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Tag: conservation of mass

  • Rip Currents and Hurricanes

    Rip Currents and Hurricanes

    When it comes to the beach, looks can be deceiving. That calm-looking water to the side of big crashing waves may actually be a rip current that carries water back out to the ocean. Rip currents are a result of conservation of mass; just as waves carry water to the shore, something has to carry that incoming water back out to the ocean. Depending on the local topography, that outflow could be below the water surface, creating an undertow, or along the surface, as a rip current.

    Even when far offshore, hurricanes can trigger unexpected and strong rip currents, largely because they create bigger waves that travel shoreward. Those waves can also change the depth and layout of the underwater shoreline, potentially exacerbating rip currents.

    For more on rip currents, including the latest guidance on how to escape one, check out this article. (Image credit: A. Marlowe; via SciAm)

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    Wind Turbine Efficiency

    Wind turbines face a paradoxical challenge: they must extract the wind’s kinetic energy while still allowing the air to pass. In this Minute Physics video, Henry gives a crash course on wind turbine efficiency, based on the restrictions of conservation of mass and conservation of energy. When the two are combined, they show that an ideal wind turbine reduces the wind speed by 2/3rds to achieve ~59% efficiency.

    Of course, actual wind turbines are far from ideal. They’re typically placed in staggered configurations in which upstream turbines can disrupt the flow seen by those downstream. And real wind turbines have to contend with dust, bugs, and other grime that builds up on the blades and disrupts air flow and their efficiency. But calculations like this one are still important for engineers seeking to make these machines as efficient as they can be. (Image and video credit: H. Reich/Minute Physics)

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    When the Mediterranean Dried Up

    Geological evidence shows that millions of years ago, the Mediterranean Sea nearly dried out. In fluid mechanics, we’d describe this problem using one of our fundamental equations: conservation of mass, also known as continuity.

    Imagine a volume containing the entire Mediterranean. To describe the amount of sea water in that volume, you need to keep track of two major quantities: how much water is flowing into the volume and how much is leaving it. For the prehistoric (as well as today’s) Mediterranean, the sources feeding the sea are 1) an inflow from the Atlantic through the Strait of Gibraltar; 2) inflows from rivers; and 3) rainfall. Water is lost primarily to evaporation.

    As explained in the video, the Mediterranean’s dry spell was heralded by tectonic changes that sealed the Strait of Gibraltar, depriving it of its largest source of inflow. At the same time, warmer temperatures and less rainfall reduced influx from rivers and the atmosphere while increasing evaporation rates. The result? Water levels in the Mediterranean dropped by hundreds of meters, creating massive salt deposits, wiping out native marine life, and allowing mass migration by land-dwelling animals. Eventually, though, the Strait re-opened, creating what might have been a massive flood. (Video and image credits: PBS Eons)

  • Physics Tattoos

    Physics Tattoos

    This is a man with great commitment to fluid dynamics. He writes:

    This, on my leg, is the incompressible form of the conservation of mass equation in a fluid, also known as the continuity equation. When people ask what it means, I say it defines flow. Sometimes I say it means you should have studied more physics, but that is only when I am feeling like being funny. What it means in more detail is that, for an incompressible fluid, the partial derivative of the velocity of the fluid in the three spatial dimensions must sum to zero. It therefore concisely states the fundamental nature of a fluid. #

    (via physicsphysics)