boundary layer

The Armstrong Whitworth AW.681, also designated the Whitworth Gloster 681 or Hawker Siddeley HS.681 - due to industry mergers - was a pretty unique transport design from Britain’s 1960s aviation industry. Fulfilling a similar short takeoff and landing specification as Lockheed’s C-130, Armstrong Whitworth produced a solid contender in the AW.681, featuring vectored thrust nozzles, boundary layer control, blown flaps, leading edges and ailerons.

Then they took things a little further, trading the four Rolls-Royce RB.142 Medway engines for four Bristol Siddeley Pegasus turbofans, to obtain VTOL capability. These were the engines which went on to power the Harrier. It would have been an interesting sight. Thanks to the swept shoulder-mounted wings and high T-tail, it would have also resembled today’s C-17 and A400M.

The entire project was scrapped in 1964 when the moment’s Labour Government announced a defence spending review, opting instead to buy the American Lockheed C-130. As a result the company closed it’s Coventry factory, making 5000 workers redundant.

She gave only what was easy for her; what came naturally and instinctively. What she had always given of herself to past lovers, but nothing more. That was usually enough for them, at least in the short run, because she was very special in her way. As a result, she never made any genuine effort to go beyond her old safe boundaries, the first layer, below the thin topsoil of her limitations. She probably believed she worked hard to make these relationships succeed. Look at what I’m doing for you and all that I’m giving of myself. But it was no different from what she had given others in the past. Had she really made any new effort, gesture or concession? No.

It is not hard for a person who knows how to waltz to waltz again. But if they have never tap danced and are asked to learn, then dancing becomes both difficult and challenging. She never attempted to dig deep within to find any latent qualities that might have helped her grow and become more whole. It takes real courage and effort to search out undiscovered parts of ourselves and then improve them. Because in truth most of the time we do not want things to change. We rarely choose to do it voluntarily. Change makes waves in our lives and the higher they are, the more they frighten and challenge us. Attempting to become better (stronger, wiser, more understanding…) than we were yesterday means swimming straight into those waves. If she had looked and found such things, such potential in herself, and then had the guts to put them to use, it might have changed everything.

—  Jonathan Carroll

A good soccer player can kick the ball from the corner of the field into the goal thanks to the Magnus effect. But if you’ve ever tried to play soccer with a smooth ball, you may have noticed that sometimes the ball bends the wrong way! This is the reverse Magnus effect and it’s caused when the boundary layers on either side of the ball switch from turbulent to laminar flow at different times. Dianna Cowern explains (with a little help from yours truly) in the video above. Want to learn more about how roughness affects boundary layers? Check out our companion video on FYFD’s YouTube channel. (Video credit: D. Cowern/Physics Girl)

The Memory of Water

“Water forms a relationship with foreign objects when it touches them and this is what you call surface tension.  So It has a structure which is uniquely different. 

Ultimately water will probably become the ultimate computer.  In my years when I was in physics, we talked about forbidden zones and boundary layers, where we took two dissimilar pieces of metal, gallium and arsenic, they would form a register where you could store information, that’s how computer chips work. 

Water does exactly the same thing, only with six orders of magnitude more accurately…  It’s a million times more efficient than our computers currently. 

When the water comes up against a foreign structure, it changes the water in terms of the way the ions align themselves and it becomes an information register.  And if you put it inside something, then that means, it’s been surrounded by all three dimensions and that takes on a whole different relationship of what water becomes. 

There’s a physicist at MIT, Mark LeClaire that would suggest that water is where God is present on earth because of its detail of resolution of information."   Dr. Richard Allan Miller.

Why I love Numb3rs
  • Larry (failing to fly a paper airplane): Okay, see? What am I doing wrong here?
  • Charlie: I'm telling you, Larry, it's that 11th critical fold. You keep -
  • Larry (regretfully): I know. I know. I keep impinging upon my laminar boundary layer.
  • Charlie: Right. Which results in a high Reynolds number.
  • Larry: I can't... no, I can't do this anymore. I can't.
  • Charlie: Hey, hey. Don't get all Fleinhardt on me. It's just the physics department paper airplane contest.
  • Larry (in outrage): Flein- Fleinhardt? Since when did my last name become a predicate adjective?

