Soap films are a handy way to create nearly two-dimensional flow fields. Previously we’ve seen them used to show wake structures of pitching foils, flapping flags, and multiple bodies. In this video, we see the dynamics of a pendulum in a soap film. Initially its length is quite long, and the ring end of the pendulum bobs side-to-side in a figure-8 motion. There are two rotational effects here: one is the standard oscillation of a pendulum about its pivot, the other is the rotation of the pendulum’s ring about its attachment point. Interestingly, they have the same frequency. The major destabilizing force for the pendulum is the periodic shedding of vortices we see off the ring. By shortening the pendulum length, the pendulum’s behavior shifts; first it loses the stationary node in its string. Eventually, the string becomes so short that the pendulum no longer oscillates. (Video credit: M. Bandi et al.)
shiftymctwizz Asks:
fuckyeahfluiddynamics Said:
I can’t give you numbers off the top of my head, but I suspect that your typical hydroelectric dam will be more reliable if not more efficient. The trouble with things like the Corryvreckan, aside from the randomness of where the vortices pop up, is that they aren’t there every single day the way, say, Niagara Falls is.
That said, there is on-going work to effectively harness ocean waves for power, with ideas like buoy generators or sea snake generators. As with most concepts one of the difficulties in implementation is determining a safe and efficient manner to transmit the electricity generated from these offshore sites (we’re generally talking miles from shore) to where it’s needed. This problem is often similarly faced by solar and wind energy producers. There are already wave farms in place around the world, though, and it’s a promising field of renewable energy. (Photo credit: Wikimedia)
The ends of an airplane’s wings generate vortices that stretch back in the wake of the plane. Most of the time these vortices are invisible, even if their effects on lift are distinctive. Here an A-340 coming in for a foggy landing demonstrates the size and strength of these vortices. Notice how the fog gets swept up and away by the vortices. Pilots will sometimes use this effect to their advantage in clearing a runway of fog by making repeated low-passes to clear the fog before landing. (Video credit: A. Ruesch; submitted by Jens F.)
Flapping flight, despite being utilized by creatures of many sizes in nature, remains remarkably difficult to engineer. In this experiment, a simple rectangular wing is flapped up and down sinusoidally. Above a critical flapping frequency, the wing—which is free to rotate—accelerates from rest to a constant speed. This rotation is equivalent to forward flight. The upper image shows a photo and schematic of the setup, while the lower images shows flow visualization of the wing’s wake. The wing moves to the right, shedding thrust-providing periodic vortices in its wake. (Photo credits: N. Vandenberge et al.)
In this still image from a student experiment, smoke visualization shows the formation of a vortex over the wing of a paper airplane during a wind tunnel test. This wing vortex is mirrored on the opposite wing, though there is no smoke to show it. At high angle of attack, the delta-wing shape of the traditional paper air plane creates these vortices on the upper surface, which helps generate the lift necessary to keep the plane aloft. (Photo credit: A. Lindholdt, R. Frausing, C. Rechter, and S. Rytman)
Any time there is relative motion between a solid and a fluid, a small region near the surface will see a large change in velocity. This region, shown with smoke in the image above, is called the boundary layer. Here air flows from right to left over a spinning spheroid. At first, the boundary layer is laminar, its flow smooth and orderly. But tiny disturbances get into the boundary layer and one of them begins to grow. This disturbance ultimately causes the evenly spaced vortices we see wrapping around the mid-section of the model. These vortices themselves become unstable a short distance later, growing wavy before breaking down into complete turbulence. (Photo credit: Y. Kohama)
The bumps—or tubercles—on the edge of a humpback whale’s fins have important hydrodynamic effects on its swimming. Here dye is used to visualize flow over a hydrofoil with tubercle-like protuberances—a sort of artificial whale fin. Dye released from the peaks and troughs of the protuberances flows straight back in a narrow line before breakdown to turbulence. But the dye released from ports on the shoulders of the protuberances twists and spirals into vortices. At angle of attack, these vortices are stronger. They may help keep flow from separating on the upper side of a whale’s fin. (Photo credits: SIDwilliams, H. Johari)
Fluid flow near a surface—inside the boundary layer—can often be unstable. This image shows one possible instability, formed when a cylinder is rotated back and forth about its longitudinal axis. This oscillation and the curvature of the cylinder destabilize flow in the boundary layer, forming vortices that line the cylinder. This particular behavior is called a Görtler instability. To visualize it, threads soaked in fluorescing dye have been embedded into slits in the cylinder. The cylinder is oscillated in a water tank and ultraviolet light is used to fluoresce the dye for the image. (Photo credit: Miguel Canals/University of Hawaii)
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 (c) 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 #)
The von Karman vortex street of shed vortices that form the wake of a stationary cylinder are a classic image of fluid dynamics. Here we see a very different wake structure, also made up of vortices shed from a cylindrical body. This wake is formed by two identical cylinders, each rotating at the same rotational rate. Their directions of rotation are such that the cylinder surfaces in between the two cylinders move opposite the flow direction (i.e. top cylinder clockwise, bottom anti-clockwise). This results in a symmetric wake, but the symmetry can easily be broken by shifting the rotation rates out of phase. (Photo credit: S. Kumar and B. Gonzalez)
