#boundary layer separation

[fuckyeahfluiddynamics]

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

[fuckyeahfluiddynamics]

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)

[fuckyeahfluiddynamics]

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)

[fuckyeahfluiddynamics]

The flapping of flexible objects like flags have long fascinated mankind. The figure above from Shelley and Zhang 2011 shows several possible flapping states.  In (a) a thread immersed in a running soap film displays the standard von Karman vortex street of shed vortices in its wake. Parts (b) and (c) show the thread in coherent flapping motion; (b) shows an snapshot of the flapping thread in the soap film whereas (c) is a timelapse of the thread showing its full range of motion.  Image (d) shows the effects of a higher flow speed--the flapping motion becomes aperiodic. Image (e) shows a stiff metal wire bent into the shape of a flapping filament; note the strong boundary layer separation around the wire compared to the thread in Image (b). As one might expect, the drag on the unflapping wire is significantly greater than the drag on the flapping thread. (Image credit: M. Shelley and J. Zhang, Shelley and Zhang 2011)

[fuckyeahfluiddynamics]

Unlike road stages in which cyclists can draft off one another to reduce drag, in the time trial a cyclist is on a solo race against the clock with nowhere to hide. As a result, the event features lots of technologies designed to reduce both pressure drag and skin friction on the cyclist. For time trials, cyclists wear skinsuits and shoe covers to eliminate any sources of flapping fabrics and to reduce skin friction. They ride bicycles designed to be as light and aerodynamic as possible. Instead of rounded tubing in the frames, these bikes consist of elongated airfoil profiles that direct air past and prevent separation that may increase pressure drag. The rims of their tires are wider and the back wheel is replaced with a disc wheel that allows no airflow aross the wheel. Like the airfoil tubing, these changes help prevent separation. Similarly, riders wear elongated helmets designed to be as aerodynamic as possible while the rider is in the "aero" position, with arms directed out over the wheels, head level, elbows tucked, and back flat. In wind tunnel tests, the rider best able to hold this position will experience the least drag. Even the addition or subtraction of a water bottle is not left to chance, with many time trial bikes designed to be more aerodynamic with a water bottle onboard (though you probably won't catch the cyclists breaking their aero position to get a drink)! (Photos by Veeral Patel)

FYFD is celebrating the Tour de France with a weeklong exploration of the fluid dynamics of cycling. See previous posts on drafting in the peloton, and pacelining and echelons, and the art of the lead-out train.

[fuckyeahfluiddynamics]

Bill Nye, Samuel Jackson, golf balls, Reynolds number, dimples, and boundary layers. It doesn’t get much better than this. - Khristopher O (submitter)

It definitely beats Jackson's other foray into aerodynamics! The dimples on a golf ball cause turbulent boundary layers, which actually decrease drag on the ball and make it fly farther. Why bluff bodies experience a reduction in drag as speed (and thus Reynolds number) increases was a matter of great confusion for fluid mechanicians early in the twentieth century, but it's not too hard to see why it happens with some flow visualization.

On the top sphere, the laminar boundary layer separates from the sphere just past its shoulder. This results in a pressure loss on the backside of the sphere and, thus, an increase in drag. On the bottom sphere, a trip-wire placed just before the shoulder causes a turbulent boundary layer, which separates from the sphere farther along the backside. This late separation results in a thinner wake and a smaller pressure loss behind the sphere, thereby reducing the overall drag when compared to the laminar case. (Photo credit: An Album of Fluid Motion)