1. How starting vortex is formed?
a) As the airfoil begins to move vortex are formed
b) As the airflows on the airfoil vortex are formed
c) As the airfoil moves against the relative wind vortex are formed
d) As the airflows on the circular body vortex are formed
Explanation: As the airfoil begins to move it carries this vortex, known as the starting vortex along with it, pioneering aerodynamicists were able to photograph starting vortices in liquids to confirm their existence. The vorticity in the starting vortex is matched by the vorticity in the bound vortex in the airfoil.
2. How airfoil increases its speed?
a) Flow over the topside
b) Flow over the leading edge
c) Flow over the bottom side
d) Flow over the trailing edge
Explanation: As the vorticity increases the bound vortex and also progressively increases and causes the flow over the topside of the airfoil to increase in speed. The starting vortex is soon cast-off the airfoil and is left behind, spinning in the air, where the airfoil left it.
3. How stagnation point on the topside of the airfoil reaches the trailing edge?
a) Due to viscous forces
b) Due to surface forces
c) Due to pressure forces
d) Due to drag forces
Explanation: The stagnation point on the topside of the airfoil then moves until it reaches the trailing edge. The starting vortex eventually dissipates due to viscous forces. As the airfoil continues on its way, there is a stagnation point at the trailing edge. The flow over the topside of the airfoil conforms to the upper surface of the airfoil.
4. Which of the following ensures flow smoothly leaving the trailing edge given the right value of circulation?
a) Kutta Condition
b) Momentum Theorem
c) Angle of Attack
d) The Shape of the Airfoil
Explanation: According to the Kutta Condition, for a given angle of attack, the value of circulation around the airfoil is such that the flow leaves the trailing edge smoothly. The velocity at the trailing edge is dependent on the shape of the airfoil.
5. It is possible to have lift without friction (i.e. in an inviscid medium).
a) True
b) False
Explanation: Nature enforces Kutta Condition through the means of the boundary layer (friction). If there were no friction Kutta condition would not be achieved and there will be no lift.
6. For which of the following Kelvin’s theorem is applicable?
a) Flow with Viscous Stresses
b) Compressible Flow
c) Inviscid, Compressible Barotropic Flow
d) Flow with Non-Conservative Body Forces
Explanation: Kelvin’s Theorem is applicable for the special case of barotropic flow while dealing with inviscid, compressible flows.
7. Which type of the following flow is characterized by density being a single-valued function of pressure only?
a) Viscous Flow
b) Barotropic Flow
c) Inviscid Flow
d) Baroclinic Flow
Explanation: A barotropic flow is a fluid where density is a function of pressure only, i.e. ρ = ρ(p). Baroclinic flow is the fluid which is not only dependent on the pressure but on other factors also. Viscous and inviscid flows are not necessarily dependent on pressure only.
8. A vortex sheet in the incompressible, inviscid fluid dies after some time.
a) True
b) False
Explanation: According to Kelvin’s theorem vortex sheet cannot die since circulation has to remain constant with time. It says a vortex sheet stays forever, in the ideal case.
9. For a fluid initially at rest, the formation of starting vortex implies ______
a) generation of lift
b) generation of circulation
c) generation of lift and circulation
d) no lift is produced
Explanation: From Kelvin’s Theorem, circulation remains constant with time. So for initial zero circulation, the formation of starting vortex means there has to be equal and opposite circulation in the form of lift.
10. Generation of lift is accompanied by a starting vortex at the trailing edge. If the flow is inviscid, this will not happen. What reason can best describe this?
a) There is no boundary layer formation, hence no vorticity
b) Kutta Condition is enforced
c) Kelvin’s Theorem is violated
d) Starting Vortex dies off instantly
Explanation: For inviscid flows, the boundary layer is not formed. Therefore, in the regions of high velocity, high viscosity is not there and hence no vortex can form. Thus, there is no lift produced. Starting vortex cannot form in inviscid medium and in the viscous medium it dies due to viscosity.