Effect of surfactants on free-surface turbulent flows

Shen, L; Yue, DKP; Triantafyllou, GS

dc.contributor.author	Shen, L	en
dc.contributor.author	Yue, DKP	en
dc.contributor.author	Triantafyllou, GS	en
dc.date.accessioned	2014-03-01T01:20:17Z
dc.date.available	2014-03-01T01:20:17Z
dc.date.issued	2004	en
dc.identifier.issn	0022-1120	en
dc.identifier.uri	https://dspace.lib.ntua.gr/xmlui/handle/123456789/15885
dc.subject.classification	Mechanics	en
dc.subject.classification	Physics, Fluids & Plasmas	en
dc.subject.other	Computer simulation	en
dc.subject.other	Data reduction	en
dc.subject.other	Elasticity	en
dc.subject.other	Kinetic energy	en
dc.subject.other	Navier Stokes equations	en
dc.subject.other	Reynolds number	en
dc.subject.other	Turbulent flow	en
dc.subject.other	Free-surface turbulent flows	en
dc.subject.other	Marangoni vorticity	en
dc.subject.other	Scaling laws	en
dc.subject.other	Turbulent boundary layer	en
dc.subject.other	Surface active agents	en
dc.subject.other	boundary layer	en
dc.subject.other	Marangoni effect	en
dc.subject.other	mathematical analysis	en
dc.subject.other	model	en
dc.subject.other	surfactant	en
dc.subject.other	turbulent flow	en
dc.title	Effect of surfactants on free-surface turbulent flows	en
heal.type	journalArticle	en
heal.identifier.primary	10.1017/S0022112004008481	en
heal.identifier.secondary	http://dx.doi.org/10.1017/S0022112004008481	en
heal.language	English	en
heal.publicationDate	2004	en
heal.abstract	In two earlier papers, we studied the statistical and mechanistic structure of the turbulent boundary layer under a stress-free (clean) free surface. Findings there, such as the presence of inner and outer surface layers, are very much the direct result of the absence of shear stresses at the surface. The latter condition is easily lost when the surface is contaminated and surface elasticity varies with space and time. In this paper we consider the effect of surfactant on features of the free-surface turbulent flow. We perform direct numerical simulations of the Navier-Stokes equations subject to surfactant-laden free-surface boundary conditions for varying Reynolds and Marangoni numbers and low Froude numbers. As expected, the Marangoni effect decreases the horizontal turbulence intensity and normal vorticity at the surface. The direct effect on the turbulent kinetic energy is an increase in the dissipation and viscous diffusion and a decrease in the production near the surface relative to the clean case. The most prominent effect of the presence (of even a small amount) of surfactant is the drastic reduction in the surface divergence and the associated sharp decrease of up- and downwelling at the surface which has direct implications to near-surface turbulent transport. The observed surfactant effects on turbulent kinetic energy budget can be attributed to the generation of Marangoni vorticity at the free surface by approaching hairpin vortices. The Marangoni effect has also a direct effect on the boundary-layer structure, causing an increase of the thickness of the boundary layer and in the maxima of the mean shear near the surface. For moderate values of the Marangoni number, up-/downwelling effectively vanishes and the flow approaches a state independent of the Marangoni number. Guided by these results and to obtain theoretical insight, we develop a similarity solution for the mean flow. The analytic solution agrees well with the numerical data and provides precise measures for the multi-layer structure of the boundary layer. Based on the theoretical model, we derive scaling laws for the thickness of the inner and the outer boundary layers, which are also confirmed by numerical simulations. © 2004 Cambridge University Press.	en
heal.publisher	CAMBRIDGE UNIV PRESS	en
heal.journalName	Journal of Fluid Mechanics	en
dc.identifier.doi	10.1017/S0022112004008481	en
dc.identifier.isi	ISI:000221652800003	en
dc.identifier.volume	506	en
dc.identifier.issue	506	en
dc.identifier.spage	79	en
dc.identifier.epage	115	en