Effect Of Reynolds Number On Drag Reduction In A Turbulent Boundary Layer By Blowing

In recent years,the turbulent boundary layer drag reduction is one of the frontier field of fluid mechanics and the effect of Reynolds number on drag reduction is also a hot research direction.Active control has been widely studied because of its great drag reduction potential and controllability.The blowing control method is characterized by convenience,operability and obvious drag reduction effect,so it is used to turbulence control method in this subject.The wall shear-stress was measured by a new movable Wollaston wall hot-wire(WWHW),which can measure the wall shear-stress under high Reynolds number of turbulent boundary layer.The rectangular cavity under WWHW was established to reduce the Wall effect and study the effect of the slot width(No slot,slot width 0.1,0.2,0.4 mm)on response frequency and measurement sensitivity of WWHW.The experimental results shows that the slot width has no significant effect on the response frequency of hot-wire,and can significantly improve the sensitivity of the measurement when the slot width is greater than 0.1 mm.The optimal slot width is 0.2 mm.The normalized wall shear-stress fluctuation measured by WWHW in l~+=15-22 is consistent with DNS results,indicating that WWHW has a high measurement accuracy.The actuator has 31 streamwise slits which aligned in spanwise direction and slit is20 mm long,0.25 mm wide,2 mm spacing.The intensity and frequency of blowing were controlled by flow valve and electromagnetic valve.The drag reduction experiment was carried out at four Reynolds Numbers,including U_?=5,10,15,25 m/s,corresponding to Re_?=924,1707,2096,2820,respectively.The Tungsten wall hot-wire and WWHW were applied to measure the wall shear-stress at x=5 mm of downstream.Experiment results shows that the exponent?of the maximum drag reduction decaying with the Reynolds number is 1.2 larger than DNS results(?=0.2-0.4).The attenuation exponent?is varies greatly across the space of control parameters(A~+=0.4,0.8,1.3),it indicating that the Reynolds number is not the only independent variable of affecting drag reduction rate.Comparison of the normal profile of flow velocity and turbulence intensity in turbulent boundary layer for without and with control that on the same blowing control parameters(A~+=1.6)under Re_?=924 and Re_?=1707,respectively.The experimental result shows that due to the increase of Reynolds number,the influence of blowing control on flow field was weakened and suppression of burst was also reduced.