Effects of tracheal microstructures on insect respiratory flows*

Insects have evolved respiratory systems that transport air inside complex microscale tracheal networks in a highly efficient way, as evidenced by the fact that their metabolic range is the highest in the animal kingdom. Insect respiratory flows are characterized by low Reynolds numbers (~0.1), but the Knudsen numbers in insect tracheae span the continuum, slip, and transitional regimes (~0.0001-1). In this work, we investigate the effects of the fine-scale internal tracheal morphology on intratracheal flows in insects in silico. We hypothesized that the helically winding taenidial microstructures found on the inner surface of the tracheal tubes determine the structure of the flow field near the wall and play a vital role in transport. We have closely reproduced the internal morphology of the tracheal tubes of the American cockroach, Periplaneta americana, in our computational geometry. To investigate this hypothesis, we performed a series of simulations at a Reynolds number of 0.1. We found that the presence of taenidia creates transverse helical flows close to the tracheal wall and concentrates streamwise velocity near the centerline of the channel, enhancing vertical transport.