Elongational Viscosity and Secondary Instability of Laminar Boundary Layer

Abstract

Drag reduction phenomena of liquid flows due to an addition of polymer molecules are caused by an interaction of the elongational viscosity of the polymer solution and the elongational-type secondary flow of the base flow. In transition form laminar to turbulent flow on a flat plate boundary layer the elongational viscosity acts on the secondary vortices appearing by amplification of Tollmien-Schlichting waves and extends the stable region of Tollmien-Schlichting waves. At the same time the wave number corresponding to the critical state of neutral stability for the waves becomes larger with an increase in elongational viscosity. By this analysis it is suggested that the addition of polymer can delay the turbulent transition of boundary layer flow.