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Observations on turbulent-drag reduction in a dilute suspension of clay in sea-water

Published online by Cambridge University Press:  29 March 2006

Giselher Gust
Affiliation:
Institut für Meereskunde an der Universität Kiel, 23 Kiel, Düsternbrooker Weg 20, Germany Present address: University of Hawaii at Manoa, Department of Oceanography, 2525 Correa Road, Honolulu, Hawaii 96822.

Abstract

Hot-wire anemometer measurements have been made in a dilute sea-water/claymineral suspension. For fully developed turbulent flows in an open channel with a smooth mud (from the North Sea) bottom, mean streamwise velocity profiles were measured for Reynolds numbers between 5400 and 27 800 (i.e. non-eroding and eroding flow rates) and compared with Newtonian flows under the same experimental conditions. For the clay-mineral suspensions, measurements of the kinematic viscosity v, Kármán's constant k and the mean streamwise velocity $\overline{u}$ of the logarithmic layer seemed to verify a Newtonian flow structure. Although the distributions of concentration showed no substantial increase towards the wall, it was found that beneath this Newtonian core there existed a viscous sublayer whose thickness was enhanced by a factor of 2–5. The friction velocity u* determined by the gradient method in the viscous sublayer was reduced by as much as 40 %. The mean flow structure exhibited an additional new layer in the region 10 < y+ < 30.

The measurements indicate that turbulent-drag reduction occurs for the experimental clay-mineral suspension at non-eroding and also at eroding velocities. Agglomeration of suspended clay-mineral particles is suggested as possible explanation of this phenomenon.

Type
Research Article
Copyright
© 1976 Cambridge University Press

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