Unsteady separated-flow dynamics over a box-girder section under sinusoidal inflow: Effects of angle of attack and Keulegan–Carpenter number

Abstract

The aerodynamic behavior of bluff bridge-deck sections in unsteady inflow depends on both the angle of attack and the time scale of the imposed velocity fluctuation. Active-control wind tunnel measurements and two-dimensional unsteady Reynolds-averaged Navier–Stokes simulations are used to examine surface pressure, aerodynamic forces, and time-averaged separated-flow topology of a streamlined box-girder section under sinusoidal streamwise inflow. The measurements validate the imposed velocity oscillation and pressure response at experimentally accessible frequencies, while the simulations span −15° ≤ α ≤ 15° and Keulegan–Carpenter numbers 0.5 ≤ KC ≤ 24.0. For |α| ≤ 8°, mean and root mean square pressure distributions vary comparatively smoothly, and the influence of KC on mean force coefficients is generally weaker than that of α. At |α| = 12° and 15°, chordwise pressure redistribution and upper-lower asymmetry become pronounced, while RMS pressure and force coefficients increase markedly, particularly at lower KC, which here represents shorter forcing periods. Pressure and force spectra remain dominated by the imposed frequency (F = f/fu = 1), with only weak second harmonics in some lift spectra. The parameter Πα,KC, incorporating KC and the angle-dependent projected dimension, approximately organizes fluctuating drag for both signs of α, whereas fluctuating lift remains strongly dependent on the sign of α. Time-averaged streamline and vorticity fields show that α and KC jointly reorganize mean recirculation regions and alter the mean-wake width, deflection, asymmetry, and streamwise organization. These findings distinguish excitation-frequency locking from amplitude modulation and clarify the coupled roles of α and KC within the present two-dimensional scope.