Here’s a ferrofluid video with a little more explanation about how ferrofluids work. Surfactants prevent the tiny magnetic particles suspended in the fluid from separating out when exposed to a magnetic field.
Month: February 2011
Marangoni Effect
Dyed milk pulls away after a drop of acetone is added. The acetone creates a gradient in the surface tension, which causes mass flow due to the Marangoni effect. See a video of the effect (or try it yourself at home!) here.
Geometrical Droplet Splashes
Sadly, this video shows no droplet impacts on a heart-shaped post, but maybe you can imagine what it would look like after seeing other geometrical shapes. Happy Valentine’s Day, guys!
High-Speed Leidenfrost Levitation
The Leidenfrost effect occurs when a liquid encounters a surface with a temperature much higher than its boiling point. Some of the liquid is instantly vaporized and then a droplet will skate across the surface on that vapor. This video shows the process at 3000 frames per second.
Volcanic Turbulence
One of the characteristics of turbulence is its large range of lengthscales. Consider the ash plume from this Japanese volcano. Some of the eddy structures are tens, if not hundreds, of meters in size, yet there is also coherence down to the scale of centimeters. In turbulence, energy cascades from these very large scales to scales small enough that viscosity can dissipate it. This is one of the great challenges in directly calculating or even simply modeling turbulence because no lengthscale can be ignore without affecting the accuracy of the results. #
Reader Question: Froude vs. Reynolds
@spooferbarnabas asks: I was wondering what the difference is between Froude’s number and Reynold’s number? they seem very similar
Fluid dynamicists often use nondimensional numbers to characterize different flows because it’s possible to find similarity in their behaviors this way. The Reynolds number is the most common of these dimensionless numbers and is equal to (fluid density)*(mean fluid velocity)*(characteristic length)/(fluid dynamic viscosity). The Reynolds number is considered a ratio of total momentum (or inertial forces) to the molecular momentum (or viscous forces). A small Reynolds number indicates a flow dominated by viscosity; whereas a flow with a large Reynolds number is considered one where viscous forces have little effect.
The Froude number, in contrast, focuses on resistance to flow caused by gravitational effects, not molecular effects. It is defined as (mean fluid velocity)/(characteristic wave propagation velocity). Initially, it was developed to describe the resistance of a model floating in water when towed at a given speed. As the boat’s hull moves through the water, it creates a wave that travels forward (and backward in the form of the wake), carrying information about the boat–much like pressure waves travel before and behind a subsonic aircraft. The speed of the wave created by the boat depends on gravity (see shallow water waves). The closer the boat’s speed comes to the water wave’s speed, the greater the resistance the boat experiences. In this respect, the Froude number is actually analogous to the Mach number in compressible fluids.
I hope that helps explain some of the differences!
The Pistol Shrimp’s Secret Weapon
The pistol shrimp (or pistol crab) is a finger-sized crustacean with a fluid dynamical superpower. When it snaps its claw, a jet of water shoots out so quickly (62 mph) that a low-pressure bubble forms in its wake. When the bubble collapses, it emits a bang and a flash of light in a process known as sonoluminescence. The whole event takes less than 300 microseconds. The light emitted suggests that temperatures inside the bubble reach 5,000 degrees Kelvin, around the temperature of the surface of the sun. #
Instability in a Jet
This photo shows the development of a flow instability in an axisymmetric jet. On the left, the jet is smooth and fully laminar, but, by the center of the photo, disturbances in the jet have grown large enough to distort the laminar profile. The jet is then in transition; by the right side of the frame, it has reached a turbulent state, as evidenced by the increased mixing (which causes the smoke to disperse more quickly) and intermittency of the flow. #
Dr. Seussian Mystery Fluid Could Have Saved Top Kill
Dr. Seussian Mystery Fluid Could Have Saved Top Kill
Wired article about using non-Newtonian fluids to plug leaking oil wells as we featured previously.
Reader Question: National Committee for Fluid Mechanics Films
lazenby asks: Have you seen these guys? http://web.mit.edu/hml/ncfmf.html
Yes, absolutely! Those videos, which date from the 1960s, are so useful that they’re still shown to undergraduates today. (Or at least they showed several of them to us when I was junior!) They can seem a bit slow by current standards, but the films are full of great demonstrations of basic fluid mechanics. If the links on that page don’t work (or, if like me, you can’t stream RealPlayer), a lot of the videos can also be found on YouTube by searching for individual titles. The Low Reynolds Number Flow video is one of my favorites because it’s hosted by G. I. Taylor, one of the the most prolific and influential fluid mechanicians of the 20th century.
