Espectros de turbulência em terreno complexo

Abstract

Throughout this paper a spectral analysis is conducted on the superficial limit layer in complex terrains. The data used were collected in the central part of the state of Rio Grande do Sul, in the Valley of Jacuí River. A 15-meter tower, with fast-response sensors and slowresponse ones, collected data at frequencies of 10 Hz and 1 Hz, in July and August 2000, respectively. The time series were numeric analyzed through a piece of program developed in Fortran. The calculated spectra were classified according to stability class, intensity and direction of the wind speed. The spectra of the wind speed vertical component have a well-defined peak for all the analyzed conditions, except for the night series in which the wind direction is transverse to the valley axis. This same spectrum is in accordance with -5/3 Kolmogorov s law, with the beginning of the inertial sub-range in f ≈ 2 for winds that are parallel to the valley axis and any wind intensity. For winds transverse to the valley, the beginning of the inertial sub-range is in f ≈ 3. The frequencies associated with the spectral maxima are inferior to those observed in the Kansas experiment. Being superior in stable conditions when compared to convective ones in parallel conditions. In transverse conditions there is higher frequencies scattering for stable conditions. For parallel winds in stable conditions, the maximum no dimensional spectra are approximately 0.4 independently of z / L , and for convective conditions these maximum vary from 0.4 to 0.6. For transversal mean winds in stable conditions the high frequency vary from 0.4 to 0.5 and for convective conditions they are approximately 0.7. The spectra of the lateral components of velocity higher than 1m/s under stable conditions showed a cut frequency of ≈ 0.06 as initial number of this frequency, for all the cases in the low-frequency region. In the high-frequency region, the aliasing for all the parallel conditions starts at ≈ 5.0 and for transversal cases the starting point was at ≈ 10.0 . The consequences of the mesoescala movements are more important in the nocturnal boundary layer. The average time of ≈ 30 minutes renders contaminations in the computed flux measurements, since it captures mesoescala movements. The lateral spectra for convective conditions show an only spectral peak, highlighting that the importance of the thermal and mechanical effects have the same magnitude. In the region of low frequencies there is a scattering of data that could be explained by the factors associated with topographical influences. In the region of high frequencies the spectrum is in accordance with Kolmogorov s law, indicating, still under not homogeneous conditions, the presence of isotropic eddies. In convective and stable conditions with winds slower than 1m/s, the Kolmogorov s law is not applicable to most of the series; therefore such conditions were not analyzed in this study. For the different classes of z / L it is shown that the reason between vertical and horizontal spectra w u S / S fast increases until it reaches its isotropic number. For the parallel condition, we have a frequency of about f ≅ 2 and for the transverse case, about f ≅ 3.