Twenty-First Symposium on Naval Hydrodynamics (1997) / Chapter Skim
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Conductivity Measurements in the Wake of Submerged Bodies in Density-Stratified Media
Conductivity Measurements in the Wake of Submerged Bodies in Density-Stratified Media
Pages 266-277
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From page 266... ...
These phenomena give rise to complex wore structures, even for a relatively simple linear body motion in an homogeneous medium. The unsteady motions such as impulsive starts and stops, changes in course or deploy give rise to additional wake widening in both the homogeneous and stratified media Hydrodynamic models, utilizing recently developed numerical techniques, attempt to predict the behavior of unsteady flow fields in the wakes of submerged bodies.
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From page 267... ...
They provide an accurate representation of the conditions that would be encountered under normal operating conditions. Of special importance is the relative conductivity which we define here as He ratio of the time change in conductivity at a given point to Hat which existed prior to the motion of the body at the Same point, i.e., RC(x,y,z)
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From page 268... ...
The latter consists of one inch thick polyurethane foam sandwiched between two perforated aluminum plates. Stratification System It had two constant head tanks (one for fresh water, one for brine)
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From page 269... ...
In any case, the maximum initial acceleration (dU/dt~o in He experiments did not exceed 1.5 ft/s2. PRESENTATION OF RESULTS Impulsive Start and Wake Collapse Examples of the ensemble-averaged relative conductivity records for the central probes, for an impulsively-started clean body (no sails)
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From page 270... ...
Re = 54,000 Fig. 3 Relative conductivity versus Nt for an impulsively-st~ clean model (N = 0.15 rad/s, U =0.76m/s)
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From page 271... ...
Only the amplitude of RC during Me period of wake growth and collapse seems to be affected. Non-Impulsive Start and Wake Collapse Examples of the ensemble-averaged rel~ive conductivity records for the central probes, for a clean body (no sails)
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From page 272... ...
The fact that there is an upstream effect for all bodies in all fluids, stratified or unstratified, is clear Mom the fact that a body generated by a suitable combination of hydrodynamic singularities (sources, sinks, doublets) and uniform flow gives rise to upstream effects, albeit small, at relatively short distances.
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From page 273... ...
However, this does not imply that other types of unsteady motions such as practical maneuvers, including sudden starts and stops, are not capable of giving rise to measurable conductivity changes far upstream of a suixnarine-lilce body. ~ r It is against this background and much unsubstantiated speculation that a series of suddenstart-stop experiments were undertaken with the clear understanding that the demonstration of an upstream effect under laboratory conditions does not necessarily imply mat it will be equally observable in the ocean environment.
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From page 274... ...
gives rise to large conductivity changes and to many modes of short internal waves [151. The relative conductivity is sufficiently large to cause measurable changes in He geomagnetic field.
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From page 275... ...
S H., "Turbulent Mixing in Stratified Fluids," Annual Reviews of Fluid Mechanics.
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From page 276... ...
The fact that the passage of a slender self-propelled body causes large conductivity changes in an otherwise undisturbed region of a stratified medium and that these changes have a decaying harmonic character is all that is needed for the understanding and interpretation of the corresponding geomagnetic anomalies. This has not been demonstrated before.
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From page 277... ...
, which lead to significantly larger signal-amplitude to up-s~eam-distance ratios above the threshold noise level. The end-wall reflections have nothing to do with the issue because the upstream effect under unsteady circumstances (impulsive starts, turns, dives, porpoising, etc.)
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