Influence of self-adaptive hairy flaps on the stall delay of an airfoil in ramp-up motion

It is known in the case of some birds that the coverts on the upper side of their wings pop-up under critical flight conditions such as the landing approach, thus acting like a brake on the spread of flow separation. Taking experimental investigations as its basis, this paper deals with the influence of various configurations of self-adaptable hairy flaplets located on the lower half of the wing and with chord-length c (dense rows of slender elastomeric flaps, L= 0.05c, 0.1c, 0.2c) on the flow around an NACA0020 airfoil at low Reynolds number flow (Re= 77 x 10(3)). Flow evolution along the airfoil when in ramp-up motion (alpha(0)=0, alpha(s)=20 degrees, reduced frequency k= 0.12) was measured with and without hairy flaps, with growth in the chord-normal thickness of the separation region above the airfoil investigated in order to determine stall onset time Ts. Whereas small flaps with L= 0.05c do not change the overall stall process, it was possible to use configurations with L=0.1c (double-row, triple-row configuration) to delay stall onset Ts by a factor of around 2-4 when compared with the clean airfoil. The motion of the flaps and the flow field were measured simultaneously at high temporal resolution using highspeed PIV. Correlation between flap motion and velocity distribution showed that backflow induced by vortex structures is indeed prevented by the hairy flaps. A significant difference was identified in the shear-layer roll-up process, which was almost regular and locked with the fundamental frequency on the covered airfoil with no signs of non-linear growth over longer periods. By way of contrast, in the case of the clean airfoil the early merging of the shear-layer vortices and a rapid increase in the thickness of the separation region were observed. It is therefore concluded that mode locking is achieved between flap rows with an interspacing of 0.15c- 0.2c, while the fundamental shear-layer roll-up wavelength measured (lambda(0) approximate to 0.15c-02c) indicates the relevance of flap row arrangement at the specific Reynolds number. Furthermore, interaction between shear-layer vortices and flaps in the row furthest downstream leads to the beneficial modification of the trailing edge flow in a way which increases bound circulation. (C) 2014 Elsevier Ltd. All rights reserved.