The hawk moth (Manduca sexta) flies quite similarly to a hummingbird, able to hover over the flowers from which it feeds by rotating its wings as it flaps. This constant change in angle of attack allows it to maintain lift while remaining stationary in space. Researchers study the stability of such miniature hovering flight by destabilizing the moths and studying how they react to disturbances like being struck with a miniature clay cannonball. By testing how the moths recover from disturbances, we can learn how to build better robots and micro air vehicles (MAVs). (via supercuddlypuppies)
Month: January 2012
Reader Question: How Airfoils Produce Lift
doughboy3-deactivated20120305 asks:
I’m a Undergrad Aeronautical Engineering student. I’m curious as to your opinion as to how airfoils produce lift. I know the usual theory told in this situation. However my aerodynamics professor says that there are many things going on during the flow around an airfoil. I’m hoping to get a better idea of the different mechanisms responsible for lift.
There’s a common misconception of Bernoulli’s principle that’s often used to explain how an airfoil creates lift (which I assume is the “usual theory” to which you refer), and while there are many correct (or, perhaps, more correct) ways of explaining lift on an airfoil, I think the only opinions involved are as to which explanation is best. After all, opinions don’t keep a plane in the air, physics does!
I tackled the air-travels-farther-over-the-top misconception and presented one of my preferred ways of looking at the situation in a previous post; in short, the airfoil’s shape causes a downward deflection of the flow, which, by Newton’s 3rd law, indicates that the air has exerted an upward force on the airfoil. There’s a similar useful video from Cambridge on the topic here.
Another explanation I have heard used concerns circulation and its ability to produce lift (see the Kutta-Joukowski theorem for the math). In this case, it’s almost easier to think about lift on a cylinder instead of lift on a more complicated shape like an airfoil. If you spin a cylinder, you’ll find that the circulation around that object results in a force perpendicular to the flow direction. This is called the Magnus effect and, in addition to explaining why soccer balls sometimes curve strangely when kicked, has been used to steer rotor ships. One of my undergrad aero professors used to do a demonstration where he’d wrap a string around a long cardboard cylinder and demonstrate how, by pulling the string, the cylinder’s spinning produced lift, making the cylinder fly up off the lectern and attack the unsuspecting students.
An airfoil doesn’t spin, but its shape produces the same type of circulation in the flow field. Without delving into the mathematics, it’s actually possible through conformal mapping and the Joukowski transform to show that the potential flow field around a spinning cylinder is identical to that around a simple airfoil shape! Although that mathematical technique is not all that useful in a world where we can calculate the inviscid flow around complicated airfoils exactly, it’s still pretty stunning that we can analytically solve potential flow around (and thus estimate lift for) a host of airfoil shapes on the back of an envelope.
In short, your aerodynamics professor is right in saying that there are many things going on during the flow around an airfoil. If you get a roomful of aerodynamicists together and ask them to explain how airfoils generate lift, you would be faced with a lively discussion with about as many competing explanations as there are participants. As you learn more in your classes, you’ll gain a better intuitive feel for how it works and you’ll learn more of the nuances, which will help you understand why there is no one simple-to-understand explanation that we use!**
** Lest I confuse someone into thinking that aerodynamicists don’t know how airfoils produce lift, let me add that the argument here is over how best to explain the production of lift, not over how the lift is produced. We have the equations to describe the flow and we can solve them. We know that lift is there and why. We simply like to argue over how to explain it to people without all the math.
Reader Question: How to Get Started in Fluid Dynamics
unboundid-deactivated20131116 asks:
Hi. I’m a freshman engineering student at UCSD, and I was hoping to get more into fluid dynamics. Could you possibly give a quick shake-down of what I should look into if I’m just kind of starting? I want to either work in studying specifically fluid dynamics or in studying interactions of oil and petroleum.
Glad to hear that you’re interested in fluid mechanics! I usually answer these kind of questions privately, but I’m going to go ahead and publish my answer here because I think the advice is useful for any undergraduates interested in fluids.
First of all, most engineering courses of study won’t cover fluid mechanics–outside of pipe flow–until the junior or senior-level courses. This is because, unlike many other engineering topics, fluid mechanics relies heavily on foundational material in other subjects. Although fluid mechanics is still essentially F = ma, writing and manipulating the fundamental equations requires advanced calculus. So you will definitely benefit from paying a lot of attention in your math courses, especially vector calculus and differential equations. I also highly recommend learning to solve differential equations numerically using tools like Matlab or Mathematica. These are super useful skills for just about any form of engineering, but they can really pay off in fluid mechanics.
Now, while this classroom work is very important, you don’t have to wait until you’ve finished four semesters of calculus and physics before getting into fluid mechanics. Look up the professors at your school and the research they do. Find some topics/projects you want to learn more about, and go meet with those professors. In my experience, professors are willing to have undergraduates–yes, even freshmen–volunteer in their labs. I can’t guarantee that you’ll get paid, but I can tell you that you will learn a lot, especially from the graduate students you will probably be assisting. As you gain experience, you’ll gain responsibility. Right now, my research group has a sophomore preparing to be the lead on a new data collection campaign in one of our best research wind tunnels.
