Turbulence spectral modeling using local similarity theory for onshore and offshore wind fields under unstable, neutral, and stable conditions

While traditional turbulent spectral models are proposed based on onshore measurements under neutral conditions, they may have limitations for structures such as wind turbines since not only the strong winds but also winds with smaller mean wind speeds are related to the structural integrity, and application to offshore wind fields are also important. This study proposes a spectral model along with a series of models to predict the model coefficients for turbulence spectra under unstable, neutral, and stable conditions based on an offshore measurement. The spectral model is proposed such that the model coefficients are related to the energy intensity and the peak frequency in each frequency region. Examination of the model coefficients reveals that the characteristics of the turbulence spectrum under unstable to neutral conditions are well expressed using the local similarity theory for 60 m-80 m height, while only behavior of the inertial subrange can be estimated accurately using the atmospheric stability for the stable conditions. Comparison between the two measurements conducted over the sea suggests the contribution of the mesoscale turbulence to the differences in the peak frequency of the microscale turbulence and the overall standard deviations for stable conditions.

Automated summary

This study proposes a spectral model and a series of models to predict turbulence spectra and model coefficients under unstable, neutral, and stable conditions based on offshore measurements. The model coefficients are related to the energy intensity and peak frequency in each frequency region. The results show that the turbulence spectrum characteristics under unstable to neutral conditions can be well described using the local similarity theory, while only the behavior of the inertial subrange can be accurately estimated for stable conditions. Comparison of measurements conducted over the sea suggests the influence of mesoscale turbulence on the differences in peak frequency and overall standard deviations for stable conditions.