Navigation Links
Following the leader can be a drag, according to student's research on flapping flags
Date:11/7/2008

From the Tour de France to NASCAR, competitors and fans know that speed is only part of the equation. Strategy -- and the ability to use elements like aerodynamic drafting, which makes it easier to follow closely behind a leader than to be out in front -- is also critical.

But in some cases, drafting happens in reverse: It's the leader of a pack who experiences reduced drag, while the followers encounter more resistance -- and have to expend more energy to keep up.

In research published in the Nov. 7 issue of Physical Review Letters (Vol. 101: No. 194502), Cornell fourth-year physics graduate student Leif Ristroph and New York University researcher Jun Zhang used a simple tabletop experiment to show that two or more flexible objects in a flow -- flags flapping in the wind, for example -- experience drag very differently than rigid objects in a similar flow.

The findings could help biologists understand a variety of phenomena, including why animals like fish and birds travel in groups.

"It's counterintuitive," said Ristroph. "People who have studied schooling fish and flocking birds always postulate that they flock because the ones downstream can save energy, and the guy who's at the front has to work harder. Here's a case where that gets turned on its head."

To test the effects of a flowing fluid on flexible objects, Ristroph created a thin film of soapy water -- the beginning of a giant soap bubble -- stretched between two fishing lines and constantly refreshed with a flow of water from the top. Into the membrane, he inserted pieces of thin rubber (the flags) -- attached to perpendicular wire "flagpoles."

To measure the forces on the flags as water flowed past them, Ristroph attached small mirrors -- actually microscope cover slips -- to the far ends of the "flagpoles." As the flags flapped in the flow, the slightly flexible poles moved correspondingly -- and by shining a laser light on the mirrors, Ristroph could see the movements magnified and traced on a far wall.

He also used optical interferometry -- a technique based on the way light waves interfere with each other -- to measure the fluid flow around the flapping flags.

Instead of finding that the front flag took the brunt of the drag and following flags experienced less resistance, he found that for two flags close together, the front flag flapped less and thus experienced less drag -- even relative to a single flag without a follower. For the follower, he found the reverse: The flag oscillated more and experienced correspondingly more drag.

"That was completely unexpected," Ristroph said. Additional experiments with multiple flags and different spacing showed that the effect is consistent for closely spaced objects and drops off as the space between them increases.

The effects aren't fully understood, Ristroph said. "It appears that the follower is sort of confining the flow at the trailing edge of the leader, so it feels like he can't flap as hard; [and therefore] the amplitude of the leader is reduced." For the follower, the oscillations of the leader likely cause a resonating effect, increasing the follower's flapping and thus its drag.

"This is now like a two-way conversation, where the fluid talks to the object, and the object talks back," Ristroph said.

"Simulating this is very difficult," he added. "The theory and the simulation really cannot handle how to deal with the flow and an object that has flexibility.

"You often have to do the experiment," he said. "And when you do the experiment you can get something that is counterintuitive."


'/>"/>

Contact: Blaine Friedlander
bpf2@cornell.edu
607-254-8093
Cornell University Communications
Source:Eurekalert

Related biology news :

1. Researchers block damage to fetal brain following maternal alcohol consumption
2. Following traumatic brain injury, balanced nutrition saves lives
3. When following the leader can lead into the jaws of death
4. MNI researchers locate neurological basis of depression following sports concussion
5. Researchers take first steps towards spinal cord reconstruction following injury
6. LSU professor studies army-ant-following birds
7. Ecologist finds dire devastation of snake species following floods of 93, 95
8. Gender biases in leadership selection during competitions within and between groups
9. World leaders in infectious diseases convene to discuss emerging global viruses
10. Growing a garden of future environmental leaders
11. USGS Coalition to honor Reps. Dicks, Regula with Leadership Award
Post Your Comments:
*Name:
*Comment:
*Email:
(Date:6/2/2016)... 2016 Perimeter Surveillance & Detection ... Physical Infrastructure, Support & Other Service  The ... offers comprehensive analysis of the global Border Security ... revenues of $17.98 billion in 2016. Now: ... leader in software and hardware technologies for advanced video ...
(Date:5/24/2016)... care by providing unparalleled technology to leaders of the medical imaging industry.  As such, ... to the range of products distributed by Ampronix. Photo - http://photos.prnewswire.com/prnh/20160524/371420 ... ... ... With ...
(Date:5/16/2016)... NEW YORK , May 16, 2016   ... authentication solutions, today announced the opening of an IoT ... to strengthen and expand the development of embedded ... provides an unprecedented level of convenience and security with ... to authenticate one,s identity aside from DNA. EyeLock,s platform ...
Breaking Biology News(10 mins):
(Date:6/23/2016)... Md. , June 23, 2016 A person ... from the crime scene to track the criminal down. ... the U.S. Food and Drug Administration (FDA) uses DNA evidence ... Sound far-fetched? It,s not. The FDA ... sequencing to support investigations of foodborne illnesses. Put as simply ...
(Date:6/23/2016)... --  EpiBiome , a precision microbiome engineering company, today ... from Silicon Valley Bank (SVB). The financing will allow ... drug development efforts, as well as purchase additional lab ... been an incredible strategic partner to us – one ... provide," said Dr. Aeron Tynes Hammack , EpiBiome,s ...
(Date:6/23/2016)... 2016  Blueprint Bio, a company dedicated to identifying, ... community, has closed its Series A funding round, according ... "We have received a commitment from Forentis Fund ... to meet our current goals," stated Matthew Nunez ... to complete validation on the current projects in our ...
(Date:6/23/2016)... ... June 23, 2016 , ... ... of intelligent tools designed, tuned and optimized exclusively for Okuma CNC machining centers ... The result of a collaboration among several companies with expertise in toolholding, cutting ...
Breaking Biology Technology: