Question

Automotive engineers plan to build a one-third scale model of a sports car and test it in a wind tunnel. The temperature of the wind tunnel is at 25°C. The drag force is measured with a drag balance and the moving belt is used to simulate the moving ground. If the sports car runs at 25°C, Wind tunnel test section L Fom Moving belt Drag balance Figure 4 Question 4 a) Propose the speed of the car that can be predicted by the wind tunnel if the maximum speed of the wind tunnel is at 300km/h. (8 marks) b) For the proposed value in (i), determine the speed of the wind tunnel to achieve similarity between the model and the prototype. (7 marks)

Answer 1

solni we are to utilize the concept of similarity to determine the speed of the wind tunnel. Assumption 1. Comprensibility of the car is negligible. drag 2. The wind tunnel walls are far enough away so as to not interface with the aerodynamic on the model cas 3. The model is geometrically similar to the prototype. 4. The wind tunnel has a moring belt to simulate the ground under the car. cSScanned with CamScanner

M = 1.849x10 kg-m/s Properties:- for air at atinospheric pressure and at T: 25°C, $=1.184 kg/m3 Analysis; since there is only one independent T in this problem. the similarity ean. halds if Tam T2, p where, SVL T2 (as If the max. speed of the wind tunnel is at 300 km/h. then we proposed the speed is 95km/h. of the car CScanned with CamScanner

(a) T2, in = Rem Sain bn → Moda ITs, p = Rep Sp V p Lp Mp which we solve for unknown wind tunned speed for the model tests, Vm & Prototype Vans - Voor de E) up - 95(1)*(*) (3) Vro - = 285 km/h where Lp/Lm = 3 (given) Thus to ensure similarity, the wind tunnd should be sun 285 km/h at Scanned with CamScanner