Tokyo 2020: Baseball Aerodynamics

For a long time, people thought baseball aerodynamics were simply a competition between gravity and the Magnus effect caused when a ball is spinning. But the seams of a baseball are so prominent that they, too, have a role to play.  (Image credit: top - Pixabay, others - B. Smith; research credit: B. Smith; see also Baseball Aerodynamics) Today wraps up our Olympic coverage, but if you missed our earlier posts, you can find them all here. Read the full article

Every four years, Adidas creates a newly designed ball for the World Cup. This year’s version is the Telstar 18, which features six glued panels (no stitching!) with a slightly raised texture. That subtle roughness is an important feature for the ball’s aerodynamics. It helps ensure that flow around the ball will become turbulent at relatively low speeds. Some previous designs, notably the 2010 Jabulani, were so smooth that flow near the ball would not become turbulent until much higher speeds. In fact, one side of the ball might have laminar flow while the other was turbulent, causing the ball to wobble and misbehave. To learn more about World Cup aerodynamics and the importance of a little surface roughness to the ball’s behavior, check out the Physics Girl video below.    (Image credit: Adidas; via APS News; video credit: Physics Girl)

Saguaro cacti can grow 15 meters tall, and despite their shallow root systems can withstand storm winds up to 38 meters per second without being blown over. Grooves in the cacti’s surface may contribute to its resilience, by adding structural support and/or through reducing aerodynamic loads. The latter theory mirrors the concept of dimples on a golf ball; namely, grooves create turbulence in the flow near the cactus, which allows air flow to track further around the cactus before separating. The result is less drag for a given wind speed than a smooth cactus would experience. 

Indeed, recent experiments on a grooved cylinder with a pneumatically-controlled shape showed exactly that; the morphable cylinder’s drag was consistently significantly lower than fixed samples. Cacti do change their shapes somewhat as their water content changes, but they don’t have the ability for up-to-the-minute alterations. Nevertheless, their adaptations can inspire engineered creations that morph to reduce wind impact. (Image credit: A. Levine; research credit: M. Guttag and P. Reis)

Compared to birds, manmade aircraft tend to be quite limited and inelegant. Fixed-wing aircraft, for example, require long, flat areas for take-off and landing, whereas birds of all sizes are adept at maneuvers like perching. This video examines the perching behaviors of large birds and extends the physics to a small unmanned aerial vehicle (UAV). As a bird approaches a perching location, it pitches its body and wings upward. This places the bird in what’s known as deep stall, where air flowing over the upper surface of the wing separates just after the leading edge. This move dramatically increases drag on the bird, slowing it for landing. At the same time, the speed of the pitch maneuver generates a vortex on the wing that helps the bird maintain lift despite the drop in speed. With the help of both forces, the bird can make a graceful, controlled landing in only a short distance. (Video credit: J. Mitchell et. al.)

Interest in micro-aerial vehicles (MAVs) has proliferated in the last decade. But making these aircraft fly is more complicated than simply shrinking airplane designs. At smaller sizes and lower speeds, an airplane’s Reynolds number is smaller, too, and it behaves aerodynamically differently. The photo above shows the upper surface of a low Reynolds number airfoil that’s been treated with oil for flow visualization. The flow in the photo is from left to right. On the left side, the air has flowed in a smooth and laminar fashion over the first 35% of the wing, as seen from the long streaks of oil. In the middle, though, the oil is speckled, which indicates that air hasn’t been flowing over it--the flow has separated from the surface, leaving a bubble of slowly recirculating air next to the airfoil. Further to the right, about 65% of the way down the wing, the flow has reattached to the airfoil, driving the oil to either side and creating the dark line seen in the image. Such flow separation and reattachment is common for airfoils at these scales, and the loss of lift (and of control) this sudden change can cause is a major challenge for MAV designers. (Image credit: M. Selig et al.)

One of the challenges of experimental fluid dynamics is capturing information about a flow that varies in three spatial dimensions and time. Experimentalists have developed many techniques over the years--some qualitative and some quantitative--all of which can only capture a small portion of the flow. The photos above are a series of laser-induced fluorescence (LIF) images of an airfoil at increasing angles of attack. The green swirls are from an added chemical that fluoresces after being excited with a laser. In this case, the technique is providing flow visualization, showing how flow over the upper surface of the airfoil shifts and separates as the angle of attack increases. The technique can also be used, however, to measure velocity, temperature, and chemical concentration. (Image credit: S. Wang et al.)

