FYFD

Category: Phenomena

  • Formula 1 Aerodynamics

    [original media no longer available]

    Computational fluid dynamics (CFD) and the advent of supercomputing have forever changed the way engineers design. Here the use of CFD in the design of Formula 1 racing cars is discussed. Although CFD is used by many companies in place of wind tunnel testing, each method has its advantages.  CFD provides information about all flow quantities at all points in the flow but can only do so with an accuracy dependent on the grid and models used.  It remains impossible to solve the equations of motion exactly for any problem of practical application because the computational cost is simply too high; instead software packages like FLUENT utilize turbulence models that approximate the physics.  Wind tunnel testing, on the other hand, is physically accurate but typically yields only limited data and flow quantities due to the difficulty of instrumentation. (Video credit: BBC News; submitted by carhogg)

    (Source: /)
  • Featured Video Play Icon

    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.

  • Featured Video Play Icon

    Didgeridoo Soap Bubble

    This high-speed video shows a soap bubble being blown via didgeridoo, a wind instrument developed by the Indigenous Australians. The oscillations of the capillary waves on the surface of the bubble vary with the frequency of note being played. High frequency notes excite small wavelengths, whereas lower notes create large wavelength oscillations. For more fun, check out what you can do with didgeridoos in space. (submitted by Christopher B)

  • Featured Video Play Icon

    When Fluids Behave Like Solids

    Many common fluids–like air and water–are Newtonian fluids, meaning that stress in the fluid is linearly proportional to the rate at which the fluid is deformed. Viscosity is the constant that relates the stress and rate of strain, or deformation. The term non-Newtonian is used to describe any fluid whose properties do not follow this relationship; instead their viscosity is dependent on the rate of strain, viscoelasticity, or even changes with time. A neat common example of a non-Newtonian fluid is oobleck, a mixture of cornstarch and water that is shear-thickening, meaning that it is resistant to fast deformations. Like the cornstarch-based custard in the video above, these fluids react similarly to a solid when struck, resisting changing their shape, but if deformed slowly, they will flow in the manner of any liquid.

  • Featured Video Play Icon

    Creating Lava

    In Syracuse, NY, artists and scientists work together to study volcanic flows by melting crushed basalt in a special furnace before releasing the lava into the parking lot.  This particular flow is very prone to boiling behavior, likely because of the cold air and ground temperatures (less than 0 C).  The outer layers of rock cool quickly, leaving bubble-shaped chambers which hotter lava can fill before melting out. (via It’s Okay To Be Smart; submitted by @jpshoer)

  • Featured Video Play Icon

    Fireball in Slow Motion

    The high-speed video above shows an atomized spray of flammable liquid being ignited using a lighter.  It was filmed at 10,000 fps and is replayed at 30 fps. Although uncontained, this demonstration is similar to the combustion observed inside of many types of engines.  Automobiles, jet engines, and rockets all break their liquid fuel into a spray of droplets to increase the efficiency of combustion.  The turbulence of the flames dances and swirls, with small-scale motions close to the sprayed droplets and larger-scale motions around the vaporized fuel. This variation in size of the scales of motion is a hallmark feature of turbulence and can be used to characterize a flow.

  • Featured Video Play Icon

    How to Escape a Whitewater Hole

    One of the perils of whitewater sports is getting stuck in what paddlers call a “hole” or a “hydraulic”. This river feature forms just downstream of large obstacles like rocks or low-level dams. As water pours over the obstacle and into its shadow, the flow forms a recirculating vortex-like zone.  Immediately next to the obstacle, water is pulled upstream toward the obstacle and then down toward the bottom of the river. This makes the hydraulic very dangerous and hard to escape.  Note in the video how the raft is held in place by the upstream motion of the water at the surface of the hydraulic.  The rafters are preventing their craft from flipping over by weighing down the side experiencing the upward flow of the vortex. Escaping a hydraulic usually requires getting near its edge, where its current is weaker.  If swimming, the best way to escape is to swim toward the bottom of the river and then downstream with the current of the hydraulic rather than against it at the surface.

  • Featured Video Play Icon

    Astro Puffs

    Microgravity continues to be a fascinating playground for observing surface tension effects on the macroscale without pesky gravity getting in the way. Here astronaut Don Pettit has created a sphere of water, which he then strikes with a jet of air from a syringe. Initially, the momentum from the jet of air creates a sharp cavity in the water, which rebounds into a jet of water that ejects one or more satellite drops.  Surface waves and inertial waves (inside the water sphere) reflect back and forth until the fluid comes to rest as a sphere once more. Note how similar the behavior is to the pinch-off of a water column. Both effects are dominated by surface tension, but on Earth we can only see this behavior with extremely small droplets and high-speed cameras! (Video credit: Don Pettit, Science Off the Sphere)

  • Featured Video Play Icon

    Soap Film Loops

    Here’s a fun demonstration of the effects of surface tension. If a loop of thread is dropped onto a soap film, as shown above, popping the soap film inside the thread will pull the thread into a circle.  This is because the surface tension of the soap film outside the thread is reacting to the sudden loss of the balancing force exerted by surface tension inside the thread loop. Surface tension arises from intermolecular forces in a fluid.  Because those forces are in balance except along the interface of a fluid–where the fluid molecules are not completely surrounded by identical molecules–there is a net force exerted at the surface.

  • London 2012: Discus Physics

    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.