DNS Study Of Rotation Effects On Turbulent Viscoelastic Plane Couette Flows

The system rotation and the fluid elasticity are often seen in the practical industrial flows.Wall turbulence under the coupling effects of the two involves the transition of complex flow states,which is an important scientific problem in the field of fluid mechanics and nonlinear science,and its physical mechanism has guiding significance for industrial applications and fundamental science.In this paper,a spanwise-rotating plane Couette flow(RPCF)is studied via a series of direct numerical simulations conducted by changing the strength of system rotation(Ro)at given Reynolds number(Re)and fluid elasticity(Wi).The research purpose is to explore the flow structure changes and turbulent dynamics characteristics under these two coupling effects.The main contents of this work include:(1)the effect of rotation on turbulence characteristics at a low Re of 1300;(2)flow transition process driven by rotation at a high Re of 5200.The main findings are obtained as folllows:1.Rotation effects on turbulence features have been examined in viscoelastic RPCF at low Reynolds number Re=1300 and weak elasticity effect Wi=5 by increasing the system rotation Ro=0.02～0.9.Based on the detailed investigation of turbulence statistics,energy budget and polymer behaviour,viscoelastic RPCF can be divided into three regimes:weak rotation for Ro ≤ 0.1,intermediate rotation for 0.1<Ro<0.4,and strong rotation for Ro≥ 0.4.With the increase of Ro,the flow changes from the inertial turbulence with large-scale roll cells to the elasto-inertial turbulence(EIT)state with a large number of small-scale turbulent vortices.The intense polymer-turbulence interaction is found to occur primarily near the stagnation points of the extensional flows between two neighboring roll cells at weak and intermediate rotation,whereas for the high-Ro EIT state it happens in the bulk region as the small-scale turbulent vortices serve to homogenize the polymer dynamics via their vortical circulations.These findings lead to a new understanding of polymer-turbulence interaction under system rotation.2.The flow transition of viscoelastic RPCF driven by rotation(Ro=0～1)at Re=5200 and Wi=10 is explored.It is found that the transition path starts from the inertial-dominated turbulence with drag reduction(0≤Ro<0.12),passes through the inertial-dominated turbulence with drag enhancement(0.12 ≤Ro<0.24),and finally reaches the elasticity-dominated turbulence with drag enhancement(0.24≤Ro≤1).Through the examination of the drag characteristics,it is found that the polymer can reduce the turbulence by inhibiting the generation of Reynolds stress,meanwhile,polymer stress acts as a drag increasing source term.The combination of these two effects leads to drag reduction/drag enhancement.By studying the energy transfer of turbulence field and viscoelastic stress field,it is found that the Coriolis force term replaces the previous pressure-strain term as the main energy source of<v’ v’>.The energy relationship of viscoelastic RPCF with universal properties is also summarized.In addition,it is also found the re-entry of turbulence state at Ro=1 under the viscoelastic conditions.This is because the polymer chain is strongly stretched by the action of vortices generated by the Coriolis force,and the polymer stress become non-negligible throughout the channel,releasing a large amount of elastic energy so that the flow become turbulent.