Executive Summary : | Flow over porous walls is a common phenomenon in natural and technological applications, such as riverbeds, urban structures, and aerodynamic applications. The presence of porous surfaces can significantly alter the boundary layer flow and the transport of mass, momentum, and energy. Numerical simulations suggest that small-scale coherent structures, such as streamwise and hairpin vortices, may be replaced by free-shear-layer-like Kelvin-Helmholtz structures over porous walls, altering the turbulence in such flows. Large-scale turbulence in a boundary layer over an impervious wall modulates the generation and transport of small-scale structures. However, the mechanisms involved in the generation of Kelvin-Helmholtz structures over porous walls and their interaction with large-scale boundary layer turbulence at high Reynolds numbers have not been thoroughly explored. To address this gap, a study aims to develop and utilize dual-view in-line holographic particle image velocimetry (PIV) to study the structure of turbulence in boundary layers over porous walls at moderately high Reynolds numbers. Sensor-based wall pressure measurements will be used to trigger image acquisition, identifying outer-layer large-scale flow events and testing the hypothesis that these structures play an important role in the generation and transport of Kelvin-Helmholtz structures over porous walls. This study is essential for understanding the mechanisms that sustain turbulence in flows over porous walls, which can help develop better models for high Reynolds numbers of practical interest. |