ISBN 978-3-8439-5820-2

978-3-8439-5820-2, Reihe Ingenieurwissenschaften

Ulrich Deck
The Influence of Atmospheric Turbulence on NLF Airfoil Behavior Investigated by Free-Flight and Wind Tunnel Measurements

273 Seiten, Dissertation Universität Stuttgart (2026), Softcover, A5

Zusammenfassung / Abstract

During the development of natural laminar flow (NLF) airfoils, inflow conditions free of any turbulence are usually assumed, and the resulting airfoils are tested in special laminar wind tunnels. Nevertheless, the atmospheric turbulence in the Earth's boundary layer during active convection is significant. This is especially relevant for general aviation aircraft, in particular sailplanes, wind turbines, and small UAVs.

This study investigates the influence of atmospheric turbulence during active convection on a nominally 2D sailplane airfoil, which is a modern natural laminar flow airfoil with a high percentage of laminar boundary layer flow. The main goal is to investigate and recognize different mechanisms that affect airfoil performance under atmospheric turbulence. While flight measurement data provides a comprehensive overview of airfoil performance under atmospheric turbulence, wind tunnel measurements are used to separate different mechanisms for a more detailed insight.

The main findings of the present study are that performance degradation under high inflow turbulence is significant only at the corners of the laminar bucket. Large-scale angle-of-attack fluctuation is a major influencing factor of this performance reduction if the airfoil pressure distribution is flat without significant gradients. For the investigated airfoil, the pressure distribution is flat at the lower side, which is responsible for the drag increase at the lower corner of the laminar bucket. Additionally, small-scale disturbances lead to higher initial amplitudes of the Tollmien-Schlichting waves, resulting in an upstream shift of the transition location. This is especially relevant if the pressure distribution allows linear growth of the Tollmien-Schlichting waves over a large part of the chord. This is the case for the upper side of the investigated airfoil at high angles of attack, which explains the increase in drag at the upper corner of the laminar bucket.