Numerical and Experimental Stability Analysis Predicting Natural Laminar Flow Extension on a Realistic Swept Wing

Abstract

It is well known that the stability theory, coupled with eN method, offers a rational approach to predict transition from laminar to turbulent flow condition in several aerodynamic applications. It is also known that stability analyses are affected by the accuracy of the mean flow that strongly depends from an adequate resolution in the boundary layer region. Unfortunately, Navier-Stokes analyses for stability investigations are often not affordable since the large number of points normal to the wall, necessary in the viscous layer to have desiderata boundary layer accuracy, makes RANS computations extremely time consuming. Furthermore, the classical definition of the boundary layer edge (the height where the boundary layer velocity reaches 99% of the no-viscous velocity) is not a straightforward task. For these reasons, in the framework of EU-funded Research Project RECEPT, a three-step approach has been applied to perform stability analyses. The first step consists of Euler computations to evaluate the mean flow. In the second step, a 3-D boundary-layer code, starting from the no-viscous flow field, allows to compute the boundary layer data necessary to perform the linear stability analysis and, finally, a 2.5D linear stability code is used to predict transition location. The reference geometry used to test this approach is the laminar wing developed by CIRA and Piaggio Aero Industries S.p.A. (PAI) in the framework of an Italian national program called VITAS (Vettore Innovativo per il Trasporto AeroSostenibile). This wing has been selected because an extended set of experimental data is available. Among the available experimental results, four different conditions have been selected and numerical analyses were performed. The selection has been driven preferring the data that presented both the best IR (Infra-Red) camera results and the pressure distributions. Euler computations have been performed using the commercial CFD (Computational Fluid Dynamic) code CFD++ and the boundary layer computations have been performed using the ONERA (Office National d'Études et de Recherches Aérospatiales) 3C3D laminar/turbulent boundary layer code. Stability computations have been performed using the compressible code NOLLI (NOn Local Linear Instability code) developed in house by CIRA and based on the application of the multiple scale technique and ray tracing theory. For stability analyses the fixed span- wise wavenumber strategy has been used.