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.

Fan Exhaust (Diffuser)

One of the issues coming up was how to attache the fan to the 150 x 150 mm cross-section? the fan is only 132 mm in diameter. To to insure that there is no backflow or dead space makes this a hard problem to solve. If the shroud goes from the testing cross-section to the cross-section of the fans 132mm the back pressure increases.

After another long search on google and looking at videos and papers from MIT and NASA found that this type of design has been used in closed wind tunnels. At the moment the main worry is that this shroud would upset the laminar flow in the testing section. Currently the tunnel has good laminar air flow. Now before the math was more of a concept to check if it could even be done at the size. This was done without the design even being done with random information. Since than the tunnel has come into life and a simple analysis of the flow, using sting on a bar showed good flow with the core of the testing section.

What is happing mathematically

In the testing section without the air-strainer. The cross-sectional area is 0.15m x 0.15m, the fans is 120mm in blade diameter and spins at 2200rpm. The air density is 1.2kg/m^3, the viscosity is 1.8 x 10^-5 Pa sec. >Assuming that the pc fan has flow "Q" of 60-80 CFM and we use the middle value of 70 CFM, converting this value to meters we get a "Q" of 0.033m^3/s of flow. Once we have this value and the cross-sectional area of the testing tunnel you can calculate the the velocity of flow. Flow speed is velocity "V" = flow rate "Q" divided by the Cross-Sectional area "A". Plugin the values and we get 1.47m/s.
This value is needed to see what the Reynolds numbers are in side the testing section. Basically Reynolds number are unit less values that are used to check for flow behavior such as laminal, transitional, and turbulent flow. This lets us see what is happening in the wind tunnel without the air-strainer. Re= ρ V D / μ plugin the values and we get 1.2 x 1.47 x 0.15 / 1.8^-5 = 14700.
What does this value tell us? It states that we have really turbulent air flow in tunnel. This will come apparent when doing the same mathematical process with the air-strainer, as this reduce the air flow, thus giving a lower Reynolds Number "Re". Using the same formula that was used be for to find out "Re" value but this time we add the air-strainer. The dimensions of the air-strainer are 150mm x 150mm and 100mm long each cell is 6mm in diameter. Looking at the air-strainer we get the following result, Re_cell = (1.2 x 1.47 x 0.006) / (1.8 x 10^-5) = 588. This is a great result as it tell me that that my string test has shown that there is laminar flow. The Re_cell value of 588 shows that indeed there is laminar flow as the Re value is less than 1000. Any value above this will be transitional laminar flow. Meaning it will straighten out again. If the Re value is greater than 2000, there is only turbulent flow.
This very interesting as it shows when the air-strainer is removed the airflow is greater but turbulent. This shows while the fan can generate airflow up to 1.4 m/s in open conditions, introducing the flow straightener significantly reduces that velocity due to added flow resistance and a drop in static pressure. After passing through the strainer, the airflow slows to approximately 0.06 m/s, which corresponds to a Reynolds number of 588. This lower Re value is well within the laminar flow regime, helping to ensure stable and uniform flow conditions in the test section.