One of the challenges of experimental fluid dynamics is gathering sufficient data in environments that can be fast-changing, visually dense, and sometimes harsh. Ideally, researchers want to gather as much data–velocities, temperatures, pressures–at as many points as possible and do so without disturbing the flow with a probe. No technique can provide everything, and thus new diagnostics are always under development. This video shows a new particle tracking method developed for fluidized granular flows where the high concentration of particles makes other techniques unsuitable. Such flows are often seen in industrial applications in chemical processing, pharmaceuticals, and powder transport. Interestingly, the technique can also be used in particle-seeded fluid flows like those normally studied with particle image velocimetry (PIV). (Video credit: F. Shaffer and B. Gopalan; submitted by @ASoutolglesias)
Tag: experimental fluid dynamics
Making Better Tags for Tracking Turtles
Tagging equipment is used on all manner of aerial and marine creatures to gather data about animal behavior in their natural environments. It can be difficult, though, for researchers to gauge what effects the tags have on an animal. A recent study by T. T. Jones et al. used drag measurements on marine turtle casts to estimate the effects of common tagging equipment. They found that, on large turtles, the equipment increases a turtle’s drag by as little as 5%, but for smaller species or juvenile turtles, the drag cost can be much larger – in some cases doubling a turtle’s drag when swimming. Such large increases in drag may significantly change a tagged turtle’s behavior and skew results or even endanger the animal. The researchers suggest a model that allows others to estimate a tag’s drag effects across species. (Image credits: T. Gray and M. Carey; research credit: T. T. Jones et al.; via PopSci; submitted by Chi M.)
Oil Flow Viz
Fluorescent oil sprayed onto a model in the NASA Langley 14 by 22-Foot Subsonic Wind Tunnel glows under ultraviolet light. Airflow over the model pulls the initially even coat of oil into patterns dependent on the air’s path. The air accelerates around the curved leading edge of the model, curling up into a strong lifting vortex similar to that seen on a delta wing. At the joint where the wings separate from the body those lifting vortices appear to form strong recirculation zones, as evidenced by the spiral patterns in the oil. Dark patches, like those downstream of the engines could be caused by an uneven application of oil or by areas of turbulent flow, which has larger shear stress at the wall than laminar flow and thus applies more force to move the oil away. Be sure to check out NASA’s page for high-resolution versions of the photo. (Photo credit: NASA Langley/Preston Martin; via PopSci)
Shock Trains
In compressible flows, shock waves are singularities, a tiny distance across which the density, temperature, and pressure of a fluid change suddenly and discontinuously. In this video, there is a wedge at the top and bottom of the frame and a Pitot probe roughly in the center. Flow is left to right and is initially subsonic. Once Mach 6 flow is established in the wind tunnel, a series of shock waves and expansion fans appear as light and dark lines in this schlieren video. Oblique shocks extend from the sharp tip of each wedge and interfere to create a normal shock in front of the Pitot probe. The air that passes through the normal shock is subsonic to the right of the shock, whereas air that goes through the oblique shocks remains supersonic. The fainter lines further to the right are weaker shock waves and expansion fans that reflect off the walls and probe. They exist to continue turning the airflow around the probe and to equalize conditions between different regions. (Video credit: C. Mai et al.)
