FYFD

Tag: flow visualization

  • Liquid Sculptures

    [original media no longer available]

    Water sculptures–a marriage of liquids, photography, and timing–are spectacular form of fluid dynamics as art. Artist Markus Reugels is a master of the form. This video captures the life and death of such water sculptures at 2,000 fps, beginning with the fall of the initial blue droplet. The droplet’s impact causes a rebounding Worthington jet, which reaches its pinnacle just as a second droplet strikes. The impact spreads into an umbrella-like skirt consisting of a thin, expanding liquid sheet with a thicker rim. The rim itself is unstable, breaking into regularly spaced filaments and tiny satellite droplets that shoot outward before the entire structure collapses into the pool. One especially cool aspect of watching this in video is seeing how the blue dye from each droplet spreads as the water splashes and rebounds. You can see the set-up Reugels uses for his photography here. (Video credit: M. Reugels and L. Lehner)

  • How Erosion Shapes a Flow

    How Erosion Shapes a Flow

    Erosion creates all manner of strange shapes as wind and water cut away at solids. But why does the interaction of the fluid and solid result in the geometries we observe? Above is a collage from an experiment in which a soft clay sphere was immersed in a water tunnel. After 70 minutes, the sphere had worn into a roughly conical body (Image A) reminiscent of a re-entry capsule. Images B and C show instantaneous streaklines around the clay at 10 minutes and 70 minutes, respectively. Images D and E show diagrams of the flowfield seen in B and C. Fast-moving flow above and below the stagnation point (SP) wears the front of the body into a conical shape, whereas the recirculating vortices aft of the separation point (SL) create a sloped shoulder and flattened back in the clay. The results are consistent with a model in which erosion tries to create uniform shear stress at the solid surface – essentially the process is keeping the frictional force between the fluid and air constant along the surface. This makes sense. If a region’s shear stress is higher, it will be worn more quickly than the surrounding solid, causing it to recede and experience decreased shear stress (relative to the surrounding area) as a result. (Image credit: L. Ristroph et al.)

  • The Vortex Under a Falling Drop

    The Vortex Under a Falling Drop

    We take for granted that drops which impact a solid surface will splash, but, in fact, drops only splash when the surrounding air pressure is high enough. When the air pressure is low enough, drops simply impact and spread, regardless of the fluid, drop height, or surface roughness. Why this is and what role the surrounding air plays remains unclear. Here researchers visualize the air flow around a droplet impact. In (a) we see the approaching drop and the air it pulls with it. Upon impact in (b) and © the drop spreads and flattens while a crown of air rises in its wake. The drop’s spread initiates a vortex ring that is pinned to the drop’s edge. In later times (d)-(f) the vortex ring detaches from the drop and rolls up. (Photo credit: I. Bischofberger et al.)

  • Mixing Flows

    Mixing Flows

    Turbulence is an excellent mixer. Here two fluorescent dyes are injected into a turbulent water jet. Flow is from the bottom of the image toward the top. The dyes are quickly mixed into the background fluid by momentum convection, their concentration decreasing with increased distance from the source. Large-scale structures like the eddies visible in this image drive this convection of momentum in turbulent flows. In contrast, consider laminar flows, where momentum and molecular diffusion dominate how fluids move. In such laminar flows, it’s even possible to unmix two fluids, a feat that cannot be accomplished in the jet above. (Photo credit: M. Kree et al.; via @AIP_Publishing)

  • Featured Video Play Icon

    Schlieren in Flight

    Schlieren photography is a common method of visualizing shock waves in wind tunnel experiments, but it’s much harder to pull off for aircraft in the sky. This video from NASA shows off some stunning work out of NASA Dryden capturing schlieren video of shock waves from a F-15B aircraft at Mach 1.38. You’ll notice that shock waves extend off the nose, wings, tail, and other parts of the airplane and extend well beyond the camera’s field of view. It’s these shock waves hitting the ground level that causes distinctive sonic booms. These tests are part of NASA’s on-going research into minimizing the effects of sonic boom so that civilian supersonic flight over land is feasible in the future. When the U.S. government shutdown ends, you’ll be able to learn more about this work at NASA Dryden’s GASPS page. (Video credit: NASA Dryden)

  • Dynamic Stall

    Dynamic Stall

    In nature, birds and other flying animals often use unsteady flow effects to enhance the lift their wings generate. When a wing sits at a high angle of attack, it stalls; the flow separates from the upper surface, and its lift force is suddenly lost. If, on the other hand, that wing is in motion and pitching upward, lift is maintained to a much higher angle of attack. The reason for this is shown in the flow visualization above. This montage shows a rectangular plate pitching upwards. Flow is left to right. Each row represents a specific angle of attack and each column shows a different spanwise location on the plate. As the plate pitches upward, a vortex forms and grows on the leading edge of the plate. Eventually, the leading-edge vortex separates, but not until a much higher angle of attack than the plate could sustain statically. This effect allows birds to maintain lift during perching maneuvers and is also key to helicopter rotor dynamics. (Image credit: K. Granlund et al.)

  • The Bathtub Vortex

    The Bathtub Vortex

    If you’ve ever watched a swirling vortex disappear down the drain of your bathtub and wondered what was happening, you’ll appreciate these images. This dye visualization shows a one-celled bathtub vortex, created by rotating a cylindrical tank of water until all points have equal vorticity before opening a drain in the bottom of the tank. A recirculating pump feeds water back in to keep the total fluid mass constant. Once a steady vortex is established, green dye is released from the top plate of the tank and yellow dye from the bottom. The green dye quickly marks the core of the vortex. Ekman layers–similar to the boundary layers of non-rotating flows–form along the top and bottom surfaces, and the yellow dye is drawn upward in a region of upwelling driven by Ekman pumping. (Photo credit: Y. Chen et al.)

    Just a reminder for those at Texas A&M University: I will be giving a talk today Wednesday, October 2nd entitled “The Beauty of the Flow” as part of the Applied Mathematics Undergraduate Seminar series at 17:45 in BLOC 164.

  • Oil Flow Viz

    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)

  • Fluids Round-up – 21 September 2013

    Fluids Round-up – 21 September 2013

    First off, I’d like to give a special shout-out to FYFD’s friends at Pointwise, who were kind enough to invite me for a visit this week. For any readers looking for CFD grid-generation software, check them out; they are a fantastic bunch and very good at what they do.

    My thanks again to everyone who donated this week to help get me to the APS conference. The campaign is still open if anyone wants to get in on the FYFD wallpapers and stickers on offer to donors. As a reminder, any funds beyond conference costs will go toward improving FYFD, including getting equipment to make FYFD videos. On to the fluids round-up!

    (Photo credit: L. Gilman)

  • Featured Video Play Icon

    Wind Tunnel Testing

    Wind tunnel testing is an important step in designing new aircraft. This video shows footage of visualization tests of the 21-ft wingspan Boeing X-48C model in NASA Langley’s Full-Scale Tunnel. The X-48C is a blended wing body design capable of higher lift-to-drag ratios than conventional aircraft, which should lead to a higher range and greater fuel economy. The video shows some smoke visualization that illustrates airflow around the airfoil-shaped craft. The long probe sticking forward from the starboard wing is used to measure air pressure, angle of attack, and sideslip angle of the model. Notice how smoke from the wand is pulled from below the leading edge of the wing up and over the top of the wing. This is because there is lower pressure over the top of the wing than the bottom, and, like an electrical charge seeking the path of least resistance, fluids flow preferentially toward lower pressures. (Video credit: NASA Langley)