Study On Active Control Of Turbulence For Drag Reduction Based On Near-wall Coherent Structures

Turbulent drag reduction is always a hot topic in fluid mechanics. Due to the closerelationship between the near-wall coherent structures and high skin-friction in wallturbulence, the active drag-reduction control has been developed by instantaneouslyinterfering with the evolution of near-wall coherent structures. Active control ofturbulent coherent structures is an attractive research issue owing to its robustperformance, great potential and high efficiency. In recent years, the development of theMEMS technology provides us the possibility for the real practical application of activeturbulence control. As a prerequisite for the design of active control systems based onMEMS, the mechanism and control schemes for drag reduction should be deeplyunderstood. In this thesis, the mechanism and control schemes for drag reduction basedon the manipulation of the near-wall coherent structures are studied via direct numericalsimulation (DNS) of turbulent channel flow. The main work and results are as thefollowing:(1) The dynamical mechanism of turbulence suppression is explored via thetransient response of Reynolds stress and enstrophy transport to the opposition control.It is found that the balance of the Reynolds stress in the wall normal direction is firstbroken by the pressure-related term which plays a key role in the subsequent globalsuppression of turbulence. In the enstrophy transport, the stretching of the vorticity isattenuated and the wall normal vorticity makes a significant contribution to thesuppression of enstrophy.(2) The relationship between the measurable flow quantities (streamwise andspanwise wall shear stressτ wx,τ wzand wall fluctuating pressure p~_w) and thenear-wall streamwise vortices is studied using the DNS databases of fully developedturbulent channel flow. It is found that all the three wall quantitiesτ wx, p~_wandτ wzareclosely related with the near-wall streamwise vortices.τ wxcorresponds to the sweep andejection motions induced by the downstream streamwise vortices.τ wzis the spanwisefootprint of the above vortical structures. Compared with the wall shear stress, thecorrelation between p~_wand the near-wall streamwise vortices is more complex. In theupstream of the detecting point, the high pressure region (p~_w>0) corresponds to the sweep motion on the down-wash side of the streamwise vortices, while the low pressureregion (p~_w <0)corresponds to the ejection on the up-wash side of the structure; in themiddle, a low pressure region is formed just below the streamwise vortices; in thedownstream, the correspondence between p~_wand the sweep/ejection motions is inopposite to that in the upstream. Considering the practical requirement of turbulencecontrol, a random blowing/suction at the wall is introduced to examine the robustness ofthe relationship. The results show that the relationship based onτ wxis destroyed whilep~_wandτ wzstill exhibit an excellent correspondence with the near-wall streamwisevortices.(3) Two new control algorithms based onτ wzand p~_ware proposed for turbulencedrag reduction and realized by the two typical actuations "blowing/suction" and "smartskin". The control schemes based onτ wzusing "blowing/suction" and "smart skin"obtained16%and5%drag reduction while the control scheme based on p~_wgets11%and2%drag reduction, respectively.In summary, the present work has deepened our understanding of the mechanismof turbulent drag reduction. The inner relationship between the wall-measurable flowquantities and the near-wall streamwise vortices has been disclosed. Two new controlschemes based on the wall-measureable local information have been constructed. Itprovides with us valuable ideas and methods for the development of active turbulencecontrol for drag reduction based on near-wall coherent structures.