Featured Research

from universities, journals, and other organizations

Lead-flapping Objects Experience Less Wind Resistance Than Their Trailing Counterparts

Date:
November 14, 2008
Source:
Cornell University
Summary:
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. New research shows that two or more flexible objects in a flow -- flags flapping in the wind, for example -- experience drag very differently from rigid objects in a similar flow.

Pattern created by a zebrafish swimming through water. (A thin layer of oil coats the water's surface). Leif Ristroph's research on flapping flags could provide insight into how fish and other animals move through and interact with their fluid surroundings.
Credit: Leif Ristroph

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.

Related Articles


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.

A 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."

Ristroph performed the research during a summer fellowship at NYU through the Interdisciplinary Graduate Training in Nonlinear Systems program, which is funded by the National Science Foundation.


Story Source:

The above story is based on materials provided by Cornell University. Note: Materials may be edited for content and length.


Journal Reference:

  1. Ristroph et al. Anomalous Hydrodynamic Drafting of Interacting Flapping Flags. Physical Review Letters, 2008; 101 (19): 194502 DOI: 10.1103/PhysRevLett.101.194502

Cite This Page:

Cornell University. "Lead-flapping Objects Experience Less Wind Resistance Than Their Trailing Counterparts." ScienceDaily. ScienceDaily, 14 November 2008. <www.sciencedaily.com/releases/2008/11/081107143618.htm>.
Cornell University. (2008, November 14). Lead-flapping Objects Experience Less Wind Resistance Than Their Trailing Counterparts. ScienceDaily. Retrieved November 23, 2014 from www.sciencedaily.com/releases/2008/11/081107143618.htm
Cornell University. "Lead-flapping Objects Experience Less Wind Resistance Than Their Trailing Counterparts." ScienceDaily. www.sciencedaily.com/releases/2008/11/081107143618.htm (accessed November 23, 2014).

Share This


More From ScienceDaily



More Matter & Energy News

Sunday, November 23, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

Toyota's Hydrogen Fuel-Cell Green Car Soon Available in the US

Toyota's Hydrogen Fuel-Cell Green Car Soon Available in the US

AFP (Nov. 21, 2014) Toyota presented its hydrogen fuel-cell compact car called "Mirai" to US consumers at the Los Angeles auto show. The car should go on sale in 2015 for around $60.000. It combines stored hydrogen with oxygen to generate its own power. Duration: 01:18 Video provided by AFP
Powered by NewsLook.com
Google Announces Improvements To Balloon-Borne Wi-Fi Project

Google Announces Improvements To Balloon-Borne Wi-Fi Project

Newsy (Nov. 21, 2014) In a blog post, Google said its balloons have traveled 3 million kilometers since the start of Project Loon. Video provided by Newsy
Powered by NewsLook.com
Raw: Paralyzed Marine Walks With Robotic Braces

Raw: Paralyzed Marine Walks With Robotic Braces

AP (Nov. 21, 2014) Marine Corps officials say a special operations officer left paralyzed by a sniper's bullet in Afghanistan walked using robotic leg braces in a ceremony to award him a Bronze Star. (Nov. 21) Video provided by AP
Powered by NewsLook.com
British 'Bio-Bus' Is Powered By Human Waste

British 'Bio-Bus' Is Powered By Human Waste

Buzz60 (Nov. 21, 2014) British company GENeco debuted what its calling the Bio-Bus, a bus fueled entirely by biomethane gas produced from food scraps and sewage. Jen Markham explains. Video provided by Buzz60
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:

Breaking News:

Strange & Offbeat Stories


Space & Time

Matter & Energy

Computers & Math

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News



Save/Print:
Share:

Free Subscriptions


Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile


Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?


Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile: iPhone Android Web
Follow: Facebook Twitter Google+
Subscribe: RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins