Thomas Streit

Complementary Numerical and Experimental Data Analysis of the ETW Telfona Pathfinder Wing Transition Tests

Within the European Project Telfona the Pathfinder Model was designed, analyzed numerically, constructed and tested with the aim of obtaining a laminar flow testing capability in the European Transonic Wind Tunnel (ETW). The model was designed for natural laminar flow (NLF) for transonic flow conditions with high Reynolds number. Results of pre-test numerical analysis demonstrated that the Pathfinder wing pressure distribution was adequate for providing calibration test points. The ETW tests provided pressure distribution data while transition positions were determined from images using the Cryogenic Temperature Sensitive Paint Method (cryoTSP). The evaluation of this data with several transition prediction tools was used to establish the transition N-factor values for ETW. In this work, after-test CFD solutions are obtained using numerical Navier-Stokes solutions. In the first part of this work, numerical results are given which verify the requirements of the Pathfinder wing as a calibration model. In the second part, it is shown that for selected flow conditions a good agreement is obtained between stability analysis based on experimental and numerical data. In the third part the correlation of experimental transition locations to critical N-factors is summarized for ETW Test Phases I and II. In the fourth part numerical analysis and experimental data are used complementarily.

Progress Toward Efficient Laminar Flow Analysis and Design

A multi-fidelity system of computer codes for the analysis and design of vehicles having extensive areas of laminar flow is under development at the NASA Langley Research Center. The overall approach consists of the loose coupling of a flow solver, a transition prediction method and a design module using shell scripts, along with interface modules to prepare the input for each method. This approach allows the user to select the flow solver and transition prediction module, as well as run mode for each code, based on the fidelity most compatible with the problem and available resources. The design module can be any method that designs to a specified target pressure distribution. In addition to the interface modules, two new components have been developed: 1) an efficient, empirical transition prediction module (MATTC) that provides n-factor growth distributions without requiring boundary layer information; and 2) an automated target pressure generation code (ATPG) that develops a target pressure distribution that meets a variety of flow and geometry constraints. The ATPG code also includes empirical estimates of several drag components to allow the optimization of the target pressure distribution. The current system has been developed for the design of subsonic and transonic airfoils and wings, but may be extendable to other speed ranges and components. Several analysis and design examples are included to demonstrate the current capabilities of the system.

Implications of Conical Flow for Laminar Wing Design and Analysis

In this work a method based on sectional conical wings is presented which allows the analysis and design of airfoil sections for swept tapered wings with a computational effort which is slightly higher than a 2D computation. Navier-Stokes analysis and re-design examples using the sectional conical wing approximation are given. They show that pressure distribution, boundary layer stability analysis and designed airfoil geometry compare better with the 3D swept tapered wing solution than results obtained using solutions based on infinite swept wing theory. In the second part of the work the transversal flow incompressible solution for a conical wing with circular airfoil section is given. For small cone angles sectional conical solutions are obtained. The crossflow instability significant stagnation line velocity is obtained. It corresponds to an effective leading edge sweep angle which is twice the cone angle. An extension of this equation is obtained and verified for usual transport aircraft 3D wings by parametric studies varying airfoil section, root to tip thickness and leading edge radius.

Transonic High Reynolds Number Transition Experiments in the ETW Cryogenic Wind Tunnel

With the goal of studying Natural Laminar Flow (NLF) wings for future ‘green’ transport aircraft, the aim of the European Research Project TELFONA is to develop and demonstrate the possibility of testing full aircraft models at large Reynolds numbers in the cryogenic Wind Tunnel ETW, with direct measurements of total drag. Two main steps were defined, first the design and test of a ‘calibration’ model, to be followed by a realistic transport aircraft model. This paper is dedicated to the first one, which was especially designed in order to allow a calibration of the Wind Tunnel transition N-factors at large values of the chord Reynolds number typical of testing in ETW. In order to do so, the wing shape was optimized so that TS and CF N-factors would show a monotonous growth over the longest possible distance. Apart from classical aerodynamic forces, two lines of pressure taps, as well as four patches of a two components cryogenic Temperature- Sensitive Paint (cryoTSP), were installed on both sides of each wing. Model surface temperatures were recorded by several CCD cameras as the intensity of light emission from the TSP, which has to be excited by suitable light sources. After the tests, stability analysis was applied in order to ‘calibrate’ the various models currently used by research labs in Europe, involving a wide range of approaches, including simplified database, local linear and non local linear stability approaches. The paper will describe these different phases of the activities, from design, testing and numerical validation, with a focus on the validation and calibration of transition prediction tools. Examples of numerical results obtained by the project partners will be confronted to the experiments.

Design of a Retrofit Winglet for a Transport Aircraft with Assessment of Cruise and Ultimate Structural Loads

In this work retrofit winglets are designed for a transonic aircraft. The used geometry is a generic twin engine aircraft. In a first step, winglets are designed using a lifting line method along with RANS solutions. L/D is optimized by taking into account a certain wing root bending moment reserve for the reference wing. The final analysis includes wing deformation studies by means of fluid-structure coupling. Therefore a finite element model has been developed with respect to standard loadings of certification authority. Using the fluid-structure coupling process RANS solutions with deformed wing shapes are obtained for cruise conditions in order to determine the influence of deformation on performance and for a 2.5g load case in order to evaluate ultimate structural loading. Comparing the results of the rigid wing with the deformed wing the wing root bending moment and the bending moment of the device is clearly reduced for the deformed wing. Thereby the advantage to which an implementation of the described method for future design processes would lead becomes apparent.

High Reynolds Number Transition Experiments in ETW (TELFONA project)

A wind–tunnel experiment on laminar-turbulent transition has been performed in ETW (the European Transonic Wind Tunnel in Koln) at high Reynolds number and cryogenic conditions. The studied geometry is a sting mounted full model in swept–wing configuration. The transition location was determined by means of Temperature Sensitive Paint (CryoTSP). The experimental observations were further analysed using different transition prediction tools, based on linear stability theory.

Wing Design Based on a Tapered Wing Natural Laminar Flow Airfoil Catalogue

In this work airfoil and wing design results using a sectional conical wing method for analysis and design are presented. This method allows the design of swept tapered wing sections. It is useful for transonic Natural Laminar Flow (NLF) design, where sweep and tapering of the wing has to be taken into account. In the first part of this work a transonic tapered wing NLF airfoil catalogue was generated. Different airfoils were designed by varying the design Mach and Reynolds numbers, lift coefficient, leading-edge sweep and airfoil thickness. In the second part a NLF wing was designed for a short range aircraft wing-body configuration. Using the airfoils from the catalogue a wing was obtained with an acceptably large transition region and small shocks on the upper side. Due to the fuselage influence the inner lower wing required further design. For this purpose a 3D inverse design method was used.