UHURA - Unsteady High Lift – Unsteady RANS Validation

This work describes the cooperative/competitive design process that led to the definition of the Krueger flap to be used in the numerical and experimental tests of the European project UHURA. The project requirements are particularly challenging because it is necessary to develop a device with good aerodynamic high-lift characteristics, but it is necessary to consider many constraints of structural and kinematic nature. Indeed, the kinematics for its deployment is quite complex and imposes hard constraints on the Krueger shape, and the structural charctacteristics must allow it to withstand considerable structural stresses in the deployment phase, which is studied in the wind tunnel.

The industrial valid design of a folding bull nose Krüger leading edge device will be presented. It serves as a geometrical basis for the UHURA project, which investigates the retraction and deployment process with numerical and experimental means. A set of realistic aerodynamic and kinematic requirements was applied, such as deflection angle range and deployed target position to enable insect shielding. In order to achieve an industrial relevant design solution of a Krüger device, realistic space allocation constraints and structural keep-out-zones were defined within the fixed leading edge. Numerical shape optimization of the Krüger device was performed to balance size, shape and deployed position with optimal aerodynamic performance, respecting the given requirements and constraints. Based on that, layout and sizing of the kinematic structural and connecting elements was carried out. Industrial viability was ensured by applying aerospace design rules, as well as relevant material selection, deformation and tolerance analysis.

The European H2020 project UHURA is focusing on the unsteady flow behaviour around high-lift systems and will first time deliver a deeper understanding of critical flow features at new types of high-lift devices of transport aircraft during their deployment and retraction together with a validated numerical procedure for its simulation. UHURA performed detailed experimental measurements in several wind tunnels to obtain a unique data set for validation purposes of Computational Fluid Dynamics (CFD) software, including detailed flow measurements by Particle Image Velocimetry (PIV) and other optical measurement technologies. Advanced CFD methods promising significant improvements in the design lead time are validated against this database to obtain efficient and reliable prediction methods for design.

This paper presents the initial assessment of one of the validation experiments of the Unsteady High-lift Aerodynamics-Unsteady RANS validation project (UHURA). The aim of the study is to investigate the unknown aerodynamic characteristics of a slotted Krueger flap during deployment and retraction phases. The DLR-F15 airfoil model was equipped with a full span actuated Krueger device. A test campaign was conducted at the ONERA L1 wind tunnel. The test included measurements of steady and unsteady pressures along with phase locked Particle Image Velocimetry to achieve high quality validation data for the UHURA project. The results highlighted the transient behaviour of the flow during the deployment of the Krueger.

Laminar wing technology is seen as the major single source for drag reduction on the airframe of a transport aircraft and will be a key technology to achieve the targets for emission reduction. In recent EC funded projects, the Krueger flap leading edge device was found to be the most promising concept of a dual-functional leading-edge device for laminar wings. While the these studies focused on the general performance and integration, the behavior of the system during its deployment or retraction proves to be a major issue due to the very different kind of motion compared to conventional leading edge high-lift devices. The risks of this concept are identified in the areas of load estimates, handling qualities and asymmetric failure cases. During the deployment, the Krueger device is deflected from the lower side against the flow, passing critical stations when perpendicular to the flow, forming large scale separated flow on the lower side when moved around the leading edge (Figure 1). Current conservative estimations require the installation of many independently driven Krueger flap elements to prevent examination of critical situations along the whole wing span. The multiplication of drive stations leads to increasing complexity, weight and maintenance costs. On the other hand, despite the great progress in numerical simulation methods in the past years, there have up to now been no investigations on the validity of the current methods for predicting the behavior regarding these critical topics. The aerodynamics during movement of high-lift devices have not yet been addressed in detail. The project UHURA is focusing on the unsteady flow behavior around high-lift systems and will first time deliver a deeper understanding of critical flow features at new types of high-lift devices of transport aircraft during their deployment and retraction together with a validated numerical procedure for its simulation. UHURA performs detailed experimental measurements in several wind tunnels to obtain a unique data set for validation purposes of Computational Fluid Dynamics (CFD) software, including detailed flow measurements by Particle Image Velocimetry (PIV) and other optical measurement technologies. Advanced CFD methods promising significant improvements in the design lead time are validated against this database to obtain efficient and reliable prediction methods for design.

The present work addresses improvements for mesh generation of unstructured and multi-block structured grids for rigid body motion applications. The activities are concerned with local re-connection and local refinement approaches in order to obtain high-quality meshes for the simulation of the full deployment of a Krueger leading-edge device. Local reconnection offers a suitable way to implement a re-meshing strategy based on the Chimera approach that eliminates the need for non-conservative interpolation. The strategy is based by replacing overlapping mesh regions by a conformal triangulation. An initial multi-block structured grid and a Chimera grid are used for the development and assessment of the local reconnection approach. The full sequence of a Krueger flap deflection has been obtained on meshes that differ significantly in mesh resolution (1:4). The mesh quality of the interfacing meshes has been assessed based on established mesh quality criterions. It shows that the local reconnection retains the anisotropy of the baseline mesh over the full deflection range of the Krueger flap. Due to the triangulation, a slightly higher size variation is observed than in the baseline mesh. The method has been shown to be robustly implemented. The block-structured local grid refinement method aims for a uniform mesh spacing by first refining the block topology and subsequently refining the grid per block. The smoothness of the locally refined grid is maintained by an appropriate smooth interpolation of the original grid-point distribution. The refined grid attached to a solid surface is mapped onto the original geometry definition in order to preserve the correct aerodynamic shape. The local grid refinement capability is combined with the Chimera approach to perform unsteady simulations of Krueger device deployment. Here, local grid refinement is employed to arrive at similar grid resolution on both sides of the Chimera interface in order to improve the interpolation accuracy. The validity of the method is demonstrated by a refined Chimera grid which has been generated around the DLR F15 airfoil with a Krueger device. The grid around the main wing has been locally refined in order to obtain a uniform grid point distribution in the region downstream of the Krueger device where turbulence must be resolved.

The feasibility of laminar flow control technology for future wing is bound to the development of a leading edge high-lift system that complies with the requirements on smooth surfaces to enable maintaining the laminar boundary layer flow, such as a Krueger flap. Although in principle the aerodynamic performance of a Krueger flap is known, the unsteady behaviour of the flow during deployment and retraction is completely unknown. This is as even more important as during deployment the Krueger flap is exposed to highly unfavourable positions perpendicular to the flow. To mitigate the risk of unfavourable aircraft behaviour, it is therefore expected that a Krueger flap has to be deflected significantly fast and may trigger unsteady aerodynamic effects. Within the European H2020 project UHURA, currently a wind tunnel test is conducted incorporating the vented foldable bull nose Krueger. A wind tunnel model based on the DLR-F15 airfoil has been designed and manufactured that features a part span and a full span Krueger device, which can be actuated at high deflection rates up to 180°/s. First wind tunnel tests will be conducted at the ONERA L1 wind tunnel in Lille in May 2020. The tests include the measurements of internal forces, steady and unsteady pressures, as well as phase locked PIV to achieve high quality validation data for comparison with numerical methods.