Reynolds Number Effect On Drag Reduction In Turbulent Boundary Layer Under Active Control

Reducing the drag of transport vehicles is important for the country’s sustainable development,and a large part of its resistance comes from the boundary layer.In addition,many related researches on drag reduction conducted by many laboratories and simulations are based on low Reynolds numbers,but the Reynolds number in real life is very large.Therefore,it is necessary to study the effect of Reynolds number on the active control drag reduction of the boundary layer.There are many ways to measure the boundary layer dr ag reduction.It is a common method to measure the drag reduction by the slope layer hot line using the slope method.However,when the hot wire is too close to the wall surface,it is affected by the wall effect,and when the Reynolds number is high,the linear region of the boundary layer is thin,etc.,which causes the slope method to be no longer applicable.The purpose of this paper is to design a new type of wall shear stress sensor(also known as wall-wire)to measure the drag reduction at high Reynolds number to study the effect of Reynolds number on the active control drag reduction of the boundary layer.In order to study the influence of the wall effect,a total of four wall-wire were designed,and the wall-wire without slit and the wall-wire with slit widths of 0.1 mm,0.2 mm,and 0.4 mm,respectively.There are three methods for verifying the wall-wire:(1)calibration of the wall-wire in the large flat plate of the wind tunnel,using the theoretically determined shear stress at different wind speeds and the actual measured voltage to calibrate the voltage and shear according to the calibration formula.Force,and determine its kurtosis,turbulence,etc.;(2)using the boundary layer hot wire to determine the drag reduction at 20 Hz and 300 Hz at wind speeds of 2.4 m/s and 5 m/s by slope method.In the case,the wall-wire designed in this paper is used for the measurement,and the drag reduction data obtained by the two hot wire are compared;(3)the pressure difference at different positions is measured by the pressure scanning valve in the channel wind tunnel to find the non-ventilating speed.The shear force is simultaneously measured at the corresponding wind speed to complete the calibration of the wall-wire.Finally,it is found that the wall-wire with the slit width of 0.1mm and 0.2mm is better than 0.4mm.The seamless wall-wire has a poor fitting effect due to the wall effect.The measured drag reduction data is measured at high wind speed and slope method.The drag reduction data is not much different.It is indicated that the wall-wire designed in this paper is feasible.Finally,the drag reduction under different Reynolds numbers is measured by the designed wall-wire.It is found that when the Reynolds number increases,the maximum drag reduction decreases.