LES of passive scalar transport in ABL flow over fractal topography and evaluation of interfacial transfer coefficient relations

Environmental flows frequently encounter topography that is composed of a broad spectrum of modes, for example evolved fluvial topography or the rough, wind-driven ocean surface. In such cases, the topographic height field may exhibit a power-law energy spectrum. Furthermore, a significant range of these topographic modes may be more finely resolved than the resolution of a large-eddy simulation computational mesh, rending part of the topography subgrid-scale (SGS). Recently, Anderson and Meneveau, 2011: JFM 679 288—314 and Anderson et al., 2012: BLM (Accepted) have presented and applied, respectively, a dynamic approach to describing momentum fluxes (pressure drag) associated with SGS topography. This dynamic approach is based on an equivalence of fluxes self-consistency argument (Germano et al., 1991: PoF A 3 1760—1765). In the present work, the author considers models for passive scalar fluxes (i.e. potential temperature, humidity) in LES of ABL flow over multiscale, fractal-like topography. Typically, similarity theory with a scalar roughness length is used to parameterize SGS scalar fluxes. Unlike momentum transport, in which inertial pressure forces associated with turbulence are exclusively responsible for losses, no such mechanism exists for scalars. For this reason, the scalar roughness length is frequently modeled as an order of magnitude smaller than the momentum roughness length. However, it is also known that for hydraulically smooth topography, the scalar roughness length can significantly exceed the momentum roughness length. A dynamic scalar roughness model based on an equivalence of (convective heat) fluxes argument is presented. Results are shown for flow over synthetic fractal topographies. The model is stable, yields physically realistic results, and is simple to implement.