Effects of wall roughness on turbulent junction flow characteristics

Abstract

Global measurements of turbulent flows at wall–cylinder junctions are employed to quantify the effects of wall roughness on the behavior of the horseshoe vortex system (HVS). Two laboratory setups were considered: one with an impermeable smooth wall and a second characterized by a porous hydraulically rough bed. The measurements were obtained using planar particle image velocimetry. Time-averaged flow topology, turbulence statistics, and instantaneous fields associated with the streamwise and wall-normal velocity components are emphasized. Proper orthogonal decomposition (POD) is also applied on the velocity signals to probe into the characteristics of the energetic flow structures. For the Reynolds numbers studied here and the specific differences in the roughness geometry of the bed, a clear trend for the increase in flow incoherence due to the rough wall is documented. It is also demonstrated that, in the presence of roughness, vorticity and turbulence spread more evenly throughout the junction. On the other hand, qualitative and quantitative agreement between the smooth and rough bed tests is found in the structure of the downflow and the near-wall jet opposing the bulk flow. The efficiency of POD in analyzing turbulent junction flows is justified based on its results and metrics of modal energy distribution. POD verified in an objective way the role of integral components of the HVS dynamics such as the vortices comprising the system and their interplay with the wall. The decomposition furnishes new evidence about energetic structures that were not captured with the other data analysis methodologies. It also confirms the aperiodic behavior of the HVS for the investigated Reynolds numbers.