ARNOLD AIR FORCE BASE, Tenn. -- To validate the latest capabilities of the wind tunnels, Arnold Engineering Development Complex team members in the Aerodynamics Test Branch conducted testing last year on standard models in 16T and 16S, 16-foot transonic and supersonic wind tunnels, and Tunnels B and C, 50-inch hypersonic wind tunnels, at Arnold Air Force Base.
“Standard wind tunnel models have been used since the early 20th century,” said Michael Mills, AEDC engineer and AEDC Technical Fellow. “The model configuration used in tunnels 16T and 16S was developed in the 1950s by the Advisory Group for Aerospace Research and Development, or AGARD.
“At that time, AGARD proposed a series of standard models when transonic, supersonic and hypersonic tunnels were under development in the United States and Europe for the purpose of sharing data from common model configurations in various wind tunnels. According to AGARD literature, this would help establish the validity of test data in various tunnels and allow test engineers to explore the effects of wind tunnel flow quality, as well as the effects of model size and manufacturing tolerances.”
The 7-degree cone selected for hypersonic speeds in Tunnels B and C is not a standard AGARD configuration, but is common with cones tested in other hypersonic facilities.
John Hopf, Aerodynamics Test Branch technical lead for Aerodynamics and Aerothermal Test and Evaluation, mentioned different model designs are used for different flow regimes, and the models are typically equipped with multiple types of instruments to better qualify test results.
“A standard model tested in supersonic or hypersonic flow will have more of a missile shape, featuring less wing area or lifting surface than a model designed for subsonic and transonic flow,” Hopf said. “The shapes are fairly representative of vehicles tested at the
speeds and offer desired sensitivity characteristics for the speed range. The geometries are not complex so that high fidelity computational analysis and simulations can be more easily managed.”
In addition to standard model tests, modeling and simulation, or M&S, tools such as computational fluid dynamics, or CFD, can be used to acquire aerodynamic data.
According to Scott Meredith, technical adviser for the Aerodynamics Test Branch, CFD typically yields “cleaner, less ambiguous results with the less complex geometry incorporated into standard model designs.”
“M&S tools help design the test matrix and validate the test data points,” Meredith said.
Standard model tests take place any time changes are made to a facility that could potentially affect data quality or expand capabilities.
Marvin Sellers, AEDC engineer and AEDC Technical Fellow, added that these standard model tests can be used as piggybacks for demonstrations of new test technologies. One such technology being tested by AEDC test teams is called unsteady pressure sensitive paint, or uPSP.
“While AEDC has been using steady-state PSP for almost 30 years, uPSP is relatively new and provides a different type of data to the customers,” Sellers said. “Unsteady PSP provides fluctuating pressure in sound pressure level at different frequencies similar to dynamic pressure transducers.
“However, uPSP provides this information over the painted area instead of several discrete locations for the transducers. The amount of information uPSP provides is tens of thousands times greater and provides better insight to the aerodynamic phenomena.”
The results from the most recent standard model tests in the wind tunnels were beneficial, showing that the tunnels are operating as expected.
“Data from 16T and 16S indicated excellent flow quality and a highly-effective test process, anchored by the productivity of the data acquisition and processing systems,” Hopf said. “Initial assessments from the standard model testing in von Kármán Gas Dynamics Facility Tunnels B and C show flow quality remains high and the new data acquisition systems to have reliably generated accurate results.”
The Aerodynamics Test Branch plans to conduct further standard model testing after annual maintenance outages, which also permits assessment of the facility’s readiness for customer testing.
“Cost and schedule at times restrict implementation at more infrequent intervals,” Hopf said. “But even if no changes are made, repeat entries of tests using a standard model are a fairly low-cost way to validate data quality without the time and expense of executing a full
tunnel calibration.”