What do you get when you combine liquid gallium, a blender, and a special probe lens? Some pretty wild slow-mo video of a liquid metal vortex, courtesy of the Slow Mo Guys. This video is almost as notable for its set-up as it is for the high-speed footage, given the lengths Gav and Dan go to in order to get the shot! (Image and video credit: The Slow Mo Guys)
A fixed cylinder will shed alternating vortices in its wake, but one allowed to oscillate forward and backward in the flow instead sheds simultaneous vortices. The shape of the wake still depends on the flow’s velocity. At low flow speeds, the two vortices are the same size when they shed. At higher velocities, the two vortices still shed simultaneously, but one will be large while the other is small. The larger vortex moves faster and travels downstream, but the smaller, slower vortex drifts inward. In the next shedding cycle, the small and large vortices switch positions, creating alternating symmetric shedding. (Image and video credit: P. Boersma et al.)
Although engineers often consider fluid mechanics through the lens of mathematics, that’s far from the only way to understand fluid physics. Today’s video is an alternative interpretation of a classic flow — the flow around a cylinder — created in a collaboration between dancers and engineers. The result is what they call a “physics-constrained dance improvisation” that shows how the flow changes as its speed increases. I love this concept! It highlights the visual and qualitative differences between flow states and maintains space for artistic creativity. Be sure to watch the full video! (Video and submission credit: J. Capecelatro et al.)
Vortex ring impacts are eternally fascinating. Here, researchers explore what happens when a vortex ring encounters a V-shaped wall. Because the outer portions of the vortex ring hit the wall sooner than the inner ones, distortions begin there first.
The vortex’s approach creates a pressure gradient that causes flow near the wall to separate, generating that first little hook in each arm of the vortex. Next, secondary vortices develop on either side and quickly get pulled into the original vortex. The whole process repeats a second time to generate tertiary vortices that continue the inward spiral. The impact appears even more complicated when viewed from the side of the valley (Image 2). Check out Image 3 for a point-by-point breakdown of the impact process. (Image and research credit: T. New et al.)
While we’re most used to seeing levitating Leidenfrost droplets on a solid surface, such drops can also form above a liquid bath. In fact, the smoothness of the bath’s surface, combined with mechanisms discussed in a new study, means that drops will levitate at a cooler temperature over a liquid than they will over a solid surface.
Researchers found that a donut-shaped vortex forms in the bath beneath a levitating droplet, but the direction of the vortex’s circulation is not always the same. For some liquids, the flow moves radially outward from beneath the drop. In this case, researchers found that the dominant force was shear stress caused by the vapor escaping from under the droplet.
With other droplet liquids, the flow direction instead moved inward, forming a sinking plume beneath the center of the drop. In this situation, researchers found that evaporative cooling dominated. As the liquid beneath the droplet cooled, it became denser and sank. At the same time, the lower temperature changed the bath’s local surface tension, creating the inward surface flow through the Marangoni effect. (Image credit: F. Cavagnon; research credit: B. Sobac et al.)
Many plants have evolved an ability to move remarkably quickly. Often, this capability is driven by water. Here we see the moss Sphagnum affine, which disperses its spores explosively. The process is triggered by the spore capsule gradually drying out; its shape changes from round to cylindrical, pressurizing the capsule. Once the internal pressure is high enough to overcome the strength of the capsule’s upper membrane, the capsule bursts, sending a plume of spores aloft. The sudden release of spore-laden air forms a vortex ring, which lifts the spores higher far more efficiently than they would be otherwise. (Image credit: capsule dry-out – J. Edwards et al., spore dispersal – J. Edwards et al. 2010; research credit: J. Edwards et al.)
Vortex ring collisions have long been admired for their beauty, but they’re now shedding light on the fundamental interactions that lead to turbulence. By dying just the cores of colliding vortex rings (Image 2), researchers observed anti-symmetric perturbations that develop along each core as they interact. These are indicative of what’s known as the elliptical instability.
But the breakdown doesn’t stop there. Instead, as the elliptical instability develops, it generates a set of secondary vortex filaments that wrap around the original cores (Image 3). Just like the original vortex cores, those counter-rotating secondary filaments interact with one another, develop their own elliptical instability, and generate a set of smaller, tertiary filaments (Image 4).
What’s exciting is that this process gives us a physical mechanism for the turbulent energy cascade. Researchers have talked for decades about energy passing from large-scale eddies to smaller and smaller ones, but this work lets us actually observe that cascade in the form of smaller and smaller pairs of vortex filaments interacting. To see more, check out some of our previous posts on this work. (Image and research credit: R. McKeown et al.; via Cosmos; submitted by Ryan M. and Kam-Yung Soh)