| Popis: |
The mechanism by which periodic blowing and suction at the leading edge improves lift and stall angle of a 70° sweep delta wing is investigated in water tunnel experiments using PIV measurements. Periodic sinusoidal blowing and suction with zero net mass flux is applied at the leading edge of the wing. The experiments were conducted at a freestream flow speed of 0.126 m/s, corresponding to a root chord Reynolds number of 25,000. The wing was kept at an angle of attack of 35° for this study. A forcing frequency of F" = 1.75 was used, which was shown in previous research to be most effective in improving lift. The momentum coefficient was kept constant at (Cjii) of 0.004. The two main vortices that dominate the unforced flow field are stationary without forcing. With forcing, however, the vortex centers travel both in spanwise and wing normal direction along an elliptic path. The streamwise vortex breakdown location is not changed as determined by measurements of the streamwise vorticity component. Instead, the forcing increases the axial velocity downstream of the vortex breakdown location, thus decreasing the local surface pressure and increasing normal force. This effect is attributed to the formation of a shear layer vortex during the blowing cycle, which carries high momentum fluid into the wake left downstream of the main vortex breakdown. The wake is weakened in synchronization with the presence of the shear layer vortex through the forcing cycle. |
