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681.
Wind-tunnel experiments were performed to study turbulence in the wake of a model wind turbine placed in a boundary layer
developed over rough and smooth surfaces. Hot-wire anemometry was used to characterize the cross-sectional distribution of
mean velocity, turbulence intensity and kinematic shear stress at different locations downwind of the turbine for both surface
roughness cases. Special emphasis was placed on the spatial distribution of the velocity deficit and the turbulence intensity,
which are important factors affecting turbine power generation and fatigue loads in wind energy parks. Non-axisymmetric behaviour
of the wake is observed over both roughness types in response to the non-uniform incoming boundary-layer flow and the effect
of the surface. Nonetheless, the velocity deficit with respect to the incoming velocity profile is nearly axisymmetric, except
near the ground in the far wake where the wake interacts with the surface. It is found that the wind turbine induces a large
enhancement of turbulence levels (positive added turbulence intensity) in the upper part of the wake. This is due to the effect
of relatively large velocity fluctuations associated with helicoidal tip vortices near the wake edge, where the mean shear
is strong. In the lower part of the wake, the mean shear and turbulence intensity are reduced with respect to the incoming
flow. The non-axisymmetry of the turbulence intensity distribution of the wake is found to be stronger over the rough surface,
where the incoming flow is less uniform at the turbine level. In the far wake the added turbulent intensity, its positive
and negative contributions and its local maximum decay as a power law of downwind distance (with an exponent ranging from
−0.3 to −0.5 for the rough surface, and with a wider variation for the smooth surface). Nevertheless, the effect of the turbine
on the velocity defect and added turbulence intensity is not negligible even in the very far wake, at a distance of fifteen
times the rotor diameter. 相似文献
682.
Jorge Fernndez Vera Markgraf Hctor O. Panarello Miguel Albero Fernando E. Angiolini Susana Valencio Mirta Arriaga 《Geoarchaeology》1991,6(3):251-272
The Barro Negro site (23°S lat., 65°37′W long.) in the Altiplano (Puna) of northwestern Argentina contains a well stratified sequence of remains of Hippidion, the American extinct horse, camelids, and archaeological materials, which is the focus of this study. In addition to establishing a reliable chronology, paleoenvironmental information was obtained based on analyses of pollen and stable isotopes (oxygen and carbon) from bone and marl. The data indicate that Hippidion was present at the site between 12,000 and 10,000 yr B. P., at a time when Altoandean grasslands had expanded to lower elevations. By 10,000 yr B.P., when modern semi-arid sub-puna scrub had replaced the Altoandean grasslands, only camelids (Lama or Vicugna) were present, simultaneous with the first evidence of local human occupation. This suggests that a climatic shift from cool and moist (winter rain regime) to warm and dry (summer rain regime) conditions took place simultaneously with the disappearance of the American horse and the appearance of camelids and man. 相似文献