Experiments in Fluids 23 (1997) 427—437 Springer-Verlag 1997
Measurement of surface velocities and shears at a wavy air—water interface
using particle image velocimetry
William L. Peirson
Measurements have been accomplished within the
aqueous surface viscous sublayer of wind-forced microscale
breaking waves using particle image velocimetry. These
measurements have been used to derive surface velocities and
tangential stresses. Assessment is made of the performance
of the experimental equipment and techniques developed to
accomplish these measurements. Comparison is made with
previous studies of viscous sublayer behaviour at the air—water
interface and at smooth solid surfaces.
Transfer of both momentum and constituents between gases
and liquids occurs at their interface. Consequently, under-
standing the detailed ﬂuid mechanics of such interfaces is
critical for a wide range of disciplines. In all open water bodies
(tanks, lakes, estuaries and oceans), wind action is a primary
source of energy and momentum input.
Studies of air—water interfaces are complicated by the
presence of surface wind waves. These are predominantly
a reﬂection of local wind conditions, fetch and the presence of
remotely generated swell. Under low wind conditions, open
water surfaces assume a glassy, unbroken appearance similar
to that shown in Fig. 1 (upper panel). Whilst wave breaking is
popularly recognised by the collapse of shoaling waves at the
shore and so-called ‘‘white-capping’’ in deeper water, wave
breaking on windswept waters occurs much more widely in the
form of microscale breaking waves (Fig. 1 (lower panel)).
Understanding the behaviour of these microscale breakers is
fundamental to an overall understanding of air—water interfa-
ces. In particular their behaviour has important implications
in determining wind stress from remotely sensed data,
William L. Peirson
Water Research Laboratory, University of New South Wales, Manly
Vale NSW 2093 Australia
Current address: Department of Meteorology, University of Reading,
P.O. Box 243, Reading RG6 6BB, U.K.
The author is pleased to acknowledge the support of the Australian
Research Council in funding this work. He would also like to
acknowledge the support and encouragement of his supervisor, Prof
Michael Banner of the School of Mathematics, University of New
South Wales and his other collegues at the Water Research Laboratory.
momentum and gas transfer at the interface, and, entrainment
of ﬂoating pollutants and organisms.
The instrumentation and experiments reported herein were
designed to resolve the large uncertainties regarding the
partitioning of the wind stress over microscale breaking waves.
Twenty years ago, a laboratory study of the air ﬂow over these
wavelets was undertaken by Banner and Melville (1976). They
reported that air ﬂow separation occurred in the presence of
wave breaking and consequently, that the applied form drag
(pressure differences between the windward and leeward faces
of the waves) was expected to be signiﬁcant. About the same
time, laboratory studies by Okuda et al. (1977) examined the
aqueous viscous sublayer structure using hydrogen bubble
lines as ﬂow tracers. They concluded that for strongly
wind-forced wavelets, the wind stress was almost entirely
supported by large skin friction or tangential surface stresses
on the crests and windward faces of the waves. This implied
that the form drag was virtually negligible. Banner (1990)
attempted to resolve these conﬂicting conclusions by measur-
ing surface pressure distributions over wind-driven, mechan-
ically triggered waves. The controversy deepened when Banner
(1990) reported that the form drag was dominant, supporting
over 70% of the total wind stress.
To resolve the inconsistency of these two sets of measure-
ments, a technique was sought which was capable of measuring
the tangential stress at the surface of strongly forced microscale
breaking waves. In particular, recent research based on particle
image velocimetry (PIV) has provided new insights into the
behaviour of ﬂuids and it was believed its application would
overcome some of the perceived weaknesses associated with
the use of hydrogen bubbles.
The purpose of this article is to document the experimental
technique in detail with particular emphasis on the novel
aspects of the instrumentation and an appraisal of its
performance. A brief description of the preliminary ﬁndings of
these studies has been reported in Banner and Peirson (1995).
A complementary article (Banner and Peirson 1997) docu-
ments the major ﬁndings of this investigation for a range
of wind speed and fetch conditions and its implications for
estimating the surface tangential stresses of wind-driven seas.
Length and time scales
To accurately determine the tangential stress exerted on wavy
boundaries, it is essential that measurements be made within
the linear sublayer (Fernholz 1978). Before reviewing other
studies that have attempted to examine this problem, it is