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650:422 AEROSPACE ENGINEERING LAB SECTION 12 MARCH 23, 2017 Supersonic Flow in a Wind Tunnel Department of Mechanical and Aerospace Engineering Rutgers University, Piscataway, New Jersey 08854 The objective of the supersonic wind tunnel laboratory is to familiarize ourselves with the physics behind Schlieren Imaging and to perform the experimental setup of a blowdown in a supersonic wind tunnel. The blowdown in conjunction with the schlieren method will allow us to visually see the shock waves formed around an object in the test section. In this particular tunnel setup, we use the Mach number 3.45 which is specific and cannot be changed. A Schlieren camera is used to examine the shock waves within the test section. INTRODUCTION A supersonic wind tunnel consists of a closed environment in which air is drawn through at supersonic speeds. The mach number and flow are determined by the geometry of the nozzle which are specific and cannot be changed. The wind tunnel is used in order to achieve a fixed flow pattern at a given instant. The apparatus given to us is a tunnel that includes a high quality schlieren imaging system, LaVision digital camera, high bandwidth pressure transducers, an Ng-YAG laser and a data acquisition within the tunnel. The apparatus includes a small window at the test section to observe the flow over an object, in our experiment we used a pointed structure. The purpose is to simulate,envision, and examine how the flow over the absorbed object affects it. The visual flows that we see over the object are called shock waves and are produced by the deflection of supersonic flow over the object. In our case, we witness an attached shock due to the sharp structure used and the low angle of attack. Table 1. Wind Tunnel Specifics Mach Number 3.45 Velocity 630 m/s Stagnation Pressure 200 psia Stagnation Temperature 60 ∘ F Test Section 6 x 6 in Run Time 10 s The Schlieren Imaging as discussed before is produced by the deflection of light rays by the changes in the index of refraction n. With n being the ratio of the speed of light in a vacuum which is 3x 108 m/s to that of the speed of light in the specific material.We are given a Z-type schlieren system as seen below. Figure 1. Z-type schlieren system In our system here we have an LED light that is reflected from one spherical mirror and passes through the test section where the light rays may be bent from the transition through the shock wave. Another spherical meets the light at the other side and reflects it towards a fixed point at the edge of a knife which interrupts the rays. These rays are reflected for the third time and brought to the image plane, the LaVision camera which creates the visual effects of the shock waves. Another image created is that of the expansion fan(s) which is a centered expansion about the pointed corner which is the production of Mach waves. The flow is accelerated at these fans. RESULTS AND DISCUSSION For the first part of our experiment we 1 MICHELLE GIRALDO 144-00-8098 TA: MAHSA MORTAZAVIThis study source was downloaded by 100000799028264 from CourseHero.com on 10-13-2022 21:39:36 GMT -05:00 https://www.coursehero.com/file/22470137/Supersonic-Wind-Tunnel/ https://www.coursehero.com/file/22470137/Supersonic-Wind-Tunnel/ 650:422 AEROSPACE ENGINEERING LAB SECTION 12 MARCH 23, 2017 test the Schlieren imaging on a table top. The tools used are a candle, a razor blade, a magnifying glass, and the mirror for reflecting. A light is shined through the candle flame and by blocking a fragment of the light that refracted from the density of the hot air we may create an image of the contrasted hot air. We blocked the light with the edge of a razor blade and placed the magnifying glass precisely at the focal point in order to create the image. Figure 2. Schematic of Schlieren System To achieve a clear image, the focal length must be 2 times the length as seen in the image above but instead of being reflected towards a camera, the image is reflected onto a white piece of paper. Because of the use of a spherical magnifying glass and mirror, the image will appear to be flipped. In order to explain how Schlieren images are made, we must first discuss the properties behind refraction. Refraction takes place when light rays pass from one medium into the next. What makes the light bend is the difference in refractive index (optical density). If light enters a medium with a lower refractive index than the previous one, then it will speed up and vice versa. The schlieren method has been used in order to visualize the fluctuations in the optical densities. Figure 3. Schlieren Imaging Table Top Setup Figure 4. Focused Schlieren Image Next, we proceeded with the Schlieren experiment using the supersonic wind tunnel. A separate system is provided to us on a bigger scale than that of the table top but consisting of the same method. They are placed in a way to result a high resolution image via a camera on one end. The goal is to obtain the schlieren images at different angles of attack. 