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# Fluid Dynamics

Media Type:
Video

Running Time: 6m 18s
Size: 36.7 MB

or

This media asset is adapted from "NASA Launchpad: Fluid Dynamics - What a Drag!"

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Working in the microgravity environment of space presents many challenges to astronauts. Using the ideas behind fluid dynamics, astronauts are better able to prepare for these challenges. This NASA Launchpad video showcases some of that preparation being done in NASA’s Neutral Buoyancy Lab and how NASA has partnered with Speedo to design a more drag-resistant professional swimsuit.

Background Essay

Newton’s Second Law, Fnet=ma, tells us that forces are related to acceleration. So, if an object experiences an imbalance of forces, it will experience an acceleration. Similarly, if an object is seen accelerating, it must be experiencing an imbalance of forces.

Forces acting in opposite directions can balance out one another. For an object under water, the downward force of weight can be balanced out by an upward force of buoyancy. When a beach ball floats in water, its weight force (downward) is balancing the water’s buoyancy (upward) force. In fact, even if an object sinks in water, it still experiences buoyancy. This is why things like rocks (and your younger sibling!) are easier to lift when you’re in a body of water than when you’re on land—the buoyancy force is helping you out by doing some of the lifting! Flotation devices increase buoyancy, so even, say, an astronaut-in-training weighed down by a heavy suit with an assortment of tools can hover without sinking if flotation devices provide an upward buoyant force that balances out the downward weight force.

Drag is a force much like friction or air resistance. It tends to slow down moving things. The drag force opposes forward motion for objects moving through a liquid or gas. Since our atmosphere is a gas, everything in our everyday lives that moves through the atmosphere experiences a drag force. That’s why cars need a constant supply of fuel to keep moving forward. When objects move slowly, the magnitude of the drag force is relatively small. As objects move more quickly, drag increases. This is why you don’t feel the air much when you move your hand to pick up a pencil but when you stick your hand out of a moving car’s window, you feel a significant push backward.

The shape of an object helps determine how it passes through a medium and how much drag its motion produces. Designers and engineers adjust the shape and configuration of mechanical systems like automobiles, boats, planes and spacecraft to maximize their efficiency of movement.

Discussion Questions

Before Viewing

• Astronauts need to train to go to space, work in space, and live in space, but obviously that training has to happen before they actually go into space. Can you think of some ideas for how NASA might train astronauts to be ready to work in space?
• Is it possible for an object to stay still under water, neither sinking down nor floating up? If not, why not? If so, what would stop the object from sinking down or floating up?
• At the end of very fast “drag races,” when cars need to decelerate quickly, a driver deploys a parachute behind his or her car. Why? How does the parachute change the car’s motion?
• While Viewing

• How does neutral buoyancy help astronauts train for space? What about it doesn’t quite replicate being in space?
• How do scientists use wind tunnels to learn about drag? Why is drag relevant for space flight?
• What does it mean for something to be "hydrodynamic"?
• After Viewing

• If you were drawing a force diagram for one of the astronauts under water, how would you change your diagram if the astronaut added more weights? More flotation devices? How do you think NASA scientists find the right balance?
• The video mentions that it’s especially important to understand the effects of drag for objects like airplanes or spacecraft moving at high velocities. Why do you think that’s the case? Why might drag be less of an issue for objects moving at slower speeds?
• Give some examples of systems that work best when drag is reduced, and give some examples of systems that work best when there’s a lot of drag. When is drag useful?
• The Speedo swimsuit featured at the end of the video has seams with a “solid state weld.” What does that mean? Why is it mentioned in connection with the Orion spacecraft?
• Bonus Question: Think about an everyday object that moves through the air or the water. Name three modifications you might test out to reduce drag.

Teaching Tips

Classroom Activity: Wind Tunnel Test

Materials

• Range of craft supplies, including dowels, felt, connectors, paper clips, paper, and other items, purchased or found
• Measuring tape
• Students break into small groups and, using the provided materials, create a test launcher. Each group releases its test vehicle into the airstream and measures its flight distance. After each round, groups have the chance to modify their designs. As a finale, the class as a whole comes up with a new design based on all the trials and tests that new design. If time allows, that design can be modified and re-tested to maximize flight distance.

Discussion Questions

• What design features seemed to make the most difference?
• What was the most surprising thing you learned from this activity?
• How would you explain the connection between this activity and the video we saw?
• Do you think it would make a difference if we launched the test vehicles vertically rather than horizontally?
• Could there be dangers or challenges associated with a vehicle being too aerodynamic or too hydrodynamic?
• Classroom Activity: Balloon Balance

Use helium balloons with paper clips hooked through the loose ends. Measure and cut pieces of construction paper and clip them to the paper clips. Try to make the weight of the paper hanging exactly equal the buoyant force of the balloon. The “winner” is the group whose balloon stays motionless in mid-air, or is neutrally buoyant.

Transcript

Justin Tully: Hey there, welcome to NASA Launchpad. I’m your host Justin Tully. You ever wonder how astronauts train for working in reduced gravity? Well, there’s actually a whole bunch of different training techniques and facilities for them. You’ve got stuff like virtual reality trainers, mock-ups, simulators.

