Investigation of flow over oscillating NACA 4421airfoils in tandem configuration at low Reynolds number

The present study is on the experimental investigation of the flow over oscillating NACA4421 airfoils in various tandem configurations at low Reynolds number of 8.6 x 103. Force and two dimensional particle image velocimetry (PIV) measurements were carried out in the NTU closed loop water tunnel with a freestream velocity, U∞ = 0.102m/s. The effects of phase angle, ϕ, stagger distance, Sd (= x/c where x is the horizontal axial spacing between the tandem airfoils and c is the chord) and gap, G (= |y|/c, where y is the vertical spacing between the tandem airfoils) were investigated for both oscillating and static tandem airfoils. In-phase oscillating tandem configuration has shown its potential to increase average combined lift coefficient, CL up to 47.7% but resulted in a corresponding increase in average combined drag coefficient, CD up to 54% at α > ±30˚ in relation to the single oscillating case. It also attained its best combined CL / CD ratio of 2.33 which is 35% lower than the single oscillating airfoil at α = ±45˚ with Sd of 2. This is attributed to the in-phase oscillating downstream airfoil’s ability to generate 89% higher average CL than the single oscillating airfoil as a result of the down-washed vortex interaction between the primary shed vortex before its first stroke reversal. However, this interaction increases the average CD up to 5 times more than the single oscillating airfoil which decreases the combined CL / CD ratio for the in-phase oscillating tandem configuration. The out-of-phase tandem oscillating configuration produces the highest combined CL / CD ratios at larger α > ±30˚ with the larger Sd of 2.5 due to the downstream airfoil’s ability to generate large lift improvements of more than 100% and exhibiting comparable drag production in relation to the single oscillating airfoil through effectively capitalizing on its ‘indirect’ down-washed vortex interaction effects. Thus, making the out-of-phase oscillating tandem configuration at Sd = 2.5 the most ideal configuration in the present study, potentially viable for hovering flight.Doctor of Philosoph