Creating a 2-Dimensional Fluid Wind Tunnel
Soap film tunnels act remarkably like wind tunnels, using a two-dimensional fluid film instead of three-dimensional wind. Film tunnels use a moving soap film (like a bubble) suspended between two pieces of nylon wire which flows around a stationary test object and can, with the right visualization techniques, produce striking examples of 2-D fluid flow as shown in the images below:
Soap film tunnels are commonly used to study 2D flow in laboratories, however we believe we can create a low-cost fluid flow visualization apparatus for a classroom setting. For this to happen, our soap film tunnel design must be inexpensive and portable. The basic structure for our design is adapted from academic papers, but has been pared down to a manageable size and cost. An important aspect of our design process will be careful documentation that outlines the components and assembly instructions to construct this soap film tunnel, as well as techniques for visualizing the flow so that it can be recreated with minimal testing and tuning. This is crucial because, while there are plenty of research and results to be found on liquid film tunnels in research papers, there is is no easily understandable and implementable instructions to manufacture a soap film tunnel that would be suitable for a classroom setting.
We will use thin-film optical interference (shining light through the soap film in a way that shows differences in film thickness) to visualize the fluid flow. The flow phenomena that will be focused on is vortex shedding (flow that oscillates back and forth when passing an obstacle in fluid flow), and we will 3D print a variety of test objects to visualize and observe how different shapes affect fluid flow.
The Design Process
We began by searching for scientific papers that used soap film tunnels to obtain their results, looking for diagrams, schematics, and dimensions to base our own design on. Over the past few decades many fluid dynamics researchers have published on their use of soap film tunnels, and we even got to see one of these academic soap film tunnels in person as a professor at Brown University uses one in his research. Here is a photo of the soap film tunnel in Professor Mandre’s lab:
The apparatus is large (maybe 10ft tall) and fixed to the table. Seeing the size in person, we realized that it would be impossible to implement in a classroom. Below is our preliminary design drawing, which focused primarily on shrinking the dimensions of Professor Mandre’s soap film tunnel while maintaining functionality.
Through further discussion, we decided that we wanted our design to be significantly cheaper and easier to build than this design which uses aluminum extrusion and a pump would allow. In order to reduce cost, we designed a frame out of wood instead of aluminum and looked into alternatives for a pump (which was one of the more costly components of the system). In addition to this, we brainstormed all sorts of different and new features for our design. We discussed frame designs, pump alternatives, and different kinds of flow phenomena to visualize and ways to attach the test objects to the frame. We especially liked an idea to use magnets to attach and detach test objects, and implemented this idea into our first prototype. Here is a photo of our post-it notes from this brainstorming session:
Through these ideas and further discussion we came up with our first full design which we modeled in SOLIDWORKS:
There are three significant differences we made from the previous design drawing. First, we changed the frame from aluminum to wood. Second, we use a magnet to attach the test object instead of a clamp. And third, we use reservoirs of water on top of the frame along with a float valve instead of the pump system. The float valve is a simple system that is designed to keep the water level (or pressure head) constant as the reservoir drains. We thought that we could make one cheaply based on this YouTube video of someone making a float valve from an empty yogurt container.
Another component of a soap film tunnel we looked into was how to clearly visualize the flow. A popular method, and the one we decided to use, is to shine a sodium lamp through a thin fabric which diffuses the light. With this technique we got some clear images of a soap film on a bottle cap; although we have yet to try this technique on a soap film tunnel.
We presented these ideas and findings in a Preliminary Design Review where we got some helpful feedback. In particular after this presentation we decided to make the frame out of laser cut plywood instead of two by four lumber. This will make our design cheaper, much lighter, as well as easier to reproduce by anyone with access to a laser cutter (the illustrator files for the frame will be provided with our documentation). Also, the laser cut a pieces greatly simplify the assembly process. We decided that, since our focus is on flow visualizing and not on taking precise measurements for experimental purposes, maintaining a precisely constant pressure head is unnecessary. This opened an opportunity to significantly simplify our design by using a ball-valve between the top reservoir and nozzle instead of a float valve. This design change allows the flow to be easily stopped and adjusted while still maintaining a reasonably constant flow rate. Below is a image of the CAD for our first prototype:
Next we got to building this design to see how well it works. We had some trouble with the laser cutter at first. We ruined one piece of plywood in the laser cutter, and then lit a second on fire because it was excessively thick. Finally we found a middle ground which was thicker than ideal, but came out usable (although a little charred). In order to make the glueing process stronger we mixed a paste out of glue and sawdust to fill any cracks. Once it dried we had to chisel and file off the excess which was much harder than we thought. Here is a photo of Will chiselling off some of the glue:
After this difficult glueing process we decided that for our next prototype we want to try to make a design that only using hardware, as this would make the assembly process easier. One option that can be easily made on a laser cutter is shown below:
Finally, we painted our frame to cover the charred edges:
Once the paint dried, we added the reservoirs of water, plastic wire, hardware, nozzle, valve, and weight to try and make our first soap film!
Our soap solution is made from a small amount of Dawn dish soap and water, as suggested by one paper. (Rutgers, 2001)
We used an aluminum rod as a test object and attached a magnet to it. Future tests will involve a 3D printed kit of test objects, but this worked for the time being.
We had some trouble sealing the hole in the top reservoir. At first we used hot glue, but there was water leaking so instead we used JB Weld which worked well.
Another thing we found difficult was drawing the soap film around the object. One method we found that works is using our fingers to wipe the film down and around the object, but this takes a fair amount of care to do so we are looking to develop an easier method. Below is an image showing the process of drawing a soap film across the wires:
Building this prototype was a fantastic learning process which will make the final design far more successful. We were able to identify multiple kinks in the assembly process, and can now focus on obtaining clear flow visualization before we construct and document our final design.
Fayed, M., Portaro, R., Gunter, A., Abderrahmane, H., and Ng, H. 2011 Visualization of flow patterns past various objects in two-dimensional flow using soap film, Phys. Fluids 23, 091104.
Ragan, S. M. (2012, April 13). CNC Panel Joinery Notebook | Make:. Retrieved April 14, 2019, from https://makezine.com/2012/04/13/cnc-panel-joinery-notebook/
Rutgers, M., Wu, X., and Daniel W. 2001 Conducting fluid dynamics experiments with vertically falling soap films, Rev. Sci. Instrum. 72, 3025.
“Vortex Shedding.” Wikipedia, Wikimedia Foundation, Inc., 19 April 2019,https://en.wikipedia.org/wiki/Vortex_shedding.