A critical assessment of turbulent flow over textured superhydrophobic surfaces
Author(s)
Jelly, Thomas
Type
Thesis
Abstract
Over the past century, a sustained effort has been expended on the research and development
of surfaces that reduce the amount of drag experienced by a fluid as it passes by, motivated by
both environmental and economic savings.
Superhydrophobic surfaces have recently emerged as an attractive means to reduce the levels
of skin-friction drag under both laminar and turbulent flow conditions. A superhydrophobic
state is attained naturally or synthetically through a combination of surface topology and surface
chemistry and can, in some cases, support a free-stress gas-liquid interface. In the presence of
bulk fluid motion, the interfaces permit a finite slip velocity which has been credited to the
reduction of the average wall shear stress. The fundamental drag reduction mechanism, however,
remains unclear.
In order to accurately resolve the full spectrum of turbulent scales, direct numerical simulations
of fully turbulent channel flow over superhydrophobic textures at a friction Reynolds
number of Reτ ≈ 180 were conducted. The instantaneous flow fields were subject to triple decomposition
which permits statistical quantities to be accumulated in a phase-averaged form. From
these phase-averaged statistics the mean, periodic and stochastic fluid motions can be considered
independently. Following a detailed statistical analysis, the contributions of the mean, periodic
and stochastic fluid motions towards the local levels of wall shear stress were determined by the
derivation and evaluation of an appropriate skin-friction identity.
In addition, a new modification to superhydrophobic surfaces is investigated by means of
meandering the surface topology in the streamwise direction. Relative to a streamwise-aligned
topology, it was anticipated that superior drag reduction would be achieved due to the addition
of an oscillatory spanwise motion to the mean flow.
of surfaces that reduce the amount of drag experienced by a fluid as it passes by, motivated by
both environmental and economic savings.
Superhydrophobic surfaces have recently emerged as an attractive means to reduce the levels
of skin-friction drag under both laminar and turbulent flow conditions. A superhydrophobic
state is attained naturally or synthetically through a combination of surface topology and surface
chemistry and can, in some cases, support a free-stress gas-liquid interface. In the presence of
bulk fluid motion, the interfaces permit a finite slip velocity which has been credited to the
reduction of the average wall shear stress. The fundamental drag reduction mechanism, however,
remains unclear.
In order to accurately resolve the full spectrum of turbulent scales, direct numerical simulations
of fully turbulent channel flow over superhydrophobic textures at a friction Reynolds
number of Reτ ≈ 180 were conducted. The instantaneous flow fields were subject to triple decomposition
which permits statistical quantities to be accumulated in a phase-averaged form. From
these phase-averaged statistics the mean, periodic and stochastic fluid motions can be considered
independently. Following a detailed statistical analysis, the contributions of the mean, periodic
and stochastic fluid motions towards the local levels of wall shear stress were determined by the
derivation and evaluation of an appropriate skin-friction identity.
In addition, a new modification to superhydrophobic surfaces is investigated by means of
meandering the surface topology in the streamwise direction. Relative to a streamwise-aligned
topology, it was anticipated that superior drag reduction would be achieved due to the addition
of an oscillatory spanwise motion to the mean flow.
Version
Open Access
Date Issued
2013-07
Date Awarded
2014-03
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Zaki, Tamer
Sponsor
Engineering and Physical Sciences Research Council
Publisher Department
Mechanical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)