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)
Tag: flow separation
Sharkskin Fluid Dynamics
Sharks have evolved some incredible fluid dynamical abilities. Instead of scales, their skin is covered in microscopic structures called denticles. To give you a sense of size, each denticle in the black and white image above is about 100 microns across. Denticles are asymmetric and overlap one another, creating a preferential flow direction along the shark. When water tries to move opposite the preferred direction, the denticles will bristle, like in the animation above. The bristled denticles form an obstacle for the reversed flow without any effort on the shark’s part. Since local flow reversal is an early sign of separation, researchers theorize that this bristling tendency prevents flow along the shark’s skin from separating. Keeping flow attached, especially along the shark’s tail, is vital not only to the shark’s agility but to keeping its drag low. Researchers have even begun 3D printing artificial shark skin to try and harness the animal’s hydrodynamic prowess. For much more shark-themed science, be sure to check out this week’s “Several Consecutive Calendar Days Dedicated to Predatory Cartilaginous Fishes” video series by SciShow, It’s Okay to be Smart, The Brain Scoop, Smarter Every Day, and Minute Physics. (Image credits: J. Oeffner and G. Lauder; A. Lang et al.; original video; jidanchaomian)
Separating Flow
Flow separation occurs when a fluid is unable to flow smoothly around an object. In the case of the photo above, fog is being used to visualize flow around an airfoil at a large negative angle of attack. The incoming flow stagnates at a point on top of the airfoil, and streamlines on either side of that point split to move around the airfoil. Those on top are accelerated to high velocity, generating smooth, low-pressure flow over the aft section of the upper surface. On the other side of the stagnation point, however, the fog is trying to flow around the curve of the leading edge but the local pressure gradient is increasing, which slows the flow. Ultimately, it separates from the airfoil, creating a large region of recirculating, turbulent flow. When this effect occurs on the upper surface of a wing at a high (positive) angle of attack, it is called stall and causes a dramatic loss in lift. (Photo credit: Wikimedia/Smart Blade GmbH)
When Turbulence Is Desirable
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)
What Makes Squids Fast
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.)
Humpback-Inspired Turbine Blades
The bumps–or tubercles–on the edge of a humpback whale’s fins have important hydrodynamic effects on its swimming. Here dye is used to visualize flow over a hydrofoil with tubercle-like protuberances–a sort of artificial whale fin. Dye released from the peaks and troughs of the protuberances flows straight back in a narrow line before breakdown to turbulence. But the dye released from ports on the shoulders of the protuberances twists and spirals into vortices. At angle of attack, these vortices are stronger. They may help keep flow from separating on the upper side of a whale’s fin. (Photo credits: SIDwilliams, H. Johari)
Stalling
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At high angles of attack, the flow around the leading edge of an airfoil can separate from the airfoil, leading to a drastic loss of lift also known as stall. Separation of the flow from the surface occurs because the pressure is increasing past the initial curve of the leading edge and positive pressure gradients reduce fluid velocity; such a pressure gradient is referred to as adverse. One way to prevent this separation from occurring at high angle of attack is to apply suction at the leading edge. The suction creates an artificial negative (or favorable) pressure gradient to counteract the adverse pressure gradient and allows flow to remain attached around the shoulder of the airfoil. Suction is sometimes also used to control the transition of a boundary layer from laminar to turbulent flow.
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.)
London 2012: Discus Physics
Like the javelin, the discus throw is an athletic event dating back to the ancient Olympics. Competitors are limited to a 2.5 m circle from which they throw, leading to the sometimes elaborate forms used by athletes to generate a large velocity and angular momentum upon release. The flight of the discus is significantly dependent on aerodynamics, as the discus flies at an angle of attack. Spin helps stabilize its flight both dynamically and by creating a turbulent boundary layer along the surface which helps prevent separation and stall. Unlike many other events, a headwind is actually advantageous in the discus throw because it increases the relative velocity between the airflow and the discus, thereby increasing lift. The headwind also increases the drag force on the discus, but research shows the benefits of the increased lift outweigh the effects of increased drag, so much so that a discus flies further in air than it would in a vacuum. (Photo credits: P Kopczynski, Wiki Commons, EPA/K Okten)
FYFD is celebrating the Olympics by featuring the fluid dynamics of sports. Check out our previous posts, including why corner kicks swerve, what makes a pool fast, how an arrow flies, and how divers avoid splash.
London 2012: Javelin Physics
Few Olympic events can boast as long as history as the javelin. Though the event has existed since the ancient Olympics, humans and our ancestors have been throwing spears for hundreds of millennia. But today’s javelin, oddly enough, is designed so that it cannot be thrown as far as those that came before. After a world record throw in 1984 that nearly reached the edge of the track, the sport’s governing body authorized new rules that shifted the weight of the javelin forward, causing the center of mass of the javelin to lie in front of its center of pressure. This causes the javelin to tip forward in flight, ensuring it will land nose down. Simultaneously, they made changes to the nose of the javelin to reduce its lift during flight, resulting in a javelin that flies only 90% of the previous distance. Since then manufacturers have introduced other innovations to try to increase the javelin’s flight, such as a roughened tail to prevent flow separation, only to later have these changes banned. (Photo credits: Getty Images, Zeenews)
FYFD is celebrating the Olympics by featuring the fluid dynamics of sport. Check out some of our previous posts, including what makes a pool fast, how divers reduce splash, how cyclists get “aero”, and how rowers overcome drag.
