For a little Friday fun, consider this microgravity experiment conducted aboard the infamous Vomit Comet: what happens when you combine Diet Coke and Mentos in microgravity? For the record, I think this is the answer. (Via jshoer, who will be flying on the Vomit Comet in 2 weeks – but sadly not with a fluids experiment)
Tag: fluid dynamics
Sailing Faster than the Wind
Is it possible to create a vehicle that uses the wind to move itself directly downwind faster than the wind does? Strangely enough, it is. The concept has been the subject of debate for years, but one team has confirmed the physics by building a vehicle that uses wind but can travel 2.8x faster than the wind does. See their article on Wired. #
Non-Newtonian Fun
Non-Newtonian fluids are a favorite for displaying odd behaviors. High-speed video simply improves the experience.
Remember, though, that non-Newtonian fluids don’t actually become solids when you hit them. They just react similarly to a solid because they exhibit a nonlinear response to deformation.
Microgravity Boiling
Boiling a liquid is a common enough phenomenon that we are all familiar with it. But, as with many aspects of fluid mechanics, removing gravity drastically changes the situation.
Bubble Art
Bubbles are all about surface tension and minimizing energy. Arrange things just right and you can even make square ones. (via JetForMe)
Flying Fish Aerodynamics
New research using wind tunnel measurements of (dead) flying fish is giving new insight into how these fish are able to fly over the waves. Lift and drag data indicates that flying fish have a gliding ability comparable to soaring birds like hawks! #
Island Vortices
The von Karman vortex street isn’t just found in the wake of cylinders in a lab. Wind moving over islands frequently creates the effect, as in this MODIS Aqua image of the coast of Baja California, Mexico. #
How Wings Create Lift
One of the topics in fluid dynamics almost everyone has come across is the explanation of how airplanes produce lift. Using Bernoulli’s principle–which relates velocity and pressure–and a picture of an airfoil, your average science text will say that a bit of air going over the top of the airfoil has to travel farther than a bit of air going under the airfoil, and that, therefore, the air over the top travels faster than the air under the airfoil.
Unfortunately, this is misleading and, depending on the wording, outright wrong! The hidden assumption in this explanation is that air that goes over the top and air that goes under the bottom have to reach the trailing edge of the airfoil at the same time. But why would that be? (As one of my profs once said, “There is nothing in physics that says there is Conservation-Of-Who-You-Were-Sitting-Next-To-When-You-Started.”)
Take a look at the video above. It shows an airfoil in a wind tunnel using smoke visualization to show how the air moves. Around the 0:25 mark, the video slows to show a pulse of smoke traveling over the airfoil. What happens at the trailing edge? The smoke going over the top of the airfoil is well past the trailing edge by the time the smoke going under the airfoil reaches the trailing edge!
It’s true that air goes faster over the top of the airfoil than the bottom and that this causes a lower pressure on top of the airfoil (as Bernoulli tells us it should) and that this causes an upward force on the airfoil. But which causes which is something of a chicken-and-egg problem.
A more straightforward way, in my opinion, of explaining lift on an airplane is by thinking about Newton’s 3rd law: for every action, there is an equal and opposite reaction. Take a look at the air’s movement around the airfoil as the angle of attack is increased around 1:00 on the video. Just in front of the airfoil, the air is moving upward. Just after the airfoil, the air is pointed downward. A force from the airfoil has pushed the air down and changed its direction. By Newton’s 3rd law, this means that the air has pushed the airfoil up by the same amount. Voila! Lift!
Rain Drops in Puddles
As those of us in Texas get drenched by Tropical Storm Hermine, it’s worth taking a moment to enjoy the beauty of each rain drop hitting a puddle.
