Spectra over complex terrain

Spectra have been measured over land downwind of a water surface, over hilltops and escarpments, and over rolling farmland. The following hypotheses can be used to explain the differences between these spectra. (1) For wavelengths short compared to the fetch over the new terrain, spectral densities are in equilibrium with the new terrain. (2) For wavelengths long compared to this fetch, spectral densities remain unchanged if the ground is horizontal. If the flow is over a steep hill, the low-frequency structure is modified by distortion of the mean flow, with the longitudinal component losing energy relative to the lateral and vertical components. Because vertical-velocity spectra contain relatively less low-frequency energy than horizontal-velocity spectra, energetic vertical-velocity fluctuations tend to be in equilibrium with local terrain

Spectra over complex terrain in the surface layer

For design and control of Wind Energy Conversion Systems (WECS), the spectral distribution of wind speeds over frequencies needs to be known as well as velocity variances. Velocity spectra and other velocity statistics are estimated over three types of complex terrain: on tops of hills or escarpments; on land a short distance downwind from a water body; and over rolling farm land. The most important characteristics of turbulence models over uniform terrain are summarized briefly. Theoretical aspects of spectral characteristics over complex terrain are discussed, followed by detailed observations over complex terrain and procedures for their estimation. A theory is presented for calculation of response of engineering systems to wind fluctuations. (LEW