The following article expands on highlights and insights from one of our Expert Series events, which are exclusive for Young Scholars and their parents.
Authored by: Scott Beaver, Ph.D.
Introduction
Physics has laws describing how matter behaves, including the movement of fluids such as air and water. Using the law of conservation of energy, physics provides equations to predict motion, for example the parabolic flight path of a launched or thrown solid object.
A second law, however, requires that entropy always increases as motion occurs. Entropy can be thought of as randomness or chaos, and it leads to unpredictable, turbulent flows. For example, an object flying through the air or a rock in a flowing river create a series of eddies, ripples, and waves that have random motions. Experimental evidence of these vortices can be obtained by using dyes or smoke in the fluids to observe the spiraling motions created by the frictional forces where the fluid contacts a solid surface. Satellite imagery shows the effect at a larger scale for “Karman vortex streets” forming in the clouds downstream of islands.
The mathematics of the Reynolds number and the concept of vortex shedding describe how these random vortices happen. Generally speaking, faster flows create more surface friction, more turbulence, and more pronounced eddies. The increased friction generally dissipates energy and slows the bulk motion. Above a certain threshold speed, however, a thin turbulent boundary layer separates from the bulk flow around an object, forming an insulating layer that markedly decreases the friction.
Over a hundred years before fluid dynamics were understood scientifically, the baseball and golf ball were designed with rough threads and dimples, respectively, which promote turbulent boundary layer separation. The result is reduced air drag on the sports balls, providing longer, straighter flight paths. The scientist von Karman described vortex streets likewise about a hundred years before modern satellites were able to confirm the effect from above. These examples show how human ingenuity allows for an understanding of the physical world long before the scientific details are precisely worked out.
Highlights
- Flight paths can be precisely calculated ignoring the effects of drag forces.
- In the real world, drag forces create random vortices, commonly referred to as eddies or a wake.
- The Reynolds number allows us to predict what type of vortices form downstream of an object.
- Vortex shedding theory shows that eddies will spin off a solid object, alternating from one side to the other.
- Karman vortex streets observed from satellites show vortex shedding occurring downwind of islands in the open ocean.
- Above a certain velocity, a drag crisis occurs, the vortex shedding stops, and drastically lowered drag forces are observed.
- The designs of the baseball and golf ball take advantage of the drag crisis effect to allow for longer, straighter flight paths of the sports balls.
- Surprisingly, Karman streets and sports balls were observed to exhibit vortex shedding and drag crises, respectively, long before these topics were developed rigorously by modern physics.
Things Students Can do to Explore This Topic Further…
Use an AI tool such as ChatGPT to research additional information on the following topics:
- How a drag crisis occurs in common sports balls.
- Additional satellite imagery of Kármán vortex streets.
- Calculating the Reynolds number in common situations involving solid objects moving through air or water.
- Investigate how an effect similar to a drag crisis improves aerodynamic efficiency on airplane wings using flaps.
- https://chatgpt.com/
Resources
Learn quickly about classic high school physics, including the equations that predict the parabolic flight paths of solid objects while ignoring drag forces. These concepts are developed and extended to atmospheric flows in our online physics video course series, available at the link below:
https://learnwithdrscott.com/best-homeschool-physics-curriculum/
To see more advanced fluid dynamics in action, check out the videos below. The first link is a simulation of ideal vortex shedding. The second is a time-lapse satellite image showing Kármán vortex streets over an island chain. Use the videos to compare theoretical predictions with actual observational data.
https://en.wikipedia.org/wiki/File:Vortex-street-animation.gif
https://www.nesdis.noaa.gov/news/noaa-20-sees-von-karman-vortices-near-the-cape-verde-islands
Speaker Bio:
“Dr. Scott” Beaver was a gifted child drawn to science, earning a Ph.D. in chemical engineering with a focus on computer modeling—an early form of AI—when the internet was still new. His passion led him to diverse projects, including air quality modeling, pollution abatement, water purification, carpet recycling, chemistry education, computer chip manufacturing, fermented foods, homeopathic medicine, and public health. A lifelong traveler, he shared his knowledge globally, creating the educational website learnwithdrscott.com covering chemistry, biology, physics, and algebra. His legacy is one of curiosity, innovation, and a commitment to making science accessible and fun.
Permission Statement
This article is provided as a service of the Davidson Institute for Talent Development, a 501(c)3 nonprofit dedicated to supporting profoundly gifted young people 18 and under. To learn more about the Davidson Institute’s programs, please visit www.DavidsonGifted.org.
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