Sub-sonic Open-return Wind-tunnel and Measurement Apparatus

Wind tunnels are an effective tool for understanding relationships between predicted and actual behavior of air foils in laminar flow. In order to improve the ability of Macalester College staff and students to observe and experiment with the effects of laminar air flow on various objects and systems, it was necessary to construct and test a sub-sonic open return wind tunnel. The purpose of the tests was to discover whether or not the air flow through the test section of the tunnel was laminar around the object being tested and also to evaluate the quality of an apparatus constructed to measure lift and drag within the wind tunnel. I present the results of my tests and analyses as well as comparisons to known data and proven relationships. 1 Adams: Sub-sonic Open-return Wind-tunnel and Measurement Apparatus Published by DigitalCommons@Macalester College, 2013 2 I . Introduction The concept of a wind tunnel was first proposed by Leonardo Da Vinci in the 16 Century. He is quoted as stating in his work, quiescent medium, or whether the particles of the medium impinge with the same velocity upon the quiescent body (Anderson 2008). Although this thought did not lead to any immediate practical applications, it was the first recorded example of human understanding of how fluid flow across objects works. The first true wind tunnel was constructed in Greenwich, England by Francis Wenham. This wind tunnel was a very primitive box design and used a steam powered engine to push air through a tunnel and then measured the lift and drag forces using weighing beams attached to the object being tested. In 1884 the second wind tunnel was built by Horatio F. Phillips. This second tunnel maintained a similar box design to air through the tunnel he used steam injectors to pull air through. Although this development lead to more accurate results, 1901 that truly useful results were obtained in wind tunnels through the efforts of the Wright Brothers. In 1901 the Wright Brothers worked towards developing the first aircraft. During many of their struggles to create an effective vehicle, they came to the conclusion that most of the data relating to airfoils was incorrect. The data was accepted as right based off of the tests of other experimentalists, but it had no practical application to flight testing and needed to be reevaluated. In response, they created their own 6 foot long and 16 inch squared wind tunnel that was driven by a two blade gas powered engine. In addition to this tunnel they also developed an apparatus 2 Macalester Journal of Physics and Astronomy, Vol. 1, Iss. 1 [2013], Art. 1 http://digitalcommons.macalester. edu/mjpa/vol1/iss1/1 3 which measured the ratio of lift to drag. Over the course of the next year and half they tested 200 different wing designs and finally found one they thought would be optimum. These results lead to the first flyable aircraft and was also the beginning of the use of modeling to assess the performance of structures before building the full design (Anderson 2008). Due to the achievements of the Wright Brothers, one of the most important aspects of contemporary engineering is the ability to model and understand how something will work/perform before constructing it. Wind tunnels have been a major advancement in the last century that allowed engineers and designers to see how different constructs behave in varied wind conditions and what relationships there are between different components of design and performance. In this project, I used many experimentally proven pieces of data to construct a sub-sonic open-return wind tunnel. Once I completed construction of the wind tunnel and built an apparatus to measure the lift and drag of an object in the tunnel, I began running tests on a ball and a flat plate that had an adjustable angle of attack. The purpose of these tests was to determine whether or not this wind tunnel was creating laminar flow within the test section, observe whether or not results could be replicated for known experiments to verify that the tunnel was useful, and to test whether or not the apparatus was useful in making the proper measurements of lift and drag. I I . Methodology I I . 1 Wind Tunnel Construction The most important component of building a wind tunnel is determining which type of wind tunnel is the most practical for your needs and your available resources. After a review of 3 Adams: Sub-sonic Open-return Wind-tunnel and Measurement Apparatus Published by DigitalCommons@Macalester College, 2013 4 multiple descriptions the open-return sub-sonic wind tunnel, Figure 1, was determined to be the most practical and simple apparatus to create. Once this particular tunnel design was selected it was necessary to know what principles governed its operation. The first and most critical component was the assumption that airflow in the tunnel was incompressible. This means that if a small parcel of air was selected and monitored throughout the entire wind tunnel, the density would change negligibly. Incompressible flow has numerous impacts on the operation of the tunnel. It primarily affects the equations that govern how the velocity in the different sections is related, but it can also have a major impact in the test section if the air is compressing around the air foil being tested. Since interpretations of experiments strongly depend on the type of fluid flow within the tunnel, it is very important to justify the reason for assuming incompressible flow. Since it takes a large amount of force to compress and increase the density of a fluid, a compressible flow would require a high volume of air to be forced into a smaller section very rapidly. To simplify this matter, the velocity in the tunnel would need to exceed the velocity of the individual particles movement within a given volume of air. Through testing it has been observed that this effect does not occur for air velocities under 134.112 m/s (Anderson 2007). Since our tunnel design does not use a powerful enough fan to achieve this velocity or even velocities greater than 10% of this value (max at 9.05 m/s), it is safe to assume that any effects due to compressibility are negligible. With this assumption (1) to show how full sized wind tunnels are operated, and through the continuity equation (2) we were able to show the relationship between the velocities in the three main sections of the wind tunnel shown in Figure 1.