Tag Archives: fluid dynamics

How to Walk Safely Coffee in Hand

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(I am just so glad that the scientists have addressed this ever so critical issue. )

Science Reveals How Not to Spill Your Coffee When Walking

Natalie Wolchover, Life’s Little Mysteries Staff Writer
Date: 11 May 2012 Time: 01:02 PM ET

 

how to keep from spilling
CREDIT: H.C. Mayer and R. Krechetnikov

Ever wondered why it’s so hard to walk with a cup of coffee without spilling? It just so happens that the human stride has almost exactly the right frequency to drive the natural oscillations of coffee, when the fluid is in a typically sized coffee mug. New research shows that the properties of mugs, legs and liquid conspire to cause spills, most often at some point between your seventh and tenth step.

So says a pair of fluid physicists at the University of California at Santa Barbara (UCSB). They investigated the science of sloshing in a new study published in the journal Physical Review Letters E, and calculated the natural frequency at which coffee sloshes back and forth when held in mugs of a variety of sizes, from a dainty espresso cup to a cappuccino behemoth. They found that a normal human gait moves at nearly the same frequency, so each step amplifies the coffee’s heave-ho motion. Stumbling or changing pace — common occurrences when you’re low on caffeine — make matters worse by causing chaos in your cup, increasing the chance of a splash over the rim.

But now, there’s hope. By modeling the fluid and walking dynamics of the situation, and comparing the math with some real-world walking-with-coffee experiments, the UCSB scientists have uncovered a few tips for bleary-eyed coffee cup carriers.

“Of course, there are ways to control coffee spilling,” study co-author Rouslan Krechetnikov told Life’s Little Mysteries.

Coffee drinkers often attempt to walk quickly with their cups, as if they might manage to reach their destination before their sloshing java waves reach a critical height. This method is scientifically flawed. It turns out that the faster you walk, the closer your gait comes to the natural sloshing frequency of coffee. To avoid driving the oscillations that lead to a spillage, walk slowly. [Why Does Room-Temperature Coffee Taste So Bad?]

Secondly, watch your cup, not your feet. The researchers found that when study participants focused on their cups, the average number of steps they took before spilling coffee increased greatly. Krechetnikov and his graduate student Hans Mayer, the primary author of the study, suggested two explanations for this result: First, focusing on one’s cup tends to engender slower walking, and second, it dampens the noise, or chaotic sloshing, in the cup. Whether focused carrying decreases the amount of noise because we perform “targeted suppression,” automatically counteracting the sloshing of the liquid with small flicks of our wrists, or because we simply hold the cup more steadily when we’re looking at it, the researchers could not say.

Third, accelerate gradually. If you take off suddenly, a huge coffee wave will build up almost instantly, and it will crash over the rim after just a few steps.

But the best way to prevent coffee spilling might be to find an unusual cup. According to Krechetnikov, ideas from liquid sloshing engineering studies, which historically were done to stabilize fuel tanks inside missiles, indicate three possibilities for spill-free cup designs: “a flexible container to act as a sloshing absorber in suppressing liquid oscillations, a series of annular ring baffles arranged around the inner wall of the container to achieve sloshing suppression, or a different shape cup.”

from:    http://www.livescience.com/20246-coffee-spill-walking.html

The Benefits of Wine Swirling

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Mechanism of Wine Swirling Explained

ScienceDaily (Nov. 21, 2011) — Wine drinkers know that swirling a good vintage around in a glass aerates the wine and releases its bouquet. Just how the process — known as “orbital shaking” — works, however, has been something of a mystery.

Wine swirling. (Credit: © Patricia Hofmeester / Fotolia)

Fluid dynamicists have long observed that orbital shaking generates a wave that propagates around the inner edge of the glass, churning the liquid as it travels. “The formation of this wave has probably been known since the introduction of glass or any other kind of cylindrical bowl, but what has been lacking is a description of the physics related to the mixing and oxygenation,” says Mohamed Farhat, senior scientist at the Ecole Polytechnique Federale de Lausanne in Switzerland.

To figure out how the mixing occurs, Farhat and his colleagues generated such waves in clear cylinders and used state-of-the-art instrumentation to track the motion of traveling waves and measure the liquid velocity.

The researchers found that “as the wave propagates along the glass wall, the liquid is displaced back and forth from bottom to top and from the center to the periphery,” Farhat explains. “This pumping mechanism, induced by the wave, is more pronounced near the free surface and close to the wall, which enhances the mixing.” The research team also discovered that, “for a given glass shape, the mixing and oxygenation may be optimized with an appropriate choice of shaking diameter and rotation speed,” he says.

“The intuitive and efficient motion of wine swirling has inspired engineers in the field of biopharmaceuticals,” Farhat says, where cell cultures can be placed in large cylindrical containers — or bioreactors — and “shaken” in a manner similar to the aeration of a glass of wine. The new work, he says, demonstrates that “such bioreactors offer better mixing and oxygenation over existing stirred tanks, provided that operating parameters are carefully optimized. Moreover, the gentle nature of orbital shaking also ensures a better viability and growth rate of the cells at reduced cost.”

Martino Reclari, a Ph.D. student and a member of the Swiss team, presented the findings in a talk at the American Physical Society’s Division of Fluid Dynamics Meeting, which takes place Nov. 20-22, 2011, at the Baltimore Convention Center in the historic waterfront district of Baltimore, Maryland

from:   http://www.sciencedaily.com/releases/2011/11/111121104142.htm