Formation of local separation zones on the wings with a rigid and varioform wavy surface
In the present paper, a method for choosing the parameters of a wing-surface waviness intended for use on small aircraft is introduced. We propose choosing the waviness geometry based on the parameters of the boundary layer for the given regime of the flow around the classical wing installed at zero angle of attack, a = 0°. We propose to check the result of the choice of the waviness parameters by the change of the flow structure in the boundary layer of the wing. For the properly chosen waviness, the continuous separation zone characteristic of the classical wing should transform into local separation zones. To substantiate and verify the proposed method, we investigate the flow around two wing models, one model being a wavy wing with a rigid surface and the other, a wavy wing with a varioform surface characterized by variable hump height. The study was carried out at wing-chord-based Reynolds numbers Re ranging from 0.35×10<sup>5</sup> to 1.4×10<sup>5</sup> at a = 0°. Using oil-film visualization, a transition of local separation zones into a continuous separation zone on the rigid-surface wing model with a decrease of Reynolds number implemented by decreasing the flow velocity was revealed. By measuring the velocity profile in the boundary layer on the varioform wing, the transformation of the continuous separation zone into local separation zones yet at a constant velocity with a change in waviness height has been also demonstrated. It is experimentally shown that the formation of local separation zones is observed provided that the waviness height f is one order of magnitude greater than the displacement thickness s <sub>1 </sub>in the boundary-layer pre-separation region, and the waviness coefficient K <sub>w</sub> is greater than unity. A distinct formation of local separation zones is revealed by oil-film visualization at K <sub>w</sub> = 2.8.