Experiments on turbulence beneath the free surface generated by an airfoil

An understanding of the structure and dynamics of free‐surface turbulence is essential for the correct interpretation of many interface phenomena, and the need for measuring turbulence characteristics beneath a free surface arises from its role in many important phenomena taking place at interfaces, such as gas and heat exchange in the ocean, which have huge influences on the balance of chemicals and energy. In many engineering and industrial problems, most exchange takes place at the interface between a gas and a fluid, and many large‐scale physical problems are governed by turbulence characteristics beneath an interface. The free surface represents a boundary for the flow domain and imposes some conditions: the material derivative of the free surface must be zero, while the tangential stresses should be zero unless a shear is exerted by the overflowing gas. The interaction between turbulence and a free surface is expected to vary with the level of turbulence. The two main measures for describing the phenomenon are the Reynolds number and the Froude number, which generally increase as a pair. At reduced Froude numbers, a free surface is essentially unaffected by the turbulence of the flow beneath, is almost flat and imposes only a reduction in the normal velocity component. In this limit, it can be described as a slip‐free rigid flat surface. At higher Froude numbers, the free surface is not flat, and an energy exchange with the fluid flow starts. Such an exchange is assumed to be initially very limited, but to be quite strong for a free surface which loses its connectivity and includes air bubbles and drops. There are a great variety of free surface patterns and several mechanisms of energy transfer at a free surface such as capillary and gravity waves. Many different ways to generate turbulence in the water side impinging the interface from below have been adopted and are documented in literature. The present experiments make use of an airofoil in water able to generate a wake partially reaching the free surface from the water side. The aim of this report is to detail all the methods, techniques, solutions to practical problems encountered during the experiments and to give all the information necessary for post processing the acquired data.