Acceleration effect on the drag of a bristled wing at low Reynolds numbers

Miniature insects use drag to provide the weight-supporting force, and the drag is produced by rapidly accelerating the wing downward at an angle of attack close to 90°. It is known that a membranous wing can generate large drag during rapid acceleration at low and high Reynolds numbers (Re). However, for the smallest miniature insects, the wings become bristled. Here, we consider the case of a bristled wing in rapid acceleration motion. It is found that, when Re is less than approximately 40, the rapidly accelerating bristled wing can generate a large drag peak, similar to that of the membranous wing, but the acceleration effect on the drag of the bristled wing decreases sharply as Re exceeds 40. This is because the interaction among the array of bristles, which causes the large drag peak, becomes weak at high Re. These findings may explain why only the smallest miniature insects (Re below 40) use bristled wings.