But what about the physical aspect of being in space, the reduced gravity, the floating around and all that? You’ve probably heard about the vomit comet, officially the KC-135, a plane that flies in parabolas causing the feeling of weightlessness during the free-fall segments of the parabolas. I mean, they actually used that plane to film some of the scenes in Ron Howard’s movie Apollo 13. This kind of training is so important for our real astronauts, that a newer version of the weightless wonder, the C9-B, is used by NASA today.

But what about something a little closer to the ground? Do you know anything about the neutral buoyancy laboratory in Houston, Texas? What is it? What does it look like? Well, it’s basically a really really big pool. It’s about 61 meters long, 31 meters wide, and roughly 12 meters deep. That’s even bigger than an Olympic-sized pool, and much deeper. It holds about 23.5 million liters of water—that’s almost 6.2 million gallons! It’s so big, it also holds full-sized mock-ups of space station modules, so astronauts can actually suit up, dive in, and train for their space walks.

But it’s not quite that simple. If you were to dive into a pool with a heavy spacesuit on, you’d probably sink. Not so weightless…. That’s where the neutral buoyancy comes into play. Neutral buoyancy is a condition in which an object’s mass is equal to the mass it displaces in a surrounding medium. In simple words, a neutrally buoyant object neither floats nor sinks. To make an object neutrally buoyant, an object’s mass needs to be equal to the mass of the water. The astronauts accomplish this with a combination of weights and flotation devices. Once neutral buoyancy is achieved, the astronaut essentially hovers in the water. But, here’s the thing. In the neutral buoyancy lab, the astronauts still feel their own weight as gravity tugs on them, so it’s not exactly the same as being weightless in space. But, it does allow the astronauts to practice moving heavy objects or practice performing fine-motor tasks with those heavy suits on. Neutral buoyancy may not be perfect, but it is the closest thing you can get to weightlessness here on Earth.

Of course, the astronauts in the big pool have to deal with another problem: water drag. Even in neutral buoyancy, the drag of moving water slows down the astronauts’ movements. Drag is a force that opposed the motion of a solid object through either liquid or gas. So an astronaut moving his arm through the water encounters resistance of the water drag, or an airplane flying through the air has to deal with the drag force of the air, or even when you’re walking around, there’s drag resisting your motion. Now, granted, you’re not going to be moving very fast, and neither is the astronaut; but in special situations, when you’re moving at higher velocities, like in an airplane or a rocket, it’s important the understand the effects of drag forces.

How do you do that? Well, NASA’s got the answer. They’re called wind tunnels, and they’re used to check the fluid dynamics of objects. Some are large enough to test full-sized planes and rockets, but most deal with smaller models. All kinds of things get tested in NASA wind tunnels, from airplanes to rockets, even NASA race cars.

But, what about a swimsuit? Because let’s be honest, for competitive swimmers, drag is a huge deal. It’s why a lot of swimmers where swim caps, shave their heads, even shave their bodies. Hair, even a little bit, increases drag. Studies indicate that viscous drag, or skin friction, is roughly 1/3 of the total restraining force on a swimmer. And for these competitive swimmers, who are working hard to improve their form and their hydrodynamics (hydrodynamics, that’s like aerodynamics but in water) every little bit helps. A few hundredths of a second can mean the difference between a gold medal and not placing at all.

And that’s why the Speedo company did testing on their LZR Racer swimsuit in NASA’s wind tunnels. The result? Well, the numbers kind of speak for themselves. In the 2008 Olympics, 94% of the races were won by competitors wearing the LZR Racer swimsuit, and in the same 2008 games, over 60 world records were broken by swimmers in LZR Racer suits. That seems pretty effective, right?

So, how did this suit come about? Well, the first step was finding the right material. NASA researchers, working in collaboration with Speedo, tested over 100 different materials for swimsuits in wind tunnels. The materials were stretched over a smooth, flat, aluminum plate, and the edges were taped down. This was all done so that nothing would interfere with the airflow over the fabric. Each material was tested at a number of different wind speeds, which, with the help of sensors, allowed researchers to measure drag across the surface of the plate. In the end, after years of study, one material emerged. It was called LZR Pulse. This material is ideal because, for one thing, it is very efficient at reducing drag. Also, it repels water and is extremely lightweight.

But the material is not the only consideration. You have to worry about seams, which cause drag as well. But, it turns out, the same technology that can be used to make perfect seams on NASA’s new Orion spacecraft can do the same thing for swimsuits. Speedo ultrasonically welded the seams on the LZR Racer, rather than using traditionally sewn areas. Never heard of ultrasonic welding before? I hadn’t either. Basically, high frequency ultrasonic acoustic vibrations, sound waves with a frequency above the capabilities of humans to hear them, are applied to pieces of material that are being held together under pressure. This creates a solid state weld, that is, the connection is made just by the material. There are no soldering materials, adhesives, nails, stitches, nothing like that.

So the LZR Racer was the first full bonded, full body swimsuit with ultrasonically welded seams, and that alone reduced drag by 6%. Also, the zipper for the suit was ultrasonically bonded but hidden inside the suit. That generated 8% less drag in wind tunnel tests than a standard zipper. So all in all, the LZR Racer suit reduced the passive drag of the fabric over the previous Speedo FS Pro suit by about 24%. When you’re talking hundredths of a second between first and last, that’s a huge deal.

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