# Matching Reynolds Number in Scale Model Wind Tunnel Testing

• Andres
In summary, the drag coefficient for a truck and trailer is a function of the Reynolds number. If you can't match the Reynolds number with the real one, you can use CFD simulations or wind tunnel testing to prove your point.
Andres
For my senior design project, I have to reduce the aerodynamic drag of a truck and trailer. I am doing CFD simulations and wind tunnel testing. The dimensions of the wind tunnel (test section) are 94 in long, 13.5 in tall and 20 in wide and a cross-sectional area of 270 in^2 and it could go up to 110 mph. The truck and trailer has a total length of 712 in and a frontal area of 10560.2175 in^2. I'm trying to figure out a way to match the Reynolds Number of the full truck and trailer using a scale down model but have a blockage ratio of less than 5%. The Reynolds Number of the full scale is 34,695,627.05 at a velocity of 65mph and 712 inches for characteristic length. A scale of 1:17 gives me a blockage ratio of about 5% but a Reynolds number of 3,137,150 at 100mph and 41.88 in long, which is nowhere close to the actual Reynolds Number. What do you guys recommend I do about the wind tunnel testing if I can't match the Reynolds Number to the real one? P.S, I need the velocity to be 65mph because at that speed, most of the fuel is used to overcome drag.

Sorry for the lengthy paragraph but I'm stuck.

The drag coefficient is the most important dimensionless parameter for your study. The drag coefficient is a function of the Reynolds number. Here are some typical relationships:

For high Re (>3500), you can usually assume the Cd is constant. The exception is for an object with not well-defined fixed separation points (like as sphere) where a dip is seen before stabilizing (usually somewhere around Re = 106).

What you can do is plot the Cd vs Re for your scale model. If you see a dip and it is below your testing Re, then you can assume that the Cd will be similar for your full scale truck (at a greater Re).

You also mentioned CFD simulations, so you can plot the Cd vs Re that way as well to further prove your point (since you won't be able to measure high Re with your scale model).

## 1. What is Reynolds Number and why is it important in scale model wind tunnel testing?

Reynolds Number is a dimensionless value that represents the ratio of inertial forces to viscous forces in a fluid flow. In scale model wind tunnel testing, it is important because it helps to ensure that the flow conditions in the wind tunnel are similar to those in the real-world scenario being simulated. This allows for accurate and reliable results from the scale model testing.

## 2. How is Reynolds Number calculated in scale model wind tunnel testing?

Reynolds Number is calculated by multiplying the characteristic length of the model (such as its wing span or body length) by the airspeed and dividing that value by the kinematic viscosity of the fluid being used in the wind tunnel. This calculation is the same for both the scale model and the real-world scenario being simulated.

## 3. Can a scale model wind tunnel test accurately represent real-world conditions?

Yes, as long as the Reynolds Number is matched between the scale model and the real-world scenario. This ensures that the flow conditions are similar and allows for accurate representation of the aerodynamic forces and behaviors.

## 4. How is Reynolds Number matched in scale model wind tunnel testing?

Reynolds Number can be matched by using a suitable scaling ratio for the model and the wind tunnel, as well as adjusting the airspeed and fluid viscosity in the wind tunnel to match those in the real-world scenario. The scaling ratio and adjustments are based on the calculated Reynolds Number for the real-world scenario.

## 5. What are the limitations of matching Reynolds Number in scale model wind tunnel testing?

While matching Reynolds Number is important for accurate results, it does not account for all factors that may affect the aerodynamic behavior of the model. Other factors such as surface roughness, model material, and model flexibility may also impact the results. Additionally, scale model testing is limited in its ability to fully represent complex fluid dynamics, making it important to validate results with other methods such as computational fluid dynamics (CFD).

• Mechanical Engineering
Replies
4
Views
2K
• Engineering and Comp Sci Homework Help
Replies
9
Views
2K
• Aerospace Engineering
Replies
10
Views
2K
• Aerospace Engineering
Replies
6
Views
12K
• Aerospace Engineering
Replies
24
Views
40K
• Mechanical Engineering
Replies
2
Views
11K
• Aerospace Engineering
Replies
4
Views
5K
• Aerospace Engineering
Replies
6
Views
21K
• Aerospace Engineering
Replies
5
Views
3K
• Aerospace Engineering
Replies
2
Views
1K