Category: Phenomena

Aircraft Contrails
[original media no longer available]
Under the right atmospheric conditions, condensation can form, even at low speeds, as moist air is accelerated over airplane wings. This acceleration causes a local drop in pressure and temperature, which can cause water vapor in the air to condense. The condensation can sometimes get pulled into the wingtip vortices shed off of the wings, tail, and ailerons of an aircraft, as in the video above, making the aerodynamics of the airplane visible to the naked eye.
August 29, 2011
Hurricane Irene
This August 25th satellite image shows Hurricane Irene over the Bahamas and Florida. Hurricanes are fueled largely by the release of heat as warm water vapor in the rising air condenses. The hurricane requires a body of warm water to sustain the process, which is why hurricanes weaken drastically after they make landfall. Over open water, the heat released by condensation fuels higher winds, which lowers the pressure at the center of the system and helps increase the rate of evaporation near the ocean surface, providing additional warm vapor for future condensation. See more photos of Irene from space, along with video from the ISS. #
August 26, 2011
The Barus Effect
Non-Newtonian fluids are full of all kinds of unusual behaviors. Here a highly viscoelastic non-Newtonian fluid exhibits the Barus effect, in which extruding the fluid causes the falling jet to swell to several times larger than the diameter of the opening through which it was extruded. This is caused by the stretching and relaxation of polymers in the fluid as it passes through the opening.
August 25, 2011
Sound and Harmonics
The vibrations we perceive as sound, whether in air, water, or any other fluid, are tiny pressure waves emanating from a source, transmitting like ripples across a pond, and finally being caught by our ears and translated by our brains. In this video, the mechanisms and mathematics of sound and harmonics are explained. Although we’re most familiar with these concepts in acoustics, the same principles are used when studying other oscillatory motions, including pendulums, mass-spring systems, disturbances in boundary layers, and the vibrations of a diving board. All of these things rely on the same fundamental principles and mathematics.
August 23, 2011
Feynman: The Universe in a Glass of Wine
Some wisdom for you this Friday from the incomparable Richard Feynman:
A poet I think it is who once said the whole universe is in a glass of wine. I don’t think we’ll ever know in what sense he meant that for the poets don’t write to be understood. But it is true that if you look at a glass of wine closely enough, you’ll see the entire universe.
There are the things of physics: the twisting liquid, the reflections in the glass, and our imagination adds the atoms. It evaporates, depending on the wind and weather. The glass is a distillation of the earth’s rocks and in its composition, as we’ve seen, the secret of the universe’s age and the evolution of the stars. What strange array of chemicals are in a wine? How did they come to be? There are the ferments, the enzymes, the substrates and the products, and there in wine was found great generalization: all life is fermentation. Nor can you discover the chemistry of wine without discovering, as did Pasteur, the cause of much disease. How vivid is the claret, pressing its existence into the consciousness that watches it?
And if our small minds for some convenience divides this glass of wine, this universe, into parts: to physics, biology, geology, astronomy, psychology and all, remember that nature doesn’t know it. So we should put it all back together and not forget at last what it’s for. Let it give us one final pleasure more: drink it up and forget about it all.
(submitted by @jerrodh)
August 19, 2011
The Spinning Underwater Vortex
Vortex rings are a topic we’ve covered before with dolphins, whales, humans, volcanoes and even moss, but this video is particularly fun thanks to the addition of a bottle cap. By sticking the bottle cap next to the ring, these swimmers are able to demonstrate the forceful spinning of the fluid near the vortex. This spinning is what helps the vortex hold its shape over distances much larger than its diameter. As you can also see, though, sticking a bottle cap in the ring causes it to break up faster than it would otherwise! (submitted by Kris S)
August 12, 2011
Airshow Wakes
Turbulent plumes stream out behind airplanes in an airshow. #
August 10, 2011
Molecular Diffusion
This video explains molecular diffusion with demonstrations in gases and liquids. Molecular diffusion is an important process in all fluids and will occur in laminar, turbulent, or quiescent fluids. Diffusion occurs more quickly in heated fluids because molecules move more energetically at higher temperatures. (via robertlovespi)
August 4, 2011
Cornstarch Monsters
[original media no longer available]
Shaking a fluid surface often results in standing waves known as Faraday waves, but with a non-Newtonian fluid like oobleck, at some frequencies it’s possible to incite other behaviors. Oobleck is shear-thickening, meaning that its viscosity increases when force is applied. This is what allows it to develop finger-like protrusions under high frequency vibrations.
August 3, 2011
Carboy Combustion
Lighting a thin layer of ethyl alcohol in a jug produces some beautiful pulse jets and a moving wall of flame that shifts and flows according to the changing pressures inside the jug. Like the video’s author, we do NOT recommend trying this combustion demo yourself.
As for the video’s questions, firstly, blowing into the jar helps the flame because humans do not exhale pure CO2. With regard to the second question, the interior of the jug is initially thinly coated in ethyl alcohol vapor. Combustion starts at the top of the jug and the sheet of flame moves downward as the fuel at the top is spent. As that flame moves downward, however, it’s heating the air inside the jug, which expands and is forced out the opening. When the flame goes out in the upper part of the jug, that does not mean all of the fuel has combusted, simply that the ratio of air/fuel is insufficient for continued combustion. I suspect the flame persists at this opening because the air/fuel mixture is concentrated at that point. Any residual ethyl alcohol in the container is forced out through that narrow opening, and the resulting concentration of fuel there may be high enough to keep the flame burning there. (idea submitted by davidbenque #)
August 1, 2011