Direct Numerical Simulation of the Aerodynamically Rough Atmospheric Boundary Layer

Direct numerical simulation (DNS) of the atmospheric boundary layer (ABL) is becoming more and more popular for its conceptual simplicity and increasing degree of realism: domain sizes and simulation durations can be attained that allow for extrapolation of results to the geophysical limit. Geophysical flows predominantly occur over rough surfaces, which significantly affects drag, mixing and transport properties of the flow. At the same time, the representation of roughness surface-layer modelling is generally based on bulk representations of roughness related to the roughness parameters for scalar and momentum exchange z_0H and z_0M. Here, we circumvent surface-layer similarity by directly imposing the intricate mechanical boundary condition resulting from a rough wall, while maintaining the efficient and tuned numerical methods for Cartesian meshes by an immersed boundary method (IBM); three-dimensional roughness elements are fully resolved at the bottom wall of a direct numerical simulation. We follow a spline-based approach for a partially staggered arrangement that was introduced by Laizet and Lamballais (J. Comp. Phys 2009, Vol 228, p.5989-6015). By this approach, the flow boundary conditions at roughness objects are fulfilled exactly which allows for a straightforward treatment of roughness effects in the scalar field. We apply this implementation to investigate the effect of fully resolved three-dimensional roughness elements in a turbulent Ekman boundary layer and obtain a priori estimations of scalar and momentum roughness parameters for canonical roughness configurations.

* This work is funded by the ERC Starting Grant ”Turbulence-Resolving Approaches of the Intermittently Turbulent Atmospheric Boundary Layer [trainABL]” of the European Research Council (funding ID 851347).