Turbulent boundary layers over surfaces with streamwise-preferential permeability

Experimental investigation into the drag and flow mechanics

Master thesis (2021)

Authors

F.H. Hartog Aerospace Engineering

Contributors

M. van Nesselrooij (supervisor 1)
F.F.J. Schrijer Aerodynamics - Aerospace Engineering (supervisor 1)
B.W. van Oudheusden Aerodynamics - Aerospace Engineering (supervisor 2)
K. Masania Aerospace Manufacturing Technologies - Aerospace Engineering (coach)
L.L.M. Veldhuis Flow Physics and Technology (coach)
Faculty
Aerospace Engineering
Copyright: © 2021 Friso Hartog
Particle Image Velocimetry (PIV) Drag reduction Turbulent boundary layer Passive flow control Anisotropic permeability Direct force measurement
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Published Date

09-09-2021

Language

English

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Faculty

Aerospace Engineering

Abstract

Turbulent drag is the largest source of fuel consumption in aviation and forms a significant contribution to climate warming. Recent numerical studies have suggested the ability of surfaces with streamwise-preferential permeability to reduce turbulent friction, and a theoretical framework behind the working mechanism has been proposed. This work is the first experimental study on this concept in air and in which the permeability requirements for predicted drag reductions are met. Three different physical realisations of streamwise-preferential permeable substrates were manufactured: two seal furs, a substrate with unidirectional fibres, and a 3D-printed structure. They were investigated through wind tunnel experiments using direct force measurements and planar (2D-2C) particle image velocimetry (PIV). Results show an increase in drag for all test specimens and suggest that drag sources other than surface friction are non-negligible. One-point turbulent statistics show an overall increase in turbulence, mainly wall-normal velocity fluctuations enabled by the wall-normal permeability, which results in higher Reynolds stresses. No significant flow modulation in terms of turbulent events or change in coherent structures is observed. The measurements and results are a first of its kind and none of the theoretical framework predictions on drag reduction or flow modulation are found. Based on analytical derivations relating characteristic length scales to permeability, the validity of the framework and its assumptions in experimental settings is questioned. It is thought that there is an inherent mismatch between the pore size assumption and the virtual origin approach taken in the framework. All things considered, it is deemed unlikely that turbulent drag reduction by means of streamwise-preferential permeable surfaces is feasible in experimental settings. Nevertheless, given the relatively small increase in drag for the 3D-printed specimen, streamwise-preferential permeable surfaces might be interesting for other (flow control) purposes involving turbulent boundary layers.