Describe how surface roughness affects the pressure drop in a pipe if the flow is (i) laminar.

Answer all questions. (Total 100 marks) Question 1 A Pitot-static probe connected to a water manometer faces a stream of air. The velocity of air is to be measured. [The conditions are: ambient air temperature 25 °C and pressure 101 kPa. Take the density of air = 1.184 kgm-3 and the density of water = 997 kgm-3. The acceleration due to gravity = 9.81 ms-2] (a) Identify the set-up with a sketch, clearly indicating the pitot-static probe, the
manometer showing its connections to the probe, the relative water levels between the two manometer tubes and the direction of the air-flow.
(5 marks) (b) Derive a suitable expression relating the velocity of air flow, V with the difference
in height, H of the two manometer tubes. (10 marks)
(c) The difference in height between the two tubes, H is 12 cm. Calculate the air
velocity, V in ms-1. (6 marks)
(d) Apply TWO (2) precautions that must be taken to ensure the accuracy of the
measurements. (4 marks)
Question 2 (a) Describe how surface roughness affects the pressure drop in a pipe if the flow is
(i) laminar.
(4 marks)
(ii) turbulent. (4 marks)
(b) How is head loss related to pressure loss? Explain how you would convert head
loss to pressure loss. (4 marks)
(c) Define the term, “frontal area of an aerofoil subjected to external flow”. When is
it appropriate to use the frontal area in drag and lift calculations? (6 marks)
(d) In the flow across an aerofoil, interpret “stall” and explain the causes for an
aerofoil to stall. (7 marks)

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