Experimental investigation of artificial boundary layer transition

Abstract

Nowadays there is a wide variety of applications for forced transition. They are all based on introducing disturbances to the flow and increasing its momentum thickness. Forced transition or bypass transition as it is also called is used to manipulate the flow by setting the transition point in such a way that the required/better flow situation is achieved. There is a variety of types of transition devices in use such as a simple wire, sandpaper, surface steps and zigzag tape. All of them have their own characteristics. In this report an investigation of the last two has been given. From practical experience it is known that the zigzag tape produces transition to turbulence with less height of the obstacle. This implies that the zigzag strip produces turbulence more efficiently than a two‐dimensional roughness. The difference in efficiency suggests that there may be differences in the mechanism creating turbulence behind these two kinds of devices. What happens in the transition phase of the flow over these obstacles has been investigated. For this investigation hot wire anemometry has been used to locate and quantify turbulence and particle image velocimetry to visualize structures in the flow. The results confirm the different behaviour (efficiency) of the two tripping device types. A clear difference is revealed in the flow comparing the zigzag strip to the two‐dimensional roughness element composed of a strip of rectangular cross‐section. The zigzag strip causes streamwise vortices which interact with the outer flow to create turbulence through mixing. This explains why it is more efficient. These vortices persist quite far downstream, hence the turbulent flow is only slowly moving to a uniform state. This means the flow is not completely uniform. The surface step creates a spanwise vortex which causes the mixing. This vortex doesn’t trail far behind the strip and thus creates a more homogeneous turbulent flow downstream of it.