ENGR217 Summer 2018 Q B3[PARTIAL] official answers Left out: Parts (a) (7 marks) and (b) (2 marks) are omitted: (a) asks for the wind-tunnel air speed of a one-fifth scale model by Reynolds-number similarity and (b) asks whether a 75 m/s tunnel can still give a fair drag coefficient. Dynamic similarity / dimensional analysis is not on the current ENGR5002 syllabus (tagged CHECK in the audit). Parts (c)-(e) do not need their answers; the stem data they use is kept below.
Answer ALL parts (c) - (e). [Parts (a) and (b), on dynamic similarity, are omitted.]
A road haulage truck has an approximate rectangular frontal area 2.5 m wide by 3.8 m high. To investigate the drag on the truck, a one-fifth scale model is to be tested in a wind tunnel. We can assume that the density and viscosity of the air are the same for the model as for the full size truck. The truck travels at 120 km/h. [From part (b): the wind tunnel available is only capable of generating an air speed of 75 m/s.]
[Data Book: air at , .]
Formulas you may need
- Drag force: , A = frontal area for road vehicles (learn this; it was on the old data sheet)
- Power to overcome drag: (learn this)
- (c)[5]For the one fifth scale model tested at an air speed of 75 m/s the drag force is 700 N. In these conditions, what is the drag coefficient?
- (d)[5]So if we assume that the drag coefficient for the full size truck has approximately the same value as (c) above, what drag force will it experience when traveling at 120 km/h?
- (e)[6]If the vehicle speed is reduced to 100 km/h, calculate the associated reduction in power required to overcome the drag force, commenting on the result.