Bottom flow patterns of high-speed trains in infinite long tunnel

Compressible WLES (wall-adapting local-eddy viscosity large eddy simulation) is used to simulate the unsteady flow field of 1:25 scale eight-coach and five-coach high-speed train models in long tunnel, validated by an aerodynamic wind tunnel test of 1:5 scale head-coach train in tunnel. Scale effect is small on surface pressure distribution between the 1:5 scale and 1:25 scale head-coach train models. Through correlation analysis of surface pressure around bogie cavities, the existence of spatially periodic bogie cavities is reckoned as the formation condition of airflow hunting motion under the intermediate coaches, while the existence of bogies strengthens the phenomenon. Blocking effect of bogies on bottom airflow dominates the hunting motion, while structural complexity of bogies is a minor factor. Comprehensive kinematic description of the bottom flow is given, where four patterns are classified: initial stage, transition stage, hunting motion, and wake flow, based on the characteristic frequency–wavenumber pairs of the fluctuating streamwise velocity. Initial stage presents high-frequency, large-wavenumber, and small-energy characteristics. Transition stage presents slightly greater coherence, with airflow over trailing edges of bogie cavities at 200 Hz. Bogie components are crucial for the coherent hunting motion, leading to alternate vortex shedding from the side gaps of bogie cavities at around 160 Hz. Switch from hunting motion to wake flow is found around the last inter-coach gap and under the tail coach.