AE 2251 –AERODYNAMICS I Two Marks Questions With Answers 2014

Anna University, Chennai


Department of Aeronautical Engineering

AE 2251 –AERODYNAMICS I Two Marks Questions.


1. Define momentum principle.

Net force acting on a mass of fluid is equal to rate of change momentum of flow in that direction.

2. Define moment of a momentum principle with reference to a turbo machine.

he torque acting on a rotating fluid is equal to the rate of change of momentum.

3. What is equation of continuity in one3 dimensional compressible flow

UA = cons

4. State equation of continuity in 1D incompressible flow


5. What is Newton 2nd law?

Rate of change of momentum is directly proportional to applied force acting on me direction


6. Types of applied force

A: load, gravitational force

B: shear force, viscous force, pressure force

7. Define circulation.

It is a flow means that the flow system could be resolved into a uniform irrotational portion and a circulating portion and if circulation is present in a fluid motion vorticity must be present even though it is restricted to a space.

8. What is aerodynamics?

It is the study of flow of gases around the solid bodies.

9. Diff control volume and control surface?

Control volume has a fixed boundary, mass, momentum and energy are allowed to cross the boundary. The boundary of the control volume is referred to as control surface.

10. Define system?

It is defined as fixed mass with a boundary however with time the boundary of a system change but the mass may remain the same.

11. Differentiate compressible and Incompressible flow.

In a compressible flow, Density of a flow density will not be change from point to point in a fluid flow, for incompressible flow density will not be change from point to point in a fluid flow.

12. What are the forces can be experienced by the flowing of fluid on a system

1. Body forces like gravity , electromagnetic forces(or)any other forces which act at a distance on a fluid inside volume.

2. Surface forces like pressure and shear stress acting on the control surface S.

13. What is the principle of conservation of mass?

Mass can be neither created nor destroyed. This is the basic principle for continuity equation.

14. Differentiate steady & unsteady flow.

In a steady flow fluid characteristics is velocity, pressure , Density etc at a point do not change with time but for unsteady flow these characteristics will change

with repeat to time.


15. Define source flow?

The flow will radialy outwards towards a fixed point.

the strength of the source flow is q/2πr

16. Define sink flow?

The flow will be radialy inwards towards a fixed point.

The source of the sink flow –q/2πr

17. Define free vortex?

A flow field with circular steam lines with absolute value of velocity varying

inversely with the distance from centre. The flow is irrotational at every point except at the centre.

18. Define force vortex?

A flow in which each fluid particle moves in a circular path with speed varying

directly as the distance from the axis of rotation.

19. Define doublet flow?

The combination of sink and source flow is called doublet flow.

20. Define vorticity?

It is defined as the value twice of he rotation and hence it is given as 2w

21. Define vortex flow?

It is defined as the flow of a fluid along a circular path or the flow of a rotating

mass of fluid is known as vortex flow.

There are two types 1) free vortex 2)force vortex

22. Define stream fn and its eqn?

Flow per unit time across the line joining two stream lines is called stram fn of stream line.

The unit of stream fn is m2/sec

Per unit dimension perpendicular to the plane of stream line stream fn gives volume rate of flow. It is a scalar fn.

23. Define potential fn and its eqn?

It is a scalar fn. It is a positive derivative with respective positive direction. it

satisfies Laplace eqn.

24. What is d’alemberts paradox?

For a stationary cylinder kept with the axis perpendicular to the flow of an ideal

fluid, no lift or drag force is felt.

25. Define Magnus effect?

For a spinning cylinder kept with its axis perpendicular to the flow of an ideal

fluid there is force only in one direction which is lift force this is called magnus effect Magnus effect is independent of the cylinder size.


1. When will conformal transformation break down?

If aerodynamic design to involve only 2D flows at low speeds the design method based on conformal transformation theory would be best choice this technique can extended to three dimensional.

2. When will joukowski transformation breakdown?

For finite angle trailing of an aerofoil the trailing edge becomes stagnation point.

For cusped trailing edge trailing edge of aerofoil no longer becomes stagnation point.

By method joukowski transformation the trailing edge becomes cusped and so the kutta condition is not satisfied. This is the draw back of joukowski transformation.

3. What is kutta trailing edge condition?

A; for finite trailing edge angle V1 = V2 = 0 strength of vortex at a a=


B; cusped trailing edge angle V1=V2

4 .state importance of Kelvin’s circulation

Circulation and hence cortex strength does not vary with time if (i) the fluid is non viscous (ii) the density is either constant or a function of pressure only (iii) body forces such as gravity or magnetic force such as gravity or magnetic force are single valued potential

5. State the importance of blasius theorem in aerodynamics.

For an assumed complex potential of a two dimensional flow past a body of given shape and orientation the aerodynamic forces per span length and moments per span length of the body are obtained in terms of complex variables of the flow .

