1 00:00:03,389 --> 00:00:09,450 This is an F/A-18 Hornet fighter jet. Look here. There are forward extensions of the 2 00:00:09,450 --> 00:00:15,020 wing called leading edge strakes or leading edge extensions. Why were the wings designed 3 00:00:15,020 --> 00:00:19,250 this way? In this video, you'll find out. 4 00:00:19,250 --> 00:00:23,810 This video is part of the Representations video series. Information can be represented 5 00:00:23,810 --> 00:00:29,859 in words, through mathematical symbols, graphically, or in 3-D models. Representations are used 6 00:00:29,859 --> 00:00:35,420 to develop a deeper and more flexible understanding of objects, systems, and processes. 7 00:00:35,420 --> 00:00:41,210 Hi, I'm Dave Darmofal, and I'm a Professor in the Department of Aeronautics and Astronautics 8 00:00:41,210 --> 00:00:46,690 at MIT. In this video, we're going to see an example that helps you to visualize the 9 00:00:46,690 --> 00:00:51,539 air velocity around an airplane body using smoke flow visualization. You will see that 10 00:00:51,539 --> 00:00:56,829 at low angles of attack, the velocity field is independent of time, while at high angles 11 00:00:56,829 --> 00:01:01,030 of attack the velocity field depends on space and time. 12 00:01:01,030 --> 00:01:05,780 After watching this video: * You will know that flow quantities around bodies are often 13 00:01:05,780 --> 00:01:12,130 analyzed using an Eulerian frame. * You will recognize that flow velocity is a vector field 14 00:01:12,130 --> 00:01:18,500 which depending on the application can be not only a function of space but also time. 15 00:01:18,500 --> 00:01:23,670 This is Dick, he's the Senior Technical Instructor here in the department of Aeronautics and 16 00:01:23,670 --> 00:01:27,880 Astronautics. He is in charge of the Wright Brothers wind tunnel. 17 00:01:27,880 --> 00:01:32,649 * Wind tunnels are used to simulate the airflow around a variety of objects including buildings, 18 00:01:32,649 --> 00:01:36,239 cars, trains, and of course aircraft. 19 00:01:36,239 --> 00:01:40,408 * The Wright Brothers Wind Tunnel is an example of a closed-circuit wind tunnel, which uses 20 00:01:40,408 --> 00:01:45,610 a fan to circulate the air. The test section of the Wright Brothers Wind Tunnel has an 21 00:01:45,610 --> 00:01:52,009 oval cross-section that is 7 feet high by 10 feet wide. The top velocities we typically 22 00:01:52,009 --> 00:01:57,780 use are around 100mph. While somewhat higher speeds are possible, the noise is increased 23 00:01:57,780 --> 00:02:04,780 rapidly with increased fan speed. 24 00:02:06,149 --> 00:02:11,450 * We will visualize the air velocity vector field using smoke visualization. We seed smoke 25 00:02:11,450 --> 00:02:16,950 in the tunnel through a handheld probe. The smoke follows the local air velocity allowing 26 00:02:16,950 --> 00:02:22,120 us to "see" where the flow is going. This smoke flow visualization techniques works 27 00:02:22,120 --> 00:02:29,120 best at lower wind speeds, so we will be testing at about 25 mph. In other words, the speed 28 00:02:29,480 --> 00:02:33,719 in the tunnel test section upstream of the model will be approximately 25 mph. 29 00:02:33,719 --> 00:02:37,269 * The model we will be using is based on an F-16 aircraft. Several years ago, Lockheed 30 00:02:37,269 --> 00:02:41,230 Martin was investigating increasing the size of the F-16 wing. A key issue with the increased 31 00:02:41,230 --> 00:02:48,230 wing size, especially at the higher speeds the F-16 flies at, is the potential for aeroelastic 32 00:02:48,810 --> 00:02:53,540 instability due to coupling between the aerodynamic forces and the wing structure. In other words, 33 00:02:53,540 --> 00:02:55,760 because the wing is not a completely rigid structure, aerodynamic forces can cause the 34 00:02:55,760 --> 00:02:57,090 wing to vibrate, which can cause big problems during flight. To study this issue, Lockheed 35 00:02:57,090 --> 00:03:00,120 developed the following simplified geometry. In the department of Aeronautics and Astronautics 36 00:03:00,120 --> 00:03:00,959 at MIT, we have also used this model for various labs and projects in our undergraduate subjects. 