An experimental study of a turbulent shear layer at a clean and contaminated free-surface Amy Warncke Lang, Carlos E. Manglano Abstract An experimental study was performed to mea- sure the flow properties of a vertically-orientated shear layer in the vicinity of a free-surface. The effect of surface contamination on the near surface flow field was also determined. Digital Particle Image Velocimetry was used to measure instantaneous and averaged velocity, vorticity, and Reynolds stresses. Results show that the presence of surfactants can cause directional shifts of the shear layer, as well as an overall damping of the turbulence in the vicinity of the free-surface, except in the vicinity of a Reynolds ridge where an increase in Reynolds stress was observed. Keywords Turbulent shear layer, Surfactants, Free-surface 1 Introduction The study of surfactants on free-surface phenomena has ranged from understanding their effects on wave damping to two-phase flows. However, observations of their effects on turbulent flows at a free-surface has been limited. Da- vies (1966) was one of the first to notice the damping of turbulent eddies at a free-surface by the presence of surfactants. One of the more recent studies is that of Tsai (1996), who computationally looked at the presence of turbulence at a free-surface due to a wind-blown interface. The turbulence in this simulation was composed of the flow field associated with a wall-bounded shear flow (containing horseshoe or hairpin structures). In the con- taminated surface case, Tsai found an overall damping in the turbulent intensities for all three components when compared to a clean one. Another recent study by Milgram (1998) focused on short wave damping in the presence of both surfactants and sub-surface grid-generated turbu- lence. In addition, a study by Hirsa et al. (1995) looked at the decay of a columnar vortex attached normally to a free- surface in the presence of both a clean and surfactant coated surface. They found that the contaminated case showed a disconnection and therefore earlier vortex breakdown in the vicinity of the free-surface. Finally, Lang and Gharib (2000) showed that the connection process of vortices shed from the wake of a cylinder in a low Rey- nolds number flow near the free-surface was significantly altered by the presence of surfactants. In fact it was shown that these vertically-orientated vortices, which connect normally to a clean free-surface, would in fact pair up and form a vortex connection below and parallel to the free- surface. The presence of a clean free-surface on a turbulent flow field is one that has received much study in recent years. One of the more recent studies was that by Shen et al. (1999) where they described two kinds of layers associated with free-surface turbulence. The first is a thicker ‘‘source’’ or ‘‘blockage’’ layer, which, due to the kinematic boundary condition of a free-surface reducing vertical fluctuations, shows a redistribution of the turbulence intensity with an increase in horizontal velocity fluctuations. This layer typically has a size of the order of the characteristic macro length scale of the flow. The second layer is defined at the ‘‘surface layer’’ in which the magnitudes of both horizontal components of vorticity, as well as the gradient in vertical vorticity, are reduced. For high Reynolds number flow this layer is much thinner, typically of the order of a few millimeters, and is due to the dynamic zero-stress boundary condition at a clean free-surface. However in this study, a shear layer, orientated verti- cally to the free-surface, was used to study the effect of surfactants on the turbulent properties and their variations between that of the bulk fluid, clean surface, and con- taminated surface. In this case, the turbulent eddies have a more coherent structure to them than that found in gen- eral homogenous or even wall-bounded turbulence. Larger coherent structures are known to form in the free shear layer, and in this case are orientated vertically to the free- surface. The clean surface case of this shear layer was first studied at a higher Reynolds number by Maheo (1998). He observed a slight directional shift in the shear layer to the Experiments in Fluids 36 (2004) 384–392 DOI 10.1007/s00348-003-0648-3 384 Received: 2 September 2002 / Accepted: 16 April 2003 Published online: 23 January 2004 Ó Springer-Verlag 2004 A. W. Lang (&), C. E. Manglano Department of Aerospace and Mechanical Engineering, Parks College of Engineering and Aviation, Saint Louis University, 3450 Lindell Blvd.St. Louis, MO63103, USA E-mail: langaw@slu.edu The authors would like to acknowledge the support of this work through Saint Louis University. First, the award of a SLU 2000 Research Assistantship to the Department of Aerospace and Mechanical Engineering paid the tuition and stipend of Carlos Manglano. Secondly, this research was also made possible by an award through the Beaumont Faculty Development Fund to the first author.