WEBVTT 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 00:00:09.450 --> 00:00:15.020 wing called leading edge strakes or leading edge extensions. Why were the wings designed 00:00:15.020 --> 00:00:19.250 this way? In this video, you'll find out. 00:00:19.250 --> 00:00:23.810 This video is part of the Representations video series. Information can be represented 00:00:23.810 --> 00:00:29.859 in words, through mathematical symbols, graphically, or in 3-D models. Representations are used 00:00:29.859 --> 00:00:35.420 to develop a deeper and more flexible understanding of objects, systems, and processes. 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 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 00:00:46.690 --> 00:00:51.539 air velocity around an airplane body using smoke flow visualization. You will see that 00:00:51.539 --> 00:00:56.829 at low angles of attack, the velocity field is independent of time, while at high angles 00:00:56.829 --> 00:01:01.030 of attack the velocity field depends on space and time. 00:01:01.030 --> 00:01:05.780 After watching this video: * You will know that flow quantities around bodies are often 00:01:05.780 --> 00:01:12.130 analyzed using an Eulerian frame. * You will recognize that flow velocity is a vector field 00:01:12.130 --> 00:01:18.500 which depending on the application can be not only a function of space but also time. 00:01:18.500 --> 00:01:23.670 This is Dick, he's the Senior Technical Instructor here in the department of Aeronautics and 00:01:23.670 --> 00:01:27.880 Astronautics. He is in charge of the Wright Brothers wind tunnel. 00:01:27.880 --> 00:01:32.649 * Wind tunnels are used to simulate the airflow around a variety of objects including buildings, 00:01:32.649 --> 00:01:36.239 cars, trains, and of course aircraft. 00:01:36.239 --> 00:01:40.408 * The Wright Brothers Wind Tunnel is an example of a closed-circuit wind tunnel, which uses 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 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 00:01:52.009 --> 00:01:57.780 use are around 100mph. While somewhat higher speeds are possible, the noise is increased 00:01:57.780 --> 00:02:04.780 rapidly with increased fan speed. 00:02:06.149 --> 00:02:11.450 * We will visualize the air velocity vector field using smoke visualization. We seed smoke 00:02:11.450 --> 00:02:16.950 in the tunnel through a handheld probe. The smoke follows the local air velocity allowing 00:02:16.950 --> 00:02:22.120 us to "see" where the flow is going. This smoke flow visualization techniques works 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 00:02:29.480 --> 00:02:33.719 in the tunnel test section upstream of the model will be approximately 25 mph. 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 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 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 00:02:48.810 --> 00:02:53.540 instability due to coupling between the aerodynamic forces and the wing structure. In other words, 00:02:53.540 --> 00:02:55.760 because the wing is not a completely rigid structure, aerodynamic forces can cause the 00:02:55.760 --> 00:02:57.090 wing to vibrate, which can cause big problems during flight. To study this issue, Lockheed 00:02:57.090 --> 00:03:00.120 developed the following simplified geometry. In the department of Aeronautics and Astronautics 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. 00:03:00.959 --> 00:03:03.450 * The model has three main parts to it: 00:03:03.450 --> 00:03:07.510 ** The fuselage, which is a body of revolution with a pointed tip and a bluff end 00:03:07.510 --> 00:03:07.620 ** The trapezoidal wing 00:03:07.620 --> 00:03:10.780 ** Leading edge strakes, sometimes also called leading edge extensions 00:03:10.780 --> 00:03:15.030 * The leading edge strakes help stabilize the flow at high angles of attack by creating 00:03:15.030 --> 00:03:18.849 a strong vortex over the wing. This vortex is a region swirling velocity and low pressure 00:03:18.849 --> 00:03:25.849 and generates a significant amount of lift. Thus, strakes can provide significant performance 00:03:27.530 --> 00:03:34.530 benefits for aircraft that require high angle of attack maneuverability. We will look at 00:03:35.560 --> 00:03:41.260 the vortices created by these strakes today. 00:03:41.260 --> 00:03:47.530 In many aerodynamic applications, engineers analyze flows using an Eulerian frame in which 00:03:47.530 --> 00:03:54.530 flow quantities such as velocity, pressure, temperature, etc are viewed as fields: i.e. 00:03:55.170 --> 00:03:59.930 functions of space and often the space of interest is fixed to the objects frame of 00:03:59.930 --> 00:04:06.019 reference. In our case, this frame of reference is also the wind tunnels frame of reference. 