Search results for: “drag”

Fluids Round-up – 13 July 2013
Prepare yourselves for lots of links in today’s fluids round-up!
- Longtime FYFD favorite Mark Stock (see here, here, and here) and his collaborator James Susinno have unveiled a new interactive art piece, “Everything is Made of Atoms” that utilizes some impressive real-time fluids simulation. NVIDIA’s blog has some details on the computing.
- ScarbsF1 takes a detailed look at the F-duct used to stall an F1 car’s rear wing to reduce drag. (submitted by Vinnie)
- Just in time for summer fun, National Geographic talks about the physics of water slides.
- SpaceX’s reusable Grasshopper rocket has set a new altitude test of over 1000 ft. Check out this feat of aerodynamic control over at io9.
- Stanford engineers are using high-speed video of birds in the wild to study the mechanics of flapping flight. If you check out their video, you’ll notice how the birds rotate their wings as they flap in order to maximize lift throughout the flapping cycle. (via io9)
- Speaking of io9, they highlighted a couple of great examples of meteorological fluid dynamics recently: roll clouds and water spouts.
- New research suggests that thresher sharks may whip their tails quickly enough to produce cavitation-induced shockwaves to stun their prey. If so, they join the pistol and mantis shrimp in utilizing this technique for hunting.
- If you’re looking for some casual games, Liquid Sketch is a fun fluids puzzle game for iOS (submitted by Keri B)
- Finally, congratulations to Toronoto’s AeroVelo for capturing the AHS Sikorsky Prize with their human-powered helicopter. Check out this video from their historic flight (submitted by Chris R).
(Photo credit: AeroVelo)
July 13, 2013
Reader Question: Drafting in Triathlons
Reader juleztalks writes:
I’ve just entered an amateur triathlon, and there’s a whole load of rules about not “drafting” in the cycle stage (basically, not sitting in other cyclists’ slipstream). However, there are no such rules for the swim or run stage; I thought the effects would be the same from drafting other swimmers and runners. Any ideas?
As in many endurance sports, it’s all a question of energy savings from drag reduction. Drag on an object, like a triathlete, is roughly proportional to fluid density (air for cycling or running, water for swimming), frontal area, and the velocity squared. Because drag increases more drastically for an increase in velocity, it makes sense one would worry most about drag when one’s velocity is highest – on the bike.
Drafting has major benefits in cycling and can reduce drag on a rider by 25-40%. Aerodynamic drag accounts for 70% or more of a cyclist’s energy expenditure, so that reduction can really add up. The energy saved by drafting during cycling can even increase a triathlete’s speed during a subsequent running leg. So it makes sense for a sport’s governing body to be concerned with it.
That said, there’s plenty of room for drag reduction in swimming as well. Even though the velocities are much lower, water’s density is 1,000 times higher than air’s, generating plenty of drag for an athlete to overcome. For swimmers at maximum speed, drafting can reduce drag by 13-26%, depending on relative positioning. Such drafting has been found to increase stroke length and may (or may not) improve subsequent cycling performance.
Although a similar reduction in drag is possible by drafting when running, drag on a runner only accounts for about 8% of his/her energy expenditure so such savings would matters very little next to the swimming and cycling legs. There could be some psychological benefits, though, in terms of pacing oneself. (Photo credit: Optum Pro Cycling p/b Kelly Benefit Strategies)
May 15, 2013
Diving Peregrines
Few animals can compete with a peregrine falcon for pure speed. There is evidence that, when diving, the falcon can reach speeds upward of 200 mph (320 kph). That the birds can achieve this by pulling their wings back into a low-drag profile is impressive, but the control they exert to do so is even more astounding. The placement and acuity of a falcon’s eyes would require tilting its head roughly 40 degrees if diving straight down on its prey. Such asymmetry increases their drag by more than 50% and creates a torque that yaws the bird. Instead, as seen in the video above, the falcon keeps its head straight and flies in a spiral-like dive, allowing it to maintain sight contact with its target and maximizing its speed despite the extended dive. (Video credit: BBC; research credit: V. A. Tucker)
April 9, 2013
Egg-Spinning Fun
If you have any leftover hard-boiled eggs, you can recreate this bit of fluid dynamical fun. Spin the egg through a puddle of milk, and you’ll find that the egg draws liquid up from the puddle and flights it out in a series of jets. As the egg spins, it drags the milk it touches with it. Points closer to the egg’s equator have a higher velocity because they travel a larger distance with each rotation. This variation in velocities creates a favorable pressure gradient that draws milk up the sides of the egg as it spins, creating a simple pump. To see the effect in action check out this Science Friday video or the BYU Splash Lab’s Easter-themed video. (Photo credit: BYU Splash Lab)
April 1, 2013
Breaking Up Falling Beads
In a stream of falling liquid, surface tension instabilities cause the fluid to break up into droplets. This video shows a similar experiment with a stream of glass beads, a granular material. The whole system is housed under a vacuum to eliminate the effects of air drag on the stream, and a camera rides alongside the stream to track the evolution of the falling material in a Lagrangian fashion. As with a liquid stream, we see the granular flow develop undulations as it falls, ultimately breaking up into clusters of beads. The authors suggest that nanoscale surface roughness and van der Waals forces may be responsible for the clustering behavior in the absence of surface tension. (Video credit: J. Royer et al.)
