Study of the Upstream Influence of the Diffuser of CICLoPE “Long Pipe” Using Oil Film Interferometry

Wall-bounded turbulence is an extremely relevant topic for engineering and natural science application even though many aspects of this physics are not clear yet. Due to the relevance of these flows, in recent times, several facilities were developed to investigate turbulent flows at high-Reynolds number under fully developed conditions. In particular, the “Long Pipe” in CICLoPE is a 111.5 m full carbon fibre pipe with a diameter of 0.9 m, minimising spatial and temporal filtering issues, a common problem that arises as the Reynolds number increases. Several measurements documenting fully developed conditions in the test section of this facility have been previously reported. A detailed investigation of the turbulent flow in this location is important not only because this is the position where measurements are typically performed; but also to exclude that there is no upstream influence of the diffuser which is mounted downstream. While previous work reported on the invariance of the velocity profiles in the near-wall region, in this work we focus on the centerline velocity and the local wall-shear stress.