Water tunnel shows smooth water leads to smooth flying

Dye-colored water reveals flow patterns around a Boeing 757 model as Aerodynamicist Pat Hayes examines flow fields from outside a water tunnel in Huntington Beach, Calif. The 757 model is being used to calibrate water tunnel instruments because of the model’s large existing database of dynamic flight characteristics. Flow visualization in the water tunnel provides detailed observation of the flow around a wide variety of configurations. The process uses a number of techniques, including dye flow through ports in the model, hydrogen bubble generation from strategic locations on the model, or laser light sheet illumination. The free-stream flow and the flow field dynamics are low speed, allowing real-time visual assessment of the flow patterns. (Anthony Romero photo)

By Bob Howard

Water tunnels have been used in one form or another to explore fluid mechanics and aerodynamic phenomena since the days of Leonardo da Vinci. Only in recent years, however, have water tunnels been recognized as highly useful facilities for critical evaluation of complex flow fields on modern vehicles, such as high-performance aircraft.

The water tunnel in Huntington Beach, Calif., is a one-of-a-kind test facility at Boeing. With movement like the slow, slow waters of a deep river, the tunnel produces a nearly perfect, undisturbed flowing column of water by taking out all flow distortions, currents and eddies before sending it through the glass tunnel.

Measuring 10 feet by 3 feet by 4 feet high (3 meters by 0.9 meters by 1.2 meters), the tunnel has been newly outfitted with sophisticated software and a sensitive, mechanical apparatus that aeronautical designers call a “force and moment balance” -- a device that essentially resembles a stick, but with miniature instruments inside. The apparatus holds the model, points it into the water flow and moves it to any flight orientation desired -- even upside down. Its sensors continuously read and record dynamic data of the physical forces the vehicle experiences every moment of the simulated flight.

The tunnel is now completing calibration tests and coming on line to test the next generation of air transports, fighters and X- vehicles, providing engineers with quick, low-cost assessments early in the design process and help them build a database of dynamic measurements.

“We will be helping our customers achieve higher quality earlier in the design cycle,” said Aerodynamicist Pat Hayes. “A quick check of the design early on greatly reduces risk, because issues are discovered and solved sooner when they are easier and less costly to address.”

Water tunnels are highly useful for seeing and evaluating complex flow fields dominated by vortices and vortex interactions. The free-stream water flow patterns created by a model’s particular design are made visible using dye flowed out through ports in the model itself.

The water tunnel is usable with a minimum of training, and its location enables engineers supporting proprietary and classified work to use it as well.

With a rapid check of the design, and with the dynamic data produced from simulated flight, the airframe designers are able to bring in the next team months earlier with high confidence they are on the right track. Guidance navigation and control designers can come on board with assurance that an early design is at a mature point for them to start. And, of course, the sooner they can get in the game the better.

“What could cost six months in effort at a vendor -- not to mention a very expensive model -- can be handled in Huntington Beach in one week using the Stereo Lithography Labs to build the model and the water tunnel for dynamic data testing,” Hayes said.