Turbulence Structure for Fully Developed Flow in Rough Pipes

Abstract

This paper presents the results of an experimental study of fully developed turbulent flow in rough pipes. The investigation was undertaken in an attempt to better understand some of the changes in turbulence structure which accompany a change in wall roughness, and the experimental results include energy spectra of the fluctuating velocities in each coordinate direction, as well as microscales and macroscales of turbulence. The results indicate that the turbulence structure in the central region of the pipe is relatively independent of surface roughness, while the structure near the wall is very much dependent on the nature of the solid boundary. For hydraulically smooth flow, the principal effect of the wall is to radially suppress the extent of the larger eddies in the flow field. In the transition regime, the longitudinal extent of both large and small eddies is disrupted by the roughness elements, and the dissipation eddies appear to be stretched radially as they intermesh with the sand grains. The large eddies are almost completely obliterated by the roughness particles in the fully rough flow regime, while a tendency toward isotropy of the small eddies is noted. Thus, very definite changes in the flow structure occur as wall roughness is varied, and it is postulated that these changes are due principally to the fact that the roughness elements occupy the space where the three-dimensional streaky structure originates in smooth pipe flow.