Experimental Study On Multi-scale Coherent Structures In Turbulent Boundary Layer Controlled By A Group Of Parallel Heating Wires On The Wall

An experimental study is carried out to investigate multi-scale eddystructure control in wall turbulence. The local heating generates a groupof organized stream-wise vortex structures. Local heating is given to theboundary layer flow by means of a set of longitudinal parallel heatingwires fixed on the surface of flat plate.In order to study the effects of drag reduction, distributions of thestream-wise mean velocity profile are measured. First longitudinalfluctuating velocity measurements have been finely carried out using hotwire anemometer at different vertical locations in the near wall region ofa turbulent boundary layer ,at two span locations. Skin friction velocityhas been calculated through fitting the logarithmic law of mean velocityprofile. According as formula τ_w=ρu~2 , the skin friction shear stress iscalculated.A phase-averaging technique is employed to capture the multi-scalecoherent structures. It bases on conditional sampling , wavelettransformation and local average technique.It is found that with locally heating, the phased-average evolutionwaveforms show a typical shape in buffer layer. At the same time, thedetected coherent structures and the flatness factors for multi-scale eddystructure are increased, the turbulent flow fluctuating kinetic energy andcoherent structure energy for main scale enhanced, the shape of PDF forwavelet coefficient broaden .The correlativity of flow structures areobviously enhanced and the flow trends to be regularity. Add up to localheating on the wall organized the multi-scale eddy structure orderly, withthe result that skin friction shear stress is reduced, in the center of twowires.