Water droplets sprinkled on a sufficiently hot frying pan will skitter and skate across the surface on a thin layer of vapor due to the Leidenfrost effect. When a solid object is much warmer than a liquid’s boiling temperature, the surface is surrounded by a vapor cloud until the solid cools to the point that the vapor can no longer be sustained. Then the vapor breaks down in an explosive boiling full of bubbles. Unless, as researchers have just published in Nature, the solid is treated with a superhydrophobic coating. The water-repellent surface prevents the bubbling, even as the sphere cools. The technique could be used to reduce drag in applications like the channels of a microfluidic device. (Video credit: I. Vakarelski et al.; see also Nature News; submitted by Bobby E)
Search results for: “drag”
Wingtip Vortices
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.
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)
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: Soccer Aerodynamics
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.
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.
London 2012: Rowing Physics
In rowing, as in any water sport, drag comes in three varieties: skin friction, form (or pressure) drag, and wave drag. Skin friction comes from the friction between the hull and water causing the boat to drag water with it as it moves. This can be mitigated with the right materials and surface finish but will never be completely negligible. In fact, the racing shells used in rowing are unusual for boats because skin friction is their major source of resistance. This is because form drag, caused by the shape of the boat cutting through the water, and wave drag, the energy lost due to the waves that form along the hull, are small in racing shells due to their long, narrow, and streamlined shape. Because skin friction dominates among the three types of drag, the force a rower overcomes to move the boat is proportional to the hull’s velocity squared, and the power required to do so is proportional to the hull’s velocity cubed. This means that it is more efficient for rowers to keep a constant hull speed throughout a race than it is to start slow and speed up or start fast and slow down because the work (power x time) needed to keep a constant speed is smaller. For more on the physics of rowing, check out Anu Dudhia’s excellent website or this video from Physics of Life. (Photo credits: Ecouterre, AP)
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, and the aerodynamics of badminton.
London 2012: Running Aerodynamics
Running is not an event typically associated with aerodynamics, though any runner will tell you that a headwind can slow them down. For comparison, a swimmer on world record pace sees 40 to 50 times the drag force of a runner over the same distance. But despite the relatively small influence of drag on a runner, there are measurable effects due to wind and altitude when races are judged by hundredths of a second. Given this, it comes as no surprise that researchers (and presumably manufacturers) are starting to considering how to optimize aerodynamics in running. The video above describes results of a study on running shoes that suggests modest savings may be derived from shoes with dimpled surfaces, much like a golf ball. Socks, on the other hand, don’t show any aerodynamic savings from special surfaces. Of course, the bulk of a runner’s drag comes from their hair and clothing; this is, in part, why runners wear form fitting clothes. While there may be some aerodynamic savings to be had, I don’t think we’ll see world records falling like crazy in Rio because of the latest new shoes.
FYFD is celebrating the Olympics by featuring the fluid dynamics of sport. Check out our previous posts on how the Olympic torch works, what makes a pool fast, the aerodynamics of archery, the science of badminton, how cyclists get “aero”, and how divers reduce splash.
London 2012: Diving Physics
Divers twist and spin gracefully in the air, but the highest marks come when they enter the water with little to no splash. This rip entry–named after paper-ripping sound characteristic of such a dive–is possible thanks to fluid dynamics. Any time a solid object enters a still liquid, it tears a cavity into the liquid. The smaller this cavity is, the less the liquid will rebound and splash when the cavity gets refilled. In diving, achieving a small splash requires a couple items. First, the diver will grab his hands over his head to form a flat surface. This will create the initial small cavity through which his body follows. When entering, the diver will keep his body straight and rigid, with arms pressed against his head; this adds stability to keep the diver from letting the force of striking the water at 35 mph affect his body’s form and create splash. Finally, the perfect dive enters vertical to the water surface. This ensures that all of the diver’s body finds its way into that cavity created by the hands without striking any undisturbed water. Once under the water, divers often extend their arms to generate enough drag to slow down quickly. All in all, the rip entry minimizes the cavity size and thus the splash, adding a great exclamation point to a beautiful dive. (Photo credits: Associated Press, Adam Pretty/Getty Images, Nigel Wade, Jed Jacobsohn)
FYFD is celebrating the Olympics by featuring the fluid dynamics of sport. Check out our previous posts on how the Olympic torch works, what makes a pool fast, the aerodynamics of archery, the science of badminton, and how cyclists “get aero”.