Gap effects on the aerodynamic characteristics around three rectangular boxes in tandem arrangement

Multiple-box structures have become increasingly important in practical engineering; for example, some recent large-span bridges were creatively designed using triple-box decks. The flow characteristics and flow-induced responses of multiple-box structures are notably complex due to the existence of gaps. Herein, we conducted a detailed investigation on the effects of gap width on the aerodynamics and flow characteristics of three boxes in a tandem arrangement, that is, a triple-box model. The test model comprised three rectangular cylinders with a side ratio of 3.7 (i.e., SR = 3.7), and it was arranged in line with the incoming airflow. The gap ratio (L/D = the ratio of the gap width L to the height of the box D) was varied from 0 to 10.260. Surface pressure measurement and smoke-wire flow visualization were conducted in the wind tunnel tests. For the pressure measurements, the Reynolds number (Re) was varied from 1.01 × 104 to 2.20 × 104. The smoke-wire flow visualization was performed at a relatively low Re of 6767. The results showed that the gap ratio significantly influenced the pressure distributions, aerodynamic forces, and surrounding flow patterns. A “dual-frequency” phenomenon was observed at low and moderate gap ratios; that is, a dominant frequency and secondary frequency were found. Moreover, the secondary frequency was closely related to the secondary vortices. Furthermore, the dual-frequency phenomenon disappeared at large gap ratios, which was different from the tandem circular cylinders. Based on the experimental results, the flow patterns around the triple-box model were categorized into four basic types, depending on the gap ratio.