Parallel ribbon vortex: A phenomenological flow feature in an atmospheric boundary layer near sloped terrain

In this study, flow behaviors and vortex morphology around the sloping terrain that is covered by an atmospheric boundary layer are investigated using a combination of numerical and experimental methods. An array of wind tunnel experiments and high-fidelity large eddies simulations have been conducted, considering different slope angles theta = 90 degrees, 45 degrees, 26.5 degrees, and 18.2 degrees. Systematic analysis of turbulence phenomena pertaining to the flow field's topography-driven distortions and accelerations was performed quantitatively via time-averaged velocity field, terrain-induced turbulence intensity, velocity spectrum, and acceleration ratio. Two recirculation zones, one originating from the upstream boundary layer's gradual separation bubble (GSB) and the other from the forced separation bubble (FSB) behind the downstream sharp leading-edge, were confirmed by detailed visualization of the flow field. An acceleration zone of considerable magnitude is also observed, connecting the GSB and FSB. Subsequently, a new vortex structure-the parallel ribbon vortex (PRV)-was disclosed. The PRVs originate from the disordered turbulence structures inside the GSB, spiraling into coherent structures. The accelerated mainstream up the step then stretches and elongates the coherence structures into ribbon-like vortices parallel to the mainstream, completing the formation process. Finally, the PRVs transport fluid up the step before encountering the strong spanwise vortices and dissipating thereafter. The investigation into the slope flow characteristics and turbulence structure conducted in this study has the potential to enhance the prediction of wind characteristics and wind load specifications in mountainous regions.