March 12, 2013
Hot Items Sink Faster
This combined video shows the fall of a heated centimeter-sized steel sphere through water. From left to right, the sphere is at 25 degrees C (left), 110 degrees C (middle), and 180 degrees C, demonstrating how the Leidenfrost effect–which vaporizes the water in immediate contact with the sphere–can substantially reduce the drag on a submerged object. In the middle video, the vaporization of the water around the sphere is sporadic and incomplete, only slightly reducing the sphere’s drag relative to the room temperature case. The much hotter sphere on the right, however, has a complete layer of vapor surrounding it, allowing it to travel through a gas rather than the denser liquid. (Video credit: I. Vakarelski and S. Thoroddsen; from a review by D. Quere)
January 22, 2013
Countertop Fliers
In this video, researcher Leif Ristroph and his colleagues have used a clever way to simulate flapping flight, not by actuating their fliers but by oscillating the flow. The flow is driven by a speaker, which causes the air above it to move up and down. Using straws to simulate the honeycomb flow conditioners often used in wind tunnels helps smooth flow. The end result is a great table-top set-up for testing and refining miniature flier designs. The best fliers stay aloft thanks to asymmetry in the streamwise direction; when the air moves upward, the flier catches the air, maximizing drag so that it is carried upward. When the flow reverses, however, the shape of the flier is more streamlined, so the drag is reduced, helping the flier stay aloft. (Video credit: Science Friday/Leif Ristroph et al.)
January 11, 2013
Sharkskin’s Secrets
Sharks are known as extremely fast and agile swimmers, due in part to the surface of their skin. Sharks are covered in very tiny tooth-shaped scales called denticles which are streamlined in the direction of flow over the shark. If you were to run a hand over a shark’s skin from head to tail, it would feel silky smooth, but rub against the grain and it’s like running your hand on sandpaper. Water encounters a similar resistance, which, according to new research, provides the shark with a passive flow control mechanism, requiring no effort on the part of the shark. When water near the shark’s denticles tries to reverse direction, an early stage in flow separation, the denticles naturally bristle, slowing and trapping the reversed flow. This prevents local flow separation which would otherwise increase the shark’s drag and hinder its agility. (Photo credit: James R. D. Scott; Research by A. Lang et al.)
November 26, 2012
Unmanned Aerial Vehicles
In recent years unmanned aerial vehicles (UAVs) have grown in popularity for both military and civilian application and are shifting from a remotely controlled platform to autonomous control. Since no pilot flies onboard an UAV, these craft are much smaller than other fixed-wing aircraft, with wingspans that may range from a few meters to only centimeters. At these sizes, most fixed-wing airfoil theory does not apply because no part of the wing is isolated from end effects. This complicates the prediction of lift and drag on the aircraft, particularly during maneuvering and necessitates the development of new predictive methods and control schemes. Shown above are flow visualizations of a small UAV executing a perching maneuver, intended to allow the craft to land as a bird does by scrubbing speed with a high-angle-of-attack, high-drag motion. (Photo credit: Jason Dorfman; via Hizook; requested by mindscrib)
October 9, 2012
Magnus Force
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)
October 1, 2012

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