Aerodynamics of a fly-swatter: how porosity pattern triggers stall

Our daily lives provide us with the perfect example of a porous object moving in a flow over a wide range of angles of attack: the fly swatter. While holes were originally introduced for durability and elasticity, their aerodynamic function is barely discussed, although they are known to strongly influence aerodynamic properties. By partially covering the holes of a square fly swatter, we experimentally investigate the effect of the porosity pattern on the aerodynamics of the fly swatter, and in particular on the development of stall in a three-dimensional context. We propose a minimum condition for stall mitigation on a partially-porous square plate based on a thorough investigation of 19 different patterns.

While the fly-swatter is an original, and enjoyable, direct application of the present work, it is also a canonical geometry (square perforated plate), that allows several generic conclusions to be drawn concerning the influence of a porosity pattern on subtle aerodynamic effects (such as the angular dependency of drag and lift, stall and bi-stable dynamics) of inclined plates.