Laser-Induced Fluorescence
As demonstrated in the video above, lasers can be used to excite molecules into a higher energy state, which will decay via the emission of photons, causing the medium to glow. This laser-induced fluorescence is utilized in several techniques for measurements in fluid dynamics, including planar laser-induced fluorescence (PLIF) and molecular tagging velocimetry (MTV). In these techniques a flow is usually seeded with a fluorescing material–nitric oxide is popular for super- and hypersonic flows–and then lasers are used to excite a slice of the flow field. The resulting fluorescence can be used for both qualitative and quantitative flow measurements. Here are a couple of examples, one in low-Reynolds number flow and one in combustion. (Video credit: L. Martin et al./UC Berkeley)
Hummingbirds Singing with their Tail Feathers
Aeroelastic flutter occurs when fluid mechanical forces and structural forces get coupled together, one feeding the other. Usually, we think of it as a destructive mechanism, but, for hummingbirds, it’s part of courtship. When a male hummingbird looks to attract a mate, he’ll climb and dive, flaring his tail feathers one or more times. As he does so, air flow over the feathers causes them to vibrate and produce noise. Researchers studied such tail feathers in a wind tunnel, finding a variety of vibrational behaviors, including a tendency for constructive interference–in other words two feathers vibrating in proximity is much louder than either individually. For more, check out the original Science article or the write-up at phys.org. (Video credit: C. Clark et al.)
Supersonic Oil Flow Viz
This image shows oil-flow visualization of a cylindrical roughness element on a flat plate in supersonic flow. The flow direction is from left to right. In this technique, a thin layer of high-viscosity oil is painted over the surface and dusted with green fluorescent powder. Once the supersonic tunnel is started, the model gets injected in the flow for a few seconds, then retracted. After the run, ultraviolet lighting illuminates the fluorescent powder, allowing researchers to see how air flowed over the surface. Image (a) shows the flat plate without roughness; there is relatively little variation in the oil distribution. Image (b) includes a 1-mm high, 4-mm wide cylinder. Note bow-shaped disruption upstream of the roughness and the lines of alternating light and dark areas that wrap around the roughness and stretch downstream. These lines form where oil has been moved from one region and concentrated in another, usually due to vortices in the roughness wake. Image © shows the same behavior amplified yet further by the 4-mm high, 4-mm wide cylinder that sticks up well beyond the edge of the boundary layer. Such images, combined with other methods of flow visualization, help scientists piece together the structures that form due to surface roughness and how these affect downstream flow on vehicles like the Orion capsule during atmospheric re-entry. (Photo credit: P. Danehy et al./NASA Langley #)
Countertop Fliers
In this video, researcher Leif Ristroph and his colleagues have used a clever way to simulate flapping flight, not by actuating their fliers but by oscillating the flow. The flow is driven by a speaker, which causes the air above it to move up and down. Using straws to simulate the honeycomb flow conditioners often used in wind tunnels helps smooth flow. The end result is a great table-top set-up for testing and refining miniature flier designs. The best fliers stay aloft thanks to asymmetry in the streamwise direction; when the air moves upward, the flier catches the air, maximizing drag so that it is carried upward. When the flow reverses, however, the shape of the flier is more streamlined, so the drag is reduced, helping the flier stay aloft. (Video credit: Science Friday/Leif Ristroph et al.)
Fluorescing Shock Waves
Wind tunnel testing plays a major role in the planning of many space missions. Here a model of the Mars Sample Return Orbiter is tested at Mach 10 to determine the heat shield’s response to aerobraking off Mars’ atmosphere. The colors are the result of electron beam fluorescence, in which an electron gun is used to ionize molecules in the flow, which causes them to emit photons (light). The technique can be used for flow visualization–as in the case of the shock waves shown here–or to measure flow characteristics like density, temperature, and velocity. (Photo credit: Thierry Pot/DAFE/ONERA)
Peering Inside the Kettle
Here natural convection is explored experimentally in a quasi-2D environment. The researchers demonstrate how this phenomenon, which is much like that seen in a boiling pot, can be investigated by measuring the refractive distortions caused by the thin heated fluid layer. They also demonstrate types of boiling that can occur. Typically, bubbles nucleate at the heated surface and then rise to pull hot fluid with them. At high enough temperatures above the liquid’s boiling point, however, an unstable layer of vapor can form over the heated surface. This “boiling crisis” or critical heat flux produces a marked reduction in heat transfer due to the insulation provided by the vapor layer. (Video credit: S. Wildeman et al.)
