Experimental study on boundary-layer transition control by spanwise discrete suction

Delaying boundary layer transition is an important way to reduce friction drag of vehicles in air and water. Introducing steady streaks upstream is a new idea proposed recently to delay transition. Available studies demonstrated that the steady streaks of higher amplitude have higher ability to depress T-S wave propagation, and thus are more effective in delaying transition. However, only relatively low amplitude steady streaks (12%U, where U is the freestream velocity) can be obtained by the roughness elements method commonly used previously. This is because the generation of higher amplitude steady streaks needs higher roughness elements, which will lead to the bypass transition due to the stronger perturbation of vortex shedding caused by the elements. Moreover, recent theoretical studies show that there are some kinds of streaks wich do not suppress but instead promote the transition because of their acceleration of secondary instability. But this has not been verified by experiments yet. Thus, at least two problems are valuable to be studied for the flow control technique. One is to develop an effective method to generate higher amplitude steady streaks; the other is to test the control effect of different streaks on transition. In our studies, a new method, called spanwise discrete suction, has been developed to generate high amplitude steady streaks. Based on this, the streak control on the artificially excited transition in flat plate boundary layer has been studied using hydrogen bubble timeline method, by detecting the variation of perturbation evolution before and after the steady streaks introduced. The influences of streak spacing (corresponding to the suction hole spacing) and streak amplitude (depending on the suction intensity) are tested. The results show that spanwise discrete suction is an effective method to generate high amplitude streaks. The maximum amplitude of steady streaks generated by this method reaches as high as 28.4%U, even higher than the upper amplitude threshold value of 26%U predicted theoretically for steady streaks. Moreover, both of the two steady streaks with spacing of 14mm and 28 mm introduced in our study suppress the flow breakdown, and thus delay the transition; The higher the streak amplitude or the narrower the streak spacing is, the more effective the streaks are in depressing the transition.