Wave--turbulence interaction in free-surface channel flows

Wave--turbulence interaction in turbulent channel flows has been studied using microbubble tracers and visualization. In the experiments, two-dimensional waves of different lengths and amplitudes have been superimposed on a turbulent channel flow via a wavemaker. The conditions were chosen to be such that the frequencies of the interfacial waves were in the range of the ejection frequencies in the undisturbed flow. The results show that the waves cause an increase in the number of wall ejections, giving rise to an increase in the measured values of the turbulence intensities and Reynolds stresses. However, the increases in ejection frequency do not appear to be directly related to wave frequency. They correlate better with wave amplitude. Conditional averaging of the velocity fields shows that while turbulence is increased in the region below the crest of the waves (extending to the wall), it is decreased in the wave troughs. The overall effect of the surface waves, over the range of conditions investigated is, however, to increase turbulence levels