Turbulence is expensive. By some estimates, overcoming the swirling drag that slows down submarines, boats, and airplanes costs billions of dollars a year. Now researchers have proposed a method that might one day harness electromagnetic fields to smooth the flow of salt water.
As water streams past a submarine's hull, the flow splits into many pairs of current called streaks. Each streak consists of a fast and a slow ribbon of water. Eddies circulate between the halves of each streak. These eddies grow rapidly until they burst, vibrating the hull and slowing the submarine.
But there may be a way to cut drag force by almost 30%, according to computer simulations of turbulent ocean-water flows reported in the 19 May issue of Science by mechanical engineer Yiqing Du of the Massachusetts Institute of Technology and applied mathematician George Karniadakis of Brown University. They modeled the ways in which electromagnetic pulses might break up the turbulence in the electrically conductive salt water passing over a hull. The researchers found in simulations and lab tests that the pulses prevent streaks from forming along the hull, so the explosive eddies never appear. "We cut the legs off the turbulence," Karniadakis says.
The technique could be a tremendous advance in fluid dynamics, if the simulations are borne out by upcoming experiments, says Richard Philips of the Naval Undersea Warfare Center in Newport, Rhode Island.