Drag and Drag Partition on Vegetated Urban Canopies

An extension of the Raupach (1992) drag and drag partition theory is proposed, which incorporates drag and flow-sheltering contributions from buildings and vegetation. The treatment is fully analytical and based on the assumption that roughness elements (buildings and trees) are randomly superposed on a given flat surface. Model predictions are compared against results from an extensive series of large-eddy simulations (LESs) of flow over idealized urban canopies, where buildings are approximated a cuboids and trees as porous cylinders. A large number of configurations of buildings and trees are considered, to span a realistic range of planar area fractions, frontal areas, and height distributions. Model predictions are in good agreement with corresponding LES results, both in terms of total drag and drag partition. The model will be used to evaluate aerodynamic roughness and zero-plane displacement height parameters of the considered surfaces, as well as those of a realistic urban environment. Strengths and limitations of the proposed treatment will also be discussed.