37 00:03:00,959 --> 00:03:03,450 * The model has three main parts to it: 38 00:03:03,450 --> 00:03:07,510 ** The fuselage, which is a body of revolution with a pointed tip and a bluff end 39 00:03:07,510 --> 00:03:07,620 ** The trapezoidal wing 40 00:03:07,620 --> 00:03:10,780 ** Leading edge strakes, sometimes also called leading edge extensions 41 00:03:10,780 --> 00:03:15,030 * The leading edge strakes help stabilize the flow at high angles of attack by creating 42 00:03:15,030 --> 00:03:18,849 a strong vortex over the wing. This vortex is a region swirling velocity and low pressure 43 00:03:18,849 --> 00:03:25,849 and generates a significant amount of lift. Thus, strakes can provide significant performance 44 00:03:27,530 --> 00:03:34,530 benefits for aircraft that require high angle of attack maneuverability. We will look at 45 00:03:35,560 --> 00:03:41,260 the vortices created by these strakes today. 46 00:03:41,260 --> 00:03:47,530 In many aerodynamic applications, engineers analyze flows using an Eulerian frame in which 47 00:03:47,530 --> 00:03:54,530 flow quantities such as velocity, pressure, temperature, etc are viewed as fields: i.e. 48 00:03:55,170 --> 00:03:59,930 functions of space and often the space of interest is fixed to the objects frame of 49 00:03:59,930 --> 00:04:06,019 reference. In our case, this frame of reference is also the wind tunnels frame of reference. 50 00:04:06,019 --> 00:04:10,700 When you watch a river flowing downstream, an Eulerian view of the water flow is to watch 51 00:04:10,700 --> 00:04:17,360 the flow through a fixed point of space, as the water flows past you. For this demonstration, 52 00:04:17,360 --> 00:04:22,490 we will be visualizing the velocity which is not just a field, but in particular a vector 53 00:04:22,490 --> 00:04:28,620 field. Further, depending on the flow conditions, the velocity can be a function of time in 54 00:04:28,620 --> 00:04:35,440 addition to space. So, we'll think of an air velocity vector field, v = v(x,t). 55 00:04:35,440 --> 00:04:40,850 * We will start by visualizing the flow at a low angle of attack that would be typical 56 00:04:40,850 --> 00:04:47,380 of a cruise condition, specifically we are at X degrees angle of attack. In particular, 57 00:04:47,380 --> 00:04:52,700 the flow at this low angle of attack is, to good approximation, "steady". This means that 58 00:04:52,700 --> 00:04:58,720 the flow quantities do not depend on time, though they do depend on space. In other words, 59 00:04:58,720 --> 00:05:00,570 the velocity field in this case is 60 00:05:00,570 --> 00:05:07,570 * I'll begin by putting the smoke probe relatively high above the model. You can see the smoke 61 00:05:10,330 --> 00:05:15,500 travels in essentially a straight line downstream. Note that the line does not change in time 62 00:05:15,500 --> 00:05:17,760 indicating the flow is steady. 63 00:05:17,760 --> 00:05:23,960 * Now, I'll move the probe slowly down toward the model. As I do, note that the smoke flow 64 00:05:23,960 --> 00:05:29,190 starts to show curvature, roughly pointing upwards in front of the model and then pointing 65 00:05:29,190 --> 00:05:34,960 downwards behind the model. This clearly shows that the direction of the velocity field changes 66 00:05:34,960 --> 00:05:41,960 depending on where we look. Further, note that the smoke lines are again steady. That 67 00:05:42,120 --> 00:05:46,930 is, when I hold the probe tip at a fixed location, the shape of the smoke line doesn't change 68 00:05:46,930 --> 00:05:53,370 in time. Thus, we see that the air velocity for this condition is a "steady" or time-independent 69 00:05:53,370 --> 00:05:58,700 vector field, i.e. v = v(x). 