EFFECT OF LOCAL FLOW CONTROL ON TRANSITION IN A LAMINAR SEPARATION BUBBLE

This work examines the effect of active flow control on stability and transition in a laminar separation bubble forming on an airfoil. Experiments are performed in a wind tunnel facility on a NACA 0012 airfoil at a chord Reynolds number of 130,000 and at an angle of attack of 2 degrees. Controlled disturbances are introduced upstream of a laminar separation bubble forming on the suction side of the airfoil using a surface-mounted Dielectric Barrier Discharge plasma actuator. Planar, time-resolved Particle Image Velocimetry is used to characterise the flow field. The effect of frequency and amplitude of introduced disturbances on flow development is examined. It is shown that excitation can lead to significant changes in separation bubble topology and the characteristics of coherent structures formed in the aft portion of the bubble. Statistical, topological and linear stability theory analysis demonstrate that significant mean flow deformation produced by controlled disturbances does not necessarily lead to substantial changes in stability characteristics compared to that in unforced baseline case. The findings provide a new outlook on the role of controlled disturbances in separated shear layer transition and instruct the development of effective flow control strategies. INTRODUCTION A Laminar Separation Bubble (LSB) is a characteristic feature of flow development over an airfoil at low chord Reynolds numbers (Rec) below approximately 500,000 (e.g., Carmichael (1981)). It typically forms on the suction side of an airfoil due to laminar boundary layer separation, followed by separated shear layer transition and subsequent reattachment. In general, the airfoil performance degrades in the low Reynolds number domain, and the degree of performance degradation largely depends on the characteristics of the laminar separation bubble (e.g., Gaster (1967); Carmichael (1981)). Consequently, significant research efforts have been directed towards gaining an improved understanding of flow phenomena within separation bubbles (e.g., Dovgal et al. (1994); Burgmann & Schroeder (2008); Hain et al. (2009); Jones et al. (2010); Marxen & Henningson (2011)) and developing effective flow control strategies (e.g., Gad-el Hak (2001); Yarusevych et al. (2007); Postl et al. (2011); Marxen et al. (2015)). Early studies (e.g., Tani (1964); Gaster (1967)) have provided a comprehensive description of the time-averaged separation bubble topology. Subsequently, significant advancements have been made towards a comprehensive characterisation of separated shear layer transition (e.g., Marxen & Henningson (2011); Boutilier & Yarusevych (2012)). It has been shown that the transition process begins with the amplification of small amplitude disturbances in the separated shear layer. The initial amplification is associated with a band of unstable frequencies and is well modelled by linear stability theory (Boutilier & Yarusevych (2012)). The process is largely governed by a convective, inviscid Kelvin-Helmholtz instability. However, Alam & Sandham (2000) have demonstrated that an absolute instability can also manifest in the separated shear layer when it is bound by a strong reverse flow region near the wall. This requires maximum reverse flow velocities to reach approximately 15% of the edge velocity, which is uncommon in separation bubbles investigated experimentally. The later stages of transition in the separated shear layer are governed by non-linear interaction between the disturbances, and the continuous growth of disturbance amplitude leads to separated shear layer roll-up and periodic shedding of shear layer vortices in the aft portion of the bubble (e.g., Marxen et al. (2013)). These structures are argued to be responsible for producing time-averaged flow reattachment (e.g., Jones et al. (2010)). The key role played by the transition process in the development of separation bubbles makes these flows sensitive to the level of background disturbances in experimental facilities (e.g., Ol. et al, 2005) and numerical simulations (e.g., Marxen & Henningson (2011)). It also enables the implementation of active flow control strategies that can impact LSB characteristics and improve airfoil performance. Periodic excitation using external acoustic waves (e.g., Yarusevych et al. (2007)), embedded synthetic jets 1 June 30 July 3, 2015 Melbourne, Australia 9 7D-2