Thursday, 5 August 2010: 4:00 PM
Crestone Peak I & II (Keystone Resort)
Turbulence within the atmospheric surface layer is composed of active and inactive motions. Active motion directly contributes to the momentum transport, and inactive motion does not. This study investigated the structural and statistical characteristics of the active turbulence developed over urban-like roughness under atmospheric conditions. To detect the spatial characteristics of the active turbulence over urban-like roughness, multi-point measurements of velocity fluctuations were conducted in a comprehensive outdoor scale model experiment for urban climate (COSMO). COSMO is made up of 512 cubical obstacles, each 1.5 m (=H) on a side, arranged in a rectangular pattern on a flat 50 × 100-m concrete plate. The spatial distribution of the velocity fluctuations were determined by aligning 10 sonic anemometers in a horizontal line at height of 2H with a constant interval of H. The datasets measured by all anemometers were collected synchronously with 50 Hz sampling rate. The velocity fluctuations observed in COSMO were decomposed into active and inactive contributions by applying a spatial-filtering method. We used a simple moving average along the spanwise direction of the mean flow as a filter function. This method could eliminate the considerable part of low frequency modes included in the horizontal velocity fluctuation while effectively preserving the Reynolds stress. Thus, the remaining part of the fluctuation is attributed to the active motion. Horizontal distribution of the active motion is characterized by high and low speed streaks elongated along the streamwise direction. The sizes of the streaks are much larger than the sizes of the individual roughness elements. The temporal mean statistics of the active turbulence closely follows the inner-layer similarity, i.e. Monin-Obukhov similarity. It is known that the horizontal velocity fluctuations do not follow the inner-layer similarity due to the influence of the inactive outer-layer motions. However, the horizontal velocity components of the active motion still follow the similarity.
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