Success Stories

Highly accurate numerical simulation tools are indispensable for the design of future climate-friendly commercial aircraft. During the takeoff and landing phases of such aircraft, flaps are extended at the leading and trailing edge of the wing to increase the achievable lift of the airfoil in low-speed flight (high-lift configuration). This results in complex flow phenomena with local separations and interactions with the engine, which are difficult to capture using classical numerical simulation approaches (turbulence models). For this reason, simulations are increasingly being used that switch to a complex turbulence-resolving large-eddy simulation (hybrid RANS/LES), at least in areas of particularly complex flow.

With the help of CARO, hybrid RANS/LES can be used for the first time for complete 3D aircraft configurations in order to evaluate the capabilities of this approach on industry-relevant flow cases and to derive possible further developments. One such investigation took place at the international 4th AIAA CFD High Lift Prediction Workshop, where the NASA Common Research Model shown in the figure was simulated in high lift configuration (CRM-HL) up to high angles of attack beyond maximum lift. It was shown that the high-precision resolution of turbulent vortex structures, which arise for example at the wing root, at the engine and at the trailing edge of the wing (see figure), enables a significantly improved prediction of important aerodynamic parameters such as lift, drag and pitching moment.

Institute of Aerodynamics and Flow Technology (Braunschweig/Göttingen), 2023

A key component in reducing an aircraft’s fuel consumption and thus CO2 emissions is minimizing its drag. Frictional drag, which is caused by characteristic physical effects in a very thin layer in the immediate vicinity of the aircraft surface, contributes about 50% to the flight resistance. In this boundary layer there are different types of flow, laminar flow and turbulent flow. The laminar flow is very calm and low in drag, while the turbulent flow is very turbulent, highly swirled and generates high drag. Nowadays, almost all commercial aircraft have exclusively turbulent boundary layers. To significantly reduce drag, future commercial aircraft should be designed with laminar flow components that are as extensive as possible. To accomplish this, one must find out at which flow conditions and states the initially laminar flow transitions to turbulent and in which regions of the aircraft surface this laminar-turbulent transition occurs. In addition, it is necessary to know in which way the transition occurs, because there are also different types of the transition.

In order to make such predictions, physical models have been derived and mathematically formulated in a suitable way to produce simulation results for the fundamental flow equations, the Reynolds-averaged Navier-Stokes (RANS) equations, which also include the information about the transition, within the framework of large, HPC-based calculations. In the result image, the transition line on the surface of the NASA Common Research Model with Natural Laminar Flow (CRM-NLF) is shown at the transition of the laminar frictional stresses (blue) to the turbulent frictional stresses (orange) compared to the measured transition lines (red, green). The dashed white lines show the exceedingly large increase in frictional stresses at the laminar-turbulent transition in four wing sections.

The calculations were performed on the HPC cluster CARO at DLR in Göttingen.

Institute of Aerodynamics and Flow Technology (Braunschweig/Göttingen), 2023