The maximum lift of wing-shaped bodies in unsteady motion converges to about C_L=2.5

摘要

An experimental study has been conducted to unravel the mechanism of the generation of unsteady high-lift forces acting on wing-shaped bodies impulsively started from rest for Reynolds numbers in the range of 103-104. Experimental results show that a maximum lift coefficient reaching about 2.5 at the incipient stage can be attributed to the reaction of downward momentum transfer driven by the starting vortex in collaboration with a successive vortex attached to the body surface. In the last four decades, many investigations have been conducted on aerodynamic forces and the flow structure around various kinds of bodies and oscillating wings with high angles of attack. In a number of papers, the maximum lift coefficient (or side force coefficient) converges to about 2.5. It is also shown that such high lift forces are insensitive to the geometry of the model, while the flow structure around the body is critical. This structure is known as strong downwash, the flow of which is perpendicular to the main stream.