| Addition of a minute amount of high molecular polymer to turbulent channel flows can lead to drag reduction and there exists a phenomenon of multistage transition.In order to study the mechanism of drag reduction,experiment method,theory analysis method and computation method are applied for studying to propose hypotheses.In study of characteristics and stability of turbulent drag reduction multistage transition,with minimal flow unit(MFU)solutions applying to direct numerical simulation,multistage transition phenomenon of spanwise box size and velocity gradient(Reynolds number equaled 3600)was found.In this paper,it's necessary to study the turbulence drag reduction with higher Reynolds number.The same effective viscosity model is derived based on both Maxwell flow model and FENE-P(finitely extensible nonlinear elastic Peterlin)molecule model respectively.The software Fluent is used to perform the numerical simulation and the udf can be applied for revising the viscosity in Navier-Stokes equation.Reynolds Stress Equation Model(RSM)is chosen,with enhanced wall treatment applied to near wall region.Drag reduction percentage and streamwise turbulent statistics are analyzed in this paper.Several riblets are set on the channel top and bottom wall to research the influence boundary layer change on characteristics of turbulent drag reduction multistage transition.Comparing to the results from experiment and DNS data,the conclusions are as follows:(1)Adjacent drag reduction multistage regimes can be divided by the transition points at which percentage drag reduction-drag reduction parameter curve slope bursts rising.It is basically corresponding to LDR(Low Drag Reduction),HDR(High Drag Reduction)and MDR(Maximum Drag Reduction)divided by percentage drag reduction.High Reynolds number leads to better drag reduction effect and a slight lag of HDR or MDR regime.(2)Mean velocity profile especially when the drag reduction parameter corresponds to MDR regime is in agreement with results of Virk's phenomenological model.The velocity difference is not obvious in the same drag reduction regime while velocity tends to change significantly when drag reduction regime transition happens.The regions in the channel wall normal direction can be verified separately by relationship type between velocity and distance to wall.(3)There is a negative correlation between Reynolds stress and drag reduction parameter.Stepping away from the wall,viscous dissipation tends to be leading role,with the flow stability improving.Turbulence intensities in wall-normal direction decrease with the drag reduction parameter increasing.Fluctuations in wall-normal direction play a pivotal role to drag reduction.The peaks of Reynolds stress and turbulence intensities move towards channel centerline with drag reduction parameter increasing,which reveals that turbulent structures are changed and buffer layer is thickened.Buffer layer is the main region where drag reduction happens.(4)Riblets destroying the turbulence boundary layer,the polymer drag reduction effect is reduced.The drag reduction effect is based on stable boundary layer.The thickness of buffer layer differs with the drag reduction regimes.When the height of riblets surpass the buffer layer,percentage drag reduction will decline apparently.The peak of wall shear stress in polymer-induced drag reduction flow approaches to that in Newtonian flow,which behaves similarly to Reynolds stress and turbulence intensities when closing to channel centerline.(5)High molecular polymer additives have negative effects on turbulent coherent structure,which results in decrease of streak structure and streamwise vortex.Destroying effects of riblets on boundary layer especially in buffer layer region can influence the turbulence structures,thus streak loses its stability and streamwise vortex occurs.Bursting process causes Reynolds stress,the sweep downward brings high drag to wall.Consequently,the polymer-induce drag reduction effects are reduced. |
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