Open-Channel Flow Turbulence and Its Research Prospect in the 21st Century

In 1883, Osborne Reynolds reported experiments in real viscous pipe flows and made the seminal distinction between laminar or turbulent flow regimes, each with quite distinct flow characteristics. Early pioneering research on turbulence emanated from the Gottingen group of Ludwig Prandtl, who developed the concept of the boundary layer and proposed a mixing-length turbulence closure, and from the Cambridge group of G. I. Taylor, who provided a theoretical basis for a statistical description of turbulence, with specific results for isotropic turbulence. Important flows, such as those in open channels, boundary layers, and pipes are, however, turbulent shear flows, so the theory of isotropic turbulence cannot be directly applied to such shear flows. The concept of local isotropy, introduced by the Russian group of Kolmogoroff has proved useful in understanding these flows. These theoretical concepts will be briefly reviewed. Experimental research in turbulent boundary-layer and pipe flows has been conducted in air flows since the 1950s using hotwire anemometry. In contrast, most basic research on openchannel turbulence has only been conducted since the 1970s, because of difficulties in applying thermal anemometry to typical water flows in hydraulic laboratories. Since the 1980s, laser anemometry has made experimental studies in open-channel turbulence much less arduous ~though still rather expensive!, permitting detailed investigations of not only basic two-dimensional ~2D! uniform flows, but also, more recently, unsteady and ~3D! channel flows. The following highlights contributions to openchannel turbulence research, particularly those by our group at Kyoto University.