EULERO-LAGRANGIAN SIMULATION OF RAINDROP TRAJECTORIES AND IMPACTS WITHIN THE URBAN CANOPY

An integrated approach is presented for numerical simulation of the wi nd-driven rain impacts on building surfaces. The numerical code combin es Eulerian simulations of the turbulent flows, Lagrangian random-flig ht simulations of heavy particle trajectories, and impacting water rat e computations. The flow is modelled using a version of CHENSI, a code based on the two-equation k-epsilon 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 modifie d to model the effects of turbulence, gravity and inertia. Three diffe rent models are derived from the work of Edson and Fairall (1994, J. g eophys. Res. 99, 25,296-25,311) and Mostafa and Mongia (1987, Int. J. Heat Mass Transfer 12, 2585-2593), and tested for application to raind rops that range in diameter from 0.2 to 2 mm. The distribution of impa cting drops on the various boundaries of the calculation domain is use d in combination with a rain distribution model to compute the amount of water that is absorbed by the street and building surfaces. A resul ting set of simulations is compared to experimental data and semi-empi rical formulations. The extension of the method to assess the impact o f other atmospheric hydrometeors, e.g. snowflakes or fog drops, is dis cussed.