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  • 1
    Electronic Resource
    Electronic Resource
    A diffusion model for the development of a boundary layer in lakes (1997)
    Springer
    Aquatic sciences 59 (1997), S. 95-114 
    Details
    ISSN: 1420-9055
    Keywords: Key words: Mixing model, diffusion, boundary layer, boundary mixing, diapycnal mixing.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: ABSTRACT. The development of a boundary layer characterised by very low gradients in temperature and salinity near the bottom boundary of a lake does not necessarily imply an increase in diapycnal mixing within the boundary layer. The results of a quasi three-dimensional diffusion model for a basin with sloping boundaries demonstrate that in lakes a boundary layer also develops when the diapycnal diffusivity is chosen to be constant.¶In the model mixing is assumed to be anisotropic and is described as an isopycnal and diapycnal turbulent diffusion process. Advective transport is not considered. Therefore, the model is restricted to the description of the purely diffusive response of a lake. It should be regarded as a contribution to the discussion of boundary mixing and not as a complete mixing model for a specific lake. The isopycnal and diapycnal turbulent diffusion coefficients are presumed to be constant in space and time. The direction of isopycnal and diapycnal density flux changes with time since mixing of the density distribution influences the orientations of the isopycnals. This interaction between mixing process and density distribution is accounted for by the model.¶According to the model the density distribution, and therefore the development of a boundary layer, only depends on diapycnal mixing while the distribution of a passive tracer depends on both, isopycnal and diapycnal mixing. The application of the model to the subalpine Lake Alpnach demonstrates that a simple diffusion model is sufficient to predict the development of a boundary layer. Considering that the model does not include advective processes and that diffusivities have been assumed to be constant in space and time, the structure of the boundary layer predicted agrees surprisingly well with experimental data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s000270050079
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  • 2
    Electronic Resource
    Electronic Resource
    Enhanced mixing in narrows: A case study at the Mainau sill (Lake Constance) (1998)
    Springer
    Aquatic sciences 60 (1998), S. 236-252 
    Details
    ISSN: 1420-9055
    Keywords: Key words: Turbulent mixing, narrows, lake basins, Lake Constance, temperature microstructure, sill, dissipation.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract: Previous work has identified bottom currents as a significant source of turbulence in stratified lakes. Sills may therefore be a major factor determining overall turbulent diapycnal (vertical) exchange in lakes with multi-basin hypolimnia.¶In order to investigate the contribution of the Mainau sill (separating Upper Lake Constance from Lake Überlingen) to the overall diapycnal mixing in Lake Constance, a series of temperature microstructure profiles was taken at Mainau Island in October 1993. From these profiles, using Batchelor's method, the rate of dissipation of turbulent kinetic energy was determined and related in an energy balance to the turbulent kinetic energy input from the wind, the energy content of internal seiches and the energy dissipation of bottom currents at the sill. Further, the vertical diffusivities were calculated using the dissipation method.¶The analysis shows that ≈ 5.5% of the wind energy flux was found in to the water column (below 2 m depth) and that energy dissipation was 8 times higher in the shear zone of the thermocline (≈ 8.4 ˙ 10-3 W m-2) than in the bottom boundary layer (≈ 1.1 ˙ 10-3 W m-2). Dissipation above the Mainau sill (≈ 9.5 ˙ 10-3 W m-2) exceeded the basin-wide average dissipation of internal seiche energy (≈ 0.3 ˙ 10-3 W m-2) by a factor of 34. However, since the areal extent of the sill is small (≈ 1% of the lake area), the sill contributes only about 40% to the basin-wide dissipation. Also vertical diffusivities within the thermocline were consistently enhanced by approximately the same amount over the sill. The synthesis of the observations implies that the sill plays a disproportionately large, but not dominant, role for small-scale diapycnal mixing in the hypolimnion of Lake Constance.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s000270050039
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  • 3
    Electronic Resource
    Electronic Resource
    Modeling the effects of anterior chamber fluid viscosities on intraocular pressure following glaucoma filtering surgery (1995)
    Springer
    Graefe's archive for clinical and experimental ophthalmology 233 (1995), S. 737-741 
    Details
    ISSN: 1435-702X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract • Background: Hypotonia, flat anterior chamber, and choroidal effusion are not infrequent and undesirable consequences of glaucoma filtering surgery. Methods developed so far to prevent or combat these complications have only been moderately successful. Viscous fluids admixed to the aqueous humor are known to influence fluid dynamics and hence intraocular pressure during variable time intervals following surgery. It was the intention of the authors to develop a mathematical model which predicts the influence of sodium hyaluronate determining the intraocular pressure following injection into the anterior chamber. • Methods: The model is based on first-order approximations and derives from the theory of rheology and fluid dynamics in narrow tubes, and is essentially based on Hagen-Poiseuille's law. • Results: The predictions of the model (to be published in a paper to follow) were verified in experiments on owl monkeys and showed a high degree of predictive power. • Conclusions: One may conclude that sodium hyaluronate, injected once or repeatedly with fine canulas into the anterior chamber, qualifies as an important tool to maintain normal on near-normal intraocular pressure in the immediate and late postoperative phase following surgery.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00164679
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    Paper (German National Licenses)
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