Introduction: WInd Tunnel Apparatus and Experiments for STEM Education
The following describes a relatively simple and robust experimental wind tunnel setup and the associated lesson activities for introducing some engineering aerodynamic principles to students of a wide range of levels. The setup is easily transportable, inexpensive, easy to assemble, and should be durable enough to survive the typical rough handling to be expected in a classroom of excited students.
The author is a retired engineer that more recently has been gaining some experience in delivering STEM programming to students. Accordingly, the wind tunnel presented here and its associated activities are still a work in progress as it continues to be used in the classroom, but is it felt the equipment and lessons are sufficiently developed to share with a larger community that may benefit from what it may teach, to reproduce it in their own way, and/or to solicit feedback on how it may be improved. While some of the content may not be different from the many STEM lesson plans that are currently available, I believe some of the ideas or approaches presented here are perhaps new and of interest to those involved in STEM education.
Step 1: Overall Description of the Wind Tunnel Apparatus
Pictured here is the fully assembled wind tunnel setup, where in this case it is configured to have a horizontal force balance for measuring drag force of a model place on the balance platform, versus an alternate option of a “drift” balance which measures a lift/drag ratio. The wind tunnel can be set up on most any flat surface of 29” inches in length.
Step 2: Wind Tunnel Construction
The construction consists mainly of cardboard and most of the parts are self-evident in their making, so no detailed description is provided here, and where in fact other Instructables provide good information on that. In this version, air movement is generated by a 12’ table top fan which has had the base removed. The test section cross-section is 9” by 9”, and with an access door opening approximately 14” wide.Throughout this document, the material selection in the apparatus presented is at times chosen more based on those that were on hand, where for those looking to reproduce this setup, may find many substitutions to be more convenient.
Step 3: Ease of Transport
While the wind tunnel design follows that described by many other do-it-yourselfers’, a more unique feature here is that the complete set-up is conceived such that it can be broken down and packed in a box for easy transport. The following steps will detail how this was achieved.
Step 4: Tunnel Assembly - 1
The assembly of the tunnel from the box starts by placing the fan and its shroud and the instrumentation section on a flat surface, and attaching these together with a clipboard type clip.
Step 5: Tunnel Assembly - 2
The next section to be clipped in is the air inlet, or the rudimentary “flow straightener” inlet section, which is held square by inserting cardboard in the form an “X” and which helps to reduce potential turbulence in the air flow. Others make this section more sophisticated, such as by using honeycomb or tube structures, but this is probably good enough for this application.
Step 6: Tunnel Assembly - 3
The next pieces to are the walls and door of the test section, which are also clipped to the instrumentation base in the same manner.
Step 7: Tunnel Assembly - 5
The door has embedded magnets that hold it shut, much like the magnetic strips on a refrigerator door. This closure method was chosen as it is deemed to better withstand the repeated and rough service as the students place and remove their models, as compared to say a latch-type or sliding door type of closure mechanism.
Step 8: Optimizing the Fan
A modification made to the fan blades in order maximize the air flow was to extend the diameter by ~0.5 inches. This was done by taping appropriately shaped pieces of plastic sheet material to the edges of each blade. The greater blade area, or the reduced gap between the blades and the shroud, or both, seemed to make a significant difference in the performance of the wind tunnel.
Step 9: The Force Balance for Measuring Drag - 1
The purpose of the horizontal force balance is to measure the wind resistance or drag forces of objects placed in the wind stream. For the experiments proposed here, the forces in question are only in order of a few grams. To achieve this relatively sensitive measurement, the horizontal force balance is based on using a very low compliance (i.e. low stiffness) design of load cell where the displacement of the moving part of the load cell is measured using the magnetometer of a smart phone. As in the design of typical load cells, a parallelogram geometry is used, and to obtain a relatively large and more easily measurable range of motion under relatively small applied forces, the two walls or “load beams” of the cell are made from plastic film (in this case plastic from blister pack packaging). This material is chosen as it is inherently very tough and tends to bounce back to shape even if overloaded.
Step 10: Force Balance Cont.
For the scale to be insensitive to the weight of the model placed on the balance and only react to the horizontal drag forces, the scale must be level. Accordingly, the scale is provided with a method for precise leveling by mounting it on base with a hinge at one end and a screw adjustment (using an old glue stick) at the other. If only relatively light objects are to be tested, such as in the lesson to be described, this complexity is probably not required.
When in use, the balance has a tendency to continually oscillate or swing back and forth, which would interfere with producing stable readings. A magnetic damper, using an aluminum plate and a strong magnet (recovered from a hard drive) is added to slow the movement, which works quite effectively.
Stops are added to limit movement of the beam. This helps to make it practically indestructible under any potential misuse or other crushing forces students may apply to the balance platform.
