Eighteenth Symposium on Naval Hydrodynamics (1991) / Chapter Skim
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Turbulence Measurements in a Submerged Jet Near a Free Surface
Turbulence Measurements in a Submerged Jet Near a Free Surface
Pages 361-372
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From page 361... ...
Measurements beneath a clean free surface show that the mean flow spreads laterally in a shallow surface current, and the turbulent velocity fluctuations become anisotropic as the surface is approached. Flow visualization reveals that the surface current contains fluid structures ejected from the jet, and the current is suppressed with the addition of surface active agents.
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From page 362... ...
Far field scaling of the free surface jet The similarity scaling in the far field of the turbulent axisymmetric jet has been discussed by several authors (e.g., Rajaratnam 1976~. If the jet momentum flux is constant, the linear growth of length scales with downstream distance implies that sufficiently far downstream compared to the jet exit diameter the mean centerline velocity, Um, can be written as: r ~ Po Um 362
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From page 363... ...
h/d=1 n h/d=3.5 h/d=1.5 Free Jet Data ~ h/d=2.5 7 6 s 4 3 2 1 O /~ ~' '1 1 1 1 1 0 10 20 30 40 50 /: a / / o x/h Figure 3. Decay of maximum mean velocity.
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From page 364... ...
The growth rate of the mean velocity profiles was characterized by the half velocity widths Lo, and Lz in the directions parallel and perpendicular to the free surface. These half velocity widths were determined with reference to the location of the maximum velocity in the profile.
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From page 365... ...
Evolution of mean velocity near the surface. TURBULENCE MEASUREMENTS Experimental Apparatus Measurements of jet turbulence beneath a free surface were made using a three-component, underwater Laser Doppler Velocimeter (LDV)
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From page 366... ...
The profiles are plotted using similarity variables, normalizing the vertical coordinate z by the downstream distance x and the velocity components by Uc, the mean velocity on the jet centerline. The vertical mean velocity component W
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From page 367... ...
Shown are vector plots of the mean velocity components V and W at various downstream stations. At x/d=16, the data show the beginnings of the surface current in the data taken at the surface, but not in that taken below it.
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From page 368... ...
Profile of streamwise mean velocity component, x/d=32, h/d~60.
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From page 369... ...
Profiles of horizontal and vertical mean velocity components, x/d=32, h/d-2. continue to propagate parallel to the surface away from the jet boundary.
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From page 370... ...
Vector plot of horizontal and vertical mean velocity components, x/d=48, h/d=2. Although we have not directly measured the vorticity, the fact that the surface puffs continue to propagate away from their origin near the jet boundary indicates that they consist of vertical fluid.
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From page 371... ...
The surface current did not form, and vertical ejections from the jet, emitted near and parallel to the free surface, were confined laterally through interaction with secondary vorticity generated beneath the surfactant layer. Acknowledgements This work is supported at the University of Michigan by the Office of Naval Research University Research Initiative Program in Ship Hydrodynamics, Contract Number N000184-86-K-0684 and at the David Taylor Research Center by the ONR Surface Ship Wake Consortium, Contract Number N0001490-WX22034.
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From page 372... ...
Were there any distinct streamwise or transverse waves visible? AUTHORS' REPLY Under the conditions investigated, the interaction of the jet flow with the free surface led to the generation of surface waves near the jet centerline.
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