Secondary vortices in turbulent boundary layers over combined roughness and temperature patterns

Earth topography is rich and diverse, ranging from smooth to highly rough topologies. Effects of varied topography not only impact local flow and ecosystems, but also extend to the atmospheric boundary layer (ABL), provoking transfer of energy, heat, and pollutants. In mountain ranges with extended valleys, cylindrical vortices propagate far above the surface, spanning the full height of the ABL in some cases. Added thermal complexity of low temperature mountain peaks compared to heated valleys affects the secondary vortices in various ways and even creates its own set of vortices. This work focuses on these secondary motions imposed by duality of roughness and thermal heterogeneities. Wind tunnel experiments considered stable boundary layer flow across surfaces of varied elevation and temperature. For smooth surfaces, temperature heterogeneity creates and strengthens secondary vortices up to a certain limit, where further heating then weakens vortices by the act of thermal stability. In rough cases, temperature and roughness effects on the secondary motions are opposite if the elevated part is kept at a higher temperature. As complex topography is common in nature, this work informs environmental and industrial systems such as wind farms, solar farms, and urban design.