Many professors recruit their future graduate students this way. And, if it turns out that you don’t want to work in that lab through graduate school, you will still have a leg up getting into grad school because you’ll have significant research experience and a professor who can write you a strong recommendation, having seen your work. You could even have co-authorship on a publication, and that sort of achievement is going to look good on your resume, whether you pursue graduate school or an industrial job.
In short: talk to professors about their research and find a lab where you can become a part of that research. The earlier you do this, the more impressive the results by the time you graduate. Good luck!
Supersonic Stellar Jets
Astronomers studying stellar jets–massive outflows of gases and particles pouring from the poles of newborn stars–are finding reasons to turn to fluid dynamicists to understand the timelapse videos they’ve stitched together from multiple exposures from the Hubble telescope. Usually astronomical events unfold on such a slow timescale that our only view of them is as a snapshot frozen in time. Stellar jets can move relatively quickly, though, with portions of the jet flowing at supersonic speeds. Over the course of Hubble’s lifetime, these jets have been imaged multiple times, allowing astronomers to create movies that reveal swirling eddies and shock wave motion previously unseen. (submitted by sakalgirl)
“Tidal Wave” vs. “Tsunami”
This is part of the trouble when the same term has a scientific meaning and a lay meaning. See also: fluid.
Colliding Jets
Two jets of sugar syrup collide and interact to form very different patterns. On the left, the two jets have a low flow rate and create a chain-like wake. The jets on the right have a higher flow rate and produce a liquid sheet that breaks down into filaments and droplets. The result is often likened to fish bones. (Photo credit: Rebecca Ing)
Vortex Ring Collision
Two vortex rings collide head-on in this video. If their vorticities and velocities are matched in magnitude and opposite in direction, their collision results in a stagnation plane–essentially a wall across which the fluid does not pass. In reality, there are slight variations that result in non-zero velocities where the vortices meet, so some mixing occurs, but the overall symmetry remains striking. The collision breaks up the vortex ring into filaments, some of which cross-link with the other vortex’s filaments, resulting in the little halo-like eddies around the perimeter. Videos of the same experiment at different Reynolds numbers can be found here. (Submitted by Charlie H; Video credit: T. Lim and T. Nickels)
Ejecting Drops
Large droplets ejected from a liquid pool do not coalesce immediately back into the whole. Instead, a thin layer of air gets trapped beneath them, much like the oil lubricating bearings. The weight of the droplet causes the air to drain away, and eventually the droplet comes in contact with the pool. Some of the droplet gets drained away before surface tension snaps the interface back into a low energy state. A new smaller droplet then bounces upward before repeating the process over again. Eventually the droplet becomes small enough that its entire mass gets sucked away by the pool. Researchers call this process the coalescence cascade.
Freezing in a Microchannel
Fluid mechanics at the microscale can behave quite differently than in our everyday experience. Microfluidic devices–sometimes known as labs on a chip–are becoming increasingly important in research and daily life. For example, the test strips used by diabetics to check their blood sugar levels are microfluidic devices. In this video, researchers use a microfluidic channel to observe the freezing of supercooled water droplets. As the droplet first passes into the cold zone of the channel, it flash freezes, filling from the inside out with ice crystals. As it continues through the cold zone, the drop freezes fully, beginning at the outside surface and working inward. As it does so, the ice droplet fractures due to stresses. (Video credit: Stan et al)
How Shock Waves Form
Most people are familiar with the Doppler effect–in which the frequency of a wave changes depending on the motion of the observer relative to the wave source–from the shifting pitch of sirens as they pass. But the effect is important for pressure waves in addition to acoustic waves. When an object moves through air, its motion disturbs the surrounding air via pressure waves, which travel at the speed of sound. If an object moves slower than the speed of sound (top right), then those pressure waves extend in front of the object, carrying information about the object and allowing the air to shift and move smoothly around it.
If the object is moving at the speed of sound (bottom left), then it arrives at the same time as the pressure waves. In essence, the object is striking a stationary wall of air–this is what was meant by “breaking the sound barrier”. At Mach 1, the physics of the problem have fundamentally shifted. Now the only way for air to deflect to allow the object’s passing is by the sudden compression of a shock wave.
Moving even faster than the speed of sound (bottom right) the pressure and sound waves created by the object’s motion stretch in a cone behind it. The cone, known as a Mach cone, is the shock wave that deflects air around the moving object. The result is that the object will actually pass an observer before the observer will hear it. This is because no information can travel forward of the Mach cone’s leading edge. That’s why the area outside of the Mach cone is sometimes called the Zone of Silence. When the Mach cone passes an observer, the shock wave will register as a boom, like when the space shuttle passes overhead while landing. (via fyeahchemistry)
