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Internal Report

Boundary Layers over Wing Sections

Author Document Type Year Download File size
Prabal Singh Negi Technical report 2017 Download 35065 kB
Id
ISSN 0348-467X
ISRN KTH/MEK/TR--17/15--SE

Abstract

The understanding of developing boundary layers over wings is an important topic from the perspective of industrial applications. An increased understanding would be consequential not only for achieving higher fuel efficiency but also in the design of aircraft control strategies. With these aims in mind, the current work aims to further the understanding of developing boundary layer over wing sections. The study is performed with two particular perspectives in mind - unsteady aerodynamic effects in a pitching airfoil and turbulent boundary layer structure in non-equilibrium boundary layers over a stationary airfoil. The boundary layer evolution in unsteady natural laminar flow airfoils undergoing small-amplitude pitch-oscillations is investigated. For high Reynolds numbers the origins of the non-linear unsteady aerodynamic response of laminar airfoils is explained on the basis of quasi-steady assumptions. Temporal non-linearities in aerodynamic forces are shown to be inherently linked to the non-linearities of static aerodynamic force coefficients and that a simple phase-lag concept can model the observed non-linear unsteady response. On the other hand at lower Reynolds numbers, when there exists an unstable leading-edge laminar separation bubble, the unsteady response is dynamically rich and changes in boundary layer characteristics can be abrupt. The quasi-steady phase-lag concepts are no longer appropriate to explain the unsteady flow physics in such a case. For the case of stationary airfoils, flow statistics for flow around an airfoil at two different Reynolds numbers are compared to assess Reynolds number effects in non-equilibrium flows. Pressure gradient effects are found to be stronger at low Reynolds numbers, leading to higher energy in the larger structures present in the outer part of the turbulent boundary layer.