Active flow control over a NACA 0015 airfoil using a ZNMF jet

The effect of using a wall-normal, zero-net-mass-flux (ZNMF) jet located at the leading edge of a NACA 0015 airfoil as an active flow control device was investigated. Experiments were conducted over a two-dimensional airfoil in a water tunnel at a Reynolds number of 3.08 x 10 for parametric investigations and particle image velocimetry (PIV) and at a Reynolds number of 1.54 x 10 for flow visualisations. The largest lift increases were observed when a non-dimensional frequency of 1.3 and an oscillatory momentum blowing coefficient of 0.14% were employed. Under these forcing conditions the stall angle of the airfoil was mitigated from an angle of attack of 10 to an angle of attack of 18, resulting in a maximum lift coefficient increase of 46% above the uncontrolled lift coefficient. Planar laser induced fluoroscopy (PLIF) and PIV revealed that the lift increments were the result of the generation of a train of large-scale, spanwise lifting vortices that convected over the suction surface of the airfoil. The presence of these structures resulted in the flow seemingly remaining attached to the upper surface of the airfoil for a wider range of angles of attack. Introduction Flow control over airfoils is primarily directed at increasing the lift and decreasing the drag produced by the airfoil. This is usually achieved by manipulating the boundary and shear layer flows in order to minimise the separation region over the suction surface of the airfoil. Active flow control refers to the process of expending energy in order to modify the flow [2]. This is distinct from passive techniques where flow control is provided without expending energy through means such as geometric shaping. One of the main advantages of active, rather than passive, flow control is that the control device can be switched on and off when required. Many techniques for implementing active flow control have been proposed previously. These include: piezoelectric devices, vibrating flaperons, oscillating wires, boundary layer suction and blowing devices and the ZNMF jet that was studied here. A ZNMF jet ‘transfers linear momentum to the flow system without net mass injection across the system boundary’ [7]. ZNMF jets are commonly formed using a sinusoidally oscillating membrane to alternatively force fluid through an orifice into the external flow field and entrain fluid back through the orifice. During the forcing stroke the ejected fluid separates at the sharp edges of the orifice, forming a shear layer that rolls up to form a vortex ring for the case of a round synthetic jet or a vortex pair for the case of a plane synthetic jet. By the time the membrane begins its intake stroke, the vortex pair is ‘sufficiently distant from the orifice that it is virtually unaffected by the entrainment of fluid into the cavity’ [3,7]. The flow around an airfoil actively controlled by a ZNMF jet can be characterised using the following non-dimensional groups. Firstly, the non-dimensionalised excitation frequency is defined as: ∞ + ≡ U fc F (1) Where c is the chord length of the airfoil and f is the excitation frequency. The second parameter of significance is the oscillatory momentum blowing coefficient; a measure of the momentum imparted on the flow field by the ZNMF jet normalised by a characteristic momentum for the unexcited flow field, namely: