Numerische Simulation von Wirbelaufrollvorgängen an Tragflügeln

• Numerical simulation of vortex roll-up processes at wings

Braun, Sebastian; Stumpf, Eike (Thesis advisor); Schröder, Wolfgang (Thesis advisor)

Aachen (2016, 2017)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2016

Abstract

The vortex roll-up process at a wing tip results from the pressure differences between the upper and the lower wing surface. Therefore, this process is a direct consequence of the generated lift. Due to the pressure differences, fluid flows from the bottom side of the wing around the wing tip to the upper side. As the flow cannot follow the contour of the wing tip it separates, which ultimately results in the roll-up of a wake vortex. The focus of this thesis is the detailed simulation of such highly three dimensional vortex roll-up processes by using the Reynolds-averaged Navier-Stokes equations. As the numerical dissipation substantially influences the results of such simulations, several studies are carried out in order to investigate the effect of the mesh resolution as well as the accuracy of the used methods. Based on these results, different turbulence models are validated for the use case. At the beginning, the Spalart-Allmaras model and the Menter-SST model are applied. Both turbulence models represent the state of the art for industrial applications. Furthermore, investigations are carried out to show how the accuracy of both models can be improved by using rotation correction methods. The applicability of an algebraic Reynolds stress model for the simulation of the vortex roll-up process is investigated as well. The main aspect of this thesis is the applicability of a differential Reynolds stress model for vortex simulations. In particular, the capability of the used SSG/LRR-w-model to capture the anisotropy of the Reynolds stresses inside a vortex more precisely and the exact solution of the turbulence production term improve the results of the simulations in comparison to eddy viscosity models. Nevertheless, deviations to experimental results taken from literature remain also for the Reynolds stress model. Therefore, complementary experimental investigations are carried out by using high resolution PIV measurement techniques. These measurements provide more detailed information about the velocity field of the vortex during the roll-up process as well as qualitative data about the Reynolds stresses within the vortex. At a wing with a rounded wing tip, the experimental results reveal a large-scale turbulent motion of the vortex core that increases the Reynolds stresses in the inner part of the vortex. In the immediate vicinity of a squared wing tip with sharp edges, a premature vortex breakdown of the secondary vortex is observed. Both effects cannot be simulated in detail by the used turbulence models but could be a starting point for an enhancement of these models.