Cellular Secondary Currents in Straight Conduit

Abstract

An investigation of cellular secondary currents, i.e., three‐dimensional flow patterns, is very important in hydraulic engineering because these currents might cause three‐dimensional sediment distributions and bed configurations such as sand ribbons in straight rivers. It is, however, fairly difficult to measure secondary currents of water flows, even by making use of hot‐film anemometers, because the velocity of secondary currents is within 5% of the mainstream velocity. Thus, on the basis of the hypothesis that the existence of free surface may not be an essential cause of secondary currents, the present study has investigated experimentally the turbulent structure of their currents in air conduit. Considering an essential interaction between secondary currents and bedform, longitudinal ridge elements were attached onto both lower and upper bottoms of the conduit, which simulated the longitudinal ridges and trough of river bedform, i.e., sand ribbons. All three components of the velocity were measured accurately by hot‐wire anemometers. The structure of secondary currents was examined through the equations of mean flow vorticity and mean flow energy.