Fluorescent oil-film applied to measure skin friction of flat plate with leading edge separation bubble in low Reynolds number

Abstract

Under low Reynolds number conditions, the airfoil is prone to a laminar separation bubble (LSB), which causes problems such as lift drop and drag increase. In this work, the fluorescent oil film was applied to study the leading edge LSB from the perspective of skin friction. The experimental study is carried out on a flat plate with the leading edge LSB under the Reynolds number of [Formula: see text] (the characteristic length of the plate chord). The camera captures the leading edge LSB that is formed by airflow separation and reattachment, and the flow in the bubble is opposite to the free flow. The position of separation and reattachment is determined by the distribution of skin friction, which is consistent with the visual image of the oil film. Applying the Poincare–Bendixson (P-B) index formula to test the topology of the separation bubble skin friction, the quantitative relations between the critical points and the boundary switching points in the closed region of several permeable boundary all confirm this topology law. The results show that the fluorescent oil film can quantitatively map the flow structure of the LSB, and further verify the advantage of this method in the visual measurement of the complex flow structure.