Thesis (PhD (Civil Engineering))--Stellenbosch University, 2008. / The velocity, density and turbulence profiles of a horizontal, saline gravity current
were measured experimentally. Stable stratfication damped the turbulence and
prevented the gravity current from becoming self-similar. The velocity and density
prfiles were measured simultaneously and non-intrusively with particle image
velocimetry scalar (PIV-S) technology. The application of the PIV-S technology
had to be extended in order to measure the continuously stratified gravity current.
Measurement of the Reynolds fluxes and Reynolds stresses revealed the anisotropic
turbulent transport of mass and momentum within the gravity current body. These
measurements also allowed the interaction between turbulence and stratification to
be studied. The measured profiles were used to evaluate the accuracy of a gravity
current model which did not assume self-similarity. The gravity current model was
based on a Reynolds-averaged Navier-Stokes (RANS) multispecies mixture model.
The Reynolds flux and Reynolds stress profiles did not show self-similarity
with increasing downstream distance. Comparison of the vertical and horizontal
Reynolds fluxes showed that gravity strongly damped the vertical flux. At a
downstream location, where the bulk Richardson number was supercritical, the
shear production profile had a positive inner (near bed) peak and a positive outer
peak, while the buoyancy production pro le had a negative outer peak. Further
downstream, where the bulk Richardson number was near-critical, the outer shear
and buoyancy production peaks disappeared, due to the continuous damping of
the turbulence intensities by the stable stratification. However, near bed shearing
allowed the inner shear production peak to remain. Sensitivity analyses of different
turbulence models for the gravity current model showed that the standard
k -e turbulence model, as well as the Renormalization Group theory (RNG) k -e
turbulence model, generally underpredicted the mean streamwise velocity profile
and overpredicted the excess density pro le. The flux-gradient hypothesis, used to
provide closure for the Reynolds uxes, modelled the vertical Reynolds ux reasonably,
but not the horizontal flux. This did not compromise the results, since the
horizontal gravity current had the characteristics of a boundary-layer ow, where the horizontal flux does not contribute significantly to the flow structure. It was
shown that the gravity current model, implementing the standard k -e turbulence
model with a constant turbulent Schmidt number of ot = 1;3, produced profiles
which were within 10% - 20% of the measured profiles.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/1458 |
| Date | 03 1900 |
| Creators | Gerber, George |
| Contributors | Basson, G. R., Diedericks, G., Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | Stellenbosch University |
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