Direct Numerical Simulation Of Viscoelastic Turbulent Spanwise-Rotating Plane Couette Flow

Wall-bounded turbulence(WBT)under the coupled effects of fluid elasticity and system rotation,two important factors widely encountered by industrial fluid flows,is one of the challenge scientific problems that are fundamental to the research area of nonlinearity and complex flows.The development of engineering technology and basic scientific research benefits a lot from its mechanism research.This dissertation is dedicated to viscoelastic spanwise-rotating plane Couette flow(RPCF)with emphasis on the flow transitions and underlying mechanisms of WBT under the coupled effects of elasticity and rotation via direct numerical simulations(DNS)and theoretical analysis.We explore the novel transition in viscoelastic WBT and the mechanistic understanding of polymer-turbulence interactions in those distinct turbulent states when varying the effect of elasticity and rotation.The following issues are addressed in this dissertation:(1)rotation-driven flow transitions;(2)polymer-induced flow intermittency;(3)turbulent flow relaminarization and drag enhancement;(4)maximum drag enhancement asymptote.The primary findings are summarized as follows:1.Spanwise-rotation-driven flow transitions in viscoelastic RPCF with weak elasticity(Wi=5)is reported for the first time.Specifically,this novel flow transition begins with a drag reduced inertial turbulent flow state at a low rotation number 0<Ro<0.1,then transitions to a rotation/polymer additive driven drag enhanced inertial turbulent regime,0.1≤Ro≤0.3.In turn,the flow transitions to a drag enhanced elasto-inertial turbulent state,0.3≤Ro≤0.9 and eventually relaminarizes at Ro=1.These transitions occur due to the competition between polymer-induced decrease of convective momentum flux and rotationrendered increase of polymer stress.The universal mechanism of the polymerturbulence interactions accounting for drag reduction(DR)is further confirmed by our simulations.In addition,two novel rotation-dependent drag enhancement(DE)mechanisms are proposed and substantiated.More importantly,a novel turbulent flow state characterized by streamwise-elongated small-scale vortices attached to the walls is identified as elasto-inertial turbulence(EIT)generated by Coriolis forces.Overall,this study represents a very significant step towards development of a mechanistic understanding of polymer-induced drag modification in spanwise rotating planar flows and it paves the way for mechanistic investigation of novel rotation driven coherent structures of elastic and elasto-inertial origin.2.Based on the above work,the elastic-effect-induced strong flow intermittency is examined in viscoelastic RPCF at a weak rotation number(Ro=0.02).This intermittency arises due to the interaction between polymer and roll cell structures.Specifically,the addition of the polymer suppresses the turbulent vortical structures.As a consequence,three pairs of weak roll cell which are stable in Newtonian RPCF keep splitting and emerging which in turn enhances the flow intermittency.At higher Wi,polymer additives suppress more vortical structures,whereas in the near-wall region the generation and deterioration of small-scale vortices result in strong flow intermittency.The Reynolds stress which dominates the turbulent self-sustaining mechanism changes dramatically with time and thus quantitatively accounts for the intense flow intermittency.Overall,the weak rotation effect weakens the drag reduction rate of viscoelastic PCF due to the presence of roll cell.Moreover,the transient properties indicate that the "activehibernating" cycle is not universal in all viscoelastic turbulence,thus making it inapplicable to explain DR and maximum drag reduction(MDR)mechanisms.3.Intriguingly,polymer-induced flow relaminarization and DE is reported for the first time in viscoelastic RPCF at moderate rotation(Ro=0.2).Specifically,the reverse transition pathway from a Newtonian turbulent RPCF to a fully relaminarized drag enhanced viscoelastic flow has been elucidated.Evidently,this transition occurs gradually by weakening and eventual elimination of small-scale vortices as polymer elasticity is enhanced,paving the way for a 2D laminar flow consisting of large-scale and highly organized roll cells.The influence of polymer additives on convective momentum exchange by large-scale roll cells and small-scale turbulent vortices,namely,the DR realized by elimination of turbulent vortices and the significant DE that results from polymer roll cell interactions has been identified as the mechanism of DE.The observed vortical changes point to a universal mechanism for the coupling of polymer chains and turbulent vortices in wall-bounded viscoelastic DE and DR flows.4.Inspired by above findings,the existence of the MDE asymptote in viscoelastic RPCF at high rotation(Ro=0.3～0.7)has been discovered for the first time.Specifically,it is shown that at sufficiently large Ro,introduction of polymer additives results in DE in the RPCF.The extent of DE gradually increases as the elastic forces are enhanced,i.e.,Wi is increased,and eventually plateaus at the MDE asymptote.Of critical interest,for the range of Ro considered,the mean velocity profiles at MDE attain a universal log law characterized by an identical logarithmic slope,i.e.,κK-1=1/0.85.Furthermore,it has unequivocally been shown that the MDE asymptote is realized in the EIT flow state that is mainly sustained by elastic forces much alike the MDR asymptote.This points to the universality of the interaction between polymer chains and vortical structures in the MDR and MDE flow states.Overall,the above observations taken together demonstrate that the asymptotic behavior seen in polymer-induced DM,i.e.,MDR and MDE is an inherent property of EIT flow state.To that end,the discovery of the MDE asymptote has paved the way for coordinated experimental/simulations/theoretical studies to establish the universality of asymptotic flow states,namely,MDR and MDE,in EIT flow regime in a broad range of turbulent flows.