Numerische Simulation des aerodynamischen und akustischen Feldes einer Düse-Flügel-Klappen Konfiguration

In this work, a far-field solver based on the linearized Euler equations and a far-field solver based on the acoustic analogy of Ffowcs Williams & Hawkings for acoustic problems with convection were implemented in the open source package Overture, and large-eddy simulations of the aerodynamic and acoustic field of an isolated airfoil and various nozzle-wing flap configurations were performed. To validate the methods and verify their implementation, special test cases were performed to evaluate CFD/CAA program code. In addition, compressible flow around a NACA0012 aerofoil with free stream Mach number of 0.4 and Reynolds number based on chord length of 408000 and a compressible jet with M = 0.9 and Reynolds number based on nozzle diameter of 3600 were simulated. In all cases, the numerical calculations showed good agreement with the analytical solutions and the results from measurements and direct numerical simulations from the literature, respectively.In addition, the jet simulation was used to find a suitable decomposition of the Lighthill stress tensor and to test whether its directional weighting allows conclusions to be drawn about the directionality of individual components of the Lighthill source based on instantaneous data. The simulations showed that the proposed evaluation of instantaneous Lighthill sources and their linear components and nonlinear components allow conclusions to be drawn about the directivity of the emitted sound field. The results confirm that the non-linear component has no preferred direction with respect to sound radiation, while the linear component has a clear preferred direction along the jet axis and only a very small acoustic radiation orthogonal to it. Furthermore, an increased value for linear components was found to coincide with regions of coherent vortex structures and large gradients in mean velocity, and the corresponding acoustic spectrum for angles of < 30°, measured from the jet axis, is characterized by a peak at the Strouhal number of 0.2.
In the second part of this work, nozzle-wing flap configurations with different flap angles and bypass velocities were simulated. It was found that the bumps in the mid-four-digit frequency range of the pressure spectrum that appeared in experiments could be due to feedback between the flap and the lower trailing edge of the wing. In the combination with airfoil and nozzle, increased values of the Lighthill source term occur on the wing and flap lower edge on average, which were not observed in the case of the isolated airfoil. In addition, strong contamination with sound of frequency around 3.6 kHz was observed in the region below the nozzle. From the analysis of the individual Lighthill source components in terms of their preferred acoustic radiation, it is suspected that directional sound from the region of the potential core is reflected from the underside of the wing and flap, whereby the velocity difference between the primary and secondary jets plays a non-negligible role in the refraction and scattering of the sound originating from the primary jet.