Studies in a vortex.
Turbulent air or water can adversely affect aircraft, sailing vessels, and motor vehicles, as well as stationary structures, such as bridges, buildings, and oil rigs. Researchers at Cornell University in Ithaca, N.Y., are using a specially designed computer-controlled x-y towing tank to better understand the nature of the vortices that moving objects create as they move through air and water. In time, this research could help improve the design of cars, planes, and structures, thus minimizing turbulence.
Vortex wakes downstream from a body affect the pressure distribution on the body itself, sometimes causing vibrations that can reduce effectiveness or, worse, lead to fatigue and failure, said Charles H. K. Williamson, an associate professor of mechanical and aerospace engineering at Cornell. Williamson is directing a group of graduate and undergraduate students in vortex wake studies at Cornell's fluid laboratories.
Typical areas in which wake studies are applicable include problems of wind force on structures such as buildings, chimney stacks, and bridges; wave and current loading on offshore structures; vibration of riser tubes that pipe oil from the seabed to the surface; fluid loading of submarines and projectiles; and forces and moments on lifting surfaces at high angles of attack, Williamson said. Government bodies (including the Office of Naval Research, the U.S. Air Force, and the National Science Foundation) along with civil, hydraulic, ocean, and aerospace engineering firms have expressed interest in the study of wakes and flow-induced vibration.
COMPUTER-CONTROLLED TOW TANK
Although two-dimensional studies of the wake left by cylindrical bodies are well documented, there has been little research on the three-dimensional aspects of a vortex wake, which is essential to developing designs that minimize the adverse effects of turbulence, according to Williamson. He designed a computer-controlled x-y towing tank to make three-dimensional vortex research possible. The tank is a modified version of a computer-controlled towing tank that Williamson designed as a research fellow at the California Institute of Technology's Graduate Aeronautical Laboratory in Pasadena before joining the Cornell faculty in 1990.
The water-filled tank is made of .75-inch glass walls and measures 26 feet long, 4 feet wide, and 4 feet deep. On top of the tank, a wheeled carriage that moves on rails tows a model in 2 degrees of freedom through the water, much like a giant x-y plotter.
The carriage is equipped with pulleys and cables to move it along the length of the tank (the x direction) and a lead screw that enables the carriage to move transversely (the y direction). Using a 386 computer, operators program the requisite carriage parameters for their study, such as constant speed, sinusoidal wave pattern, or elliptical orbits.
A joystick control enables operators to manually and electronically control the carriage and, thus, the model. A sting, or fine cable, suspended from the carriage is attached to the object being studied. In one study, a delta wing - essentially a triangular piece of metal - is painted with a laser-fluorescent dye. A 15-watt laser outside the tank follows the wake of the model and illuminates it.
Motor-driven, high-speed cameras capture the illuminated vortices on film. For some studies, Williamson takes stereoscopic pictures of the vortices by taking synchronous photos from two cameras at different angles. The illuminated vortex images are typically captured on photo prints (and sometimes on video).
The images captured by the video cameras are sent back to the computer. Using digital-based Particle Image Velocimetry, the researchers can see on a display monitor the entire velocity field caused by the vortices in a small area of the fluid. Williamson described the vortex dynamic studies at Cornell as embryonic, but he added that they also represent a clear advance over previous research efforts. "Scientists used to move an object through water and inject dye to mark its movement. About all they saw is vague movement. What we are doing involves extensive flow visualization to make discoveries that could not previously be made."
Williamson is also using the x-y towing tank to study the free-flight gliding of a wing in the water. Unlike other fluid dynamic tests, the Cornell equipment does not tether the model, which can affect the results. The Cornell researchers have also dampened carriage vibrations to the point where they are negligible.
One of the two primary goals of the wake vortex research at Cornell is to understand the vertical extent of wakes. "There has been extensive work done on the fluid dynamics on top of the wing, but as far as we know, there are almost no laboratory studies of the wake evolution further downstream," said Williamson.
The other research goal is to understand the mechanics of vortex decay. The Cornell research team has learned that the vortex structure lying between two trailing tip vortices (the "braid" wake) is an essential feature of flow. This braid wake rapidly imposes its turbulent length scale on the two vortices, influencing the decay of the turbulence into vortex rings far downstream.
"A lot of scientists are looking at the trailing vortices, but the wake between the vortices is also important. That can have some surprising effects as well," Williamson said. "We have shown there is a link between the turbulence scales in the primary vortices and in the wake joining the principal vortices. One may perhaps induce different decay rates of vortices."
Knowledge of vortex decay rates could be used to improve aircraft design and help prevent air crashes. For example, the strong turbulence left by an arriving or departing jet plane can cause a smaller airplane flying into the disturbance to roll over. "All this can occur even more violently on a cloudless day with what appear to be perfect conditions for taking off or approaching a runway," Williamson said. Currently, pilots wait two or three minutes before taking off or landing after another aircraft does so. That gives the training vortices a chance to dissipate.
Although the Cornell group's work has focused primarily on wake vortex dynamics, they have expanded their research to other flows as well, including the three-dimensional transition to turbulence in general shear flows, unsteady aerodynamics, automobile aerodynamics, and fluid loading of submarines. This work is being conducted for the North Atlantic Treaty Organization, the U.S. Department of Defense, and other groups. Some of this research involves collaboration with groups in Canada, France, and Switzerland.
In the area of automotive aerodynamics, Williamson and his student colleagues have experimented on reducing the drag on a cylindrical body (representing a car) by interfering with its wake. They placed a splitter plate parallel to the oncoming flow and behind the body to disrupt the formation of wake vortices, reducing drag by 20 percent. And placing a small plate upstream from the cylinder reduced drag on the entire system by 55 percent. "Designing automobiles or aircraft with even a modest reduction in drag would have a significant impact on performance," Williamson said.
The fluid dynamic research being conducted at Cornell could also improve the design of sailing vessels, a fact not lost on Williamson, an avid yachtsman. The researcher was a member of Cambridge University's sailing team while engaged in his doctoral studies, and he has represented Great Britain in world and European championships.
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|Title Annotation:||vortex wakes|
|Date:||Apr 1, 1995|
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