The physics of wall turbulence

The behaviour of wall-bounded turbulent flows is briefly reviewed, with emphasis on areas which remain open and which are distinct from the problem of turbulence in general. It is argued that the near-wall region is reasonably well understood, at least for smooth walls, but that its interactions with the outer flow are not, including the question of its asymptotic behaviour at large Reynolds numbers. The similarity properties of the logarithmic region are addressed next, in view of the recent controversy about its validity. It is concluded, from an analysis of experimental data for the fluctuation intensities, that the classical matching argument for the logarithmic law is probably correct. Finally, wall flows are identified as the seat of a second, spatial, energy cascade, different from the classical Kolmogorov one. It is conjectured that the large-scale intermittency of boundary layers might reflect a pattern-forming instability of this cascade, possibly related to certain anomalies observed in boundary layers over rough walls.