Multi-frequency numerical approach for determining two-dimensional aerodynamic admittance functions of a box-girder

The aerodynamic admittance function (AAF) is a basic transfer function for evaluating the buffeting response of long-span bridges. This study proposes an efficient multi-frequency numerical approach to determine the two-dimensional (2D) AAF of a box-girder under fully span-wise correlated sinusoidal flow fields. In contrast to conventional single-frequency wind tunnel tests or numerical simulations, the proposed method superimposes multiple prescribed harmonic components in one simulation, enabling the simultaneous identification of longitudinal and vertical AAFs over a broad frequency range. The numerically determined 2D AAFs of the box-girder are then compared with the empirical and theoretical results. Notably, the longitudinal and vertical AAFs of the lift and drag forces exhibit opposite variation trends for all angles of attack, indicating different contributions from the longitudinal and vertical turbulences to the aerodynamic admittance. These results suggest that the traditional concept of equivalent aerodynamic admittance may be inadequate for the present bluff box-girder section under the considered sinusoidal flow fields.