LES exploration of alternative coflow jet flow control application on a NACA 6421 wing

Abstract

This work employs Large Eddy Simulations (LES) of coflow jet (CFJ) active flow control on a NACA 6421 airfoil to examine the characteristics of the CFJ wing without flow control and explore an additional two modes of operation. The first section of this work compares the modified NACA 6421 wing geometry without flow control (i.e. Cmu=0.0) to the baseline wing and the CFJ wing with steady flow control activated at a jet momentum coefficient of Cmu=0.2 to assess the change in wing performance. The second part of this study explores use of a sinusoidal pulsed jets instead of steady jets at non-dimensional frequencies of St=2.0, 10.0, and 25.0, and an additional case for St=10.0 with a 50% duty cycle to examine the effect forcing frequency and mass flow rate have on flow control effectiveness. To explore the ability to sustain a significant increase in lift, the third section studies reversing the direction of the flow control at a steady angle of attack of 12 degrees which generates a dynamics stall like vortex. The flow conditions for this work was a free stream Mach number of 0.1 and a Reynolds number based on the airfoil chord of 500,000. The FDL3DI solver was employed for the LES using a sixth-order compact spatial discretization scheme in conjunction with an eighth-order filter. The first study showed the CFJ wing without flow control produced 22% less lift than the baseline NACA wing and developed a larger separation region at the wing trailing edge. The second study revealed that oscillating the jet flow control at the non-dimensional frequencies of St = 2, 10 and 25 generated different temporal flow structures over the CFJ wing and in its wake, however, the overall time-mean lift was almost unchanged. The case of St = 10 with a 50% duty cycle employed half the mass flow rate of the steady jet and oscillatory flow cases resulting in a 19% loss of lift. Although it generating less lift than the steady CFJ case, it did produced 60% more lift than the baseline airfoil. The preliminary case with upstream blowing on the wing at a static 12 degrees incidence was able to produce about 250% more lift than the baseline wing, but the effect was short term since the vortex separated from the wing after about five characteristic times. After shedding the vortex, a complex transition occurred which after about six characteristic times resulted in a flow that was developing a jet-induced vortex similar to that at the beginning of the simulation. This approach generated less lift than CFJ flow control with steady blowing for about 2/3 of the time simulated.