Mechanisms of boundary layer transition due to isolated roughness on swept wings

An experimental study

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In the context of the ongoing research regarding boundary layer transition, this project aims to study the effects of isolated cylindrical roughness elements on the transition from laminar to turbulent flow in a swept wing boundary layer. The project entails experimental campaigns: the 45 degrees swept wing model is placed inside the test section of the Low Turbulence Tunnel (LTT) and millimeter-sized roughness elements are attached to it, so that their interference with the boundary layer flow can be studied. The main goal is to investigate the flow features generating aft the roughness elements and their development into turbulent streaks. Moreover, this project aims to understand and quantify the effects of different roughness sizes on transition mechanisms, in the specific case of a three-dimensional boundary layer. This is achieved by means of different experimental techniques, such as PIV, HWA and thermographic flow visualization.
The obtained results lead to a comprehensive description of this type of flows, with several phenomena changing their characteristics due to a change in parameters (freestream speed, roughness diameter and roughness height). Velocity field, spectral analysis of the signal, wedge width evolution, instability modes and velocity fluctuations in time were investigated with different parametric conditions, in order to understand how the flow was affected by
them. Furthermore, the study allowed to evaluate the use of non-dimensional parameters such as roughness Reynolds number and aspect ratio for the prediction of the flow topology. The complexity of the phenomena involved is underlined in the conclusions of this research, with the interaction of several flow features making it a broad and interdisciplinary field of research.