Resumen de PIV applications in vortex rings and oscillatory boundary layers
Particle Image Velocimetry (PIV) is one of the most used experimental techniques in fluid mechanics to obtain the velocity field of a flow. One of its most celebrated characteristics is that it does not have interference on the phenomenon of study which makes it suitable to describe qualitatively and quantitatively many phenomena either micro or macroscopic. This thesis presents the PIV technique applied to two different fluid mechanic problems: vortex rings impinging permeable boundaries and oscillatory boundary layers in the laminar-to-turbulent regime. The first part of this thesis focuses on the impingement of vortex rings towards permeable boundaries and compares the results with the interaction of a vortex ring with a solid boundary. Assuming that a vortex ring is an axisymmetric structure, 2D PIV experiments are performed over boundaries on 4 different permeabilities and a solid boundary. When a vortex approaches a solid boundary, three different phenomena are clearly visible: the vortex ring decelerates when the distance between the core and the wall is in the order of the initial diameter of the ring. At the same time, the diameter starts increasing producing a stretching effect and, finally, secondary vorticity appears after the ring has reached the minimum distance from the wall. Experimental results lead to some interesting conclusions when the permeability of the boundary increases: the deceleration of the vortex ring starts later, the diameter does not increase as much and, finally, secondary vorticity is weaker and has shorter life. The second and third part of this thesis focus on the study of oscillatory boundary layers over smooth and rough walls. Experimental measurements were conducted over smooth and two different rough beds spanning the laminar, transitional and turbulent flow regimes. A multi-camera 2D-PIV system was used in an experimental oscillatory-flow tunnel. Characteristic variables like boundary layer thickness and friction factor were computed using different methods. Results obtained experimentally in smooth wall experiments are consistent with theoretical work. For the rough wall cases different formulations have been compared. Finally, results show how the phase lead between wall velocity and free-stream velocity is better defined when the integral of momentum equation is used to estimate the friction velocity. The observed differences are highly sensitive to the zero level definition. Finally, a detailed analysis of the structures present in such oscillatory boundary layers yield to a description of four different features: vortex tubes present in oscillatory flows over smooth beds, and vortices, turbulent spots and shear layers present in oscillatory flows over rough beds. The inception of vortex tubes is consistent with the state-of-art predictors as a result of the Kelvin-Helmholtz instability. Furthermore, structures present in rough wall experiments are a little bit more complicated because their inception and evolution are clearly influenced by the position of the sediment grains forming the bed. Vortices are created behind a kink in the bed sediment profile during the wall flow reversal and are shed from the wall when flow starts its acceleration cycle. Both the vertical and horizontal evolutions of the vortex position depend on the ratio between the amplitude of oscillation and roughness of the sediment bed. Turbulent spots are defined as structures which are born vortices but lose their shape in an early stage. They follow the same trajectories as vortices but reach lower heights before dissipating. Finally, shear layers were only detected in the larger bed roughness and are described as a sum of vortices that are shed consecutively from the same sediment. These shear layers are linked to vortices during the wall flow reversal when a big vortex is formed in the same place as the shear layer.
