Effects of flow orientation on the onset of motion of flooded vehicles

Flood risk in urban areas is recognized as a crucial issue as the population living in cities is constantly increasing with a consequent rise in the value of exposed asset. It has been demonstrated that, in developed countries, most of the fatalities during a flood occurs inside vehicles. In fact, the combination of water depth and velocity acts so that a car can be swept away also for very low water depth. This work aims at identifying the critical conditions for vehicles incipient motion in order to support flood risk mapping and management and to promote people’s education. The dimensionless mobility parameter ΘV derived in a previous study is modified to account for the effect of different flow orientations. The dimensionless mobility parameter ΘV accounts for both flood and vehicle characteristics, thus it overcomes the scatter of diagrams showing the critical dimensional pairs of water depth and velocity. 3D numerical simulations in the OpenFOAM framework have been carried out to understand the role of the angle of incidence of the flow with respect to the car. The hydrodynamic forces are evaluated for different flow regimes, both subcritical and supercritical. The results of the simulations highlight that not only the area exposed to the flow matters for the onset of motion, but also the flow field under the vehicle planform. Existing flume experiments on small-scale models are used for the numerical model validation and as a reference for the interpretation of the numerical results.