Capability extension expected to have major impact in hypersonic testing
By Bradley Hicks, AEDC/PA
/ Published November 05, 2018
ARNOLD AIR FORCE BASE, Tenn. -- With the surging demand for hypersonic development, many of the tools used in the transonic and supersonic regimes to provide analysis and measurements must be extended to meet the new capability.
One such tool under development is Department of Defense High Performance Computing Modernization Program CREATE™-AV Kestrel, an easy-to-use software used to calculate fluid flows via computational fluid dynamics, or CFD.
In addition to providing data from aircraft models, Kestrel is now being used to provide solutions for the thermodynamic properties of equilibrium air at AEDC test facilities. Equilibrium air occurs when O2 and N2 in the air begin to disassociate at extremely hot temperatures, a chemical reaction that leaves oxygen and nitrogen atoms in the air along with O2 and N2 molecules.
A demonstration of Kestrel for equilibrium air was recently completed for a test article in the arc-heated test facilities at Arnold Air Force Base.
In the arc-heated facilities simple thermodynamic models, such as calorically perfect air, are inadequate; higher-fidelity thermodynamic models are essential. A need also exists to compute such flows for geometrically-complicated test articles. Older codes utilized simple-structured meshes, while Kestrel allows the use of highly-unstructured meshes which can resolve very complex shapes. This combination of high-fidelity thermodynamic models with more flexible mesh capability is what makes Kestrel the current code of choice at AEDC.
According to AEDC Modeling and Simulation Engineer Ken Tatum, the mesh is a set of points in the flow field. A set of differential equations is solved to determine data such as pressure, temperature, density and speed. Algorithms within Kestrel solve these equations at the points within the mesh. From these fundamental quantities, which include pressure, temperature, density and speed, engineers can also calculate the heat transfer on a test article.
To accurately compute the amount of heat transfer at higher speeds, such as those in the hypersonic regime, engineers must use an equilibrium air model rather than a simple perfect gas model.
Kestrel is used in other AEDC facilities to analyze flow fields below the hypersonic regime. The comprehensive multi-physics tool was originally developed to simulate fixed-wing aircraft in the transonic speed regime and is regularly used for test articles in the Propulsion Wind Tunnel where simpler thermodynamic models are acceptable.
AEDC Senior Staff Engineer Dr. Greg Power said the equilibrium air model assumes this disassociation is going to happen “infinitely fast,” with atomic nitrogen and oxygen taken into account in the calculations. He said this provides engineers with a good approximation of the heat transfer at these extreme conditions.
The result of this extension is the calculation of surface heat flux, even near the flow stagnation point, can now be completed more quickly, using the Kestrel tool and thermodynamic models. Solutions previously requiring up to 100,000 iterations can now be obtained in less than a day.
The CREATE™-AV Kestrel tool is funded by the DOD High Performance Computing Modernization Program. CREATE™, short for Computational Research and Engineering for Acquisition Tools and Environments, is a software suite that provides production-quality design and analysis.
Power, Tatum and several members of the AEDC team at Arnold AFB, along with Dr. Ryan Bond, a research professor at the University of Tennessee Space Institute, and developers of the tool are working together to further develop complex thermodynamic models for Kestrel.
The multi-year plan is to further refine the work involving Kestrel and hypersonics in the H3 facility so the Kestrel equilibrium air and more complex reacting flow models can be used in other AEDC facilities, such as the Aerodynamic and Propulsion Test Unit at Arnold and the AEDC Hypervelocity Tunnel 9 in White Oak, Maryland. This will provide facilities with a single tool to provide CFD analyses in the hypersonic flow regime.
“There are many other facilities and many other applications,” Power said. “For example, Tunnel 9 is another one that we’re shooting for. The capability that we need for Tunnel 9 isn’t quite there but should be available in the next few months.”
Tatum said Kestrel will replace previous-generation computational tools that, in comparison, lack either geometrical complexity or advanced thermodynamic models.
“The nice thing about it is the methodology that allows us to implement equilibrium air is the same methodology that will allow us to implement the Tunnel 9 physics as well,” he said.
The work in H3 represented an important step in spreading the Kestrel extension capability to these other locations.
“This is something we’ve got to have in order to do that in the long run,” Tatum said. “Where Kestrel started out as just a basic transonic fixed-wing methodology, this is trying to get it to where it’s applicable to the hot topics like hypersonics.”