The thermal structure of a wind-driven Reynolds ridge Supathorn Phongikaroon, K. Peter Judd, Geoffrey B. Smith, Robert A. Handler Abstract In this study, we investigate the nature of a Reynolds ridge formed by wind shear. We have simul- taneously imaged the water surface, with a deposit of a monolayer of the surfactant, oleyl alcohol, subject to different wind shears, by using a high-resolution infrared (IR) detector and a high-speed (HS) digital camera. The results reveal that the regions around the wind-driven Reynolds ridge, which have subtle manifestations in visual imagery, possess surprisingly complex hydrody- namical and thermal structures when observed in the infrared. The IR measurements reveal a warm, clean region upstream of the ridge, which is composed of the so called fishscale structures observed in earlier investi- gations. The region downstream of the ridge is composed of colder fluid which forms two counter-rotating cells. A region of intermediate temperature, which we call the mixing (wake) region, forms immediately downstream of the ridge near the channel centerline. By measuring the velocity of the advected fishscales, we have determined a surface drift speed of about 2% of the wind speed. The spanwise length-scale of the structures has also been used to estimate the wind shear. In addition, a comparison of IR and visual imagery shows that the thermal field is a very sensitive indicator of the exact position of the ridge itself. 1 Introduction The surfaces of natural bodies of water, such as lakes, streams, and oceans, are typically contaminated with thin films of surface active agents, or surfactants. The presence or absence of these surfactants is of importance in many diverse fields, including oceanography and remote sens- ing. These films are frequently found to be stretched and compressed by water currents, wind forcing, and other processes. When the forcing is sufficiently strong, a por- tion of the water surface may actually be cleaned of sur- factant. Under these circumstances, a thin line of demarcation can be observed between the contaminated and uncontaminated regions. It has been found that this line is, in reality, a slightly elevated region of the surface, and has become known as a Reynolds ridge (Reynolds 1900; Langton 1872; Edser 1926; Satterly and Turnbull 1929; McCutchen 1970). The first report of the visual appearance of a line of demarcation between a surfactant contaminated region and an uncontaminated region was that of Franklin et al. (1774). Much later, Reynolds (1900) showed that surface tension gradients were critical in understanding the ori- gin of the line, which is formed as a result of the surface contaminant being trapped against a barrier by an oncoming stream. More recently, theoretical models have been developed (McCutchen 1970; Harper and Dixon 1974) based on boundary layer theory, which attempt to predict the shape and size of the ridge. These theories were largely confirmed by recent experimental studies in which Schlieren methods (Sellin 1968) or specular reflection-based methods (Scott 1982) were used to measure the surface slopes in the region of the ridge. Other recent experiments (Warncke et al. 1996; Vogel et al. 2001) have employed advanced imaging and digital particle image velocimetry (DPIV) techniques to further explore the details of the hydrodynamics beneath the ridge. The investigations described above were principally devoted to the relatively simple case in which a Reynolds ridge is formed by water flow compressing a surface film against a barrier. In the present work, we investigate the more complex case in which wind shear drives the flow. We suspect that this is a relatively common situation in natural environments, where surface slicks are formed on bodies of water due to the compression of surfactant material by the wind (Woodcock 1941; Scott 1972; Romano 1996). Our goal is to elucidate the formation and dynamics of the wind-driven Reynolds ridge. Imagery of the free surface flow in the vicinity of the ridge was cap- tured simultaneously using an infrared (IR) detector and a high-speed (HS) digital camera. We show below that high- resolution IR methods are effective in elucidating the complex fluid physics in the vicinity of the Reynolds ridge. In particular, by comparing IR imagery with HS digital imagery, we conclude that IR methods can be used to accurately locate the position of the ridge. Experiments in Fluids 37 (2004) 153–158 DOI 10.1007/s00348-004-0794-2 153 Received: 12 August 2003 / Accepted: 25 January 2004 Published online: 19 March 2004 Ó Springer-Verlag 2004 S. Phongikaroon (&), K. Peter Judd, G. B. Smith, R. A. Handler Remote Sensing Division, Naval Research Laboratory, Washington, DC20375, USA E-mail: supathorn@nrl.navy.mil We thank Professor K.A. Flack at the United States Naval Acad- emy for lending us the wind tunnel. We also want to thank Dr. C. Trump for help in constructing the wind-wave tunnel. This work is supported by the National Research Council through the Naval Research Laboratory, and the Office of Naval Research.