Large-scale PIV measurements of the boundary layer beneath a towed structure

We present measurements of the developing boundary layer beneath a towed structure. As a validation, we initially tow a flexible (but flat) structure across stagnant water. This results in a comprehensive visualization of the classical case of a developing boundary layer: an initial laminar part, followed by a transition to a turbulent boundary layer. Repeated experiments (N = 100) show that the transition position is highly variable: for some cases, the boundary remains completely laminar, while for others it already transitions as soon as Rex = 1.5 × 105 (equivalent to a downstream distance of 1 m of the total length of 5 m). By binning the results according to the transition point, average velocity fields can be obtained for various transition points. The growth of the boundary layer in each region matches with the established predictions (∼x1/2 and ∼x4/5). After this validation, the experiments are repeated with surface waves, to investigate whether these waves affect the growth and transition behavior of the boundary layer. 1. Background Our study is part of an ongoing effort to understand the hydrodynamic forces that act on large, thin, flexible floating structures (Pot et al., 2025; Uksul et al., 2025). Examples of such structures range from floating solar panel arrays to ice floes. 2. Facility and methods The experiments are carried out in Towing Tank No. 2 at the Department of Maritime and Transport Technology of Delft University of Technology. This towing tank, measuring 85 m in length and 2.75 m in width, provides a well-regulated environment for hydrodynamic studies. During the experiments, the water depth h was maintained at 1.22 m and the water temperature - while not actively controlled - was recorded to be 18 oC.