| The discovery of turbulent coherent structures, which exhibits some internalorders included in disordered turbulent motions, changes the traditional view thatturbulence is completely random process. The near-wall turbulent coherent structuresplay an important role in the formation of boundary layer resistance and the heat andmass transfer of the fluid. Therefore a better understanding of near-wall turbulentcoherent structures has an important practical significance for people to effectivelycontrol and utilize turbulent boundary layer flow. In the view of turbulent flowresistance and heat transfer, active and passive control methods are employed tocontrol the turbulent boundary layer flow, and the influence of them on the turbulentboundary layer flow and heat transfer characteristics is investigated in this paper.Firstly, the flat channel and wavy wall channel are numerically simulated. Aninitial velocity perturbation based on the feature of turbulent coherent structures isimposed on the velocity field in order to obtain the fully developed turbulent flowfield. The streamwise vortices are the dominant structures appearing in the flow fieldof the simulation results, at the same time, the typical hairpin vortice structures arecaptured well. The simulation results, which are statistically averaged, are comparedwith the calculated results of corresponding DNS and experimental correlations,where excellent matching between the results can be observed.Secondly, the passive control methods are adopted to control the turbulentboundary layer flow. The controls are implemented by using circular hole surface,furrowed surface, furrowed surface based on sinusoidally shaped wave and corrugatedsurface with streamwise and spanwise waves. In the circular hole surface models, thefluid under the circular hole surface can absorb part of momentum impact which isfrom the outer boundary layer and makes the outer momentum buff in the inner layer.In the momentum transmission process, the ejections and bursts of the coherentstructures are impeded that makes drag reduction. The furrowed surface makes thefluid boundary layer separate and fragmentate, after that a large number of inducedvortex are generated. The vortex structures, which mainly include ejections andsweeps, change the flow structures near the furrowed surface. Recirculation zones, which are accompanied with generation of lots of streamwise vortices, appear in thevicinity of furrowed wall which is based on sinusoidally shaped wave. Therecirculation zones and new generated streamwise vortices change the flow structuresnear the wall. The corrugated surface with streamwise and spanwise waves convergesthe near-wall vortices into the spanwise troughs and pushes them away from the wallbecause of the waves in the spanwise direction. Comparing with the traditional wavywall, the waves in the spanwise direction make the drag reduce and enhance thecomprehensive thermal performance factor.Finally, the active control on the wall-normal and spanwise velocity fluctuationsin the turbulent boundary layer are investigated. The wall-normal velocity fluctuationcontrol damps the ejections and bursts which have greater contributions to Reynoldsstress and that makes the flow resistance reduce. The spanwise velocity fluctuationcontrol inhibits the effects between the high and low speed streaks, impedes the streakinstability and prevents the generation of streamwise vortex that makes the dragreduce.The basic elements of turbulent boundary layer coherent structures are streakstructures and vortex structures. And the ejections, sweeps and bursts are the mainmotions of coherent structures. The results of this paper indicate that to control anypart of coherent structure events can achieve the control aim of turbulent flowresistance and heat transfer. Every event of coherent structures has differentcontribution to flow resistance and heat transfer, so to control different event ofcoherent structures can get the better comprehensive effect of fluid flow and heattransfer. |
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