WHEN DOES MOTION RELATIVE TO NEIGHBORING SURFACES ALTER THE FLOW-THROUGH ARRAYS OF HAIRS

Many animals from different phyla use structures bearing arrays of hai rs to perform a variety of important functions, such as olfaction, gas exchange, suspension feeding and locomotion. The performance of all t hese functions depends on the motion of water or air around and throug h these arrays of hairs. Because organisms often move such hair-bearin g appendages with respect to their bodies or the substratum, we assess ed the effects of such motion relative to walls on the fluid flow betw een neighboring hairs. We compared flow fields near dynamically scaled physical models of hairs moving near walls with those calculated for such hairs in an unbounded fluid. Our results suggest that the methods an organism can use to change the flow through a hair-bearing appenda ge differ with Reynolds number (based on hair diameter). When Re is 10 (-2) or below, changing speed does not alter the proportion of the flu id that moves through rather than around the array, whereas moving rel ative to a wall increases it. In contrast, when Re is between 10(-2) a nd 1, changes in speed have a big effect on the proportion of fluid mo ving through the array, while moving near walls makes little differenc e.