For although only the disciplined motion is recognized in military tactics, troops have another manner of motion when anything disturbs their order. And this is precisely how it is with water: it will move in a perfectly direct disciplined manner under some circumstances, while under others it becomes a mass of eddies and cross streams, which may be well likened to the motion of a whirling, struggling mob where each individual particle is obstructing the others. The larger the army, and the more rapid the evolutions, the greater the chance of disorder; so with fluid, the larger the channel, and the greater the velocity, the more chance of eddies.
Tag: laminar flow
Reynolds on Transition
Laminar Flow Control
On Wednesday, March 30, 2011 at 3:00 EDT NASA engineers are holding an online chat about a current project to achieve laminar flow control on business jet-class airplanes. Keeping flow over an airplane’s wings laminar could decrease the total drag on an airplane by as much as 15%. In particular, this project involves placing tiny hockey-puck-shaped discrete roughness elements (DREs) along the front of the wing. These DREs are positioned such that they perturb the mean-flow over the wing at a higher frequency than the naturally most unstable frequency; as a result, flow actually remains laminar over a greater extent of the wing than would normally be the case. For more on the technical ideas, see this NASA blog post or feel free to ask questions in the comments. #
Full disclosure: This project is being conducted in joint with professors with whom I work, and the subject matter is related to my own research.
Pouring Paint
In this artwork by Holton Rower, paint (typically a non-Newtonian fluid) is poured down a rectangular prism; the result is a neat demonstration of shearing in laminar flows. Paint is usually shear-thinning, meaning that its viscosity decreases under shear; this is why the color stripes on the vertical panels expand more than those on the horizontal surfaces do. # (submitted by Stephan)
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. #
Microgravity Water Films
In this video astronaut Don Pettit demonstrates some interesting laminar flow effects using a water film in microgravity. By using a film, fluid motion is essentially confined to two dimensions. This is important because it prohibits the development of turbulence, which is a purely three-dimensional phenomenon. Doing the experiment in microgravity allows Pettit to leave the experiment for a long period of time without buoyant effects or similar disturbances. When he first stirs the film, the tracer particles show some signs of what looks like turbulent mixing, but soon the film rotates uniformly with streaks of gray caused by different concentrations of tracer particles. Pettit notes that he allowed the film to rotate overnight and it eventually all turned milky white. This is the effect of molecular diffusion of the tracer particles; without turbulence, the only way for mixing to occur is through the random motion of molecules. See more of Pettit’s Saturday Morning Science videos for additional microgravity fluid mechanics.
Wavy Vortices
Shown above is the flow between two concentric cylinders (Taylor-Couette flow). In the laminar regime, the velocity profile between the two cylinders is linear. As the rate of rotation of the inner cylinder increases, the flow develops toroidal vortices known as Taylor vortices, seen in the video above after 9 seconds or so. This is a fluid instability exhibited by transitional flow. Increasing the rotational rate further can result in wavy Taylor vortex flow. At high enough speeds, the flow will become completely turbulent.
Swimming in Corn Syrup
Highly viscous laminar flows exhibit kinematic reversibility, meaning: if you move the fluid one direction and then execute the same motion in the opposite direction, every fluid particle will return to its initial, undisturbed position. Above, you see a swimming device attempting to move through corn syrup by flapping. Because of this kinematic reversibility, it cannot propel itself. For the same reason, many microscopic organisms do not utilize flapping to move.
Effects of Viscosity
[original media no longer available]
Today’s video demonstrates the effect of viscosity, which measures a fluid’s resistance to deformation. On the left is a column of highly viscous fluid; the fluids become less viscous as one moves right. When a jet of dye is released into the highly viscous fluid, the jet is very slow to penetrate, whereas, in the rightmost column, the dye expands quickly into a turbulent jet. Between these extremes, we see a laminar dye jet entering the liquid. The mushroom-like shape the laminar jet takes is the result of the Rayleigh-Taylor instability, which occurs when a denser fluid is on top of a lighter fluid in a gravitational field.
Three Flows in One
These plumes of smoke demonstrate the three types of fluid flow: laminar, transitional, and turbulent. At the bottom of the photo, the plumes are smooth and orderly, as is typical for laminar flow. At the top, the smoke’s movement is chaotic and intermittent, full of turbulent eddies. Between these two stages, the flow is in transition; there is still some semblance of order to it, but disturbances in the plume are getting amplified and breaking down into turbulence.
Photo credit: J. Russo
Un-mixing a Flow
Laminar flow (as opposed to turbulence) has the interesting property of reversibility. In this video, physicists demonstrate how flow between concentric cylinders can be reversed such that the initial fluid state is obtained (to within the limits of molecular diffusion, of course!)
For more examples, see the first half of this video.
