Turbulence Distributions and Flow Structure in the Coastal
Ocean from PIV Data
W.A.M. Nimmo Smith, L. Luznik, W. Zhu, J. Katz and T.R. Osborn
The Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland, USA
Email: alexns@jhu.edu
Web: http://cloudbase.me.jhu.edu/spiv/
A submersible PIV system has been developed to measure flow structure and turbulence
distributions in the coastal ocean. The system has evolved over several years from a
configuration using one 1Kx1K camera with a limited storage capacity (Bertuccioli et al.,
1999; Doron et al., 2001), which is capable of profiling up to 1m above the bed, to a system
utilising two 2Kx2K resolution cameras each with massive image acquisition systems, and a
profiling range of 10m.
The present version of the submersible system comprises two 2Kx2K pixels, 12bits/pixel
digital cameras operating simultaneously, each with a sample area of up to 0.5x0.5m. In the
present configuration we record two exposures within each frame of the digital cameras. A
hardware based 'image shifter' creates a known fixed offset between exposures on the CCD
array to remove directional ambiguity. The cameras can capture up to 4 frames/s, requiring a
total image acquisition rate of 64Mb/s. The data is stored using ship board hard disk arrays.
When the two sample areas are aligned horizontally in the same plane, and spaced 1m apart,
they enable us to resolve turbulent scales ranging from about 8mm (the vector spacing) to
1.5m. The light source of the PIV system is a pair of flashlamp pumped dye lasers located at
the surface, whose beams are transferred to each of the sample areas using two independent
optical fibres (see Figure 1). Submerged probes are used for expanding the beams into light
Figure 1. (top) The PIV components mounted on the small sampling
range platform. (right) The new 10m sampling platform which utilises
the same optical setup as the small platform. (bottom) Components of
the ship board laser and data acquisition systems.
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sheets. Naturally occurring particles are used as tracers. Data analysis consists of enhancing
each image using a modified histogram equalization method followed by auto correlation
analysis (Roth and Katz, 2001). The calculated velocity distributions are then corrected for
optical distortions in the original images. Errors associated with the out of plane component
of the velocity are minimised by limiting the thickness of the light sheets (to 2.5mm),
restricting the sample areas (to about 35cm square), setting a minimum of the camera to
light sheet separation of about 1m, and by subtracting the resulting contamination of the
radial velocity component.
Figure 2. Sample distributions of the instantaneous vorticity and velocity
fluctuations of the same sample area 0.6s apart. The values of the mean horizontal
and vertical velocities subtracted to give the fluctuation velocities are shown at the
top. A large structure can be seen to evolve in form and be advected by the mean
current and wave motion between the two frames.
The components of the PIV system are mounted on a rigid sea bed platforms, which enable
us to align the sample areas with the direction of the mean current. In the original platform,
the elevation was controlled using a hydraulic scissor jack, which enabled data collection
from the bed up to an elevation of 1.5m. This profiling range has been extended with a new
platform so that data collection is now possible from very close to the bottom up to 10m
above the bed (see Figure 1). The elevation is controlled using a rugged, double acting,
telescopic hydraulic cylinder mounted vertically on a heavy tripod base. The instrumentation
is mounted on a rigid framework suspended from a turntable at the top of the cylinder. The
system also includes a CTD, transmissometer, precision pressure transducer (for detecting
surface waves), compass and video camera for monitoring the flow direction. During recent
deployments we also installed airfoil turbulence probes, and profiled the entire water column
using ship board CTD and ADCP. A 60m umbilical, containing hydraulic, power, fibre
optic, control and data lines, links the submerged instrumentation to the support vessel.
Further details of the new system are given in Nimmo Smith et al. (2001).
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The submersible PIV system was deployed at two locations close to the Longterm
Ecosystem Observatory (LEO 15) site off the coast of New Jersey in regions with depths of
12 and 20m. Data were collected at different elevations and under different mean flow and
wave conditions for periods in excess of 20min each, and at rates of up to 3.3Hz. Figure 2
shows two sample distributions of the instantaneous fluctuation velocity (calculated by
subtracting the instantaneous mean velocity from each vector) superimposed on the vorticity
field. It can be seen that the flow consists of many vortices (O 5cm) which combine to form
much larger coherent structures (O 25cm). It is possible to follow the time evolution of
these structures both within the same sample area, and between the sample areas of the two
cameras, sometimes for times in excess of 5s.
Figure 3 shows sample time series of the mean streamwise velocity (of each vector map)
and vorticity intensity for two 20min periods under conditions of moderate and low overall
mean current, plotted in red and blue respectively. In both series the amplitude of the wave
induced motion is about 8cms 1 with a period of about 10s. Wave groups are also in
evidence. The time series of vorticity intensity for the moderate flow case shows that the
flow is characterised by extended periods of relatively quiescent flow, interspersed by short
intense burst, or 'gusts', of high vorticity (sample 'gusts' are marked as 'X' in Figure 2). The
Figure 3. Sample time series of instantaneous mean streamwise velocity of each vector
map (top) and normalised RMS vorticity (bottom) during 20min periods of moderate
mean flow (red, 7.7cms 1) and low mean flow (blue, 0.9cms 1).
vorticity intensity for the low overall mean current case is, on average, about 70% of the
moderate current case, with only a few distinct 'gust' structures. The obvious exception is
structure 'Y', which is most probably a fish wake.
Vertical distributions of mean streamwise velocity in the coastal boundary layer are shown
in Figure 4. It can be seen that the vertical profile from the site with high mean flow and
low wave motion has a logarithmic form. In contrast, the profile from the site with low
mean flow and high wave motion does not exhibit a logarithmic form, regardless of the
phase of the wave induced motion. This observation is consistent with laboratory
experiments.
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Figure 4. Vertical distributions of longterm mean streamwise velocity from (a) a
site with a longterm mean current of 12cms 1, and wave motion amplitude of
10cms 1 and (b) a site with a longterm mean current of 35cms 1, and wave motion
amplitude of 5cms 1. The distributions in (b) have been conditionally sampled
according to the phase of the wave induced motion.
References
Bertuccioli, L. Roth, G.I., Katz, J. and Osborn, T.R. (1999). Turbulence measurements in the
bottom boundary layer using Particle Image Velocimetry. J. Atmos. Ocean. Technol. 16,
1635 1646.
Doron, P., Bertuccioli, L., Katz, J. and Osborn, T.R. (2001). Turbulence characteristics and
dissipation estimates in the coastal ocean bottom boundary layer from PIV data. J.
Phys. Ocean. 31, 2108 2134.
Nimmo Smith, W.A.M., Atsavapranee, P., Zhu, W., Luznik, L., Baldwin, D.R., Katz, J. and
Osborn, T.R. (2001). PIV measurements in the bottom boundary layer of the coastal
ocean. 4th International Symposium on Particle Image Velocimetry, Gottingen,
Germany, Sept 17 19, 2001.
Roth, G.I. and Katz, J. (2001). Five techniques for increasing the speed and accuracy of PIV
interrogation. Meas. Sci. Technol. 12, 238 245.
Funded in part by ONR and in part by NSF.
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