1. Using their space-time correlation of fluctuating velocity and normal gradients,Kreplin and Ecklemann12found the streamwide velocity component leading the ejected streak outward to be «13.8wT,which is essentially the mean velocity at v+= 40. Eckelmann25found that the sweep traveled forward with anearlyconstant velocity almost corresponding to the friction velocity WT. Morrison et al.12measured the two-dimensional power spectra andfound thecharacteristicnear-wallstreamwiseandspanwisewavelengths to be A+ -630 and A+ -135,respectively.The tan /?=A+/A+ -0.21,oryff « 12 deg,can be considered to be a half-angle between one upstream sweep and two downstream ejections and vice versa. Bandyopadhyay26depicted the wall layer as an agglomeration of successively formed hairpin vortices arranged in a pattern with /? « 15.3 deg. This overall geometry has an important implication for the control of microturbulent events. Murlis et al.27showed that the burst spacingincreasesasyld decreases,indicating that near the wall the burst spacing rangesfrom 3d to 5<5for ReG1880to 4820, whereSisthe boundary-layer thickness and 9isthe momentum thickness.
2. In summary,selection of the event sequence depends on the application. For fixed point measurement and detection of microturbulent events, the BASED ON: MORR!SONETAL(1971) KIMETAL.(1971) KL!NEETAL(1967) NAKAGAWA & NEZU (1981) BLACKWELDER &ECKLEMANN (1979)
3. ACCORDINGTO: BROWN &THOMAS (1977)
4. Citing past studies of structures of turbulence that have been hampered by a limited number of sensors and limited field of view,flow visualization reveals only the consequencesnot the cause and the noncorrespondence of spatial and temporal measurements.Moin32illustrated vianumerical simulations the errors of experimental measurements of near-wall turbulence intensities. More importantly, Moin showed that there are two kinds of vortices which can be identified in his numerical simulation results: those with tipsabove the legs (ejection) and otherswith the tipnear the wall and upstream of the legs (sweep).His results also showed that there are more solitary vortices than counterrotating vortex pairs,which have a relatively short streamwise extend (100-200 wall units), long lifetimes, travel downstream without losing their identity, and appear to be the fundamental structures associated with regions of high turbulence production. Using a directnumericalsimulationofaturbulentchannelflow,Kimet al.33not only reproduced most of the experimentally established wall turbulence structure, but also showed the most interesting quantitative illustration that sweeps dominates in the near-wall region, i.e.,y+< 10, whereas ejection dominates at y+> 10 through 100. This implies that when considering effective methodologies for skin-friction control those directly focused on inhibiting sweeps should be aprimary candidate.