When supersonic flow is achieved through a wind tunnel or rocket nozzle, the flow is said to have “started”. For this to happen, a shock wave must pass through, leaving supersonic flow in its wake. The series of images above show a shock wave passing through an ideal rocket nozzle contour. Flow is from the top to bottom. As the shock wave passes through the nozzle expansion, its interaction with the walls causes flow separation at the wall. This flow separation artificially narrows the rocket nozzle (see images on right), which hampers the acceleration of the air to its designed Mach number. It also causes turbulence and pressure fluctuations that can impact performance. (Image credit: B. Olson et al.)
We take for granted that drops which impact a solid surface will splash, but, in fact, drops only splash when the surrounding air pressure is high enough. When the air pressure is low enough, drops simply impact and spread, regardless of the fluid, drop height, or surface roughness. Why this is and what role the surrounding air plays remains unclear. Here researchers visualize the air flow around a droplet impact. In (a) we see the approaching drop and the air it pulls with it. Upon impact in (b) and (c) the drop spreads and flattens while a crown of air rises in its wake. The drop’s spread initiates a vortex ring that is pinned to the drop’s edge. In later times (d)-(f) the vortex ring detaches from the drop and rolls up. (Photo credit: I. Bischofberger et al.)
Surface properties can have surprising effects on fluid behavior. This image shows the evaporation of several droplets over time. All of the initial droplets are of the same volume, but they are placed on a surface which is a) superhydrophobic, b) hydrophobic, or c) hydrophilic. The more hydrophobic the surface, the larger the initial contact angle between the droplet and surface and the smaller the wetted area of the surface. Yet despite this seemingly large surface area exposure to air, the droplet on the superhydrophobic surface is the slowest to evaporate. (Photo credit: C. Choi)
If you have any leftover hard-boiled eggs, you can recreate this bit of fluid dynamical fun. Spin the egg through a puddle of milk, and you’ll find that the egg draws liquid up from the puddle and flights it out in a series of jets. As the egg spins, it drags the milk it touches with it. Points closer to the egg’s equator have a higher velocity because they travel a larger distance with each rotation. This variation in velocities creates a favorable pressure gradient that draws milk up the sides of the egg as it spins, creating a simple pump. To see the effect in action check out this Science Friday video or the BYU Splash Lab’s Easter-themed video. (Photo credit: BYU Splash Lab)
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.
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