Despite the strange shapes of the arms on this container, the fluid inside will always settle to a common height. This is because each interconnected section is open to the outside air. The fluid’s surface has to reach a static equilibrium with the atmosphere–i.e. the surface of the fluid must be at atmospheric pressure–and the pressure at the lowest level in each section must match because the arms are connected. When fluid is added, the height of the columns oscillates some because the momentum of the added fluid carries the column past its equilibrium position, much like a perturbed mass hanging from a spring will oscillate before settling.
Month: January 2011
Ants as a Fluid
The collective behavior of ants can mirror the flow of a viscous fluid. It would be interesting to see if any such parallels carry over to the flocking of birds or schooling of fish. The latter two behaviors are thought to increase aero- and hydrodynamic efficiency for the group. #
Plugging an Oil Leak
Recent research indicates that adding cornstarch to drilling mud increases the likelihood that a “top-kill” procedure will plug a leaking oil well. Adding cornstarch to water (or mud) turns it into a non-Newtonian fluid with viscoelastic properties that prevent the instabilities that lead to turbulent breakup. On the left, an underwater photo of the Deepwater Horizons leak; in the center, colored water breaks into turbulence when descending into oil; on the right, water with cornstarch maintains its coherence when pumped downward into the oil. # (PDF of research paper)
Wake of a Rising Sphere
This flow visualization shows the wake left by a freely rising sphere. Observations of rising and falling spheres date at least back to Newton, who observed that the inflated hog bladders he used “did not always fall straight down, but sometimes flew about and oscillated to and fro while falling”. That vibration is caused by the vortices seen here in the wake. There are actually four vortices shed per oscillation cycle–two primary vortices (marked P) and two secondary vortices (marked S). #
Supercritical Fluids
Supercritical fluids live in the region of a phase diagram beyond the critical point. At these temperatures and pressures, a substance is neither strictly liquid nor a gas but exhibits behaviors from both. A supercritical fluid can effuse through a solid like a gas does but can also dissolve substrates like a liquid. As noted in the video above, supercritical fluids are useful substitutes for organic solvents in many industrial applications. Carbon dioxide, for example, is used as a supercritical fluid in the decaffeination process.
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Freezing Soap Bubbles
This is what it looks like when a soap bubble freezes. Perhaps not strictly fluid mechanical in nature, but it’s a nice thermodynamics demonstration.
Saturnian Storm
Back in mid-December, amateur astronomers discovered an enormous new storm on Saturn. The Cassini spacecraft captured this image early in the storm’s history (it now stretches farther around the planet). The fluid dynamics of Saturn’s atmosphere are incredibly complex and well beyond our current understanding, but we can certainly appreciate the majesty of a swirling, turbulent storm half the size of our entire planet. (via APOD, Martian Chronicles)
Reader Question: Surface Tension vs. Viscosity
lazenby asks:
How can superfluid liquid Helium have zero viscosity while retaining surface tension? (assuming something like surface tension is required for a liquid to form drops)
The short answer is that surface tension and viscosity are two totally separate properties for a fluid. To illustrate how one can exist without the other in a superfluid, we’ll imagine two different scenarios. For the first, imagine that you have a narrow vertical pipe. Any fluid you put in the pipe will flow downward due to the force of gravity. If you put water through the pipe, you’ll get some rate of outflow. Now imagine putting something like molasses through the pipe. Even with the same external forces on it, the molasses will never move through the pipe as quickly as the water does. This is because the molasses has higher viscosity and resists flowing. In a force balance, viscosity would act like friction, opposing the downward motion of the fluid.
Surface tension arises from a different balance of forces. Now imagine that you have a stationary droplet of one fluid (A) floating in a different fluid (B). Deep inside the droplet, each molecule of Fluid A is being pulled on all sides by other identical molecules of Fluid A. A molecule at the surface of the droplet, though, doesn’t experience that neighborly pull on all sides; it experiences different intermolecular forces from Fluid B. Our imaginary droplet is stationary, though, so all the forces on it and all the forces on its individual molecules have to balance, otherwise there’d be acceleration. Surface tension acts along the interface by pulling molecules of Fluid A in toward one another–much like the elastic of a balloon–thereby balancing the forces in the droplet and equalizing the force across the interface between Fluid A and Fluid B. (Illustration credit: Wikipedia)
In the superfluid, this balance of forces across the interface between air and helium-3 must still exist, despite the superfluid’s lack of viscosity.
Superfluid Helium Leaks from its Container
Below a temperature of 2.17 Kelvin, helium becomes a superfluid, a state of matter boasting several unique properties including zero viscosity (resistance to flow). In this video, scientists demonstrate that property. When they pull the glass “bucket” of helium out at 2:50, the helium starts to leak out. The glass is solid but it contains numerous tiny spaces between its atoms. In its normal state, the viscosity of helium prevents it from escaping through those holes. But as a superfluid, its resistance to flowing goes to zero and it leaks right through the solid glass.
Flying Paint
High speed footage of flying paint demonstrates a world of viscosity and surface tension, as well as another great example of fluid dynamics as art. (via Gizmodo)
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