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The unusual hydrodynamical behaviour of freshwater oncolites
Algal concretions are frequent in calcareous environments, occuring as individual particles (oncolites) that are easily transported during floods. Several parameters have been measured and calculated for a population of 127 particles from French streams. These oncolites have a high porosity (mean 42.7%) and a low density (mean 1.65), in respect to quartz and calcite. Oncolites have a settling velocity 1.5 times lower than that of quartz grains with the same nominal diameter. The computation of multiple correlations results in a classification in terms of decreasing efficiency: shape coefficients (Corey, ψ, and S/I of Sneed and Folk), nominal diameter, density, edge sharpness and roughness. A Cd (drag coefficient) versus Re (Reynolds number) plot of these oncolites displays two different clusters with 1000 < Re < 10,000. One cluster, with 0.6 < Cd < 2.4 is named 'oncolites'. The other, with a Cd around 0.1, represents 'encrusted snails'. In the oncolite cluster, no preferential arrangement of points appears and the regression line is horizontal (parallel to the x-axis). In this part of the diagram, the equation has the form Cd = 0Re + b. This b coefficient is an important parameter, because it varies with the values of nominal diameter and density. The various shape coefficients tested (Corey shape factor, sphericity and the Sneed and Folk triangle), the roundness (expressed by the Powers index) and the roughness also exert a slight influence. Multiple correlation calculations allow them to be ranked in function of their influence on the drag coefficient (in decreasing order): density, edge sharpness, nominal diameter, shape coefficients, and roughness. This study is a preliminary approach and many additional measurements on living material are needed to obtain a more pertinent set of data.
oncolites, travertine, settling velocity, drag coefficient, Reynolds number, multiple regression
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