Pushing Air

Re-motoring transonic and supersonic wind tunnels

—by Richard Mandel

Creating air movement is a fairly straight forward affair. Nature does it with simple tools — just a hydrogen fusion reactor placed 93 million miles away from a rotating body massing about 6.57×10²¹ tons. The resultant effects between the two gently stirs paper scraps in parking lots and flattens homes in trailer parks. All in the same day.

For research studies, though, wind tunnels answer questions not covered by FEA software. Originally devised by Alexandre Gustave Eiffel (he of the Parisian tower fame) and used by the Wright Brothers in studying their wing designs, today’s tunnel is a sophisticated industrial machine, capable of producing varying high velocities and simulating altitude extremes.

Tennessee breezes

Test facilities at the Arnold Engineering Development Center (AEDC), at Arnold Air Force Base, TN, include nearly 60 aerodynamic and propulsion wind tunnels, rocket and turbine engine test cells, space environmental chambers, arc heaters, ballistic ranges and other specialized units. Twenty-seven of the test units have capabilities unmatched elsewhere. They can simulate flight conditions from sea level to altitudes around 100,000 feet, and from subsonic velocities to those well over Mach 20.

Of these units, the Propulsion Wind Tunnel (PWT) facility, constructed in the 1950s, is devoted to aerodynamic and propulsion integration testing of large-scale aircraft models. In some cases, the propulsion systems and inlets are tested simultaneously to make sure they are aerodynamically designed to provide adequate airflow. Other tests involve store separation investigations — making sure the bombs, missiles or other stores separate cleanly from the parent aircraft when released.

The facility has a four-foot transonic tunnel used primarily to support design, development and improvement of aircraft stores. Primary to the PWT are two closed-circuit wind tunnels (one to simulate transonic speeds — Mach 0.06 to Mach 1.6 — and the other supersonic — Mach 1.5 to Mach 4.5) with 16 sq. ft. test sections, used for conventional aerodynamic tests and for combined aerodynamic/propulsion system tests. Pressure of the airflow through these test sections can be varied to simulate altitude conditions from sea level to about 150,000 feet.

Swapping motors

The air within the two 16 sq. ft tunnels is moved through five multi-blade, axial flow compressors. Each stainless steel fan disc within the compressors is 30 ft in diameter, and are all being spun at 600 rpm. To drive either tunnel, PWT boasts four large electric motors that rank amongst the most powerful ever built — tall as a two-story house and as heavy as a railroad locomotive. Two of the motors are 83,000 hp, 12-pole, 600 rpm, 13,800V synchronous units with an efficiency of 97.5%. Those synchronous motors are not capable of starting themselves, and must be brought up to operating speed by the two starting motors. Each synchronous motor stands 21½ ft high and weighs 225 tons, using 31 miles of copper wire in the motor windings. When put together in tandem, the motor drive system is almost two football fields long — among the largest rotating machines in the world. 

Both starting motors are 10-pole, 6,900V, wound rotor induction motors, each originally rated at 600 rpm for 25,000 hp continuous, and 52,500 hp for two hours. The motors were designed for starting duty and peaking power for the wind tunnel compressors. Speed control is provided by two liquid rheostats (adjustable resistors with separable electrodes in a water/sodium-carbonate electrolyte). In 1980, those starting motors were rebuilt to a rating of 35,000 hp, service factor 1.15 (i.e., 42,500 hp) continuous duty. Since 1980, they have been used for adjustable speed operation of the 16T Compressor over the speed range of 200 to 500 rpm. The efficiency was only 80% at 600 rpm, dropping below 30% at 200 rpm. Also, since 1980 these aging motors have had an increasing failure rate attributable to the severe stresses of adjustable speed operation

They will be replaced by two new synchronous ABB Medium Voltage motors with electronic adjustable speed drives. TECO-Westinghouse Motor Company, Round Rock, TX is supplying the new motors, and ABB of New Berlin, WI, the prime contractor, is supplying power electronics and transformers. Each of the new motors and power electronics sets (i.e. drives) will be rated at 69,000 hp. The increased power will allow 80% of the wind tunnel tests to be performed with only three motors in operation, or all four motors can be run for a total output of 304.000 hp at 600 rpm. The new motors and drives will have efficiencies ranging from about 90% at low speed to 96.3% at 600 rpm, achieving considerable energy savings over the existing wound rotor induction motors. Since the wind tunnels operate about 1,600 hours per year, the annual power savings was an important factor driver for the project, along with increased reliability. The improvements will also allow the compressors to be brought up to and down from test speeds quicker, improving the time it takes to load a model into the test cell, run it, remove it, make design changes, and run it again.

“The project is turnkey,” according to Ram Bhatia, director of Medium Voltage Drives for ABB. “with ABB providing transformers, power converters, motors, switchgear harmonic filters, facility controls, plus all construction and installation services. The drives add the ability to run the fans in adjustable-speed mode, or to start up and synchronize the two fixed-speed motors to run at a constant speed, depending on the test protocols and parameters.”

For more information:
Connect directly to AEDC's website via the Online Reader Service Program at www.rsleads.com/204df-195

Connect directly to ABB at www.rsleads.com/204df-196