Geoscience Reference
In-Depth Information
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Fig. 28 Turbulent energy flux. Experiments identified as in Fig. 11
The results presented in this study provide a more complete set of parameters to
interpret such phenomena. The following conclusions can be drawn from the text:
1. The movement of the finer size fractions may induce changes in the turbulent
structure of the flow.
2. The fundamental structural differences between fixed and mobile beds were
detected on the sweep/ejection ratio and, more important, on the averaged
maximum instantaneous stress, uv max .
3. Under mobile bed conditions, uv max , for both quadrants II and IV, is larger in the
wall region (cf. Fig. 27 ).
4. The observation of the sweep/ejection ratio reveals differences in the transported
momentum, namely that the sweep event looses importance to ejections in the
presence of a mobile bed (cf. Fig. 26 ); this feature represents a smoothening of
the bed, consequence of the reduced porosity.
5. Combining the last two points, one sees that, in the presence of fine mobile
fractions, the magnitude of the instantaneous hydrodynamic actions upon the
largest grains increases, thus increasing its entraining probability; the apparent
largest grain mobility for the coarsest grains is thus explained; as a consequence,
the observation of the ratio t *i /t *ci is not sufficient to predict the composition of
the transported sediment.
6. As pointed out before by Gyr and Schmid ( 1997 ), it appears that the interaction
between near-bed sediment transport and turbulent structures is reflected by an
increased degree of coherence of the latter; this observation should explain the
differences found in the distribution of the third moments; the main link should
be the decrease of the skweness of the distribution of v in the presence of fine
mobile sediments; as a result, the mobile bed data show a less steeper decrease in
the turbulent energy flux near the bed (cf. Fig. 28 ).
The last point highlights the fact that a consistent phenomenological frame,
capable of becoming a predicting model, is required to understand the interactions
between turbulent flow structures and the transport of sediment grains. It is prudent
that it should be attempted only in the presence of further data concerning other
granulometric mixtures.
 
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