Here laminar and turbulent flows, basic concepts in fluid mechanics, are demonstrated in the kitchen sink! While laminar flow is often desirable for decreasing drag due to friction, most practical flows are turbulent. The hissing the video author associates with the onset of turbulence is not a coincidence either. The chaotic motion of turbulent flows can produce aerodynamic noise like the roar produced by airplane propellers or the hum of electrical lines in the wind.
Tag: turbulence
Simulating Turbulence
Turbulent flows are complicated to simulate because of their many scales. The largest eddies in a flow, where energy is generated, can be of the order of meters, while the smallest scales, where energy is dissipated, are of the order of fractions of a millimeter. In Direct Numerical Simulation (DNS), the exact equations governing the flow are solved at all of those scales for every time step–requiring hundreds or thousands of computational hours on supercomputers to solve even a small domain’s worth of flow, as on the airplane wing in the video. Large Eddy Simulation (LES) is another technique that is less computationally expensive; it calculates the larger scales exactly and models the smaller ones. The video shows just how complicated the flow field can look. The red-orange curls seen in much of the flow are hairpin vortices, named for their shape, and commonly found in turbulent boundary layers.
Vortex Ring Collisions
Gorgeous color schlieren photography reveals the development and interaction of ring vortices. (Photo credit: Rebecca Ing)
It’s Schlier-tastic!!
These are my invisible wonders! Gas flows and fluid interactions. Nothing but hot air, metho and acetone, yup, humble old nail polish remover.
The images were captured using a colour indicating z-system schlieren optical array, an open shutter and a flash duration of 125 microseconds.
Un-Mixing a Fluid Demo
Not only is this demonstration one of my favorites, it’s a reader favorite, too. Even though I posted it nearly a year ago, I’ve had it resubmitted over and over. Here’s what I originally wrote:
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.
The results of those videos might be surprising, but they highlight the difference between laminar flow and turbulence. In laminar flow, the motion of the dye is caused by molecular diffusion and momentum diffusion, the latter of which is exactly reversible. In turbulence, much of the fluid motion is tied up in momentum convection, which is irreversible. This is why you can “unstir” the glycerin but not the milk in your coffee.
Saturn’s Storm Stretches All the Way Around
This picture captured by Cassini in February shows a storm on Saturn stretching all the way around the planet. Unlike Earth and Jupiter, which have numerous storms virtually all the time, Saturn tends to store energy in its atmosphere for decades and then release it all at once in mega-storms like this one. #
Osborne Reynolds and Transition
How and when flow through a pipe becomes turbulent has been a conundrum for fluid mechanicians since the days of Osbourne Reynolds (~1870s):
Typically, the laminar-to-turbulence transition is studied mathematically by linearizing the Navier-Stokes equations, the governing equations of fluid dynamics, then perturbing the system. These perturbations will gradually disappear in laminar flow, but if the flow is turbulent, they’ll grow and produce chaotic motion. The transition, then, is the critical point between these two.
However, for pipe flows, this linearized approach shows that the perturbations decay for all Reynolds numbers, even though this doesn’t happen in actual experiments. In the real world, as the Reynolds number increases, small, turbulent puffs begin to split and interact, and their lifetimes increase. Eventually, these puffs carry enough turbulence to transition the flow entirely. # (submitted by David T)
Tank Shock Waves
High-speed video of a tank firing at 18000 fps shows shock waves made visible due to light distortion. When the air density changes (due to temperature or compression), it’s index of refraction changes, causing the background to appear distorted. Most of the video shows the subsonic development of the turbulent exhaust plume. Note the speed at which the exhaust moves relative to the airborne shrapnel. (submitted by Stephan)
Coronal Waves
NASA’s Solar Dynamics Observatory has found evidence of Kelvin-Helmholtz waves in the sun’s corona. These waves, which occur between two fluids of different densities or moving at different speeds, are similar to the iconic waves surfers ride. Researchers suspect that this turbulent motion may help explain why the corona is 1,000 times hotter than the surface of the sun. #
Brazilian Barrier Islands
Barrier islands are in a constant state of flux due to the currents, tides, and winds that surround and shape them. This satellite image of islands off the Brazilian coast shows meandering waterways and the mixing of sediment from the land into the sea. Often, secondary flows are responsible for shaping of these sorts of geographic features. #