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 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, 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 00:04:22.490 --> 00:04:28.620 field. Further, depending on the flow conditions, the velocity can be a function of time in 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). 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 00:04:40.850 --> 00:04:47.380 of a cruise condition, specifically we are at X degrees angle of attack. In particular, 00:04:47.380 --> 00:04:52.700 the flow at this low angle of attack is, to good approximation, "steady". This means that 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, 00:04:58.720 --> 00:05:00.570 the velocity field in this case is 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 00:05:10.330 --> 00:05:15.500 travels in essentially a straight line downstream. Note that the line does not change in time 00:05:15.500 --> 00:05:17.760 indicating the flow is steady. 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 00:05:23.960 --> 00:05:29.190 starts to show curvature, roughly pointing upwards in front of the model and then pointing 00:05:29.190 --> 00:05:34.960 downwards behind the model. This clearly shows that the direction of the velocity field changes 00:05:34.960 --> 00:05:41.960 depending on where we look. Further, note that the smoke lines are again steady. That 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 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 00:05:53.370 --> 00:05:58.700 vector field, i.e. v = v(x). 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 00:06:03.770 --> 00:06:10.770 if there is any evidence of a vortex. [Ad lib] 00:06:14.440 --> 00:06:21.440 UNSTEADY FLOW VISUALIZATION: TIME-DEPENDENT VECTOR FIELDS 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 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 00:06:35.680 --> 00:06:42.350 the probe location is fixed. This indicates that the velocity vector field is time-dependent, 00:06:42.350 --> 00:06:44.090 i.e. 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 00:06:51.360 --> 00:06:53.290 and steady smoke line. 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 00:06:57.400 --> 00:07:04.330 smoke lines. Relative to the low angle of attack case, note that the curvature has increased. 00:07:04.330 --> 00:07:08.300 The smoke is still steady as we approach the body, however, ... 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 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, 00:07:20.190 --> 00:07:22.000 v = v(x,t) 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 00:07:31.020 --> 00:07:38.020 characterized by the swirling or corkscrew-like behavior of the smoke line. Also note that 00:07:44.120 --> 00:07:51.120 the smoke line is changing shape with time, again indicating that v = v(x,t) 00:07:53.050 --> 00:07:59.190 Flow visualization is used to help engineers understand what is happening in a flow. Usually, 00:07:59.190 --> 00:08:03.710 flow visualization is combined with other measurements such as force and moment measurements 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 00:08:09.000 --> 00:08:16.000 explored, a key question is when the strake vortex becomes unsteady, where is the unsteadiness? 00:08:16.860 --> 00:08:23.670 For example, is it over the fuselage, over the wing, etc? This is important because the 00:08:23.670 --> 00:08:29.280 unsteady velocity is usually tied to unsteady pressures acting on the aircraft, which can 00:08:29.280 --> 00:08:35.909 then drive the aeroelastic instability and lead to decreased life of parts of the aircraft. 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 00:08:40.520 --> 00:08:46.150 subject to unsteady forces caused by fluctuations in the strake vortex. 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 00:08:51.300 --> 00:08:57.030 to the thousands of flight hours the Navy desired. A flow visualization of the F-18 00:08:57.030 --> 00:09:02.080 strake vortex is shown here. 00:09:02.080 --> 00:09:05.640 Here's a quick summary of what we saw in this demonstration: 00:09:05.640 --> 00:09:10.660 * Flow quantities around bodies are often analyzed using an Euler frame. 00:09:10.660 --> 00:09:15.930 * The flow velocity is a vector field which depending on the application can be not only 00:09:15.930 --> 00:09:19.200 a function of space but also time. 00:09:19.200 --> 00:09:24.410 * For the aircraft model, we saw that at low angles of attack, the velocity vector field 00:09:24.410 --> 00:09:30.990 was steady (though did depend on space). At high angles of attack, the velocity field 00:09:30.990 --> 00:09:37.990 was unsteady, i.e. depending on space AND time.