In this case, the dark red area along the front of the wing represents higher-speed airflow as the TTBW's wings, which are thinner than those of today's commercial airliners, pierce the air. The tan-colored area shows the relatively smooth wake generated by the aerodynamic wings.
A TTBW aircraft produces less drag due to its longer, thinner wings supported by aerodynamic trusses. In flight, it could consume up to 10% less jet fuel than a standard airliner.
The Advanced Supercomputing Division of NASA's Ames Research Center in California created this image as part of an effort by the Transformational Tools and Technologies project to develop computational tools for TTBW research.
In January, NASA selected a TTBW concept from The Boeing Company for its Sustainable Flight Demonstrator project.
Predicting Buffet Onset on the Transonic Truss-Braced Wing
NASA and Boeing are collaborating to develop a Transonic Truss-Braced Wing (TTBW) aircraft concept design, which contains advancements in technology with the potential to improve fuel efficiency for commercial aircraft.
The unconventional configuration of the TTBW-which includes a high aspect ratio wing, in addition to wing and jury struts-leads to complex flow phenomena such as transonic buffet, separated flow, and a turbulent wake.
Current industry best practices tend to employ Reynolds-Averaged Navier-Stokes (RANS)-based computational fluid dynamics (CFD) analysis for buffet onset prediction, but accurate prediction of the onset of buffet and the development of the separated flow may require more accurate scale-resolving CFD simulations.
To provide more insight into the best practices for using scale resolving simulations to predict transonic buffet onset, among other challenges, NASA's Advanced Air Transport Technology Project launched a collaborative multi-center effort to develop new methods for simulating the TTBW to better predict its performance and that of similar truss-braced wing configurations.
Project Details
Visualization of the concept Transonic Truss-Braced Wing aircraft's free-air configuration showing time-averaged surface pressure coefficient contour
Researchers in the NASA Advanced Supercomputing (NAS) Division's Computational Aerosciences Branch are simulating the TTBW's Mach 0.8 cruise configuration to validate and develop their scale-resolving CFD models.
A wind tunnel experiment was conducted on a half-span model of this configuration in the NASA Ames Unitary Plan 11- by 11-foot Transonic Wind Tunnel in January 2022, and large angle-of-attack sweeps at different Mach numbers were run. This data is being used to validate the simulations, with a goal to be able to accurately reproduce experimental results using CFD.
The NAS Division's Launch, Ascent, and Vehicle Aerodynamics (LAVA) team initially chose Hybrid RANS/Large Eddy Simulations (HRLES) for the scale-resolving simulation approach. As the name suggests, this hybrid method models turbulence inside the boundary layer to solve the RANS equations, while the turbulence is resolved outside the boundary layer with LES. The dominant transonic buffet phenomena can largely be captured by steady-state RANS or unsteady RANS.
However, neither is suitable for investigating the phenomena in the turbulent wake (including deep stall and high-lift configurations) due to the excessive dissipation of turbulent motion, and the team has demonstrated that RANS alone is an unreliable tool when simulating the maximum value of lift (CLmax) and the onset of stall. Scale-resolving simulation approaches like HRLES are better able to resolve the turbulent content and enable more accurate predictions.
Results and Impact
The LAVA team showed that utilizing RANS-type grids for HRLES simulations would likely produce a result that is inferior to that of a RANS simulation, so they purpose-built HRLES grids with low aspect ratio grid cells appropriate for LES outside the boundary layer. The aerodynamic loads (lift, drag, and pitching moment) and surface pressure data from the HRLES simulations were then compared with steady and unsteady RANS simulations and the experimental wind tunnel data.
They discovered that at higher angles-of-attack, the RANS simulations tended to overpredict the location of the shock in the chordwise direction in both the midboard and outboard regions of the wing when compared to the experiment, while the HRLES simulations showed much better agreement with the experiment in the predicted shock location. These initial simulations provided important insight into the behavior of transonic buffet-in particular, the spanwise (root-to-tip) development of the unsteady shock motion-and indicated where the computational grids need to be refined further.
Why HPC Matters
The high-performance computing resources at the NAS facility have made it possible to run the HRLES simulations within a short turnaround time. Scale-resolving simulations are computationally expensive due to the small timestep size and grid cell sizes needed to resolve the unsteady turbulent flow, and HRLES approaches have a lot of tuning parameters. Being able to run a parameter study to test their sensitivity using the efficient throughput provided by NAS systems enabled quick decision-making and accelerated the development of simulation best practices.
What's Next
The LAVA team will move to more complex TTBW configurations, including studying deep stall, and high lift configurations where devices such as slats and flaps on the aircraft are deployed. Due to the T-tail configuration of the empennage proposed for the TTBW, it may be prone to deep stall-where the turbulent wake from the stalled main wing and strut blankets the tailplane and renders the elevators ineffective and prevents the aircraft from recovering. Accurately capturing the turbulent wake coming from the main wing and strut of the TTBW and maintaining the turbulent fluctuations until reaching the tail will be paramount in accurately predicting this behavior.
Related Links
Transformational Tools and Technologies project
Aerospace News at SpaceMart.com
| Subscribe Free To Our Daily Newsletters |
| Subscribe Free To Our Daily Newsletters |
