ANISOTROPIC TURBULENCE IN THE ATMOSPHERIC SURFACE-LAYER

Within the atmospheric surface layer the turbulence that effects low r ise buildings tends to be highly anisotropic. It is shown that while l ow level spectra match the expected form in the inertial subrange ther e also exists a range of frequencies within the energy containing rang e where the horizontal spectra can be matched by a power law. The grad ients of the log-log graphs of spectra in this range tend to diminish with decreased height. This power law range seems to be a bridge betwe en a very low frequency range where horizontal spectra scale in propor tion to the square of the mean velocity and frequencies a decade below the low frequency end of the inertial subrange. This power law range is quite narrow at higher levels but broadens at low levels. The level of the normalised power spectral density in the inertial subrange is observed to be similar at all heights and does not vary in the manner suggested by Harris and Deaves. Turbulence spectra obtained in the Uni versity of Auckland wind tunnel are shown to exhibit similar patterns to those obtained in full scale. The variation in the size of eddies i nvolved in the generation of turbulence at various heights is illustra ted by considering the cospectra. A functional form for the cospectra is proposed.