Lagrangian analysis on routes to lift enhancement of airfoil by synthetic jet and their relationships with jet parameters

Using Lagrangian Coherent Structures, the influences of synthetic jet parameters, namely, velocity and frequency, on lift at post-stall attack angle are studied in detail, from viewpoint of mass transport. First, the pressure coefficient distribution and the Lagrangian Coherent Structures evolution process of the non-controlled airfoil and the controlled airfoil, at a post-stall attack angle, are compared with each other through numerical simulation, and hence the influences of the mass transport process on the lift coefficient are discussed. Then, the mass transport process and its influences on the lift coefficient under different jet velocities and frequencies are analyzed further. As the conclusion, it can be drawn that there are mainly four routes to enhance the lift of airfoil at post-stall attack angle using synthetic jet, that is, (a) The synthetic jet can directly improve the fluid momentum of the boundary layer in the middle section of the suction surface; (b) The suction phase of the synthetic jet significantly reduces the pressure on the front section of the airfoil suction surface; (c) The synthetic jet could carry the high momentum fluid from the mainstream into the separation zone at the rear section of the airfoil, thus reducing the suction surface pressure of the rear section; (d) The tail vortex reduces the pressure on the suction surface of the airfoil tail. In particular, the jet parameters affect the lift coefficient by these four routes. Specifically, the fluid momenta of the boundary layer in both the front and middle sections of the suction surface are greatly increased with the jet velocity, and the contributions of the tail vortex and the high momentum fluid from the mainstream to the lift are reduced with the jet velocity. As for jet frequency, the contributions of the tail vortex and the high momentum fluid from the mainstream to the lift are reduced with the increase of jet frequency.