# Fuck Yeah Fluid Dynamics

Celebrating the physics of all that flows. Ask a question, submit a post idea or send an email. You can also follow FYFD on Twitter and Google+. FYFD is written by Nicole Sharp, PhD.

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In this still image from a student experiment, smoke visualization shows the formation of a vortex over the wing of a paper airplane during a wind tunnel test. This wing vortex is mirrored on the opposite wing, though there is no smoke to show it. At high angle of attack, the delta-wing shape of the traditional paper air plane creates these vortices on the upper surface, which helps generate the lift necessary to keep the plane aloft. (Photo credit: A. Lindholdt, R. Frausing, C. Rechter, and S. Rytman)

Almost everyone has tried skipping rocks across the surface of a pond or lake. Here Professor Tadd Truscott gives a primer on the physics of rock skipping, including some high-speed video of the impact and rebound. In a conventional side-arm-launched skip, the rock’s impact creates a cavity, whose edge the rock rides. This pitches the rock upward, creating a lifting force that launches the rock back up for another skip. Alternatively, you can launch a rock overhand with a strong backspin. The rock will go under the surface, but if there’s enough spin on it, there will be sufficient circulation to create lift that brings the rock back up. This is the same Magnus effect used in many sports to control the behavior of a ball—whether it’s a corner or free kick in soccer or a spike in volleyball or tennis. (Video credit: BYU Splash Lab/Brigham Young University)

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.

Sit back, relax, and enjoy some science-y goodness with Bill Nye as he explains fluids. Happy holidays, everyone!

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)

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)

Any finite length wing produces wingtip vortices—potentially intense regions of rotational flow downstream of the wing’s ends. These vortices are associated both with the production of lift on the wing and with unavoidable induced drag. The tabletop demonstration above shows the region of the vortices’ influence and how strong the rotation is there. Note also that the two vortices have opposite rotational senses—the left side induces a clockwise rotation, whereas the right side induces an anti-clockwise rotation. The larger an aircraft, the stronger and longer lasting its vortices; this can be a source of danger for smaller aircraft passing through the wake. If a pilot crosses one wingtip vortex and overreacts to compensate, crossing the second counter-rotating vortex can cause even greater damage.

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.

Corner kicks and free kicks are tough to defend in football (soccer for Americans) because the ball’s trajectory can curve in a non-intuitive fashion. Known as the Magnus effect, the fluid dynamics around a spinning ball cause this curvature in the flight path. When an object spins while moving through the fluid, it drags the air near the surface with it. On one side of the spinning ball, the motion opposes the direction of freestream airflow, causing a lower relative velocity, and on the opposite side, the spin adds to the airflow, creating a higher velocity. According to Bernoulli’s principle, this causes a lower pressure on the side of the ball spinning with the flow and a higher pressure on other side. This difference in pressure results in a force acting perpendicular to the direction of travel, causing the unexpected curvature in the football’s path. In the case of the corner kick above, the player kicks the ball from the right side, imparting an anti-clockwise spin when viewed from above. As the ball travels past the goal, air is moving faster over the side nearest the goal and slower on the opposite side. The difference in velocities, and thus pressures, creates the sideways force that drives the ball into the goal even without touching another player. The same effect is used in many other sports to complicate play and confuse opponents. In tennis and volleyball, for example, topspin is used to make the ball drop quickly after passing the net.

ETA: Check out this other great example of a free kick sent in by reader amphinomos.

FYFD is celebrating the Olympics by featuring the fluid dynamics of sport. Check out some of our previous posts including the flight of a javelin, how divers reduce splash, and what makes a racing hull fast.

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.