Eighteenth Symposium on Naval Hydrodynamics (1991) / Chapter Skim
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Nonintrusive, Multiple-Point Measurements of Water Surface Slope, Elevation and Velocity
Nonintrusive, Multiple-Point Measurements of Water Surface Slope, Elevation and Velocity
Pages 349-360
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From page 349... ...
These flows are modeled after the observed diverging surface flow field in the wake of surface ships producing a persistent, dark centerline wake in SAR images (Figure 2~. The modeled flow fields are buoyancy driven, with bubbles playing a significant role in the observed persistence of the vertical structure.
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From page 350... ...
To fully describe the "sea" surface implies that the morphology of the fluid surface and the associated velocity field are known in sufficient detail to compute the scattering properties and other signatures which are desired. This constraint has led to the development of an instrument system, the HMF, which provides a spatial description of the surface wave height, slope and velocity fields in two-dimensions.
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From page 351... ...
Surface Wave Height and Velocity Observations Tracking of Laser Induced "Warm Spots" The surface wave height and velocity of the water behind a ship or in the laboratory behind a model are of great importance in defining the wake characteristics. It is imperative to observe the height and velocity fields with spatial and temporal scales that match those of the waves of interest.
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From page 352... ...
Wave making and ancillary data collection capabilities of the Ship Hydrodynamics Laboratory have been extensively upgraded to accommodate the experimental opportunities brought about by the development of the HMF. INITIAL EXPERIMENT: BUOYANCY MAINTAINED VORTICES IN To SllPlFACE SHIP WAKE The inaugural experiments utilizing the HMF in the Ship Hydrodynamics Laboratory at The University of Michigan as part of the URI funded Program in Ship Hydrodynamics, have sought to elucidate the effects of bubbles in maintaining the flow patterns in surface ship wakes.
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From page 353... ...
Hydrodynamic Monitoring Facility design and demons~ated capabilities 81 x 81 10 x 5 0.1 ~1.0 ~ 0.5 m diem 25 x 18 cm (-4 mm)
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From page 354... ...
This vortex pair produces diverging surface currents to simulate the centerline wake region of a large displacement vessel traveling in the cross-tank direction. Measurements of surface velocity, wave height, slope and radar crosssection are made across the interaction region.
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From page 355... ...
The implication of this set of observations is that bubbles produced by the passage of surface ship wake appear to play a substantial role in the maintenance of diverging surface currents and the persistence of centerline wake, very far downstream of high speed vessels. To investigate the effect that these persistent and sustained diverging surface currents have on the anticipated radar return for the centerline portion of the wake, wind generated waves produced by a near surface fan were propagated across the spatially varying current pattern.
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From page 356... ...
Measured C-band doppler spectra for bubble source only (left column) , bubble source and fan-generated waves (center column)
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From page 357... ...
The spectrum in Figure 1 lady, on the left, which was collected with the radar footprint approximately centered on the bubble source, is rather complicated but nearly symmetric, indicating a zero mean surface current. The peak on the left may be due to a set of receding Bragg waves on the far side of the radar footprint which are Doppler shifted by a current of approximately 19.6 cmIs away from the center, while the peak on the right is due to the corresponding set of approaching Bragg waves on the near side of the footprint.
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From page 358... ...
The corresponding wavenumber at the location where u1 = -7 cm / s would be k1 = 3.0 red / cm. Assuming that the incident wave action spectrum falls off as k , the Bragg wave spectral density at the end point is then a factor of (1.4 / 3.0~ ~ =.03 smaller than that at the wave source, which is in reasonable agreement with the observed reduction in backscatter.
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From page 359... ...
5. Wright, J.W., "A New Model for Sea Clutter," IEEE Trans.
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