Are wind-powered cars the future?
Sarah Kamdem and Matias Kemper-Tapia
Embedded Files
IMG_1555.MOVAbstract Video
Abstract Video
Welcome to our 2022 DYO! Here is our abstract video.
Introduction
Before diving into the specifics of our experiment, we need to understand, where exactly does the wind come from? Wind is caused by differences in atmospheric pressure. When a difference in atmospheric pressure exists, air flows from areas of high atmospheric pressure to areas of low atmospheric pressure. This means that the wind is essentially air particles moving at various speeds around the Earth due to geography. Fast-moving particles means kinetic energy, so have you ever wondered what a wind-powered car would look like? Although the sail car is a quite popular science fair experiment, there is much less information about a real life sail car due to many technicalities which would make the use of a real life sail car difficult, which is why most wind-powered vehicles to date are powered by turbines rather than sails. However, it is interesting to know that the fastest wind-powered vehicle is actually sail-powered! The model is called the 2009 Greenbird, and uses a rigid vertical wing rather than a conventional sail to generate thrust, similar to how an airplane wing generates lift. It is the fastest wind-powered vehicle ever built, with a record speed of 126 mph!
In our experiment, our main goal was to investigate the effect of the addition of the various-sized sails on the velocity of the carts. Therefore, by using collision carts and a dynamics track, we were able to eliminate the possibility that the wind would sway the cart in various directions, as the wheels of the collision cart are perfectly aligned to the dynamics track to keep the cart on track. Since our sail car used a traditional sail rather than a wing-like sail, our model is not identical to that of the Greenbird. However, we were able to capture the impact of the addition of a sail on a cart's velocity, and understand how exactly the sail works to increase the car's velocity. The working mechanism of the sail can be explained with the aid of the equation for pressure gradient.
Figure 1: 2009 Greenbird
Figure 1: 2009 Greenbird
Its lightweight and aerodynamic design help the vehicle achieve speeds up to five times that of the wind.
Its lightweight and aerodynamic design help the vehicle achieve speeds up to five times that of the wind.
Figure 2: How a sail works
Relevant Background Theory
So, how does a sail work? As you can see in our model, the sail is curved and faces the wind source at an 'angle of attack'. As air follows the curved path along the windward side of the sail, there is a pressure gradient perpendicular to the direction of airflow, with lower pressure on the outside of the sail and higher pressure on the inside of the sail. The equation below:
dp/dz = (ρ*v^2)/r
shows how the pressure gradient relates to the velocity of the air, and thus that of the cart. In the equation, p is the pressure; dp/dz is the pressure gradient, a quantity that shows the gradient of pressure as a function of position; ρ is the density of the fluid (air in this case); v is its velocity. R is the radius of curvature of the airflow - it equals the radius of the circular arc that best approximates a curve, in this case the curve of the sail. The equation shows that without a curved sail (R = infinity), there would be a pressure gradient of zero, thus hindering the sail's efficiency. The difference in pressure (presence of a pressure gradient) is necessary because the high pressure on the inner surface pushes harder than the reduced pressure on the outer surface, as shown in the image on the left, and the net result is lift which propels the vehicle forward. This pressure gradient is best achieved with a curved sail, and thus a small value of R, as this would create a big pressure gradient as can be deduced from the equation above. This theory explains how the curved nature of the sail causes a vehicle to move, and helped us in designing our carts.
Methods
In order to make the carts, we started by cutting out rectangular pieces of cardboard which we stacked on the top of the collision carts to provide a stable base for the mast. We poked a hole in the cardboard stack through which we slid the mast, and we used tape to secure the cardboard stack onto the cart. For the sails, we cut out pieces of cardstock of various areas, in which we poked holes for the mast to fit into. We equally made a control cart, which had the cardboard and the mast but no sail attached, to keep the mass of all carts relatively constant, as we assume the mass of the cardstock to be negligible. Our setup included the dynamics track which we leveled using a bubble level tool, and the box fan which we placed 15 centimeters away from the track, as well as a measuring tape which we sealed onto the track.
In order to carry out our procedure, we set the fan to level three and placed the carts at 0 centimeters on the measuring tape. We turned the fan on to the third setting, and the timer began recording the time just as the fan pushed on the cart. A stopper was placed at the 200 centimeters mark on the measuring tape to stop the cart. Once the cart collided with the stopper, the timer stopped taking time. The timer recorded the time it took for the cart to travel 200 centimeters. This process was repeated ten times for every sail area. The sail areas were the following: 0 square millimeters, 100 square millimeters, 225 square millimeters, and 400 square millimeters. Overall, there were a total of 40 runs for four different sail areas. The data was exported to Microsoft Excel, and the times for each run were tabulated. We calculated the average time for each sail area, and then used this time to calculate the average speed for each sail area, using the formula for speed, which is distance traveled/time taken.
