Effect of a bioinspired upstream extended surface profile on flow characteristics and a drag coefficient of a circular cylinder

Abstract

<abstract> <p>In the current work, the passive drag reduction of a circular cylinder for the subcritical Reynolds number range of 5.67×10<sup>4</sup> to 1.79×10<sup>5</sup> was computationally and experimentally investigated. First, inspired by nature, the aerodynamic drag coefficient of a whole Peregrine Falcon was measured in a subsonic wind tunnel for various angles of attack and Reynolds numbers (<italic>Re</italic>) and compared with the bare cylinder. At a 20° angle of attack and <italic>Re</italic> = 5.67×10<sup>4</sup>, the whole falcon model had a 75% lower drag coefficient than the bare cylinder. Later, with the moderate Falcon model, in which the falcon's beak and neck were linked to the cylinder as an extended surface, the drag coefficient decreased up to 72% in the subcritical Reynolds number zone. Finally, the extended surface with a falcon beak profile was connected to the cylinder with a stem and investigated both numerically and experimentally for various stem lengths, angles of attack, and Reynolds numbers. It was found that at low <italic>Re</italic>, the drag coefficient can be reduced by up to 47% for the stem length of 80 mm (<italic>L</italic>/<italic>D</italic> = 1.20) with an angle of attack 10°. The computational investigation yielded precise flow characteristics, and it was discovered that the stem length and the <italic>Re</italic> had a substantial influence on vortex generation and turbulent kinetic energy between the beak and cylinder, as well as downstream of the cylinder. Investigation revealed that percentile drag reduction was much lower for the whole Falcon model over a wide range of Reynolds numbers and positive angles of attack, which exist in nature. Similarly, when compared to the other stem lengths, the 60 mm stem length (<italic>L</italic>/<italic>D</italic> = 0.97) produced similar results to the whole Falcon model. The numerical results were well validated with the experimental results.</p> </abstract>