastal discontinuity imposes strong signals to the atmospheric conditions over the sea that are important for wind-energy potential. Here, we provide a comprehensive investigation of the influence of the land–sea transition on wind conditions in the Baltic Sea using data from an offshore meteorological tower, data from a wind farm, and mesoscale model simulations. Results show a strong induced stable stratification when warm inland air flows over a colder sea. This stratification demonstrates a strong diurnal pattern and is most pronounced in spring when the land–sea temperature difference is greatest. The strength of the induced stratification is proportional to this parameter and inversely proportional to fetch. Extended periods of stable stratification lead to increased influence of inertial oscillations and increased frequency of low-level jets. Furthermore, heterogeneity in land-surface roughness along the coastline is found to produce pronounced horizontal streaks of reduced wind speeds that under stable stratification are advected several tens of kilometres over the sea. The intensity and length of the streaks dampen as atmospheric stability decreases. Increasing sea surface roughness leads to a deformation of these streaks with increasing fetch. Slight changes in wind direction shift the path of these advective streaks, which when passing through an offshore wind farm are found to produce large fluctuations in wind power. Implications of these coastline effects on the accurate modelling and forecasting of offshore wind conditions, as well as damage risk to the turbine, are discussed.

First comparison of LES of an offshore wind turbine wake with dual-Doppler lidar measurements in a German offshore wind farm

Large-Eddy Simulations (LES) are more and more used for simulating wind turbine wakes as they resolve the atmospheric as well as the wake turbulence. Considering the expenses and sparsity of offshore measurements, LES can provide valuable insights into the flow field in offshore wind farms. However, for an application of LES wind fields to assess offshore wind farm flow, a proper validation with measured data is necessary. Such a proper validation requires that the LES can closely reproduce the atmospheric conditions during the measurement. For this purpose, a representation of the large-scale features that drive the wind flow is required. Large-scale-forcing and nudging of the LES model PALM is tested with reanalysis data of the COSMO-DE model for a case study during one particular day in the beginning of 2014 at a German offshore wind farm. As wind and temperature profiles of the LES prove to follow the large-scale features closely, the wake of a single wind turbine is simulated with an advanced version of an actuator disc model. Measurement data is provided by processed dual-Doppler lidar measurements during the same day in the same wind farm. Several methods have been investigated at the University of Oldenburg to compare LES wind fields and lidar measurements. In this study a dual-Doppler algorithm was applied in order to estimate the horizontal stationary wind field. The raw data originate from Plan Position Indicator (PPI) measurements, which have been performed with two long-range wind lidars installed at different opposing platforms at the border of the wind farm.

Large-eddy simulation of multiple wakes in offshore wind farms

In this study, high-resolution large-eddy simulations of two German offshore wind farms are performed with the LES model PALM in which the turbines are parameterized with an actuator disk approach. The simulation of a single wind farm is realized by a stationary model domain with a turbulent inflow. Different atmospheric stratifications from slightly stable to unstable have been simulated and their effect on the wake characteristics is investigated. The results show a clear development of the flow within the wind farms with large differences between single, double and triple wakes. The wake deficit is increasing up to the second wake for the neutral and stable boundary layer and up to the third wake in the convective case before reaching a nearly constant value. This is explained with the turbulence intensity being much higher behind the second and subsequent turbines compared to the wake of the first turbine. With increasing atmospheric stability the wake deficits are shown to be stronger due to reduced turbulence.

Footprint Evaluation for Flux and Concentration Measurements for an Urban-Like Canopy with Coupled Lagrangian Stochastic and Large-Eddy Simulation Models

A footprint algorithm, based on a Lagrangian stochastic (LS) model embedded into a parallelized large-eddy simulation (LES) model, is used for the evaluation of flux and concentration footprints of passive scalars in flow in and above an urban-like canopy layer of a neutrally stratified

Atmospheric Impacts on Power Curves of Multi-Megawatt Offshore Wind Turbines

440 \hbox { m}

Large-eddy simulation study of wind farm active power control with a coordinated load distribution

440m deep boundary layer. The urban-like canopy layer is realized by an aligned array of cuboids whose height H is

An analysis of offshore wind farm SCADA measurements to identify key parameters influencing the magnitude of wake effects

40\hbox { m}