Low Speed Aerodynamics Questions

Page 2 DEN233 (JUNE 2020)

For air: r=1.225 kg/m3 and n =1.46´10-5 m2/s

Question 1

Part a: Only one answer is most appropriate in each of the following questions. For each question write the number of your answer in your Answer Booklet. Please note that you must provide a clear reasoning and explanation why you selected a particular answer, otherwise you will not get any marks.

i) Induced drag on a wing is: 1) Zero; 2) a function of circulation; 3) always negative; 4) negative or positive depending on the direction of the lift force.

ii) Which of the following statements is true?

1) The drag of a blunt object is less than the drag of an equivalent streamlined object at low Reynolds number;

2) The drag of a blunt object is always more than that of an equivalent 3) streamlined object; 4) At high Reynolds numbers the drag of a blunt object is always less than that

of an equivalent streamlined object; 5) None of the above.

iii) The transition of the laminar flows over wings can be delayed by:

1) Applying adverse pressure gradient; 2) Reducing the surface roughness; 3) Heating the flow; 4) Increasing free stream turbulence.

iv) Which of the following statements is false?

1) Turbulent Boundary Layers are less resistive to adverse pressure gradients; 2) Flow separation increases the form drag; 3) Reattachment of the boundary layer enhances the lift; 4) None of the above.

Question 1continues on the next page

or

DEN233 (JUNE 2020) Page 3

Question 1 continued v) In an aircraft having laminar boundary layer over the wings is preferable:

1) During the take-off; 2) During the landing; 3) During the cruise; 4) During the climb.

[2.0 marks each, 10.0 marks in total] Part b:

1- In aeronautical applications explain what is meant by flow control. Describe passive and active control methods and provide an example for an active control method commonly used for the control of boundary layer separation. Use a sketch to illustrate and explain your answer.

[6 marks]

2- The current aims of the European Commission in relation to the Greening the Air transport is to fund research in the areas related to the drag reduction of the aircraft by maintaining laminar flow as much as possible over different parts of the aircraft. Explain what the direct and indirect benefits of laminar flows are and which areas of an aircraft are targeted for laminar flows.

[6 marks]

Part c: Provide a clear picture on the effects of the adverse pressure gradient on the boundary layer flows; in particular explain how adverse pressure gradient affects the boundary layer physical thickness and velocity distribution. Use sketches with proper labelling to illustrate your answer. During the cruise flight of a typical civilian aircraft some parts of the aircraft could be exposed to adverse pressure gradient. Provide two examples and use sketches to illustrate your answer.

[6 marks] Part d: A thin two-dimensional aerofoil with a chord of 2.0 m is subjected to an external flow in which is changing linearly with distance ( corresponds to the leading edge of the aerofoil) according to equation 1:

(Equation 1)

where and are two constants to be determined. Note that the constant has a unit of meter.

Question 1 continues on the next page

eU 0.0=x

)1( A xUU refe +=

refU A A

Page 4 DEN233 (JUNE 2020)

Question 1 continued

1) Surface flow visualisation and flow measurement indicate that the boundary layer on the one side of the aerofoil does separate at a point where the local free stream velocity and the local boundary layer momentum thickness are 13.153 m/s and 0.9359 mm respectively. Based on the measured data determine .

and . [8 marks]

2) Determine the wall shear stress, boundary layer shape factor and boundary layer displacement thickness at point A ( m). Compare the boundary layer shape factor at point A with its corresponding value at the separation point. Clearly discuss the reason(s) behind the increase in shape factor from point A to the separation point. You should provide a sketch with proper labelling to justify and illustrate your answer. For this part you may assume m/s and ;

[6 marks]

3) With clear explanations provide two flow control methods (one active and one passive) that can be used in order to delay the boundary layer transition over the wings. Use sketches in order to illustrate your answer

[4 marks]

4) Clearly explain the effects of free stream turbulence, favourable pressure gradient and noise level on the boundary layer separation.

[4 marks]

Turn Over

refU

A sepx

5.0=Ax

15=refU 0.10-=A

To