Turbulence structure in the wake region of a meteorological tower

A meteorological tower significantly modifies the air flow, the mean wind speed and wind direction as well as the turbulence structure of the air. Such changes can be noticed in particular in the wake region of the tower. Measurements on the 200 m tower of Forschungszentrum Karlsruhe were carried out using Solent sonic anemometers in the lee of the tower and cup anemometers on both sides. In the wake region, spectral energy density is increased in the high-frequency range. Superposition of this disturbance spectrum on the undisturbed spectrum yields a 'knee' in the resulting spectrum. In the case of low turbulence intensity with stable stratification, a plateau with a constant energy content is observed in front of the knee. This effect is caused by the new production of turbulence energy from the mean flow as well as by an energy transfer from larger to smaller vortices. Power spectra in strongly stable conditions show a more rapid decrease of intensity in the region where the inertial subrange is expected. The relevant scales of wake turbulence are derived from the maximum of the disturbance spectrum. Locations of the high-frequency peak do not depend on atmospheric stability, but are controlled mainly by mean wind speed. Apart from the reduction of the mean wind speed, the spectra and cospectra exhibit a strong anisotropy for such cases. The results demonstrate the significant influence of a tower on turbulence spectra in the wake region.