Characteristics of aerodynamic force and flow structure behind single box girder under isolated slit control

An isolated slit was placed in a single box girder to obtain passive leading-edge suction and trailing-edge jet flow to control the unsteady aerodynamic force and modify the flow structure. The Great Belt East Bridge was used as a physical model at a geometric scale of 1:125. Wind tunnel experiments were conducted at an incoming airflow speed of 10 m/s, and the Reynolds number was calculated as 2.3×10⁴ using the test model height and wind speed. The surface pressure distribution was measured, and the aerodynamic force acting on the test model with and without the isolated slit was calculated by integrating the pressure result. It was found that the control using an isolated slit can dramatically decrease the fluctuating surface pressure distribution and aerodynamic force. An analysis on the power spectral density of the lift force revealed that the isolated slit accelerated vortex shedding. Moreover, high-speed particle image velocimetry was used to investigate the wake flow structure behind the test model. A vortex separated from the upper surface was pushed to a lower location and the wake flow structure was modified by the isolated slit. A proper orthogonal decomposition (POD) of the flow field showed that the first two POD modes in the controlled case contributed less energy than those in the uncontrolled case, indicating that more energy was transferred to higher modes, and small-scale vortices had more energy. A secondary instability structure was found in the wake flow for a nondimensional jet momentum coefficient J of 0.0667.