Effects of wall roughness on turbulent junction flow characteristics

Abstract

Global measurements of turbulent flows at wallcylinder 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 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 N. Apsilidis · C. L. Dancey · P. Bouratsis ( ) Baker Environmental Hydraulics

Influence of turbulent horseshoe vortex and associated bed shear stress on sediment transport in front of a cylinder

“…The blockage (i.e. flume width to diameter) ratio in the present experiment is 6.25, which is near the lower limit of the acceptable range indicated by referencing previous relevant studies [7,13,16]. The origin of the coordinate system is defined as the center of the cylinder on its bottom surface, with the longitudinal x-axis parallel to the flume bed and oriented with the main flow, the y-axis vertical, and with the z-axis normal to the side wall of the flume.…”

“…A number of related studies of THV have focused on the time-averaged geometrical characteristics [5][6][7][8], the dynamics of THV utilizing particle image velocimetry (PIV) [9][10][11][12][13][14], eddy-resolving large eddy simulation (LES) [15][16][17], or detached eddy simulation (DES) [18]. These studies mainly focus on the flow field induced by the THV, however, little literature reports on the relevant influence and relationship of the THV on sediment transport and erosion, which is essential to identify the mechanisms of pier scour and to further improve the prediction accuracy of scour depth.…”

Quantitative Spatio-Temporal Characterization of Scour at the Base of a Cylinder

“…The groove has also been observed in [39,40] and it has been attributed to the existence of a corner vortex that is located at the intersection of the bed with the cylinder and rotates in the opposite (counterclockwise) direction compared to the primary horseshoe vortex [41].…”

Influence of Scour Development on Turbulent Flow Field in Front of a Bridge Pier

“…The sediment bed was set up on the bottom with median diameter d 50 = 0.6 mm and specific mass ρ s = 2650 kg/m 3 . The blockage ratio (i.e., flume width to pier diameter) in the present test is 6.25, which is in the proposed acceptable range [12,28,29]. The origin of the coordinate system is defined at the bottom center of the cylinder, with the longitudinal x-axis parallel to the flume bed, the y-axis normal to the side wall of the flume, and the z-axis vertical to the flume bottom.…”