Geoscience Reference
In-Depth Information
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Figure 10.19. (a) Left-to-right propagating vertical mode 1 internal waves generated using horizontally oscillating flat plates, as
in Gostiaux et al. [2007]. (b) Internal wave beam downstream of hill placed in front of the oncoming mode 1 waves. Adapted from
Figures 2a and 3a of Peacock et al. [2009].
sinusoidal variation in a fixed plane so that rotating the
shaft produces a mode 1 wave in a uniformly stratified
medium [ Mercier et al. , 2010]. In the absence of topog-
raphy, PIV is used to reveal the mode 1 wave structure,
which at one point in the phase exhibits forward motion
at the surface and bottom and retrograde motion at mid
depth. The flow directions reverse a half period later.
When this incident waveield encounters a Gaussian hill,
the structure of the wavefield changes significantly down-
stream. Just as an oscillating body creates internal wave
beams in a stratified fluid, oscillations resulting from the
incident low-mode internal waves create beamlike struc-
tures downstream. Thus energy from low modes is effi-
ciently converted into higher modes. These ideas have
recently been extended to the examination of internal
waves incident upon the continental shelf [ Klymak et al. ,
2011].
stratiied media. When used to examine spanwise-uniform
and axisymmetric disturbances, it has provided a useful
check on the limitations of linear, inviscid theory, particu-
larly with respect to the generation of internal waves from
oscillating and steadily translating bodies.
Just as MRI revolutionized medicine, inverse tomog-
raphy for schlieren has the potential to measure fully
three-dimensional disturbances nonintrusively and con-
tinuously in time, provided the disturbances do not involve
turbulent mixing and, hence, random scattering of light.
However, several logistical obstacles remain to be over-
come. In order to reconstruct relatively fine scale fea-
tures, multiple perspectives from many angles must be
recorded simultaneously or in rapid succession. But syn-
thetic schlieren requires looking through a fluid at an
object image on the opposite side. To have a large num-
ber of perspectives one must devise a method in which
multiple cameras are not obstructed by multiple object
images. Alternately, like MRI, one could construct a sys-
tem in which the camera and object image rotate about
a cylindrical tank on a fast time scale compared with
that of internal waves. After image correction for the
curvature of the tank there would remain the theoreti-
cal challenge to reconstruct the three-dimensional distur-
bance field from the images recorded continuously from
changing perspectives.
10.6. DISCUSSION AND CONCLUSIONS
Several technological innovations have provided new
tools for the study of internal waves in the laboratory.
Here we have focused mostly upon the use of synthetic
schlieren as a nonintrusive way to measure perturbation
density gradients due to internal waves in continuously
 
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