Cephalopods like the octopus or squid are some of the fastest marine creatures, able to accelerate to many body lengths per second by jetting water behind them. Part of what makes its high speed achievable, though, is the way the animal changes its shape. In general, drag forces are proportional to the square of velocity, meaning that doubling the velocity increases the drag by a factor of four. The energy necessary to overcome such large drag increases generally prevents marine animals from going very fast (compared to those of us used to moving through air!) But drag is also proportional to frontal area. Like the bio-inspired rocket in the video above, jetting cephalopods begin their acceleration from a bulbous shape and then shrink their exposed area as they accelerate. Not only does this shape change help mitigate increases in drag due to velocity, it prevents flow from separating around the animal, shielding it from more drag. The result is incredible acceleration using only a simple jet for thrust. For example, the octopus-like rocket in the video above reaches velocities of more than ten body lengths per second in less than a second. (Video credit: G. Weymouth et al.)
Tag: drag
Holiday Fluids: Santa’s Aerodynamics
Today we have some holiday-themed fluid dynamics: visualization of flow around Santa’s sleigh! This is a flowing soap film visualization at a low speed (author Nick Moore has some other speeds as well). Santa’s sleigh is what aerodynamicists call a bluff body–a shape that is not streamlined or aerodynamic–and sheds a complicated wake of vortices. Like any object moving through a fluid, Santa’s sleigh generates drag forces made up of several components. There is viscous drag, which comes from friction between the sleigh’s surface and the fluid, and form drag (or pressure drag), which comes from the shape of the sleigh. That wake full of complicated vortices significantly increases the sleigh’s pressure drag, requiring Rudolph and the other reindeer to provide more thrust to counter the sleigh’s drag. Speaking thereof, the visualization does not take into account the aerodynamics of the reindeer, who, in addition to providing the sleigh’s thrust, would also affect the flowfield upstream of the sleigh. This post is part of this week’s holiday-themed post series. (Video credit: N. Moore)
Making Better Tags for Tracking Turtles
Tagging equipment is used on all manner of aerial and marine creatures to gather data about animal behavior in their natural environments. It can be difficult, though, for researchers to gauge what effects the tags have on an animal. A recent study by T. T. Jones et al. used drag measurements on marine turtle casts to estimate the effects of common tagging equipment. They found that, on large turtles, the equipment increases a turtle’s drag by as little as 5%, but for smaller species or juvenile turtles, the drag cost can be much larger – in some cases doubling a turtle’s drag when swimming. Such large increases in drag may significantly change a tagged turtle’s behavior and skew results or even endanger the animal. The researchers suggest a model that allows others to estimate a tag’s drag effects across species. (Image credits: T. Gray and M. Carey; research credit: T. T. Jones et al.; via PopSci; submitted by Chi M.)
Ski Jumping Aerodynamics
Last summer we featured fluid dynamics in the Summer Olympics and there’s more to come for Sochi. Winter athletes like ski jumper Sarah Hendrickson are hard at work preparing, which can include time in wind tunnels, as shown here. There are two main diagnostics in tests like these: drag measurements and smoke visualization. The board Hendrickson stands on is connected to the tunnel’s force balance, which allows engineers to measure the differences in drag on her as she adjusts equipment and positions. This gives a macroscopic measure of drag reduction, and reduced drag makes the skier faster on the snow and lets her fly longer in the jump. The smoke wand provides a way to visualize local flow conditions to ensure flow remains attached around the athlete, which also reduces drag. (Video credit: Red Bull/Outside Magazine; submitted by @YvesDubief)
Falcon vs. Raven
Earth Unplugged has posted some great high-speed footage of a peregrine falcon and a raven in flight. Notice how both birds draw their wings inward and back on the upstroke. By doing so, they decrease their drag and thus the energy necessary for flapping. On the downstroke, they extend their wings fully and increase their angle of attack, creating not only lift but thrust. The falcon boasts an incredibly streamlined shape, not only along its body but also along its wings. In contrast, the raven has broader wings with large primary feathers that fan out near the tips. Splaying these large feathers out decreases the strength of the bird’s wingtip vortices, thereby reducing downwash and increasing lift, much the same way winglets do on planes. That extra lift and control the big primaries provide is important for the raven’s acrobatic skill. (Video credit: Earth Unplugged; via io9)
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)
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)
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)
Flapping Flags
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 © show the thread in coherent flapping motion; (b) shows an snapshot of the flapping thread in the soap film whereas © 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)
Martian Landing Physics
A little over a week ago, NASA’s Curiosity rover landed on Mars, the culmination of years of engineering. The mission’s landing, in particular, was the subject of intense scrutiny as Curiosity’s size necessitated some new techniques in the final segments of the landing sequence. As it hit the Martian atmosphere at 13,000 mph, the compression of the carbon dioxide behind the capsule’s shock wave slowed the descent. At roughly 1,000 mph–speeds still large enough to be supersonic–Curiosity deployed its parachute. Shown above are the parachute in numerical simulation (from Karagiozis et al. 2011), wind tunnel testing at NASA Ames, and during descent thanks to the Mars Reconnaissance Orbiter. The simulation shows contours of streamwise velocity at different configurations; note the bow shock off the capsule and the additional shocks off the parachute. These help generate the drag needed to slow the capsule. For an interesting behind-the-scenes look at the wind tunnel testing for Curiosity’s parachute check out JPL’s four–part video series. Congratulations to all the scientists and engineers who’ve made the rover a success. We look forward to your discoveries! (Photo credits: K. Karagiozis et al., NASA JPL, NASA MRO)
