Turbulence structure in stably stratified open-channel flow

TLDR

Stable stratification alters turbulence structure in open‑channel flow, and a spectral‑equation model has been used to investigate these effects. The study measured velocity and temperature fluctuations in the outer layer of a stably stratified open‑channel flow with laser‑Doppler velocimetry and cold‑film probes, then analyzed turbulent intensities, flux correlations, and coherence‑phase relationships as functions of the local gradient Richardson number and compared them with a spectral‑equation model and other laboratory data. In strongly stable conditions, turbulence becomes wavelike and exhibits negative heat and momentum transfer against the mean temperature and velocity gradients.

Abstract

The effects of stable stratification on turbulence structure have been experimentally investigated in stratified open-channel flow and a theoretical spectral-equation model has been applied to the stably stratified flow. The measurements were made in the outer layer of open-channel flow with strongly stable density gradient, where the wall effect was small. Velocity and temperature fluctuations were simultaneously measured by a laser-Doppler velocimeter and a cold-film probe. Measurements include turbulent intensities, correlation coefficients of turbulent fluxes and coherence–phase relationships. These turbulent quantities were correlated with the local gradient Richardson number and compared with the values calculated using a spectral-equation model and with other laboratory measurements. In stable conditions, turbulent motions approach wavelike motions, and negative heat and momentum transfer against the mean temperature and velocity gradient occurs in strongly stable stratification.