6. What is barotropic fluid?

a fluid having density either constant or a function of its pressure is said to be a barometric fluid .

7. Explain the roll of the starting vortex (cast of vortex) in establishing the lift on an


The starting vortex builds and grows up to just the right strength such that equal and opposite clockwise circulation around the airfoil leads to smooth flow from trailing edge (at this situation kutta condition is successfully satisfied)when the point is reached the vorticity shed from the L.e becomes zero, the starting vortex no longer grows in strength and steady circulation exists around the airfoil.

8. Show that as per kutta condition the strength of vortex at trailing edge I zero (or)

what is kutta trailing edge condition

1)for finite t.e angle v1=v2=0 therefore strength of vortex at a = (t.e=0)

2)for cusped t.e v1=v2≠0 but as v1=v2 strength of vortex at a= (t.e ≠0 ) Kutta condition makes the the strength of vortex at t.e must be equal to zero.

9.state importance of kelvin circulation.

Circulation around a curve is un altered is transformation.

10. what is joukowski hypothesis ?

In order that the velocity at the t.e of airfoil is zero or stagnation point of the t.e , the circulation is increased to the flow past the airfoil to such a magnitude that a st.point coincides with the t.e .this method of specifying the magnitude of circulation is known as joukowski hypothesis

11. When a transformation is called conformal?

Transformation is conformal if small elements of area are unaltered in shape but altered in size position and orientation.

12. What is transformation?

It is a mathematical process by which a figure or network may be disorted or altered in size it is effected by means of an algebraic relationship between the orginal co rdinates (x,y)

And coordinates of each new position (δ,Ψ)


1. Assumption of thin airfoil theory

A; airfoil is assumed infinitely thin so that it may be represented by cambered line.

B; camber is assumed to small also angle of incidence also small.

2.State analogical electromagnetic theory to Biot- Savart law

The vortex filament is visualized as a wire carrying current ‘I’ then the magnetic

field strength dB induced at a point P by a segment of wire ‘dl’ with current in the direction of wire is

dB = µ I d l x r

clip_image0014 π | r | 3

3. Why Karman treftz equation is used?

The joukowski profile is suffer an important practical defect in that the trailing edge are

always cusped and hence this equation is used.

4. Define Helmholtz theorem

(i) Strength of vortex filament is constant along its length.

(ii) A vortex filament cannot end in a fluid .it must extend to the boundary of the fluid a form a closed path

5. Define prandtl lifting line theory

The flow past a finite wing if assumed to represent a vortex configuration as a vortex

filament of strength which somehow bound to a fixed location in a flow

6. Define aerodynamic center

It means the moment around the center is zero

7. Define centre of pressure

It means the moment is constant around its centre

8. Define potential flow of fluid

The irrotational motion of an incompressible fluid is called potential flows

9. Relate vorticity and circulation

Vorticity is the circulation around an element divided by its area

10. Assumption of horse shoe vortex

The wing is replaced by a single bond span wise vortex of constant strength which turn at

right angle at each end to trailing vortices which extend to infinity behind the wing these two trailing vortices i) each of which must provide the same total lift

ii) Each must have same magnitude of circulation and same circulation at mid span

11. State Limitation of lifting line theory.

A straight narrow wing with smooth pressure distribution theory agrees well.

Theory gives correct value of down wash along the centre of pressure of any distribution of left i.e.; symmetrical ahead and behind a straight line at right angles to the direction.

For curved or yawed lifting line of law aspect ratio theory is not adequate.

12. Define slender body of revolution

The radius of body is very small then length is knows as slender body revolution.


1. What is meant by boundary layer?

A narrow belt or layer of relatively slow moving fluid next to the surface of the body moving in the fluid is called boundary layer. There is a continuous change of velocity

through the layer from zero at surface to free stream value of outer edge of boundary value.

2. Define Boundary layer thickness?

That is distance in y character from boundary surface at which local velocity u=0.99U

where U is the stream velocity outside boundary layer.

3. Define Displacement thickness?

It is the distance by which boundary surface would have to be displaced outwards so that total actual discharge of fluid would be same as that of ideal fluid past the displaced boundary.

4. Define Momentum thickness?

It is defined as the distance along y direction from the boundary surface such that the momentum flux corresponding to the main stream. It is defined as the distance measure perpendicular to the boundary of the solid body by which the boundary should be displaced to compensate for the reduction in momentum of the flowing fluid on account of boundary layer formation

5. Define Energy thickness?

It is defined as the distance measured perpendicular to the solid body by which the boundary should be displaced to compensate for the reduction in kinetic energy of the flowing fluid on account of boundary layer formation.