Drag reduction mechanisms

Proposed polymer and surfactant drag reduction mechanisms Summary and future development... 

PROPOSED POLYMER AND SURFACTANT DRAG REDUCTION MECHANISMS... 

Turbulent drag reduction in homogeneous flows by polymer or surfactant additives is a striking phenomenon with both theoretical and practical implications. On the theoretical side, it remains a challenge to fully understand the drag reduction mechanism and the interaction details between DRAs and the turbulent flow field. New methods, especially computational ones, have been developed to solve this problem, such as direct numerical simulations and stochastic simulations. On the application... 

The combined pol5mier additives and riblets are expected to cancel out the drawbacks associated with applying each of these systems and to produce a positive synergistic effect. The effect of combined experiment would also elucidate the respective drag-reduction mechanism. Choi and co-workers (190) experimented on a ship model with riblets and then coated the model with polymer. The... 

K. S. Choi, K. K. Prasad, and T. V. Truong, Emerging Techniques in Drag Reduction, Mechanical Engineering Publications Ltd., London, U.K., 1996. 

Bechert D, Bartenwerfer M, Hoppe G. (1986) Drag reduction mechanisms derived from the sharkskin. ICAS Congress, 15th, London, 1044-1068. 

Ram Prakash Singh, India, Drag Reduction, Mechanical Performance of... 

The concepts of boiling in micro-channels and comparison to conventional size channels are considered in Chap. 6. The mechanism of the onset of nucleate boiling is treated. Specific problems such as explosive boiling in parallel micro-channels, drag reduction and heat transfer in surfactant solutions are also considered. 

Loitsianskii LG (1966) Mechanics of liquid and gases. Pergamon, Oxford Lumley JL (1969) Drag reduction by additives. Ann Rev Fluid Mech 1 367-384 Ma HB, Peterson GP (1997) Laminar friction factor in microscale ducts of irregular cross section. Microscale Thermophys Eng 1 253-265... 

Watanabe K, Udagawa Y, Udagawa H (1999) Drag reduction of Newtonian fluid in a circular pipe with a highly water-repellent wall. J Fluid Mech 381 225-238 White FM (1994) Fluid mechanics, 3rd edn. McGraw-HiU, New York... 

One of the mechanisms of drag reduction is that transmission of eddies can be damped by the viscoelastic properties of fluids. The transfer process of an isolated eddy in Maxwell fluids with viscoelastic properties was studied, and the expressions describing such phenomena were obtained [1103]. The results of the study showed that eddy transmission was damped significantly with an increase of the viscoelastic properties of the fluids. 

Savins, J.G. "A Stress-Controlled Drag-Reduction Phenomenon," SPE paper 1724, 1966 SPE Symposium on Mechanics of Rheologically Complex Fluids, Houston, December 15 16. 

Although the exact mechanism is debatable, Darby and Chang (1984) and Darby and Pivsa-Art (1991) have presented a model for turbulent drag reduction based on the fact that solutions of very high polymers are visco-... 

A. Gyr and H.-W. Bewersdorff, Drag Reduction of Turbulent Flows by Additives, Vol. 32 of Fluid Mechanics and its Applications, Kluwer Academic Publishers, Dordrecht, 1995. 

The proposed mechanisms of models to explain the drag reduction phenomenon are based on either a molecular approach or fluid dynamical continuum considerations, but these models are mainly empirical or semi-empirical in nature. Models constructed from the equations of motion (or energy) and from the constitutive equations of the dilute polymer solutions are generally not suitable for use in engineering applications due to the difficulty of placing numerical values on all the parameters. In the absence of a more generally accurate model, semi-empirical ones remain the most useful for applications. 

The non-ionic surfactants have an advantage over all the drag-reducing additives studied so far. They are both mechanically and chemically stable, do not precipitate in the presence of calcium, and hence can be used in all impure waters, sea water, brackish water, or concentrated brine solutions. They have an excellent potential for drag reduction at high temperatures. 

In order to obtain a higher effectiveness of drag reduction, it was therefore more convenient to produce the actual experimental solution from the more diluted stock solution. This is already an indication that the state of solution of macromolecules plays an important role in the interpretation of the results. However, a molecular weight of 10.1 106 g/mol and a concentration of 1000 ppm is just below the critical concentration c (this c is based on viscometric measurements) (Kulicke 1982). Above this critical concentration, mechanical entanglements ensure between the macromolecules. Beneath this limit, interactions between the molecules come into... 

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