Mixing into a turbulent boundary layer with freestream turbulence

We experimentally explore the ability of freestream turbulence (FST) above a turbulent boundary layer (TBL) to facilitate the transport of a passive scalar into the TBL. Previous studies have demonstrated that increasing turbulence intensity facilitates mixing of a passive scalar in a flow. Moreover, studies focussed on how FST influences a TBL have demonstrated that increasing the intensity of the FST generally increases the wall friction velocity and the boundary layer thickness. In contrast to the growth of the boundary layer thickness, a TBL in the presence of FST sees its coherent shear layers (demarcating, for instance, uniform momentum zones) pushed closer to the wall. The question that arises is: does the manipulation of shear layers help or hinder the mixing? An experiment was conducted where a passive scalar was introduced at a fixed height (outside the TBL) in an open channel flow, and then an active grid was used to vary the FST. It is demonstrated that indeed the FST facilitates the mixing of fluid from outside the TBL into the TBL, but perhaps not for the most intuitive reasons. The scalar appears to pass straight through the velocity/shear based interfaces. The talk will explain the mechanics of what is happening.