Self-adaptive flaps on low aspect ratio wings at low Reynolds numbers

Self-adaptive flap is a bio-inspired passive flow control device which emulates the covert feathers of birds in a most rudimentary form. An exploratory study to ascertain the effectiveness of these self-adaptive flaps in delaying flow separation on low aspect ratio wings of three different planforms were carried out in a low speed wind tunnel at a Reynolds number of 105 based on their root chord. Flap widths between 0.08c and 0.15c; placement location between 0.4x/c and 0.8x/c were investigated. The flapped configurations exhibit good stall margin and higher CLmax in most cases. The chordwise placement location of adaptive flap for better lift enhancement varies with planform. Adaptive flap placed at the maximum span location on varying span planform exhibits better lift characteristics than flap placed at other chordwise location. However, for a constant span planform like rectangle, chordwise location close to trailing edge seems to be optimal for flap placement. No detrimental effects are observed in the drag characteristics of the single flapped configurations of rectangular and inverse Zimmerman planform. However, significant drag increase is observed for the flapped configurations of Zimmerman model and double flapped configurations. The temporal behavior of the adaptive flap is aperiodic. Spectral analysis of the flap oscillation shows multiple dominant frequencies between 10 Hz and 50 Hz for different angles of attack for both rectangular and inverse Zimmerman planform. The peaks occur at same frequencies for most angles of attack irrespective of planform. Skin friction pattern obtained from oil flow visualization experiments shows that the separation bubble which extends the entire chord for clean model at post-stall angles of attack, is contained upstream of the flap which leads to better post-stall lift characteristics.