This simple demonstration shows the power of surface tension, especially at small lengthscales. Another way to break the surface tension holding the water in the sieve would be to spray the top of the jar with soapy water. The soap acts as surfactant, decreasing the surface tension such that the water is unable to counteract the force of gravity.
Year: 2011
Impinging Without Coalescing
Three impinging jets of silicone oil rebound without coalescence due to thin-film lubrication between the jets. The motion of the oil replenishes the thin layer of air separating the streams. The same phenomenon keeps droplets from coalescing as well. (Photo credit: BIF Lab, Department of Engineering Science and Mechanics, Virginia Tech) #
Blast Waves
[original media no longer available]
Watch closely in this high-speed video of a bomb exploding and you will see the spherical blast wave moving outward as a visual distortion. The increase in temperature caused by the leading shockwave changes the index of refraction of the air, bending the light and distorting our view of the background. The mechanism is similar to schlieren photography, which has been used for more than a century to capture images of compressible flows.
2D Convection
This simulation shows 2D Rayleigh-Benard convection in which a fluid of uniform initial temperature is heated from below and cooled from above. This is roughly analogous to the situation of placing a pot of water on a hot stovetop. (In the case of the water on the stove, the upper boundary is the water-air interface, while, in the simulation, the upper boundary is modeled as a no-slip (i.e. solid) interface.) The simulation shows contours of temperature (black = cool, white = hot). In general, the hot fluid rises and the cold fluid sinks due to differences in density, but, as the simulation shows, the actual mixing that occurs is far more complex than that simple axiom indicates.
Reader Question: Faucet Physics
jessecaps-blog-deactivated20170 asks:
With respect to the laminar/turbulent flow in the faucet, at the end he explains that the diameter is smaller inside the valve compared to the nozzle and therefore the velocity is greater and turbulence is achieved there before it leaves the nozzle. But turbulence is characterized by the Reynolds number not the velocity, so a larger velocity with a smaller diameter will yield the same Reynolds number, why would we expect turbulence in the nozzle before the stream?
ETA: As pointed out in the comments, I made a very silly mistake when calculating the Reynolds number last night. While most of what I say below is still true in general, it’s not in the case in the faucet, and so I’ve edited the entry to reflect that.
Great question! A quick control volume analysis of an incompressible fluid shows that, while the flow speed is higher through the faucet’s valve, the Reynolds number (based on diameter) at the valve is the same as higher than the Reynolds number at the nozzle by a factor of (nozzle diameter)/(valve diameter). Thus transition can occur at the valve before the nozzle. A word of caution, though: although we often use Reynolds number as a method of characterizing when a flow becomes turbulent, it is not a hard and fast rule.
As undergraduates we learn that pipe flow transitions to turbulence at a Reynolds number of 2,300 based on the pipe’s diameter. However, under the right laboratory conditions, it’s possible to maintain laminar flow in a pipe to a Reynolds number an order of magnitude larger. (#) It all depends on the initial conditions of the flow and the influence of factors like surface roughness. What this means in the case of the faucet is that the same Reynolds number (based on diameter) may not correctly indicate whether the flow is laminar or turbulent at a given point.
Now, while it may be possible that the contraction at the valve introduces some small turbulence that decays prior to the flow’s exit from the nozzle, that does not seem overly likely to me. Even though, by Reynolds number, transition can occur at the valve before the nozzle, I suspect most of the sound we hear comes from the increased flow rate caused by turning the faucet. It may also be that the sound is associated with the onset of turbulence at the valve but the turbulence is still slight enough that we do not notice it by eye in the external flow.
Laminar and Turbulent Flows from a Faucet
Here laminar and turbulent flows, basic concepts in fluid mechanics, are demonstrated in the kitchen sink! While laminar flow is often desirable for decreasing drag due to friction, most practical flows are turbulent. The hissing the video author associates with the onset of turbulence is not a coincidence either. The chaotic motion of turbulent flows can produce aerodynamic noise like the roar produced by airplane propellers or the hum of electrical lines in the wind.
Disrupting the Coalescence Cascade
When a droplet contacts a pool, a thin layer of air can get trapped beneath the droplet, delaying the instant when the liquids contact and surface tension pulls the droplet into the pool. If the pool is being vibrated, air flows more easily into the gap, keeping droplets intact longer. It’s even possible to make them dance.
Cloud Streets
Cloud streets–long rows of counter-rotating air parallel to the ground in the planetary boundary layer–are thought to form as a result of cold air blowing over warm waters while caught beneath a warmer layer of air, a temperature inversion. As moisture evaporates from the warmer water, it creates thermal updrafts that rise through the atmosphere until they hit the temperature inversion. With nowhere to go, the warmer air tends to lose its heat to the surroundings and sink back down, creating a roll-like convective cell. (Photo credits: NASA Terra, NASA Aqua, and Tatiana Gerus)
Rocket Engine Testing
Rocket engine tests usually feature a distinct and steady pattern of Mach diamonds in their exhaust. This series of reflected shock waves and expansion fans forms as a result of the exhaust pressure of the rocket nozzle being lower or higher than ambient pressure. A rocket will be most efficient if its exhaust pressure matches the ambient pressure, but since atmospheric pressure decreases as the rocket gets higher, engines are usually designed with an optimal performance at one altitude.
Water Spray from a Tire
The spray thrown up by a rolling tire is simulated in the lab by running a single-grooved tire (top) against a smooth tire (bottom) that simulates the road. A supply of water flows from the left at the speed of the rolling tires (6 m/s). The resultant sheet of water is a familiar site to motorists everywhere. Holes in the the sheet of water collide to form the smallest droplets, whose diameters are comparable to the thickness of the sheet, of the order of 100 microns. Thicker parts of the sheet form ligaments and break down into large droplets through the Plateau-Rayleigh instability. (Photo credit: Dennis Plocher, Fred Browand and Charles Radovich) #
