Numerical study on the aerodynamic admittance of moving trains under harmonic wind fields

The aerodynamic admittance function (AAF) is essential for accurately evaluating unsteady aerodynamic forces on moving trains. This study numerically investigates the AAFs of a CRH2 (China Railway, High-Speed train) model [including the head car (Car_h), middle car (Car_m), and tail car (Car_t)] in harmonic pulsating flow. The results show that aerodynamic admittance amplitudes and buffeting forces of trains exhibit excellent superposition in harmonic wind fields. Aerodynamic admittances of moving trains increase with train speed, with the Car_h being least sensitive and the Car_t being most affected. The lift admittance exceeds those of the drag and moment admittances for moving trains. Compared to the Car_m and Car_t on moving trains, the Car_h has the smallest aerodynamic admittance. When the reduced frequency (Fr = fB/U) exceeds 0.4, aerodynamic admittances tend to stabilize. For stationary trains at low Fr (Fr < 0.35), the drag and moment admittances fluctuate near 1. However, at high Fr (Fr ≥ 0.4), substituting the Car_h aerodynamic admittance of moving trains with that of stationary trains overestimates the unsteady forces on trains. Aerodynamic admittances for the Car_m and Car_t of stationary trains are lower than those of moving trains at high speeds. Using the Car_m and Car_t aerodynamic admittances of stationary trains to estimate aerodynamic characteristics of moving trains adversely affects the safety assessments. A fitting formula for the aerodynamic admittance of trains in harmonic wind fields is proposed. Compared to an empirical formula, it shows better agreement with simulations, particularly at low Fr (Fr < 0.25).