Tag: boundary layer

Peering Into the Gap
This video offers a glimpse into turbulence developing in a classic flow set-up, a Taylor-Couette cylinder. The apparatus consists of two upright, concentric cylinders; the outer cylinder is fixed, and the inner one rotates. This video shows the gap between the cylinders, and it’s rotated so that the inner cylinder is at the bottom of the frame. Gravity points from left to right in the video. The fluid in the 8-cm gap between the cylinders is water, seeded with rheoscopic particles to visualize the flow.
The video begins as the inner cylinder has just begun to rotate, dragging nearby fluid with it. A thin, laminar boundary layer forms at the bottom of the frame, growing as time goes on. A few seconds in, the boundary layer transitions to turbulence; look closely and you’ll see hairpin-shaped vortices appear. Just after that, the boundary layer becomes entirely turbulent and continues to slowly move upward to take over the full gap. The video is available in a full 4K resolution if you really want to get lost in the flow. (Video credit: D. van Gils)
September 14, 2022
The Tea Leaves Effect
If you’ve ever stirred a cup of tea with loose leaves in it, you’ve probably noticed that the leaves tend to swirl into the center of the cup in a kind of inverted whirlpool. At first, this behavior can seem counter-intuitive; after all, a spinning centrifuge causes denser components to fly to the outside. In this video, Steve Mould steps through this phenomenon and how the balance of pressures, velocities, densities, and viscosity cause the effect. (Note that Mould uses the term “drag,” but what he’s really referring to is the boundary layer across the bottom of the container. But who wants to explain a boundary layer in a video when they can avoid it?) (Video and image credit: S. Mould)
When liquid in a cup is stirred, the densest layers move to the center.
August 10, 2022
Tokyo 2020: Baseball Aerodynamics
For a long time, people thought baseball aerodynamics were simply a competition between gravity and the Magnus effect caused when a ball is spinning. But the seams of a baseball are so prominent that they, too, have a role to play. Here’s a baseline image of flow around a non-spinning baseball:
An non-spinning baseball with a straight, unaltered wake.
As in our previous post on golf, the colors indicate the direction of vorticity but don’t matter much to us here. What’s important is that the wake behind the ball is straight, indicating that there is no additional force beyond gravity and drag acting on the ball. Contrast this to the spinning baseball below:
Flow around a baseball spinning clockwise.
This ball is spinning in a clockwise motion, which causes flow to separate from the ball earlier on the advancing (bottom) side and later on the retreating (top) side. As a result, the wake is tilted downward. This indicates an upward force on the ball, caused by the Magnus effect.
But what if the seams fall in a place where they affect the flow? Here’s another baseball that’s not spinning:
Flow around a non-spinning baseball with a seam-shifted wake caused by early separation on the top surface of the baseball.
Notice that seam sitting just past the widest point on the top of the baseball. Flow around that wide point (called the shoulder) is very sensitive to disturbances essentially because the boundary layer is just barely hanging on to the ball. The blue arrow marks where the boundary layer separates from the ball on the top, which takes place earlier than the flow separation on the bottom, marked by the red arrow. As a result, the wake of the ball is tilted upward, indicating a downward force on the ball. The researchers who first proved this effect call it a seam-shifted wake, and it turns out to be a very common effect in baseball. They’ve got a great blog dedicated to baseball aerodynamics where you can learn tons more if you’re interested. (Image credit: top – Pixabay, others – B. Smith; research credit: B. Smith; see also Baseball Aerodynamics)
Today wraps up our Olympic coverage, but if you missed our earlier posts, you can find them all here.
August 6, 2021
Tokyo 2020: Volleyball Aerodynamics
Like footballs and baseballs, the trajectory of a volleyball is strongly influenced by aerodynamics. When spinning, the ball experiences a difference in pressure on either side, which causes it to swerve, per the Magnus effect. But volleyball also has the float serve, which like the knuckleball in baseball, uses no spin.
In this case, how the ball behaves depends strongly on the way the ball is made. Some volleyballs use smooth panels, while others have surfaces modified with dimples or honeycomb patterns, and researchers found that these subtle changes make a big difference in aerodynamics. A float serve’s trajectory is unpredictable because the ball will swerve whenever air near the surface of the ball on one side goes turbulent or separates. And without spin to influence that transition, everything comes down to the ball’s speed and its surface.
Researchers found that volleyballs with patterned surfaces transition to turbulence at lower speeds, which makes their behavior more predictable overall. But players who want to maximize the unpredictability of their float serve might prefer smooth-paneled balls, which don’t make the transition until higher speeds. (Image credit: game – Pixabay, volleyballs – U. Tsukuba; research credit: S. Hong et al., T. Asai et al.; via Ars Technica)
Stick around all this week and next for more Olympic-themed fluid physics!
July 26, 2021
Updating Undergraduate Heat Transfer
For many engineering students, their first exposure to fluid dynamics comes in a heat transfer class. The typical focus in these classes is not on the underlying physics but on learning to use empirical formulas and correlations that are used in engineering heat exchangers, computer fans, and other applications.
As part of this, students are presented with an extremely simplified view of classical flows like flow over a flat wall, known as a flat-plate boundary layer. Students are told that there are two main features of this and other flows: a laminar region where flow is smooth and orderly, and a turbulent region where flow is chaotic and better at mixing. The transition between these two, according to the undergraduate picture, takes place at a particular point that can be calculated as part of the correlation.
The problem with this picture is that it grossly oversimplifies the actual physics, and for students who may not take dedicated, graduate-level fluid dynamics courses, leaves future engineers with a false understanding that may impact their designs. The truth of transition is far more complicated and nuanced. Transition from laminar to turbulent flow rarely takes place at a single, predictable point; instead it takes place over an extended region and where it begins depends on factors like geometry, vibration, and the level of turbulence already present in the flow.
