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Turbulence characteristics in the coastal ocean bottom boundary layer are measured using a submersible Particle Image Velocimetry (PIV) system with a sample area of 20 x 20 cm. Measurements are performed in the New York Bight at elevations ranging from about 10 cm to about 1.4 m above the sea floor. Data for each elevation consists of 2 minutes of image pairs recorded at 1 Hz. The data provides instantaneous spatial velocity distributions within the sample area, and is extended to larger scales using Taylor’s hypothesis.
The vertical distribution of mean velocity indicates the presence of large-scale shear even at the highest measurement station. It also undergoes variations with time scales longer than the present data series.
The turbulence spectra calculated from the data cover about three decades in wavenumber space, and extend well into the dissipation range. The results indicate, that the turbulence near the bottom is anisotropic, not only at the scale of the energy containing eddies, but also in the inertial and dissipation ranges. The vertical component of velocity fluctuations at energy containing scales is significantly damped as the bottom is approached, while the horizontal component maintains a similar energy level at all elevations.
Different methods of estimating the turbulent energy dissipation are compared. Several of these methods are possible only with 2-D data, such as that provided by PIV, including a "direct" method, which is based on measured components of the dissipation tensor. Estimates based on assumptions of isotropy exhibit deviations from those based on the "direct" method.
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| Turbulence Structures. Sequence
of velocity fluctuations vector maps measured in
14m deep water, at 1s intervals. u'(x,z,t)=u(x,z,t)-U(z),
w'(x,z,t)=w(x,z,t)-W(z); u, w: Velocity components
of a vector; U, W: Averages of u, w at constant
z, over the entire data set. Note the structures
designated "1" and "2" being convected across the
sample area. Data from New York Bight, June 1998. |
| Turbulence Characteristics. (a)
Vertical profiles of RMS velocity fluctuations; (b) Vertical
profiles of Reynolds stress and estimate for the Turbulent
kinetic energy, assuming v’2=0.5(u’2+w’2).
Umax is the largest value of U(z); Averages of
u’2, w’2, u’w’ are at constant z,
over the entire data set. |
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This
project is funded by the ONR, Dr. L. Goodman, Program
Manager, under grant number N00014-95-1-0215. Some of
the instrumentation was purchased with DARPA funding.
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Department
of Mechanical Engineering
The Johns Hopkins University
200 Latrobe Hall, 3400 N. Charles Street
Baltimore, MD 21218
Tel: (410) 516 5427; Fax: (410) 516 4316
lefd@titan.me.jhu.edu
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