Peter Eliasson

Investigation of a Half-Model High-Lift Configuration in a Wind Tunnel

An investigation is carried out for a half model high lift configuration inside the European Transonic Wind tunnel ETW. The influence of the wind tunnel walls and model installation is investigated and the numerical results are compared to measured data and free flight CFD results. The investigated model is a three element take-off configuration with full span slat and flap. A CFD solver for unstructured grids is used for the calculations. The computed results in the wind tunnel are in good agreement with uncorrected experimental data with maximum lift predicted at the same angle of attack. Corrected experimental and numerical tunnel data, however, deviate slightly from numerical results in free flight for which about 10% higher drag is predicted. In addition, the free flight maximum lift is predicted at a higher incidence. The lower drag in the in-tunnel results is due to a lower pressure at the leading edges of the slat and main wing close to the fuselage. This is a consequence of the current mounting of the wind tunnel model in the tunnel which causes a redistribution of the velocity field due to cross flow velocity components in the plane of symmetry of the half model. I. Introduction ALCULATING viscous fluid flows over high lift configurations is still a challenge in CFD. The difficulties in simulating these flows come from the complexity of both the geometry and flow physics. In particular, the multiple elements with small gaps give rise to multiple wakes, flow separation, laminar/turbulent transition, shock/boundary layer interaction etc., where many of these phenomena interact with each other. Since the fluid dynamics is dominated by viscous phenomena, only high-fidelity simulations based on the Navier-Stokes equations can provide the required accuracy to obtain realistic CFD solutions. The numerical simulation of the flow field around high lift configurations based on the Reynolds Averaged Navier-Stokes (RANS) equations has made significant progress during the last decade 1 . Until the beginning of the EUROLIFT project in early 2000, most of the European high lift activities had been devoted to two-dimensional computations 2 . The need for an extension to three dimensions as well as a state-of-the-art experimental database stimulated the launch of the EUROLIFT programme that was funded by the EC as part of the 5 th European framework program. A close coupling and harmonization between experimental and numerical activities was attempted in the project. CFD was brought into a more daily use in EUROLIFT and introduced in three dimensions. Mainly through hybrid Navier-Stokes technology, it has become possible to compute viscous flows about take-off and landing configurations within a reasonable time frame and with sufficient reliability. The work carried out in the EUROLIFT project has resulted in several publications, an overview is given in references 3-6 . EUROLIFT II 7 is a European High Lift Programme in the 6 th European framework following the work initialized in EUROLIFT. The numerical and experimental investigations in EUROLIFT left many important questions unanswered being pursued in EUROLIFT II. The investigation described here is concerned with the installation effects inside the cryogenic wind tunnel ETW (European Transonic Wind tunnel) in which experiments at high Reynolds numbers are conducted. In particular, the influence of the wind tunnel walls and model installation is investigated by conducting CFD calculations inside the wind tunnel in comparisons with measurements and with computational results from free flight calculations. Numerous CFD calculations in free flight have been carried out with comparisons to experimental results from ETW in which reasonable agreement is reached in lift between numerical results and experiments. The maximum lift, however, is often predicted too late at a higher angle and, in particular, the drag is almost always over-predicted. This investigation has been conducted to establish if this

Transition Prediction and Impact on a Three-Dimensional High-Lift-Wing Configuration

The evolution of maximum lift coefficient of a transport aircraft as a function of Reynolds number can be linked to modifications of the laminar-turbulent transition process. In the framework of European project EUROLIFT (I), a task was dedicated to the physical understanding and the numerical modeling of the transition process in high-lift configurations. Then, in the follow-on project EUROLIFT II, a major step is the integration of transition prediction tools within Reynolds-averaged Navier-Stokes (RANS) solvers in order to estimate the impact of transition on performance. This paper presents an overview of the different activities dealing with transition in the EUROLIFT II project.

Drag Prediction for the DLR-F6 Wing-Body Configuration Using the Edge Solver

Numerical investigations are reported on the DLR-F6 wing-body configuration with and without fairing. The configurations have been adopted as test cases for the Third AIAA Drag Prediction Workshop. The addition of the fairing is to eliminate the flow separation bubble in the junction between the wing trailing edge and the fuselage. The computations have been carried out using two groups of unstructured grids with different sizes. In addition to the effect of incidences, studies of grid convergence have also been performed. The computational fluid dynamics solver Edge is used for the investigation. The calculations confirm that the flow separation can be removed in the wing-fuselage junction with the fairing. For this configuration, the results obtained with the two groups of grid are very similar. Without fairing, however, one group of grids has pronounced a lower lift and produced a more extended trailing-edge separation. Because no experimental data are available for the flow condition as swerved in Drag Prediction Workshop-3, additional calculations have been carried out with the clean wing-body configuration at the Drag Prediction Workshop-2 Reynolds number to validate the numerical results against available experimental data. Very good agreement is obtained, in particular, for the global forces and moments. The calculation indicates that, as compared with experimental data, the grid which predicts a relatively large separation region provides improved predictions.

Influence of Transition on High-Lift Prediction with the NASA Trap Wing Model

A computational analysis on the influence of the transition for the NASA Trap Wing Model has been carried out, which is an extension of the work presented for the 1 st AIAA High Lift Prediction Workshop. The transition prediction is based on stability analyses with a database method in spanwise sections. Comparisons with experimental data are made to find appropriate N-factors for the e N method leading to the estimated interval 5< N <10. The computed transition locations are used to specify laminar and turbulent regions in the 3D calculations. Including transition improves the results, especially with locations from higher N-factors, and good agreement with experimental data for aerodynamic forces, moments and pressure distributions is obtained.

