Turbulent flow around a 3D stepped cylinder: statistical and modal analysis of different flow regimes

The present study focuses on the numerical investigation of the turbulent external flow around a 3D stepped cylinder. This flow consists of two cylinders with different diameters joint at one extremity and it represents a good model for real-world applications, e.g. the foundations of offshore wind turbines. We perform direct numerical simulations with the adaptive mesh refinement technique in the spectral elements code Nek5000. The Reynolds number varies from Re_D=150 to Re_D=5000, which is the highest considered for a DNS for that flow. The case is ideal for performing a methodological study of the spectral error indicator (SEI), which is used to measure interpolation and quadrature errors. Furthermore, this configuration shows interesting physical behaviours, as the junction vortex dynamic strongly depends on the local Re_D and three different cell structures are developing in the wake regions. We present new physical insights into the irregular wake regime, looking at the time-averaged first and second orders statistical moments. Eventually, we perform a proper orthogonal decomposition to quantify the most energetic coherent structures and establish a reduced-order model. A new mode responsible for the updraft has been identified for the higher Re_D.