Velocity, vorticity and turbulence measurements in the surf zone.
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Date
1999
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Abstract
The coastal regions of the world play a significant role with regards to social and economic
development, and with the ever increasing human activities along the coast, it has become
necessary to understand and protect this vast resource. This requires a thorough
understanding of the hydrodynamic processes that occur in the surf zone as a result of wave
breaking. Laboratory investigations fonn an essential part of unravelling any physical
process. It is the aim of this thesis, therefore, to investigate via laboratory measurements,
the hydrodynamics of plunging and spilling waves in the surf-zone with the view to obtaining
greater physical insight and also to obtain real data for model validation purposes. To achieve
the above goal it was also necessary to develop new measuring techniques that were suitable
for surf zone application. since conventional water level and fluid velocity measuring devices
perfonn poorly in the presence of wave breaker related aeration, a common feature in the
surf zone.
The experimental investigation of processes occurring in the surf zone was carried out using
a plane beach in a 20 m long wave flume. Both spilling and plunging waves are considered.
The measurement of the spatial and temporal fluid velocities, water level and aeration area
was accomplished using digital video imaging and analysis techniques in conjunction with
particle image velocimetry.
The diagnostic equipment. employed for the measurement of water levels, consists of a
monochrome video camera connected to a video cassette recorder (VCR) and to a video
frame grabber residing in a Pc. The video machine is used for mass storage of video data
and easy replay of the experiment. The frame grabber is used to digitise and grab a sequence
of video frames. The frame grabber can accept a video signal directly from the camera or
from the VCR during playback. Fluid velocities were measured using a non-interlaced digital
ceo camera connected direct to a frame grabber residing in a pentium Pc. Thus, images
were stored direct to the hard disk.
Water level measurements were performed with the aim of measuring wave heights as well
as the mean water level set-down prior to wave breaking and the subsequent set-up in the
surf zone. A time series of wave evolution was obtained by tracking the air-water interface
along a vertical line of pixels at a fixed horizontal position in the video image. This method
has the advantage of being non intrusive, and is considered more reliable than some of the
more traditional techniques employed in the surf zone. Mean water levels were measured
every 0.1 m along the measurement section of the flume, which is approximately 8 m in
length.
The spatial and temporal particle velocities were measured initially by tracking individual
neutrally buoyant particles. This technique is referred to as particle image velocimetry (PIV) .
More extensive measurements were later accomplished using digital cross correlation
techniques, referred to as digital correlation image velocimetry (DCIV). A longitudinal
section of the flume was illuminated with a sheet of light and the flume was seeded with
neutrally buoyant particles. Velocities are then estimated by tracking individual seeds or a
group of seeds in consecutive video frames. The velocity flow field was measured at the
break point and in the surf zone. The average and turbulent velocities are estimated through
phase ensemble-averaging. The undertow current and shoreward mass flux are derived from
the flow field through averaging over the wave phase. A sensitivity analysis on this data is
done to ensure statistical convergence.
The measured velocity flow fields were analyzed and compared with linear wave theory and
measurements made using Laser Doppler anemometry (LDA). The turbulent structures
generated in the surf zone are investigated through the computation of the phase ensembleaveraged
turbulence intensities and vorticity. The vorticity of the averaged flow field is
computed and analyzed. The time averaged Reynolds stress and the turbulent kinetic energy
is also computed. The Reynolds stress near the bed is shown to increase almost linearly with
distance above the bed. Spectral analysis of the spatial velocities was used to obtain estimates
of the scale lengths, and a comparison with the Kolmogorov frequency scaling law in the
equilibrium region of the spectrum was also performed.
The measurement of the aeration/roller geometry for spilling and plunging waves was
accomplished using the video techniques used for water level measurements. The normalised
wave roller area, in spilling waves, is shown to be nearly constant through the inner surfzone,
increasing slightly in the middle of the surf-zone. The roller slope tends to show a
decreasing trend in the surf zone.
Description
Thesis (Ph.D.)-University of Natal, 1999
Keywords
Hydrodynamics., Water waves., Turbulence., Navier-Stokes equations., Fluid mechanics., Theses--Physics.