DRAG REDUCTION BY POLYMER ADDITIVES IN A TURBULENT PIPE-FLOW - NUMERICAL AND LABORATORY EXPERIMENTS

In order to study the roles of stress anisotropy and of elasticity in the mechanism of drag reduction by polymer additives we investigate a turbulent pipe flow of a dilute polymer solution. The investigation is carried out by means of direct numerical simulation (DNS) and laser D oppler velocimetry (LDV). In our DNS two different models are used to describe the effects of polymers on the flow. The first is a constitut ive equation based on Batchelor's theory of elongated particles suspen ded in a Newtonian solvent which models the viscous anisotropic effect s caused by the polymer orientation. The second is an extension of the first model with an elastic component, and can be interpreted as an a nisotropic Maxwell model. The LDV experiments have been carried out in a recirculating pipe flow facility in which we have used a solution o f water and 20 w.p.p.m. Superfloc A110. Turbulence statistics up to th e fourth moment, as well as power spectra of various velocity componen ts, have been measured. The results of the drag-reduced flow are first compared with those of a standard turbulent pipe flow of water at the same friction velocity at a Reynolds number of Re-tau approximate to 1035. Next the results of the numerical simulation and of the measurem ents are compared in order to elucidate the role of polymers in the ph enomenon of drag reduction. For the case of the viscous anisotropic po lymer model, almost all turbulence statistics and power spectra calcul ated agree in a qualitative sense with the measurements. The addition of elastic effects, on the other hand, has an adverse effect on the dr ag reduction, i.e. the viscoelastic polymer model shows less drag redu ction than the anisotropic model without elasticity. Moreover, for the case of the viscoelastic model not all turbulence statistics show the right behaviour. On the basis of these results, we propose that the v iscous anisotropic stresses introduced by extended polymers play a key role in the mechanism of drag reduction by polymer additives.