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| Titel |
Strongly Stratified Turbulence Wakes and Mixing Produced by Fractal Wakes |
| VerfasserIn |
Natalia Dimitrieva, Jose Manuel Redondo, Yuli Chashechkin, Philippe Fraunié, David Velascos |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250138484
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| Publikation (Nr.) |
 EGU/EGU2017-1518.pdf |
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| Zusammenfassung |
| This paper describes Shliering and Shadowgraph experiments of the wake induced
mixing produced by tranversing a vertical or horizontal fractal grid through the
interfase between two miscible fluids at low Atwood and Reynolds numbers. This is a
configuration design to models the mixing across isopycnals in stably-stratified
flows in many environmental relevant situations (either in the atmosphere or in the
ocean.
The initial unstable stratification is characterized by a reduced gravity: g′ = gΔρ
ρ where g
is gravity, Δρ being the initial density step and ρ the reference density. Here the Atwood
number is A = g′ _
2 g . The topology of the fractal wake within the strong stratification, and the
internal wave field produces both a turbulent cascade and a wave cascade, with frecuen
parametric resonances, the envelope of the mixing front is found to follow a complex non
steady 3rd order polinomial function with a maximum at about 4-5 Brunt-Vaisalla
non-dimensional time scales: t∕N δ = c1(t∕N) + c2g Δρ
ρ (t∕N)2 −−c3(t∕N)3. Conductivity
probes and Shliering and Shadowgraph visual techniques, including CIV with (Laser
induced fluorescence and digitization of the light attenuation across the tank) are used
in order to investigate the density gradients and the three-dimensionality of the
expanding and contracting wake. Fractal analysis is also used in order to estimate the
fastest and slowest growing wavelengths. The large scale structures are observed to
increase in wave-length as the mixing progresses, and the processes involved in this
increase in scale are also examined.Measurements of the pointwise and horizontally
averaged concentrations confirm the picture obtained from past flow visualization
studies. They show that the fluid passes through the mixing region with relatively
small amounts of molecular mixing,and the molecular effects only dominate on
longer time scales when the small scales have penetrated through the large scale
structures.
The Non-stationary dynamicss and structure of stratified fluid flows around a wedge were
also studied based of the fundamental equations set using numerical modeling. Due to
breaking of naturally existing background diffusion flux of stratifying agent by an
impermeable surface of the wedge a complex multi-level vortex system of compensatory fluid
motions is formed around the obstacle. The flow is characterized by a wide range of values of
internal scales that are absent in a homogeneous liquid. Numerical solution of the
fundamental system with the boundary conditions is constructed using a solver such as
stratifiedFoam developed within the frame of the open source computational package
OpenFOAM using the finite volume method. The computations were performed in parallel
using computing resources of the Scientific Research Supercomputer Complex of MSU
(SRCC MSU) and the technological platform UniHUB. The evolution of the flow pattern of
the wedge by stratified flow has been demonstrated. The complex structure of the fields of
physical quantities and their gradients has been shown. Observed in experiment are multiple
flow components, including upstream disturbances, internal waves and the downstream
wake with submerged transient vortices well reproduced. Structural elements of
flow differ in size and laws of variation in space and time. Rich fine flow structure
visualized in vicinity and far from the obstacle. The global efficiency of the mixing
process is measured and compared with previous estimates of mixing efficiency. |
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