Like many sports balls, the American football's shape and construction make a big difference in its aerodynamics. Unlike the international football (soccer ball), which undergoes significant redesigns every few years thanks to the World Cup, the American football has been largely unchanged for decades. The images above come from a computational fluid dynamics (CFD) simulation of a spiraling football in flight. Although the surface is lightly dimpled, the largest impact on aerodynamics comes from the laces and the air valve (just visible in the upper right image). Both of these features protrude into the flow and add energy and turbulence to the boundary layer. By doing so, they help keep flow attached along the football longer, which helps it fly farther and more predictably. For more, check out the video of the CFD simulation. (Image credits: CD-adapco; via engineering.com)

Maintaining consistent air flow along the contours of an object is key to aerodynamic efficiency. When air flow separates or forms a recirculation zone, the drag increases and efficiency drops. On wind turbine blades, flow often separates on the root end of the blade near its attachment point. This behavior is apparent in the video above at 0:34. The tufts in the foreground on the turning blade flap and flutter with no clear pattern because the air flow has separated from the surface. In the subsequent clip, a line of vortex generators has been attached near the leading edge of the blade. These structures--also commonly seen on airplanes--trail vortices behind them, mixing the flow and generating a turbulent boundary layer which is better able to resist flow separation. The effect on the flow is clear from the tufts, most of which now point in a consistent direction with little to no fluttering, indicating that the air flow has remained attached. (Video credit: Smart Blade Gmbh/Technische Universität Berlin)

In the transonic speed regime the overall speed of an airplane is less than Mach 1 but some parts of the flow around the aircraft break the speed of sound. The photo above shows a schlieren photograph of flow over an airfoil at transonic speeds. The nearly vertical lines are shock waves on the upper and lower surfaces of the airfoil. Although the freestream speed in the tunnel is less than Mach 1 upstream of the airfoil, air accelerates over the curved surface of airfoil and locally exceeds the speed of sound. When that supersonic flow cannot be sustained, a shock wave occurs; flow to the right of the shock wave is once again subsonic. It's also worth noting the bright white turbulent flow along the upper surface of the airfoil after the shock. This is the boundary layer, which can often separate from the wing in transonic flows, causing a marked increase in drag and decrease in lift. Most commercial airliners operate at transonic Mach numbers and their geometry is specifically designed to mitigate some of the challenges of this speed regime.  (Image credit: NASA; via D. Baals and W. Corliss)

Yesterday we discussed some of the basic mechanics of a frisbee in flight. Although frisbees do generate lift similarly to a wing, they do have some unique features. You've probably noticed, for example, that the top surface of a frisbee has several raised concentric rings. These are not simply decoration! Instead the rings disrupt airflow at the surface of the frisbee. This actually creates a narrow region of separated flow, visible in region B on the left oil-flow image. Airflow reattaches to the frisbee in the image after the second black arc, and the boundary layer along region C remains turbulent and attached for the remaining length of the frisbee. Keeping the boundary layer attached over the top surface ensures low pressure so that the disk has plenty of lift and remains aerodynamically stable in flight. A smooth frisbee would be much harder to throw accurately because its flight would be very sensitive to angle of attack and likely to stall. (Image credits: J. Potts and W. Crowther; recommended papers by: V. Morrison and R. Lorentz)

Since 2006, Adidas has unveiled a new football design for each FIFA World Cup. This year's ball, the Brazuca, is the first 6-panel ball and features glued panels instead of stitched ones. It also has a grippy surface covered in tiny nubs. Wind tunnel tests indicate the Brazuca experiences less drag than other recent low-panel-number footballs as well as less drag than a conventional 32-panel ball. Its stability and trajectory in flight are also more similar to a conventional ball than other recent World Cup balls, particularly the infamous Jabulani of the 2010 World Cup. The Brazuca's similar flight performance relative to a conventional ball is likely due to its rough surface. Like the many stitched seams of a conventional football, the nubs on the Brazuca help trip flow around the ball to turbulence, much like dimples on a golf ball. Because the roughness is uniformly distributed, this transition is likely to happen simultaneously on all sides of the ball. Contrast this with a smooth, 8-panel football like the Jabulani; with fewer seams to trip flow on the ball, transition is uneven, causing a pressure imbalance across the ball that makes it change its trajectory. For more, be sure to check out the Brazuca articles at National Geographic and Popular Mechanics, as well as the original research article. (Photo credit: D. Karmann; research credit: S. Hong and T. Asai)

