Applied Scientific Research 51: 217-222, 1993. E T.M. Nieuwstadt (ed.), Advances in Turbulence IV. (~) 1993 Kluwer Academic Publishers. Printed in the Netherlands. 217 Mixing in Horizontally Heterogeneous Flows J.M. REDONDO and I.R. CANTALAPIEDRA Dept. Ffsica Aplicada. Universitat Politdcnica de Catalunya, J. Girona 31, Barcelona 08034, Spain. Abstract. An experimental study of mixing across density interfaces produced by laterally hetero- geneous turbulence is presented in this paper. The turbulence is generated by a flow or air bubles rising through a density interface produced by brine and fresh water. The mixing efficiency, r/, of the process is measured comparing the increase in potential energy with the available kinetic energy. We find that there is a decrease in the global mixing efficiency of the process with the length of the tank, the shape of ~7(Ri) depends also on the air flow producing the turbulence, showing a geometrical limit to the ammount of kinetic energy which may be used for mixing. Key words: stratified flows - mixing efficiency - bubble-driven flows 1. Introduction Mixing across stratified fluids in geophysical situations, often takes place in hor- izontally heterogeneous processes, where the available kinetic energy is confined to a small region of the fluid, In these situations, vertical mixing is afected by the horizontal transport of mass. Important mixing proceses are associated with the formation of fronts, such as sea and mountain brezes, tidal stirring, etc. In most cases, and due to the intermittency of energetic processes, mixing is localized in time and space. There are conflicting views on the shape of the mixing efficiency versus Richard- son number curve for homogeneous flows, some authors, I1][2],have found a de- crease in mixing efficiency at high Richardson numbers while others [3], find a monotonic relationship between mixing efficiency and Richardson number. 2. Description of the experiment The experimental apparatus is shown in figure 1. Similar experiments using grid generated turbulence have been reported by [4]. The experiments used brine and fresh water in order to form a density interface. The density interface and the turbulence are characterized by means of a local 9Apg Richardson number Ri = put2 , where 9 is gravity, Ap the density step, p the reference density. The integral lengthscale of the turbulence, g, and the r.m.s. turbulent velocity u ~, are related to the air flow Q, which induces horizontally heterogeneous mixing across the interface. After filling up to 15 cm of the box with brine of a preset density, another layer of 15 cm of fresh water was carefully poured onto a floating sponge in order to avoid