Define circulation.

A)
Line integral of airfoil
B)
Line integral around an in loop enclosing the airfoil
C)
Line integral around a closed loop enclosing the airfoil
D)
Line integral around an open loop enclosing the airfoil

Correct Answer :   Line integral around a closed loop enclosing the airfoil


Explanation : Circulation is defined as the line integral around a closed loop enclosing the airfoil of the component of the velocity of the fluid tangent to the loop. To calculate the force on an airfoil, outside the boundary layer the vorticity is zero, everywhere the circulation is the same around every circuit.