Transition Prediction and Impact on 3D High-Lift Wing Configuration

The evolution of maximum lift coefficient of a transport aircraft as a function of Reynolds number can be linked to modifications of the laminar-turbulent transition process. In the framework of European project EUROLIFT (I), a task was dedicated to the physical understanding and the numerical modeling of the transition process in high-lift configurations. Then, in the follow-on project EUROLIFT II, a major step is the integration of transition prediction tools within Reynolds-averaged Navier-Stokes (RANS) solvers in order to estimate the impact of transition on performance. This paper presents an overview of the different activities dealing with transition in the EUROLIFT II project.

Computations from the Fourth Drag Prediction Workshop Using the Edge Solver

Results carried out for the 4th AIAA Drag Prediction Workshop with the flow solver Edge are summarized. Simulations have been carried out for wing–body–tail configurations with three horizontal tail incidences and for one tail-off configuration. The computations consist of a grid-refinement study and a downwash study with polar calculations to explore the trimmed condition and delta effects. An investigation of the sensitivity to the grid and turbulence model is carried out from calculations on two sets of unstructured grids with three turbulence models, including the explicit algebraic Reynolds stress model, the k-ω SST model, and the Spalart–Allmaras model. The grid-refinement study, for which the flow is attached, shows rather small differences due to different grids and models. The major difference is obtained from the turbulence models where about 10 drag counts between the highest and lowest drag values are predicted, respectively, by the explicit algebraic Reynolds stress model and the Spalart–Allm...

Improving the Prediction for the NASA High-Lift Trap Wing Model

Results presented at the 1 st AIAA High Lift Prediction Workshop (HiLiftPW-1) using the flow solver Edge are summarized for the trap-wing model with two different flap settings. A comparative study of three different turbulence models is carried out, including the Explicit Algebraic Reynolds Stress Model (EARSM), the k-ω SST model and the Spalart-Allmaras (SA) model. The comparison shows that the overall best agreement with experimental data is obtained with the SA model, which has also predicted the maximum lift at about the correct experimental incidence. The two other models predict a larger flap trailing edge separation and, consequently, resulting in an under-prediction of lift. A grid refinement study has been undertaken, indicating that the unstructured grids are of a high quality. A good prediction of the wing tip flow is obtained with the full viscous operator. A thin-layer approximation changes the tip vortex structure with large deviations from experimental pressure distribution. The inclusion of the flap and slat support systems gives improved predictions of the integrated forces and moments, as well as of pressure distributions. There is a small under-prediction of lift at higher incidences due to an under-prediction of the rear main wing and flap suction peaks causing an earlier lift break down. Transition prediction has also been carried out based on stability analysis in several typical span-wise sections. The output is used to specify laminar regions in the 3D calculations, which has improved the results further in good agreement with experimental data for aerodynamic forces, moments and pressure distributions.

Results from the Second AIAA CFD High-Lift Prediction Workshop Using Edge

The results presented at the Second AIAA High-Lift Prediction Workshop, using the flow-solver Edge, are summarized for the DLR, German Aerospace Center F11 model. A comparative study of the results, using three turbulence models, is carried out, including the Spalart–Allmaras model, an explicit algebraic Reynolds-stress model, and a curvature correction to the explicit algebraic Reynolds-stress model. The comparisons include a grid-convergence study on a simplified model without slat- and flap-track fairings, and polar calculations including the fairings. The grid-convergence study shows relatively small differences due to different grid resolution and turbulence models, but the differences are larger than those obtained in the first workshop for the NASA trap wing. The prediction has fairly large discrepancies from experimental measurements at low Reynolds numbers for which the computations were carried out assuming fully turbulent flow. The explicit algebraic Reynolds-stress model (with or without curva...

Coupling of the Edge CFD Solver with External Solvers

This paper presents a new development in the FOI CFD code Edge which enables using Edge as a part of a system in which a number of modules are interconnected. Each of the modules can for instance be dedicated to specific computations for the simulation of a complex system. The article describes implementation of the communication in the CFD code Edge and the use of communication and synchronization utilities.

Application of Reynolds Stress Models to High-Lift Aerodynamics Applications

A recently proposed explicit algebraic Reynolds stress model (EARSM) based on a nonlinear pressure strain rate model has been implemented in an industrial CFD code for unstructured grids. The new EARSM was then used to compute the flow around typical three element high-lift devices used on transport aircraft both in 2D and 3D. For 2D mean flow, various angles of attack have been investigated. Two different grids have been used, one coarse grid with 35,000 nodes and fine grid with 340,000 nodes. Furthermore, a 3D take-off configuration including fuselage was computed using a computational grid with about three million grid points. For the 2D case and pre-stall angles of attack, the new EARSM makes fair predictions. For higher angles of attack, the new EARSM and the baseline EARSM show a large sensitivity to the transition point location. The original transition setting leads to a premature stall while an alternative transition setting gives predictions that are in good agreement with experiments. For lower angles of attack, there are indications on minor improvements. One angle of attack close to the maximum lift was computed for the 3D case and compared with previous computations. No significant differences were found with the new EARSM compared with the baseline EARSM. Also the convergence rate and computational effort by using the new EARSM are comparable with the baseline EARSM.