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Boundary layer dynamics and drag reduction in flows of high cohesive sediment suspensions
Author(s) -
Li Michael Z.
Publication year - 2000
Publication title -
sedimentology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.494
H-Index - 108
eISSN - 1365-3091
pISSN - 0037-0746
DOI - 10.1046/j.1365-3091.2000.00277.x
Subject(s) - drag , shear velocity , parasitic drag , boundary layer , drag coefficient , laminar sublayer , geology , turbulence , suspension (topology) , shear stress , geotechnical engineering , mechanics , dilatant , flow separation , physics , mathematics , homotopy , pure mathematics
Drag reduction has been observed in suspension flows of low clay concentrations in previous studies. Here, velocity profiles and bed shear stresses, expressed as shear velocities, are measured using epoxy‐coated hot‐film sensors to evaluate drag reduction and controlling factors in suspension flows of high clay concentrations (4 and 8 g l –1 ). The directly measured shear velocity in the viscous sublayer is found to be reduced by as much as 70% relative to the profile‐derived shear velocity in the logarithmic layer. Drag reduction is found to increase with increasing clay concentration and decreasing flow strength. Density profile data indicate that the suspension flows were not stratified, and examinations of particle size distributions suggest that flocculation was not significant in causing the observed drag reduction. Measurements of the velocity profiles and of the shear velocity in the viscous sublayer indicate significant thickening of the inner wall layer and show turbulence damping in the viscous sublayer. These effects become stronger for higher concentrations and lower flow strength, suggesting that they are responsible for drag reduction in flows of clay suspension. Empirical relationships have been derived that can be used to predict the magnitude of drag reduction and the reduced shear stress in mud suspensions for both laboratory and field cohesive sediment transport studies.