Turbulent fluctuations above the buffer layer of wall-bounded flows

摘要

The behaviour or the velocity and pressure fluctuations in the logarithmic and outer layers or turbulent flows is analysed using spectral information and probability density functions from channel simulations at R-e tau <= 2000. Comparisons arc made with experimental data at higer Reynolds numbers. It is round, in agreement with previous investigations, that the intensity profiles or the streamwise and spanwise velocity components have logarithmic ranges Una one traced to the widening spectral range of scales as the wall is approached. The same is true for the pressure. both theoretically and observationally, Nu not For Me normal velocity or For the tangential stress cospectrum. although even those two quantities have structures will lengths of the order of several hundred times the wall distance. Bemuse the logarithmic range grows longer as the 1 Reynolds number increases, variables which am 'attached' in this sense scale I the buffer layer in mixed units. These results give Strong Support to the attached-eddy scenario Proposed by Townsend ( 1976), but they are not linked to any particular eddy model. The scaling or me outer nudes is also examined. The intensity of the streamwise velocity at fixed y/h increases with the Reynolds number. This is traced to the large-scale modes, and to an increased intensity of the ejections but not or me sweeps. Several differences are Found between Me outer structures of different flows. The outer modes of the spanwise and wall-normal velocities in boundary layers am stronger 01,111 in internal flows, and their streamwise velocities penetrate closer to the wall. As a consequence, their logarithmic layers am thinner, and some or their logarithmic slopes are different. The channel statistics am available electronically at http://torroja.dmt.upm.es/ftp/channels/.