Similarity law and turbulence intensity profiles in a bubbly boundary layer at low void fractions

Abstract

This paper follows on from the research recently published by Moursali et al. (1995). While the first data reported were relative to the void migration, the wall shear stress and in a very limited extent to the liquid velocity, the present results provide new information on the kinematic and turbulent structure of a bubbly boundary layer at low air concentrations. The mean liquid velocity is shown experimentally to obey a modified logarithmic law of the wall in the presence of millimetric bubbles. An expression for this modified law is derived by simple analytical considerations and non-dimensional scaling. The skin friction calculated on this theoretical basis fits the measurements satisfactorily. Longitudinal turbulence intensity profiles are also obtained. They show that the turbulence is increased by two relatively uncoupled mechanisms: a modification of the wall production and the creation of pseudo-turbulence in the external layer. Finally, the mixing length inferred from the data is given and compared with some of the models which are proposed in the literature.