2 MICHELLE GIRALDO 144-00-8098 TA: MAHSA MORTAZAVIThis study source was downloaded by 100000799028264 from CourseHero.com on 10-13-2022 21:39:36 GMT -05:00 https://www.coursehero.com/file/22470137/Supersonic-Wind-Tunnel/ https://www.coursehero.com/file/22470137/Supersonic-Wind-Tunnel/ 650:422 AEROSPACE ENGINEERING LAB SECTION 12 MARCH 23, 2017 Figure 5. Schematic Design of Supersonic Wind Tunnel (NASA) Figure 6. Initial startup of wind tunnel As described, the image from figure 6 represents the visual schlieren image from the test section of the tunnel during start up. As one could see, the air in the tunnel seems to be flurrying around without an orderly manner. Around the tip of the structure one can start to detect the formation of a constant streamline and what is to be predicted. At the moment we get this glimpse of weak waves entering the section. Figure 7. Wind tunnel at full speed The next figure is the image of the wind tunnel at its full speed. One can see the shock waves forming in parallel patterns at diagonals. From here we can associate the term “attached shock” and “Prandtl-Meyer expansion fan” which are noticed at the tip and the shoulder of the structure, respectively. One may also detect the Mach lines that are a result of imperfections or dents on the structure. Thanks to the schlieren setup we can capture these images of the streamlines and shock waves. Otherwise, in person, it appears as though nothing is really happening within the wind tunnel. Figure 8. Shut down of wind tunnel At the shutdown of the wind tunnel we can see from figure 8 the remaining diagonal shock waves that linger; as the top portion of the image begins to look like a flurry of smoke as in the startup image. Under the theory that the half angle of the cone model used was 11 degrees, we may calculate the shock angle: [1] where, β -shock angle θ -half angle of cone M 1 -Mach number γ = 1.4 (air) By using figure 9 below, we made estimate the 3 MICHELLE GIRALDO 144-00-8098 TA: MAHSA MORTAZAVIThis study source was downloaded by 100000799028264 from CourseHero.com on 10-13-2022 21:39:36 GMT -05:00 https://www.coursehero.com/file/22470137/Supersonic-Wind-Tunnel/ https://www.coursehero.com/file/22470137/Supersonic-Wind-Tunnel/ 650:422 AEROSPACE ENGINEERING LAB SECTION 12 MARCH 23, 2017 shock angle to be about 20 degrees for a half angle of 11 degrees. Figure 9. NACA Technical Memorandum 1135 shock angle vs. cone half-angle Figure 10. Supersonic flow encountering a wedge and is deflected, forming an oblique shock From equation 1 we calculate the shock angle for a half Static pressure can be measured by recording its movement along the fluid element at the same velocity. In our case, we must work backwards from the stagnation pressure in order to achieve the static pressure. The stagnation pressure is simply the opposite, it measured when the flow has come to a stop isentropically. The static pressure can be calculated by the following formula: [2] where, pt -stagnation pressure p-static pressure M-mach number Through plug and chug, static pressure came out to be 2.816 psia. As for static temperature, we may simply use the relation: [3] where, T t -stagnation temperature T-static temperature Again, through plugging in previously given numbers and the result for static pressure, we may solve for static temperature. Static temperature came to be 17.75 ∘ F. ERROR ANALYSIS Since the specifications were given and the wind tunnel was handled by a professional that leaves less room for error in this experiment. That does not mean there was no error at all. The wind tunnel itself was established in the mid-1990s and has been running for nearly 3 decades. Error may come from age of the tunnel or even the deflections from dents on the cone model that may not be taken into consideration for the shock angle. Because, this environment is very much maintained at certain specifications, error will be miniscule, otherwise human-related or due to outdated equipment. Another source of error may pertain to a fluctuation of temperature and thus producing vapor content. 4 MICHELLE GIRALDO 144-00-8098 TA: MAHSA MORTAZAVIThis study source was downloaded by 100000799028264 from CourseHero.com on 10-13-2022 21:39:36 GMT -05:00 https://www.coursehero.com/file/22470137/Supersonic-Wind-Tunnel/ https://www.coursehero.com/file/22470137/Supersonic-Wind-Tunnel/ 650:422 AEROSPACE ENGINEERING LAB SECTION 12 MARCH 23, 2017 CONCLUSIONS This experiment employed a supersonic (M>1.0) wind tunnel with known criterion in that allowed for the study of flow over a pointed structure at a zero angle of attack. This process was examined using the schlieren technique/images. Through examination it may be concluded that as the angle of attack on the structure increases, the shock angle will decrease. REFERENCES http://www.physicsclassroom.com/class/refrn/Less on-1/The-Cause-of-Refraction https://en.wikipedia.org/wiki/Stagnation_pressure https://en.wikipedia.org/wiki/Oblique_shock 650:433 Supersonic Wind Tunnel Lab Manual 5 MICHELLE GIRALDO 144-00-8098 TA: MAHSA MORTAZAVIThis study source was downloaded by 100000799028264 from CourseHero.com on 10-13-2022 21:39:36 GMT -05:00 https://www.coursehero.com/file/22470137/Supersonic-Wind-Tunnel/ Powered by TCPDF (www.tcpdf.org) https://www.coursehero.com/file/22470137/Supersonic-Wind-Tunnel/ http://www.tcpdf.org