Introduction: scaling and structure in high Reynolds number wall-bounded flows.

On bou *A According to Lighthill (1995), Prandtl’s (1904) boundary layer has had the same transforming effect on fluid dynamics as Einstein’s 1905 discoveries had on other parts of physics, which, by the way, were celebrated in 2005 as the World Year of Physics. That the boundary layer becomes turbulent was formally known to Blasius (1908), though, of course, the origin of turbulence in a pipe was studied earlier by Reynolds (1883). The problem of the turbulent boundary layer has since been a paradigm in the field of turbulence. Its practical importance in flows over air and water vehicles as well as in geophysical fluid dynamics has been recognized for nearly a century now. Advances in our understanding of the boundary-layer scaling and structure can be expected to shed further light on the complex and multiscale flow dynamics, and also offer basic input to flow control strategies for practically relevant problems such as reducing large vehicle drag (and hence, by implication, emission levels). The first systematic account of turbulent boundary layer was given probably by Prandtl (1942), followed by Schlichting (1956) and a more modern version by Monin & Yaglom (1971). Advances in the subject, particularly its scaling properties, have been the subject of various classical reviews such as Clauser (1956) and Coles & Hirst (1969). The importance of the flow structure was highlighted, with different emphasis, by Townsend (1956, 1976) and Kline et al. (1967), and it is fair to say that, in one form or the other, the interaction between the flow structure and scaling properties of the turbulent boundary layer has been the subject of major study since then. This interplay has been the subject of reviews such as Cantwell (1981) and Sreenivasan (1989). The last major review of structure and scaling in boundary layers, and in wall-bounded flows in general, appeared approximately 10 years ago (Panton 1997). In the intervening period, experimental attention has focused on the form of the boundary-layer scaling at Reynolds numbers much higher than before. The debate on logarithmic versus power-law scaling of the mean velocity (Barenblatt et al. 1997; Zagarola & Smits 1998) and on whether, in general, the flow close to a wall can be regarded as universal—in the sense that the influence of the outer flow is negligible—have Phil. Trans. R. Soc. A (2007) 365, 635–646 doi:10.1098/rsta.2006.1952 Published online 16 January 2007