Aerodynamic overshoot of rectangular cylinders under accelerating inflow conditions

The overshoot phenomenon, where aerodynamic coefficients of bluff sections under accelerating flows exceed those in steady flows, is critical and controversial issue in the study of non-synoptic wind effects. Experimental simulation of accelerating flows requires complex dedicated facilities, which makes the investigation of key acceleration parameters affecting the aerodynamic overshoot still a challenging task. Therefore, numerical methods that allow easy control of acceleration parameters are urgently needed. In this study, aiming at verifying the accuracy of existing force evaluation approaches, we consider four types of accelerating inflows, namely, Linear, Hyperbolic, Sinusoidal, and Turbulent. The unsteady aerodynamic forces of rectangular cylinders with aspect ratios ranging from 3 to 16 are investigated using Unsteady Reynolds-averaged Navier-Stokes and Large Eddy Simulations. Sixteen different accelerating cases are considered to examine the effects of dimensionless acceleration parameters (ap) that defined by flow acceleration relative to velocity and deck height. Results show that accelerating flows cause drag overshoots well described by Morison equation but reduce vortex-induced lift forces. The ap is the key parameter governing drag overshoot, as it increases the absolute values of windward positive and leeward negative pressures. Additionally, the added mass coefficients are identified and an empirical formula is proposed to accurately estimate drag overshoot. The study provides guidance for the numerical simulation and drag estimation for bluff sections impinged by non-synoptic winds.