Lattice Boltzmann Method Contribution to the Second High-Lift Prediction Workshop

This paper presents the results of the computational-fluid-dynamics code XFlow for the test case proposed in the 2nd High-Lift Prediction Workshop. The target of this international workshop is to assess the accuracy of numerical predictions provided by a wide selection of computational-fluid-dynamics codes and applied to the aeronautical industry. It deals with a full aircraft in high-lift configuration with unstowed flap and slat, including slat tracks, flap track fairings, and slat pressure tube bundles. XFlow is a particle-based kinetic solver, based on the lattice Boltzmann method. It is well suited for such low-speed flows because the standard lattice Boltzmann method is limited to low Mach number. It employs large-eddy simulation turbulence modeling with the wall-adapting local eddy model, coupled with a generalized law of the wall to model the boundary-layer prediction. Traditional computational-fluid-dynamics software requires a time-consuming meshing process that is prone to errors, which significantly affects the mesh quality and thus the outcome of a simulation. Furthermore, the complex geometries such as elements in small gaps add an extra complication to generate the mesh. In contrast, the method employed by XFlow avoids the traditional meshing process using an octree structure, with spatial refinement related by a factor of 2 based on the input geometry. The discretization stage is strongly accelerated, thus reducing engineering costs and complex geometry computations are affordable in a straight forward way. The research presented in this paper starts with a grid validation study (case 1) with the DLR F11 geometry in configuration 2. Because the lattice Boltzmann method approach used by XFlow simplifies the integration of geometrical details, the full polar characteristic is directly investigated with configuration 5 (case 3) that includes the actual model configuration on the aerodynamic performances. The configuration 5 geometry includes slat tracks and flap track fairings, as well as slat pressure tube bundles. This is the highest-fidelity geometry in comparison with wind-tunnel testing. This study is conducted for both low-and high-Reynolds-number conditions, 1.35 and 15.1 million, respectively. In addition, the study focuses on the poststall region and the influence of geometrical details on turbulence and flow patterns. The interest for the poststall region is justified by the lack of consistency in the stall prediction found during the 2nd AIAA High-Lift Prediction Workshop by all the participants.