Time-resolved flow dynamics and Reynolds number effects at a wall–cylinder junction

Abstract

This study investigated the physics of separated turbulent flows near the vertical intersection of a flat wall with a cylindrical obstacle. The geometry imposes an adverse pressure gradient on the incoming boundary layer. As a result, flow separates from the wall and reorganizes to a system of characteristic flow patterns known as the horseshoe vortex. We studied the time-averaged and instantaneous behaviour of the turbulent horseshoe vortex using planar time-resolved particle image velocimetry (TRPIV). In particular, we focused on the effect of Reynolds number based on the diameter of the obstacle and the bulk approach velocity, Re D . Experiments were carried out at Re D : 2.9 × 10 4 , 4.7 × 10 4 and 12.3 × 10 4 . Data analysis emphasized time-averaged and turbulence quantities, time-resolved flow dynamics and the statistics of coherent flow patterns. It is demonstrated that two large-scale vortical structures dominate the junction flow topology in a time-averaged sense. The number of additional vortices with intermittent presence does not vary substantially with Re D . In addition, the increase of turbulence kinetic energy (TKE), momentum and vorticity content of the flow at higher Re D is documented. The distinctive behaviour of the primary horseshoe vortex for the Re D = 12.3 × 10 4 case is manifested by episodes of rapid advection of the vortex to the upstream, higher spatio-temporal variability of its trajectory, and violent eruptions of near-wall fluid. Differences between this experimental run and those at lower Reynolds numbers were also identified with respect to the spatial extents of the bimodal behaviour of the horseshoe vortex, which is a well-known characteristic of turbulent junction flows. Our findings suggest a modified mechanism for the aperiodic switching between the dominant flow modes. Without disregarding the limitations of this work, we argue that Reynolds number effects need to be considered in any effort to control the dynamics of junction flows characterized by the same (or reasonably similar) configurations.

Experimental Study on Scour at a Sharp-Nose Bridge Pier with Debris Blockage

“…This finding differs markedly from observations for rectangular/circular piers for which the maximum scour depth d s is seen to occur at the upstream face of the pier (e.g., Melville 1975;Breusers et al 1977;Lagasse et al 2010). The different locations of maximum scour depth for rectangular/circular piers and sharp-nose piers may be due to the following two factors related to the flow around the pier: First, the turbulent horseshoe vortex (THV) system, which is known to be the primary factor causing scour at the upstream face of rectangular/circular piers (Kirkil et al 2008;Escauriaza and Sotiropoulos 2011;Link et al 2012;Apsilidis et al 2015;Bouratsis et al 2017;Chen et al 2017), has been observed by previous researchers (Shen et al 1966;Breusers et al 1977) to be not as well pronounced at the leading edge of sharp-nose piers. Second, the flow separation and the shedding of the two shear-layers at the two side corners of a sharp-nose pier increases bed shear stresses at these corners (Khosronejad et al 2012;Vijayasree et al 2017) and may thereby lead to maximum scour occurring at these locations.…”

“…Experimental and computational research, particularly over the last few decades, have enhanced significantly the understanding of the scour process around bridge piers by describing the underpinning science, particularly of the flow field and the turbulent horseshow vortex (THV) system upstream of circular and rectangular piers (e.g., Melville and Coleman 2000;Kirkil et al 2008;Escauriaza and Sotiropoulos 2011;Link et al 2012;Apsilidis et al 2015;Unsworth 2016;Bouratsis et al 2017;Chen et al 2017;Ettema et al 2017).…”

Impact Assessment of Pier Shape and Modifications on Scouring around Bridge Pier

“…Experimental and computational research have significantly enhanced the understanding of the local scour process around bridge piers by describing the underpinning science, particularly of the vortex systems and scour patterns around circular and rectangular pier shapes [2,[14][15][16][17][18][19][20]. Recently, Manes and Brocchini [21] derived a new predictive formula to calculate the scour depth at piers, which merges the phenomenological theory of turbulence with empirical observations.…”

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

“…In an effort to better understand the physics behind scour, fluid flows around wall-mounted obstacles, such as cylinders or wings, have been described from experimental observations [3][4][5][6] or numerical simulations [7][8][9]. These studies examined the time-averaged and time-resolved characteristics of flows around the obstacles.…”