70 00:05:58,700 --> 00:06:03,770 * Finally, let's take a look at the flow that travels near the leading edge strakes to see 71 00:06:03,770 --> 00:06:10,770 if there is any evidence of a vortex. [Ad lib] 72 00:06:14,440 --> 00:06:21,440 UNSTEADY FLOW VISUALIZATION: TIME-DEPENDENT VECTOR FIELDS 73 00:06:23,740 --> 00:06:30,550 * Now, let's increase the angle of attack to Y degrees. At this angle of attack, we 74 00:06:30,550 --> 00:06:35,680 will see that the smoke lines in some regions will no longer be fixed in time, even though 75 00:06:35,680 --> 00:06:42,350 the probe location is fixed. This indicates that the velocity vector field is time-dependent, 76 00:06:42,350 --> 00:06:44,090 i.e. 77 00:06:44,090 --> 00:06:51,090 * We'll start by placing the probe again relatively high above the model. We again see straight 78 00:06:51,360 --> 00:06:53,290 and steady smoke line. 79 00:06:53,290 --> 00:06:57,400 * As we again move the probe slowly toward the body, we again see the curvature of the 80 00:06:57,400 --> 00:07:04,330 smoke lines. Relative to the low angle of attack case, note that the curvature has increased. 81 00:07:04,330 --> 00:07:08,300 The smoke is still steady as we approach the body, however, ... 82 00:07:08,300 --> 00:07:13,080 * When we start to get much closer to the body, we start to see that the smoke lines 83 00:07:13,080 --> 00:07:20,080 change in time. In fact, they change so rapidly that the smoke tends to "mix out". Clearly, 84 00:07:20,190 --> 00:07:22,000 v = v(x,t) 85 00:07:22,000 --> 00:07:29,000 * Let's look at the flow by the leading edge strake. Now we can see the vortex, which is 86 00:07:31,020 --> 00:07:38,020 characterized by the swirling or corkscrew-like behavior of the smoke line. Also note that 87 00:07:44,120 --> 00:07:51,120 the smoke line is changing shape with time, again indicating that v = v(x,t) 88 00:07:53,050 --> 00:07:59,190 Flow visualization is used to help engineers understand what is happening in a flow. Usually, 89 00:07:59,190 --> 00:08:03,710 flow visualization is combined with other measurements such as force and moment measurements 90 00:08:03,710 --> 00:08:09,000 on the body to arrive at a more complete picture. In the high angle of attack condition we just 91 00:08:09,000 --> 00:08:16,000 explored, a key question is when the strake vortex becomes unsteady, where is the unsteadiness? 92 00:08:16,860 --> 00:08:23,670 For example, is it over the fuselage, over the wing, etc? This is important because the 93 00:08:23,670 --> 00:08:29,280 unsteady velocity is usually tied to unsteady pressures acting on the aircraft, which can 94 00:08:29,280 --> 00:08:35,909 then drive the aeroelastic instability and lead to decreased life of parts of the aircraft. 95 00:08:35,909 --> 00:08:40,520 An example of this actually happened to the F-18 in which the tail of the aircraft was 96 00:08:40,520 --> 00:08:46,150 subject to unsteady forces caused by fluctuations in the strake vortex. 97 00:08:46,150 --> 00:08:51,300 This caused the first F-18's to be limited to only a few hundred flight hours as opposed 98 00:08:51,300 --> 00:08:57,030 to the thousands of flight hours the Navy desired. A flow visualization of the F-18 99 00:08:57,030 --> 00:09:02,080 strake vortex is shown here. 100 00:09:02,080 --> 00:09:05,640 Here's a quick summary of what we saw in this demonstration: 101 00:09:05,640 --> 00:09:10,660 * Flow quantities around bodies are often analyzed using an Euler frame. 102 00:09:10,660 --> 00:09:15,930 * The flow velocity is a vector field which depending on the application can be not only 103 00:09:15,930 --> 00:09:19,200 a function of space but also time. 104 00:09:19,200 --> 00:09:24,410 * For the aircraft model, we saw that at low angles of attack, the velocity vector field 105 00:09:24,410 --> 00:09:30,990 was steady (though did depend on space). At high angles of attack, the velocity field 106 00:09:30,990 --> 00:09:37,990 was unsteady, i.e. depending on space AND time.