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fuckyeahfluiddynamics:

Any time there is relative motion between a solid and a fluid, a small region near the surface will see a large change in velocity. This region, shown with smoke in the image above, is called the boundary layer. Here air flows from right to left over a spinning spheroid. At first, the boundary layer is laminar, its flow smooth and orderly. But tiny disturbances get into the boundary layer and one of them begins to grow. This disturbance ultimately causes the evenly spaced vortices we see wrapping around the mid-section of the model. These vortices themselves become unstable a short distance later, growing wavy before breaking down into complete turbulence. (Photo credit: Y. Kohama)

When a fluid flows next to a solid, the fluid right next to the solid is always stuck to its surface. This means it has zero velocity with respect to the solid. This is called the no-slip boundary condition, and it’s why boundary layers exist.

The picture above is a special situation meant to make a beautiful and illustrative image. However, boundary layers exist everywhere: the wind blowing across your face, the water right next to you when you’re swimming, the air right next to your car when it’s driving. Since air is hard to see, in the picture they use smoke so you can see it.

As boundary layers continue across objects, they always go through a transition from well-behaved and layer-like to disordered and gnarly-looking. The word for layer-like is “laminar.” The word for disordered and gnarly is “turbulent.” In the picture you can see the laminar-to-turbulent transition very well.

fuckyeahfluiddynamics:

The flames surrounding a burning tree stump flicker and billow in this image from photographer Serdar Ozturk. The chaotic motion of the flames is indicative of turbulence, a state of fluid flow known for its many scales. Note the range of lengthscales and structures in the fire. In turbulent flows, kinetic energy cascades from large scales, like the width of the top of the plume, down to the small scales, which may be even smaller than the wisps of flame at the edges of the fire. At the largest scales, the structures and behaviors we observe are all flow- and geometry-dependent, but theory predicts that, at the smallest scales, all turbulent flows look the same. (Photo credit: trashhand/Serdar Ozturk)

Per our recent Clear Science discussions about the color and brightness of flames, check out this excellent analysis of the shapes of fire, which are determined by the fluid dynamics of gases.

Here at the Clear Science labs we’ve got Hurricane Sandy going over in the next few hours. The motion of cyclones like Hurricane Sandy can be understood by a branch of physics called fluid dynamics. After all, the atmosphere is a fluid stuck to a rigid, spinning sphere (the Earth).

In the map above (taken from this excellent website) you see paths of Atlantic hurricanes, which tend to do two things: move toward the North Pole, and switch direction from west to east around 30 degrees latitude. The math of why these two things happen is a bit in the weeds, but it has to do with 2 kinds of angular momentum a hurricane has:

• It spins around in a circle on the Earth (in other words around an axis normal to the Earth)
• It spins around in space as the Earth spins around, just like everything else on the planet

The angular momentum of a closed system (the hurricane) has to remain constant. Ever done the experiment where you spin in a chair and pull your arms in? It makes you speed up, because your angular momentum has to stay constant. Same kind of thing happens as the hurricane moves north: the earth is less big around up there, so the hurricane has to adjust its velocity. If you’d like to see the math go here and it describes why the laws of physics dictate this characteristic hurricane path.