My first sketchs

CAD Shots

CNCing the Main testing tunnel 1.0

Here, I decided to CNC the main testing tunnel before starting the Fab machining assignment. Since the material I wanted to use was unavailable, I modified the file to suit the materials that were on hand. This is true prototyping in its purest form. Now that some of the major parts are slowly taking form, and having done all the math in the simplest way, the important part is that the tunnel is built to fit a Pinewood Derby car. I also included the math for scaling, trying to determine how much airflow would be needed to maintain the scaled speed. To give an idea of scale, the wheel speed of a Pinewood Derby car at 1/8th scale is 261 mph, while the real speed of the car is 12 mph (just under 20 km/h). This provides the basic framework for the wind tunnel. The pages do have titles, but the quest for solutions is somewhat random, as the flow of questions and ideas came to mind. I will provide a PDF, which could be updated throughout the project.
This version of the test tunnel is still missing a finer mesh to further decrease turbulence within the testing area. However, for current testing purposes and to get an overall idea of functionality and size, it is sufficient. If, for instance, materials are not easy to source, I can recommend using cardboard as a testing material or even as a final material. Cardboard is a great resource as it is easy to cut and locally available. Below, you will see that in this version, I am using M3 bolts to hold the tunnel together. And yes, as long as there are no loads acting on the tunnel pulling it apart, and the bolts are not over-tightened, the threads cut into the wood will hold.

The PCB for the Tunnel

Now that I have finally made my first PCB, which serves as the main control for the tunnel and will also help in collecting data, the next step is to determine an effective method for measuring drag using a load cell. Currently, my thought process is as follows: The tunnel is not very large, so placing a load cell inside would create unwanted turbulence. However, looking at my overall design, I can mount the load cell beneath the main tunnel. This setup would allow for a string and pulley system, where an object could be attached via a hook. Once the fan is turned on, we could obtain a force reading acting on the object. The data collected here would provide a measurement of drag force, which, along with airflow velocity and object dimensions, can be used to analyze the Reynolds number and aerodynamic properties.
The idea is simple Drag is a force that can be measured. This is the Resistant force as an object moves through a viscous fluid. This is great as it allows us to use a load cell to measure the resistances. Unlike a scale where a weight is placed on top. Here the theory is to have a wire pull up. In assignment 9 I am working on calibrating a load cell. Sofar I have at least gotten it to show some reading but not ones that could be useful. But let take a look at the design that I mentioned here.
To make this work the design calls for two pulleys placed so that one is under the tunnel, where a small hole will be needed to pass the wire/string through. After talking to couple of people This may not be great idea. The reason is that the more bends and angles are involved the less accurate the results may be. Looking over the idea to have a small rode being pushed or pulled could work as well. Another idea is to have the sensor placed inside the tunnel.
In assignment 10 finally got to test the fan. The 120mm pc fan puts out about 10 cubic meters of airflow per minute. Even with the fan not bing fully enclosed it was pulling air efficiently to show Laminar flow. Here is a short video of the flow being blocked by my and hand. Later on I looked how close to the walls I could get with the help of the Professor William Megill. The flow was best in the middle of the tunnel. Showing laminar flow. The closer the test strings got to the wall the more turbulence appeared. With in about 25mm of the back wall it showed that the laminar flow was turning turbulent. This may be cause of the fans size of 120mm and the tunnel being 150mm. For now the result is good.

How to test your laminar flow

To visually see if the flow is laminar or turbulent, I did look into getting a old fog maker. This was easy as there is one in the lab but its seems to be defective. So rather than wast time trying find the problem, the simple solution is old school but will work even on modern cars and bikes. Before wind tunnel testing we would just attach strings to the car and take it down the road to see how the air flows around the surface. This method is still used on sails of sailing boats. So here is a small list of parts needed.

The math

How I came to the idea of building wind tunnel and how I decided on the scale and size. As a kid I was really into airplanes and in 8th grade I had a science class where we explored winds and lift. That same year this class got to Embry-Riddle Aeronautical University, Daytona Beach Florida. This is where I saw my very first and only wind tunnel and I have wanted one ever since. As I also teach MINT (STEM) in germany and I want thought this to be a cool project, I grabbed all the masters papers I could get my hand on and started reading. So I looked at the very basics of the math needed to design a small version of a wind tunnel. I wanted to add in all this math that was done to this point and have a all the explanations for what does what. Unfortunately I never did find a way of writing all the math equations. So at this point for anyone who would like to to have a look of what I did, here is a link to day download the zip file of photos. It is nothing crazy but it helped in finding out some this questions that came up long the way on designing the test chamber and the air-strainer. It also let me figure out what to expect when placing a Pin wood Derby car into the tunnel.