Drag reduction by polymer addition

Modeling of Coherent Structure in Boundary Layer Turbulence.

“…The theoretica l model will be formulated on basis of the two-scale model proposed by Landahl (1973). The fundamental i deas underlyin g this model and some of the conclusions wh i ch can be drawn from it have been discussed earl ier (Landahl, 1975(Landahl, , 1977.…”

“…Later findings (Landahi, 1975(Landahi, , 1977 indicate , however , that other types of disturbances besides waves of the Tollmien -Schlichting type must be incorporated in order to model properly the fluctuati on field. On basis of a two-scale model (first proposed in Landahl , 1973), i n whi ch the ma i n non l inear i nteracti on was assumed to occur through a coupl i ng between small and large scal es of moti on , it was concluded that large-scale motion in a localized region , a "coherent structure d , woul d resu l . t from nonhomogeneous mixing in a patch of secondary instability of the inflectional type .…”

Workshop on Coherent Structure of Turbulent Boundary Layers.

“…The theoretical model will be formulated on basis of the two-scale model proposed by Landahl (1973). The fundamental ideas underlying this model and some of the conclusions which can be drawn from it have been discussed earlier ).…”

“…Later findings inoicate, however, that other types of disturbances besides waves of the Tollmien-Schlichting type must be incorporated in order to model properly the fluctuation field. On basis of a two-scale model (first proposed in Landahl, 1973), in which the main nonlinear interaction was assumed to occur through a coupling between small and large scales of motion, it was concluded that large-scale motion in a localized region, a "coherent structure", would result from nonhomogeneous mixing in a patch of secondary instability of the inflectional type. In addition to waves of scales typical of the dimension of the patch, the large-scale eddy produced by the mixing will also contain a convected portion which will move downstream with the local mean velocity.…”

Drag reduction in turbulent flow due to additives: a report on Euromech 52

“…The subject is interesting because of potential engineering applications (typically 50% drag reduction is achieved with only a few parts per million by weight in the case of the most effective polymer additives) and also because an unravelling of the drag-reduction mystery would carry with it an improved understanding of the mechanics of ordinary Newtonian shear-flow t'urbulence. Several extensive reviews of the subject have appeared in the last few years, emphasizing different aspects of the phenomenon (Lumley 1969;Landahl 1972a;Hoyt 1972aHoyt , 1974.…”