The Largest Scales in Turbulent Pipe FLow

A large amount of the energy needed to push fluids through pipes worldwide is dissipated by viscous turbulence in the vicinity of solid walls. Therefore the study of wall­bounded turbulent flows is not only of theoretical interest but also of practical importance for many engineering applications. In wall­bounded turbulence the energy of the turbulent fluctuations is distributed among different scales. The largest energetic scales are denoted as superstructures or very­large­scale motions (VLSMs). In our project we carry out direct numerical simulations (DNSs) of turbulent pipe flow aiming at the understanding of the energy exchange between VLSMs and the small­scale coherent structures. While the near­wall small­scale structures scale in viscous units, the outer flow VLSMs scale in bulk units. Hence the range of scales increases as the Reynolds number of the flow increases. In order to study the interaction between these structures, we carried out DNSs of friction Reynolds numbers up to ReΤ=2,880, where ReΤ=uΤR/ν is based on the friction velocity, the pipe radius and the kinematic viscosity. Besides a large Reynolds number, required for large scale separation, a sufficiently long computational domain is needed for VLSMs to settle. In a preliminary study the required computational domain length was estimated to L=42R