Figure 3: Picture of one of our sail cars
Theoretical Work
Theoretical Work
In the background section, we learned that the difference in pressure created by the curved shape of the sail produces a force that propels the vehicle forward. This helped establish the background knowledge that the curved nature of a sail is essential for the sail’s effectiveness. In the procedure above, we measured the time taken for carts withs sails of various sizes to travel a distance of 200m. Our results will confirm our hypothesis, showing that a larger sail area makes the cart go faster as it provides more room for the car to gain more kinetic energy from the wind. The wind pushes on the sail and transfers its particles' kinetic energy to the sail and the cart, so a larger sail area results in a higher velocity. From the equation k.e. = 1/2*m*v^2, we know that an increase in kinetic energy signifies an increase in the cart's velocity. Later in the discussion, we will analyze how much of an impact increasing the sail area has on increasing the velocity.
Results
Results
Figure 4 is a graph that displays the relationship between a sail’s surface area and a car’s speed. Throughout the experiment, sail surface area was the independent variable, and the time taken to complete the distance of 200cm was the dependent variable. After collecting our raw data, we calculated the mean time for each sail-size, and used the formula for speed, distance traveled/time taken, to calculate the average speed for each sail size in cm/s.
Figure 5 is a table that also shows the relationship between a sail’s surface area and a car’s speed. As seen in the figure, the runs for each sail size were conducted 10 times in order to make the data reliable. The average time for each sail size and the average speed for each sail size was calculated. Figure 5 presents the data for each trial and run, as well as average absolute deviation for car speed, and percent average absolute deviation for car speed. Absolute Average Deviation (AAD) and % Absolute Average Deviation (% AAD) both measure how the data varies. If AAD and % AAD have low values, the data is relatively precise.
In the discussion section below, we will analyze how the impact on the speed of the cart varies with the increase in sail area.
Discussion
Discussion
You may now be wondering, if a sail-powered vehicle can go that fast, and sails are proven to increase velocity, why has the use of sail-powered cars still not been popularized more than a decade later? For starters, as seen in the results of this experiment, a bigger sail results in a higher velocity for the car. Therefore, the design of an effective sail-powered vehicle is not ideal, as its huge sail would take up an unreasonable amount of space for a car. In order to maximize the car's efficiency, the body of the car needs to be small enough compared to the sail, meaning that sail-powered cars such as the Greenbird usually do not have much room for passengers. As seen in Figure 1 earlier, the Greenbird can has a narrow frontal area which can only house one person, the driver, in order to provide the smallest possible frontal area (a streamline shape) and to minimize drag.
In our experiment, we used a regular-shaped cart in order to affect the effects of only the sail on the cart's velocity. Our results show that a larger sail area results in a higher cart velocity, as a larger sail provides more room for the cart to gain more kinetic energy from the wind's moving particles. The sail obstructs the blowing wind, and as a result the wind pushes the sail forward. This is how the boat gains speed. The bigger the sail, the more the obstruction to the wind, so the wind push area on the sail increases and the sail is able to pick up more energy from the wind. This is why a bigger sail helps the cart move faster. Although the movement of our sail car does not represent that of a real life sail car move as our 'wind' is coming from only one direction, our results prove that the addition of a sail to a car would have a significant enough impact on it.
The car with the smallest sail had an increase in speed of 117% from the cart with no sail. That is a significant increase in speed, showing that the addition of a sail has a significant impact on the cart's speed. From 100 to 225 squared mm of sail area, the speed increased by 18.0%, and from 225 to 400 squared mm, the speed increased by 3.39%. This shows that although increasing the area of the sail increases the speed of the cart, larger sails have decreasingly significant impacts on the velocity of the cart, as can be seen on the graph in Figure 4 above, as the curve of points flattens out towards the end. This analysis helps us assert that although a sail can hardly suffice for all the power needs of a car, the addition of one to a car, although not the most aesthetically pleasing, would provide a non-negligible contribution to the power needs of a car and help reduce its carbon footprint.
RPReplay-Final1650592753.movConclusion Video
Conclusion Video
Thank you for making it this far, we hope you enjoyed our DYO!
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