Interest in micro-aerial vehicles (MAVs) has proliferated in the last decade. But making these aircraft fly is more complicated than simply shrinking airplane designs. At smaller sizes and lower speeds, an airplane’s Reynolds number is smaller, too, and it behaves aerodynamically differently. The photo above shows the upper surface of a low Reynolds number airfoil that’s been treated with oil for flow visualization. The flow in the photo is from left to right. On the left side, the air has flowed in a smooth and laminar fashion over the first 35% of the wing, as seen from the long streaks of oil. In the middle, though, the oil is speckled, which indicates that air hasn’t been flowing over it–the flow has separated from the surface, leaving a bubble of slowly recirculating air next to the airfoil. Further to the right, about 65% of the way down the wing, the flow has reattached to the airfoil, driving the oil to either side and creating the dark line seen in the image. Such flow separation and reattachment is common for airfoils at these scales, and the loss of lift (and of control) this sudden change can cause is a major challenge for MAV designers. (Image credit: M. Selig et al.)


Just to elaborate on my previous post about our offshore buoys…

The video above is a boundary layer developing over a stationary surface. The dye in the fluid (moving to the left) just gives you a visual aid. When the injector is moved up through the water column (? is this water, I don’t know) you can see clearly the differences in the speed as a function of distance from the bottom. In fluid dynamics, we call this the “no-slip condition” because we assume that the velocity of the fluid approaches zero at the solid surface at the bottom due to friction. Right as the injector starts to move up, the dye line races off to the left with the rest of the fluid, while the dye at the floor is pretty much stationary, thus creating a boundary layer.

Now, you can imagine this happens at a much larger scale when we have the Earth’s fluid atmosphere flowing over the ‘flat plate’ of the Earth (land crust, or even the ocean). The velocity of the wind is equal to the velocity of the surface at the point where it makes contact. On the ocean, this might not necessarily equal zero, because it’s moving, but you get the idea. We like to represent the boundary layer using arrows, like this:

External image

Where the arrows are how fast it’s going at that particular height. Obviously, it’s not always that simple. If you introduce roughness (like terrain, or water movement) into the 'flat plate’ above, the flow over these areas is more complex. My SODAR on Cape Hatteras is able to measure the wind speeds at these particular heights, so we can get a nice representation of what the boundary layer looks like at any given time. It’s further complicated by the fact that the terrain at Cape Hatteras is not uniform. The wind that reaches my instrument could be flowing from Pamlico Sound, over a campground, to the airport (where we’re setup) or it could be coming straight from Hatteras Bight (the Atlantic) and over a small maritime forest if it comes from the other direction. These variations in surfaces do not allow the boundary layer to fully develop without introducing a second boundary layer. It will be nice to get our buoys out onto the ocean so we can get a clean depiction of what the boundary layer over the Atlantic looks like, and start looking at some other parameters.

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)


While we typically think about boundary layers as a small region near the surface of an object–be it airplane, golf ball, or engine wall–boundary layers can be enormous, like the planetary boundary layer, the part of the atmosphere directly affected by the earth’s surface. Shown above is a flow visualization of the boundary layer in an urban area; note the models of buildings. In these atmospheric boundary layers, buildings, trees, and even mountains act like a random rough surface over which the air moves. This roughness drives the fluid to turbulent motion, clear here from the unsteadiness and intermittency of the boundary layer as well as the large variation in scale between the largest and smallest eddies and whorls. In the atmosphere, the difference in scale between the largest and smallest eddies can vary more than five orders of magnitude.

Smoke introduced into the boundary layer of a cone rotating in a stream highlights the transition from laminar to turbulent flow. On the left side of the picture, the boundary layer is uniform and steady, i.e. laminar, until environmental disturbances cause the formation of spiral vortices. These vortices remain stable until further growing disturbances cause them to develop a lacy structure, which soon breaks down into fully turbulent flow. Understanding the underlying physics of these disturbances and their growth is part of the field of stability and transition in fluid mechanics. (Photo credit: R. Kobayashi, Y. Kohama, and M. Kurosawa; taken from Van Dyke’s An Album of Fluid Motion)

Virginia Layden Tucker (1909-1985) was an aeronautical and mechanical engineer who became a trailblazer for women in STEM fields, especially in aviation research. She was one of five women who worked as the first ‘human computers’ at the Langley Research Center, performing complex mathematical calculations which enabled engineers to create and improve airplane designs.

Tucker was a key figure in employing many more women at the Research Center thanks to her recruitment trips. She went on to work as an aerodynamicist at Northrop Corporation, and her research on boundary layers significantly improved aircraft efficiency. She was actively involved with the Society of Women Engineers, serving as director of its Los Angeles section, and as Chair of the Finance Committee.