One of the common themes in aerodynamics, especially in sports applications, is that tripping the flow to turbulence can decrease drag compared to maintaining laminar flow. This seems counterintuitive, but only because part of the story is missing. When a fluid flows around a complex shape, there are actually three options: laminar, turbulent, or separated flow. An object's shape creates pressure forces on the surrounding fluid flow, in some cases causing an increasing, or unfavorable, pressure gradient. When this occurs, fluid, especially the slower-moving fluid near a surface, can struggle to continue flowing in the streamwise flow direction. Consider the video above, in which the flow moves from left to right. Near the surface a turbulent boundary layer is visible, where fluid motion is significantly slower and more random. Occasionally the flow even reverses direction and billows up off the surface. This is separation. Unlike laminar boundary layers, turbulent boundary layers can better resist and recover from flow separation. This is ultimately what makes them preferable when dealing with the aerodynamics of complex objects.  (Video credit: A. Hoque)

Long track speed skating is a race against the clock. Skaters reach speeds of roughly 50 kph, so drag has a significant impact. This is why skaters stay bent and spend straightaways--their fastest segments on the ice--with their arms pulled behind them. It's also why their speedsuits have hoods to cover their hair. This year the U.S. speed skaters are wearing special suits designed by Under Armour and Lockheed Martin especially for their aerodynamics. The suits feature a mixture of fabrics including raised surface features on the hood and forearms. These bumps are designed to trip turbulent flow in these regions. It seems counterintuitive, but drag is actually lower for a turbulent boundary layer than a laminar one at the right Reynolds number range. This is because turbulent boundary layers are better at staying attached to non-streamlined bodies. The longer flow stays attached to the skater, the smaller the pressure difference between the air in front of the skater and the air in his wake. The suit's seams and even its hot-rod-like flames were placed with this effect in mind. Only time will tell whether the suits really give skaters a competitive edge, but since Sochi's low-altitude increases drag on skaters, they will appreciate some extra speed. For more, NSF has an inside look at the suit's development. (Photo credits: Under Armour)

FYFD is exploring the fluid dynamics of the Winter Olympics. Check out previous posts on how lugers slide fast and why ice is slippery, and be sure to stay tuned for more!

When supersonic flow is achieved through a wind tunnel or rocket nozzle, the flow is said to have "started". For this to happen, a shock wave must pass through, leaving supersonic flow in its wake. The series of images above show a shock wave passing through an ideal rocket nozzle contour. Flow is from the top to bottom. As the shock wave passes through the nozzle expansion, its interaction with the walls causes flow separation at the wall. This flow separation artificially narrows the rocket nozzle (see images on right), which hampers the acceleration of the air to its designed Mach number. It also causes turbulence and pressure fluctuations that can impact performance.  (Image credit: B. Olson et al.)

Ever look out an airplane's window and wondered why a row of little fins runs along the upper side of the wing? These vortex generators help prevent a wing from stalling at high angle of attack by keeping flow attached to the surface. Airflow over the vanes creates a tip vortex that transports the higher-momentum fluid from the freestream closer to the wing's surface, increasing the momentum in the boundary layer. As a result of this momentum exchange, the boundary layer remains attached over a greater chordwise distance. This also increases the effectiveness of trailing-edge control surfaces, like ailerons, on the wing. (Photo credit: Mark Jones Jr.)

This numerical simulation gives a glimpse of flow inside an unsteady rocket nozzle.  The nozzle is over-expanded, meaning that the exhaust's pressure is lower than that of the ambient atmosphere. A slightly over-expanded nozzle causes little more than a decrease in efficiency, but if the nozzle is grossly over-expanded, the boundary layer along the nozzle wall can separate and induce major instabilities, as seen here. In the first segment of the video, turbulent structures along the nozzle wall boundary layer are shown; note how the boundary layer becomes very thick and turbulent after the primary shock wave (shown in gray). This is due to the flow separating near the wall.  The second half of the video shows the unsteadiness this can create. The primary shock wave splits into two near the wall, creating a lambda shock wave, named for the shape of the lower case Greek letter. This shock structure is indicative of strong interaction between the boundary layer and shock wave. (Video credit: B. Olson and S. Lele)

Physics students are often taught to ignore the effects of air on a projectile, but such effects are not always negligible. This video features several great examples of the Magnus effect, which occurs when a spinning object moves through a fluid. The Magnus force acts perpendicular to the spin axis and is generated by pressure imbalances in the fluid near the object's surface. On one side of the spinning object, fluid is dragged with the spin, staying attached to the object for longer than if it weren't spinning.  On the other side, however, the fluid is quickly stopped by the spin acting in the direction opposite to the fluid motion. The pressure will be higher on the side where the fluid stagnates and lower on the side where the flow stays attached, thereby generating a force acting from high-to-low, just like with lift on an airfoil. Sports players use this effect all the time: pitchers throw curveballs, volleyball and tennis players use topspin to drive a ball downward past the net, and golfers use backspin to keep a golf ball flying farther. (Video credit: Veritasium)