Aerospace Sciences

A new challenge set for CFD

The Fluid Dynamics Technical Committee focuses on the behaviors of liquids and gases in motion, and how those behaviors can be harnessed in aerospace systems.

Researchers worldwide have been tackling the challenges faced when using computational fluid dynamics to simulate complex flows that occur during aircraft take off, landing and maneuvers. High­-order CFD methods are well­ suited for these types of problems because they provide a more accurate representation of the fluid compared to low-order methods used in the majority of production CFD codes.

Participants at the 4th International Workshop on High-Order CFD Methods demonstrated that high-order methods can be effectively applied to complex, time-accurate and steady-state cases of interest to industry. For example, researchers showed that high-order schemes are more computationally efficient relative to second-order schemes for transitional flow over low-pressure turbine blades. Meshing also emerged as an important consideration with presenters showing the advantages of adaptive schemes and highlighting the importance and difficulties of generating curved meshes, particularly for three- dimensional problems requiring anisotropy. NASA has set a 2017 goal to improve the numerical methods used to simulate a wide range of physics by identifying the most promising techniques and further developing them to reduce their predictive error by 40 percent.

To meet this challenge, NASA worked closely this year with the AIAA Turbulence Modeling Benchmark Working Group to identify a set of test cases that are simple enough to be useful but still possess the relevant flow physics. CFD developers will use these cases to tune and refine their codes, increasing the community’s ability to accurately predict this class of problems.

Researchers at Missouri University of Science and Technology (Missouri S&T) and NASA’s Langley Research Center in Virginia developed a database of direct numerical simulations of broadband acoustic radiation from turbulent boundary layers ranging from Mach 2.5 to Mach 14. Such simulations will advance the understanding of flow in conventional high­-speed wind tunnels. The simulations and database will improve how wind tunnel data are scaled up to match the actual flight conditions and will eventually enable better modeling of the laminar/turbulent transition process.

Fluids researchers are also investigating landing high­-mass payloads on Mars. The large mass of these vehicles, combined with a thinner atmosphere, means that conventional technology such as rigid aeroshells and parachutes aren’t as effective at slowing a craft for landing as they would be on Earth. One way around the problem is to use inflatable devices that significantly increase the drag on the vehicle. The devices are designed using complex simulations of the spacecraft re-entry. Engineers from NASA and Missouri S&T collaborated to implement efficient approaches to characterize the sensitivity and the level of uncertainty in these entry simulations due to variations in physical phenomenon and how they are modeled. The ultimate goal of the work is to validate and improve the physical models that contribute to the design of reliable thermal protection systems for inflatable aerodynamic decelerators.

Speakers at the “Future of Fluids” special session of the 2016 AIAA Aviation Forum discussed some of the key areas of fluid dynamics research and reached the following four conclusions: fluid dynamics will play an increasingly important role in the future of environmentally responsible aviation; turbulence research needs to address more complex situations such as unsteady flows; exa-scale computing power could be used to build data-driven, reduced-order flow models that include system nonlinearity; and a fundamental understanding of fluid dynamics in energy and propulsion is necessary to improve safety and reliability in domestic replacement for Russian rocket engines. ★

A new challenge set for CFD