In an effort to bring undergraduate heat transfer correlations more in line with actual physics — as well as with real, experimental data — a new study revamps the mathematical models. Personally, I applaud any effort to add some nuance to the introduction of this important topic. (Image and research credit: J. Lienhard; via phys.org)
May 25, 2020
Fiery Streaklines
Embers fly through the Kincade wildfire leaving streaks of light that reveal the strong winds helping drive the fire. This unintentional flow visualization mirrors techniques used by researchers to understand how flows are moving. The shutter of the camera remains open for a fixed time, so the length of each streak tells us about the speed of the flow. Longer streaks occur where embers moved faster.
Here we see the longest streaks in the upper left side of the image, which tells us that the wind was moving faster there than it did at lower heights, like near the photographer in the picture. That’s in keeping with what we would expect. In general, winds move faster above the ground than they do near the surface. That speed difference is one of the reasons wildfires are so difficult to contain; a single ember caught by high winds is easily carried to unburnt areas, allowing the fire to spread more quickly than if it had to burn along the ground. (Image credit: J. Edelson/Getty Images; via Wired)
October 28, 2019
The Telstar 18
Every four years, Adidas creates a newly designed ball for the World Cup. This year’s version is the Telstar 18, which features six glued panels (no stitching!) with a slightly raised texture. That subtle roughness is an important feature for the ball’s aerodynamics. It helps ensure that flow around the ball will become turbulent at relatively low speeds. Some previous designs, notably the 2010 Jabulani, were so smooth that flow near the ball would not become turbulent until much higher speeds. In fact, one side of the ball might have laminar flow while the other was turbulent, causing the ball to wobble and misbehave. To learn more about World Cup aerodynamics and the importance of a little surface roughness to the ball’s behavior, check out the Physics Girl video below. (Image credit: Adidas; via APS News; video credit: Physics Girl)
July 11, 2018
The Coexistence of Order and Chaos
One of the great challenges in fluid dynamics is understanding how order gives way to chaos. Initially smooth and laminar flows often become disordered and turbulent. This video explores that transition in a new way using sound. Here’s what’s going on.
The first segment of the video shows a flat surface covered in small particles that can be moved by the flow. Initially, that flow is moving in right to left, then it reverses directions. The main flow continues switching back and forth in direction. This reversal tends to provoke unstable behaviors, like the Tollmien-Schlichting waves called out at 0:53. Typically, these perturbations in the flow start out extremely small and are difficult or even impossible to see by eye. So researchers take photos of the particles you see here and analyze them digitally. In particular, they are looking for subtle patterns in the flow, like a tendency for particles to clump together with a consistent spacing, or wavelength, between them. Normally, researchers would study these patterns using graphs known as spectra, but that’s where this video does something different.
Instead of representing these subtle patterns graphically, the researchers transformed those spectra into sound. They mapped the visual data to four octaves of C-major, which means that you can now hear the turbulence. When the audio track shifts from a pure note to an unsteady warble, you’re hearing the subtle disturbances in the flow, even when they’re too small for your eye to pick out.
The last part of the video takes this technique and applies it to another flow. We again see a flat plate, but now it has a roughness element, like a tiny hockey puck, stuck to it. As the flow starts, we see and hear vortices form behind the roughness. Then a horseshoe-shaped vortex forms upstream of it. Aside from the area right around the roughness, this flow is still laminar. But then turbulence spreads from upstream, its fingers stretching left until it envelops the roughness element and its wake, making the music waver. (Video and image credit: P. Branson et al.)
June 4, 2018
Bouncing Off a Moving Wall
There are many ways to repel droplets from a surface: water droplets will bounce off superhydrophobic surfaces due to their nanoscale structures; a vibrating liquid pool can keep droplets bouncing thanks to its deformation and a thin air layer trapped under the drop; and heated surfaces can repel droplets with the Leidenfrost effect by vaporizing a layer of liquid beneath the droplet. But all of these methods will only work for certain liquids under specific circumstances.
More recently, researchers have begun looking at a different way to repel droplets: moving the surface. The motion of the plate drags a layer of air with it; how thick that layer of air is depends on the plate’s speed. (Faster plates make thinner air layers.) Above a critical plate speed, a falling droplet will impact without touching the plate directly and will rebound completely. This works for many kinds of liquids – the researchers used silicone oil, water, and ethanol – across many droplet sizes and speeds. The key is that the air dragged by the plate deforms the droplet and creates a lift force. If that lift force is greater than the inertia of the droplet, it bounces. (Image and research credit: A. Gauthier et al., source)
May 25, 2018
Rain on Car Windows
As a child, I loved to ride in the car while it was raining. The raindrops on the window slid around in ways that fascinated and confused me. The idea that the raindrops ran up the window when the car moved made sense if the wind was pushing them, but why didn’t they just fly off instantly? I could not understand why they moved so slowly. I did not know it at the time, but this was my early introduction to boundary layers, the area of flow near a wall. Here, friction is a major force, causing the flow velocity to be zero at the wall and much faster – in this case roughly equal to the car’s speed – just a few millimeters away. This pushes different parts of large droplets unevenly. Notice how the thicker parts of the droplets move faster and more unsteadily than those right on the window. This is because the wind speed felt by the taller parts of the droplet is larger. Gravity and the water’s willingness to stick to the window surface help oppose the push of the wind, but at least with large drops at highway speeds, the wind’s force eventually wins out. (Image credit: A. Davidhazy, source; via Flow Viz)
March 12, 2018