Eulero-Lagrangian simulation of raindrop trajectories and impacts within the urban canopy

An integrated approach is presented for numerical simulation of the wind-driven rain impacts on building surfaces. The numerical code combines Eulerian simulations of the turbulent flows, Lagrangian random-flight. simulations of heavy particle trajectories, and impacting water rate computations. The flow is modelled using a version of CHENSI, a code based on the two-equation κ-ε model developed to simulate flows in urban canopies and dispersion within streets and around buildings. Particle trajectories are computed by means of a Markov chain modified to model the effects of turbulence, gravity and inertia. Three different models are derived from the work of Edson and Fairall (1994, J. geophys. Res. 99, 25,296–25,311) and Mostafa and Mongia (1987, Int. J. Heat Mass Transfer 12, 2585–2593), and tested for application to raindrops that range in diameter from 0.2 to 2 mm. The distribution of impacting drops on the various boundaries of the calculation domain is used in combination with a rain distribution model to compute the amount of water that is absorbed by the street and building surfaces. A resulting set of simulations is compared to experimental data and semi-empirical formulations. The extension of the method to assess the impact of other atmospheric hydrometeors, e.g. snowflakes or fog drops, is discussed.