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On the coherent drag-reducing and turbulence-enhancing behaviour of polymers in wall flows



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c 2004 Cambridge University Press J Fluid Mech 2004 vol 514 pp 271 280 DOI 10 1017 S0022112004000291 Printed in the United Kingdom 271 On the coherent drag reducing and turbulence enhancing behaviour of polymers in wall ows By Y V E S D U B I E F1 C H R I S T O P H E R M W H I T E2 V I N C E N T E T E R R A P O N2 E R I C S G S H A Q F E H2 3 P A R V I Z M O I N1 2 A N D S A N J I V A K L E L E2 4 1 Center for Turbulence Research 2 Mechanical Engineering Department 3 Department of Chemical Engineering 4 Department of Aeronautics and Astronautics Stanford University CA 94305 USA Received 15 December 2003 and in revised form 2 June 2004 Numerical simulations of turbulent polymer solutions using the FENE P model are used to characterize the action of polymers on turbulence in drag reduced ows The energetics of turbulence is investigated by correlating the work done by polymers on the ow with turbulent structures Polymers are found to store and to release energy to the ow in a well organized manner The storage of energy occurs around near wall vortices as has been anticipated for a long time Quite unexpectedly coherent release of energy is observed in the very near wall region Large uctuations of polymer work are shown to re energize decaying streamwise velocity uctuations in highspeed streaks just above the viscous sublayer These distinct behaviours are used to propose an autonomous regeneration cycle of polymer wall turbulence in the spirit of Jime nez Pinelli 1999 1 Introduction The addition of small amounts of long chain polymer molecules to wall bounded ows can lead to dramatic drag reduction Although this phenomenon has been known for about fty years the action of the polymers and its e ect on turbulent structures are still unclear Detailed experiments have characterized two distinct regimes Warholic Massah Hanratty 1999 referred to as low drag reduction LDR and high drag reduction HDR The rst regime exhibits similar statistical trends to Newtonian ow the log law



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