Investigation of local winds in a closed valley: An experimental insight using Lagrangian particle tracking

An experimental study of two-dimensional katabatic and anabatic flows, and their interaction with an urban heat island centred in a closed valley is presented. Down- and up-slope flows are generated via cooling and heating 20° inclined plates. The urban heat island is simulated by an electric heater centred in the valley. In order to understand the main features of the circulation established by thermal effects in an initially stably-stratified environment, an advanced Lagrangian particle tracking technique (Hybrid Lagrangian Particle Tracking) is employed. This allows one to obtain the velocity and acceleration of passive tracer particles as the first and second derivatives of a moving spline function that filters the particle trajectory coordinates. Experiments show the dependence of mean quantities and turbulent statistics on different slope heating. Furthermore, the effects of the slope flows on the circulation in a large city located in a narrow valley appear significant. During day-time simulations the urban heat island circulation is opposed by anabatic winds, creating critical situations for pollutant dispersion. During night-time simulations, the katabatic winds increase the city updraft motion.