Step 11: Force Balance - Instrumentation
Displacement of the beam, and thus the horizontal force, is measured using the magnetometer of a smart phone together with an application called PhyPhox (free!) that performs the required signal processing and display. Used here is an obsolete Huawei P10, however most any old smartphone will work as many of these have a magnetometer. Essentially, the smartphone serves a similar function to a Linear Variable Displacement Transducer (LVDT), as the sensor signal will be nearly proportional to the distance of the magnet to phone over the range of motion experienced here (2-4 mm). The smartphone is mounted to an accessible and easily viewed location on the instrumentation base, and a magnet that is physically connected to the moving load beam is brought in proximity to interact with the smartphone. As the location of the magnetometer sensor in the phone is not consistent across different models, the exact geometry of how to achieve this for the best sensitivity, that is by having the magnet aligned close to the sensor, will be specific to the phone used.
The PhyPhox app used to perform the data processing and to display data allows the user to write the associated code, or as it is referred to in the app, the “experiment”. Accordingly, there are many different ways of how to customize the experiment depending on the activity to be done with the students. Chosen here is to display an “uncalibrated” measurement of the drag force (the magnetometer raw value less a zero offset), as well as a relative drag force value, that calculates a percentage of this latter value relative to a reference value that is entered using the app interface.
In practice, despite the use of a magnetic damper, it was observed that the raw values from the magnetometer still waver a lot, making it difficult to take a measurement. Fortunately, the app has a processing function that allows to user to apply and control data smoothing of the sensor signal, such that a stable reading can be achieved.
Step 12: Using the Wind Tunnel to Perform Vehicle Aerodynamic Testing
In this proposed activity, students will learn about vehicle aerodynamics and apply their knownledge by creating a shape of a model vehicle that is tested in the wind tunnel..
Step 13: Vehicle Aerodynamic Lesson - 1
In a first part of the activity, the wind tunnel and the force balance is used to show students how wind or drag resistance is depend on the shape of an object. Foam models of different shapes but having similar frontal areas can be tested and compared as a demonstration to the students. In the example shown here, four shapes were tested; square plate, pyramid, half-sphere, and full sphere. The display is showing the force value (top number) based on the raw magnetometer reading, along with relative value (bottom number) as a percentage of the force on the square plate.
With this new knowledge of how geometric shape can affect wind resistance, students are then tasked with sculpting an aerodynamically shaped vehicle from a block of Styrofoam. For this, the students are provided with shaping tools such as small saws, files, and sandpaper. One constraint the students are given is that the height of the car must retain at some location along its length, the height of original block, since in their design the car needs to accommodate an occupant in a sitting position. This also forces the student to make use of aerodynamic shaping, rather than simply reducing the front projected area, to reduce wind drag.
Step 14: Vehicle Aerodynamic Lesson - 2
To set up the apparatus, a foam block without any shaping is tested, and the measured drag force is used as the reference value against which the student vehicle designs will be compared and displayed (a calculation done by the app).
Step 15: Vehicle Aerodynamic Lesson - 3
To help inspire the students, the testing of a poorly aerodynamically designed car can be demonstrated, as here, only achieving a reduction of 35% in wind resistance.
Step 16: Vehicle Aerodynamic Lesson - 4
Then a more aerodynamic car can be tested, noting the improvement in drag reduction, such as here at 64%.
Step 17: Vehicle Aerodynamic Lesson - 5
This is a model done by a Grade 5 student, which actually performs rather well on testing even with minimal sculpting. In this case, incremental improvements were suggested to the student, such rounding some corners, smoothing some areas, etc., where on retesting, the student can observe the effect of the changes on wind resistance. Tests can be performed very quickly, allowing the students to use an iterative approach in creating their designs. As shown in the banner image of this page, the students can come up with many different designs.
Step 18: The Drift Balance - Overview
The idea for the drift balance presented here for studying the forces of lift and drag on various paper airplanes is derived by one of the instruments used by the Wright brothers in their wind tunnel experiments. To study the performance and efficiency of a large variety of wing shapes, the Wright brothers developed a “Drift Balance” that can accurately measure the lift to drag ratio of a wing model as a function of angle of attack.
In their studies, they constructed an elegantly simple mechanism in the shape of a parallelogram using old saw blades and bicycle spokes.
