Longitudinal Mixing of Heated Water in Open-Channel Flow

摘要

A laboratory flume was specially designed and constructed, and was used to study the longitudinal dispersion of heated water in an open channel. An instantaneous heated water source, uniformly distributed over the cross sections, was measured at various points downstream of the source. The sides of the flume were smooth, but gravel was glued to the bottom to produce turbulence. Three-dimensional effects were found to be important. Velocity variations in the lateral direction increased the ambient flow dispersion coefficient almost threefold over the theoretical value for a two-dimensional flow with a logarithmic velocity distribution. The most important buoyancy-related phenomenon was a significant increase of the initial convective period, and a delay of the onset of the final diffusive period, of the longitudinal dispersion process. With appropriate scaling factors, all experimental curves approximated the corresponding relation for a neutrally buoyant two-dimensional flow. Lagrangian turbulence properties permitted rapid estimation of the dispersion coefficient. Scaling factors and lagrangian quantities both depended on the initial Froude number.