Experimental investigation of upstream cube effects on the wake of a wall-mounted cylinder: Wake rising reduction, TKE budget and flow organization

Abstract

The development of Marine Renewable Energy based on tidal currents at a selected area needs to first analyze the flow characteristics and thus the bathymetry induced turbulence in this area. The present study aims to investigate turbulence characteristics around two wall mounted obstacles in tandem, which represent seabed obstacles in a high Reynolds number flow. 2D planar PIV measurements are performed in multiple planes to access the main flow organization. Based on successive analyses of mean flow characteristics as well as of the flow organization, it is demonstrated that the presence of an upstream cube clearly impacts the cylinder wake development by preventing the rising of the large scale flow structures observed for an isolated wall-mounted cylinder in the same flow conditions. The TKE budget analysis shows that the energy is mainly produced in the upstream cube shear layer, and TKE exchanges are very small in the far wake field, in comparison with a single cylinder flow configuration. A Fourier analysis shows that the cube–cylinder interaction generates a low frequency peak in the far wake field which persists for long downstream distances. This particular frequency is the signature of the cylinder wake coherent structures. A statistical analysis based on the two-point correlation velocity tensor confirms that energetic flow structures present in the far wake are mainly of 2D nature and develop horizontally. Elongated structures are identified in the wake and seem to correspond to hairpin-like structures. Instantaneous vortex structures are also characterized with local vortex identification swirling strength criterion, confirming previous FFT and statistical results. The impact of the upstream cube is strongest in the near wake in the symmetrical transverse plane and prevents the large scale flow structures from rising in the water column.