Step 19: The Drift Balance - Theory
As seen in this plan (i.e. top down) view of their balance, a wing model fixed to one arm of the balance, and under the action of the airflow, the relative values of the lift and drag forces acting on model will determine the angle taken on by the balance arms. The entire balance is within the air flow of wind tunnel, and being symmetrical, the only forces causing the balance to move are those from the lift and drag forces of wing model. The values of the angle of attack and the angle indicated by a dial on the balance are then used to determine the lift to drag ratio of the model using the equation:
L/D = arcTan (angle of attack + balance dial angle)
This L/D value is also synonymous with glide ratio, which is another way to present this parameter. While this device does not provide a measurement of the absolute values of the forces on the model, this ratio is very important to understanding the wing performance, which for the Wright brothers was crucial to achieving a successful flight. A more detailed detailed description of the theory and mathematics of the balance is offered in the Nasa website: https://www.grc.nasa.gov/www/k-12/airplane/wrights/balanced.html
Step 20: The Drift Balance - Construction
Proposed here for the testing of paper airplanes is a drift balance that is similar that of the Wright brothers, but is turned on its side. In so doing, this has the advantage in that the paper airplane is tested in a horizontal orientation, the way we usually see it, and thus is more intuitive for students when considering the angle of attack, and observing the reaction of the balance to the lift and drag forces. Also, this orientation change accommodates other changes that allow a better fitting the balance in the relatively small space of the current wind tunnel. Unlike in the Wright brothers case, effort is made here due to space constraints to try and locate most of the balance’s mechanism out of the air flow, and only have the model in the air flow, so that the measurements are reflective of only the forces on the model. In moving the balance on its side, the weight of the upper and lower parts of he mechanism must also be neutrally balanced with the model attached, which is achieved with a fine weight adjustment on a threaded rod. By testing only paper airplanes made from a same stock of paper, the balance need only be calibrated once.
While here is used a combination of wood, band iron, and small rods to make low friction moving connections, there would exist many other ways and geometries to make a successful balance. For example, symmetry of the balance arms about the centre axle as done here is not really required.
Step 21: The Drift Balance - Construction - Cont.
The plane is held in the air stream using a clip design where the planes are simply slid in and out of the clip. This clip, which ironically is also made from a bicycle spoke, accepts most paper airplane shapes. The clip keeps the plane’s wings parallel relative to drift balance as well as the dial arm controlling the angle of attack. Despite needing to be nimble enough to react to the relatively small forces produced by the airplanes, the balance is relatively robust and is able to survive the repeated manipulation of students loading and removing their plane creations. As well, during testing the needle on drift angle indicator is seen to move back and forth slightly, suggesting friction is not limiting the motion of the device and true equilibrium is achieved.
Step 22: Paper Airplane Testing Activity
Paper airplanes are particularly interesting to study in the wind tunnel, as there exists copious plane designs having different wing shapes, claiming to be either fast, or having long flight times, or both, by their designers. This latter characteristic to fly far and long is largely determined by the L/D ratio as measured here. The shape of the plane's wings, as well as other characteristics such as neatness of folds, use of tape, etc. can be studied by the students by measuring the L/D of different planes that they chose to make. Perhaps like the Wright brothers, they too will come to the conclusion that in general, wings with a high aspect ratio (length to width), will have a higher lift to drag ratio.
Step 23: Paper Airplane Testing
An internet search will uncover a vast number of wildly different plane designs that can be made and tested by the students, with some examples in the picture provided. The testing of the planes simply involves opening the test chamber door, inserting the plane in the clip, and closing the door. Then starting at a zero angle of attack, where the balance should be fully collapsed indicting only drag forces, the lever controlling the angle of attack is set to a specified value (for example, 10 degrees seems to works well). The students should immediately see the plane rise up as a result of the lift force that is now created. A reading is taken from the dial gage on the balance, and the student can calculae the L/D ratio from the formula given earlier, or from a table perpared by the instructor that provides the calculation for the specific angle of attack being tested.
Step 24: Paper Airplane Testing - Cont.
As an example, the Easy Glider airplane is a design having a relatively large aspect ratio for its wings. Here it tested showing a drift angle of approximately 1 degree, at an angle of attack of 10 degrees. This would compute as an L/D ratio of 5.1 for this plane.
Step 25: Paper Airplane Testing - Cont.
Testing of a Delta wing design plane, having a low aspect ratio for its wings, indicates a drift angle of 11 degree at an angle of attack of 10 degrees. This would suggest a relatively poorer L/D ratio of 2.6. Doing test flights of these two planes by launching them by hand would corroborate that the Easy Glider does in fact stay in the air longer than the Delta.
By encouraging many different plane designs to be tested, results amoung the students can be shared and compared to understand how the plane geometry contributes to its performance, and how the wind tunnel tests might corroborate with hand launched flight testing. More advanced studies could be performed by studying the effect of angle of attack on L/D, where students could discover this has an optimum value, and that it drops off rapidly at some angle, indicative of a “stall” condition. These can be very advanced concepts for the students to try and comprehend, but in my experience so far, just to perform the manipulations of the experiment seemed to create some excitement with the students, which has value in itself.
