A CFD-based Spectral Synthesis Method for Decoupling Longitudinal and Vertical Turbulence Effects on Box Girder Aerodynamics

Conventional aerodynamic analyses for long-span bridges often assume equivalence between longitudinal (u-) and vertical (w-) turbulence components, potentially compromising accuracy. This study introduces a novel numerical approach to decoupling u- and w-turbulence effects. By combining Adjustable Harmonic Multi-Frequency Spectral Synthesis (AHMFSS) with CFD simulations, we independently generated and analyzed u-/w-turbulence components, enabling separate control of their intensities (Iu, Iw) and length scales (Lu, Lw). This framework highlights the sensitivity of a box girder's total aerodynamic forces, pressure distributions, and flow characteristics to these individual parameters. The key findings are that the aerodynamic forces increase with Iu but decrease with Iw, whereas they increase with Lu but remain largely unaffected by Lw. The effects of u- and w-turbulence on surface pressure may align or diverge across different regions of the girder surface. This research establishes a refined framework that could significantly improve the accuracy of aerodynamic analysis for long-span bridges by explicitly distinguishing u- and w-turbulence contributions.