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Consider the turbulent flow around a truck. Say that the truck is 10m long, 2m wide and 2m

high. You have to determine the drug force acting on this truck as it moves with a speed of

100 km/h

a. You will make an experimental measurement with a 1/10 model of the truck.

i. What should be the minimum size of the wind tunnel you are using?

ii. What is the wind speed at the inlet of the wind tunnel?

iii. What measurements would you take (pressure, velocity, etc.? )

iv. How do you post-process the recorded measurements to calculate the drug

force on the actual truck.

b. You will make a simulation to calculate the drug force.

i. Would you use the actual 3D model of the truck, or would you simplify the

geometry? Why?

ii. What should be the minimum size of the flow domain?

iii. What are the boundary conditions set?

iv. Would you prefer RANS, unsteady RANS, LES or DNS? Why?

v. Assume you have to use RANS, which turbulence model would you use, and

why?

vi. What should the first wall distance (∆????????1) be in your mesh? What is the

corresponding ???????? +? Do you use a wall model?

vii. Which data do you extract from the results of the simulation to calculate the

total drag force?

c. Estimate the time and money you would need to conduct

i. Experiment

ii. Simulation

iii. A back-of-the-envelope type estimation based on analytical methods.

d. How would these methods compare in their accuracy?

A)

a) Experimental Measurement:

i. The minimum size of the wind tunnel can be determined using the principle of dynamic similarity. The length scale of the model truck is 1/10 of the actual truck. Therefore, the wind tunnel should have a minimum length that is 10 times the length of the model truck, which is

ii. The wind speed at the inlet of the wind tunnel should be scaled down according to the model scale. Since the model truck is of the actual truck, the wind speed of . Therfore, the wind speed at the inlet of the wind tunnel should be

iii. The measurements to be taken in the wind tunnel experiment would typically include:

Pressure measurements using pressure taps placed at different locations on

the surface of the model truck to capture the pressure distribution.

Velocity measurements using anemometers or hot-wire probes to measure the

velocity field around the model truck.

Flow visualization techniques such as smoke or dye injection to visualize the

flow patterns.

Drag force measurements using force balances or strain gauges to measure

the aerodynamic forces acting on the model truck.

iv. To calculate the drag force on the actual truck from the recorded

measurements, you would need to perform scale-up calculations. The drag

force acting on the model truck can be scaled up to the actual truck size using

appropriate scaling laws, such as the drag coefficient. By knowing the

dimensions of the model truck and the actual truck, along with the measured

drag force on the model, you can estimate the drag force on the actual truck.