Setup and characterization of a wind tunnel for turbulence manipulation studies

The development of flow control techniques to reduce aerodynamic drag is a topic of significant interest within the fluid dynamics research community. Drag reduction is, in fact, a key factor in lowering the emissions and operating costs of transport vehicles. The portion of drag resulting from the viscous forces arising in the proximity of surfaces, referred to as skin friction, is particularly relevant in aerodynamic bodies such as aircraft wings and fuselages. A promising passive flow control method that enables skin friction drag reduction consists of applying microscopic-sized grooves, known as riblets, to the surface. Experimental observations of riblet-manipulated boundary layers are usually carried out on riblet-equipped tiles installed within the surface of a bigger flat plate. The requirements for successfully conducting this type of experimental study are thus that the flat plate flow is canonical, adhering to the zero pressure gradient condition, and that the boundary layer behaves correctly. In this work, the last steps of the assembly of the recently renewed boundary layer observation facility of the Politecnico di Torino are covered. The experimental setup preparation involved the design of new components and solutions to enhance the facility's operating field, including different turbulence-tripping devices and a terminal flap for pressure gradient management. The wind tunnel has also undergone a preliminary characterization, where the flow quality and the effect of the previously mentioned devices have been tested. Static pressure measurements through the flat plate pressure taps have been employed to study the effect of the flap on the pressure field, obtaining a map of the realizable operating conditions for different flow velocities and flap deflection combinations. Hot wire anemometry was used to carry out boundary layer observations in multiple positions along the flat plate at different velocities, applying two different types of turbulence tripping devices. The natural transition of the boundary layer was thus highlighted, and the development of the artificially generated turbulent boundary layers was examined. Recently proposed diagnostic approaches have been applied to the study of the behavior of the turbulent boundary layers. This study therefore serves to test the reconfigured experimental facility, making sure that the previously introduced requirements are met, while also comparing the experimental results with data from previous works.