Influence of shallowness on growth and structures of a mixing layer

Abstract

chapter discusses the development of two shallow mixing layers with different water depths that is analyzed experimentally by means of laser doppler anemometry (LDA). The experiments show that bottom friction plays an important role in the growth of the mixing layer width, and in the strength and dimensions of the large quasi-two-dimensional turbulent structures therein. The initial growth rate of both mixing layers is found similar to what it is found for deep-water plane mixing layers. Further downstream, the reduction of the growth rate can be ascribed to the decrease of the velocity difference between the two ambient streams in combination with the suppression of the growth of the large turbulence structures. In most shallow mixing layers, the influence of the bottom friction is dominant, impeding the further growth of the mixing layer width. The reduced mixing layer growth is related to a loss of coherence in the large turbulence structures. Experimental data show that the mixing layer development is severely affected by bottom friction. The growth of the width and of the large scale structures in the mixing layer depend on the turbulence generated at the bottom. Regarding the modeling of such flows, it is important to have a correct estimate. The successful inclusion of the effects of bottom friction in the scaling of the global mixing layer properties does not imply a correct scaling of the large-scale structures present in the mixing layer.