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.

[1] A new photogrammetric technique has been developed for monitoring the morphology of evolving stream beds. A pair of commercial cameras is used to record the evolution of the bed, and a computational approach that consists of a set of computer-vision and imageprocessing algorithms is employed to analyze the videos and reconstruct the instantaneous 3-D surface of the bed. Time-and space-resolved measurements are obtained to generate accurate representations of the bed. The required setup for the implementation of the technique is relatively simple and minimally intrusive. A thorough description of the algorithms that were used and detailed instructions for the implementation of the technique is provided. High-resolution measurements of a gravel bed in a clear-water, bridge scour experiment were carried out to demonstrate the operation and validate the capabilities of the technique. The new technique shows advantages compared to existing methods in terms of spatial resolution, temporal resolution, simplicity, and cost.

Abstract:The measurement of the morphologic characteristics of evolving sediment beds around hydraulic structures is crucial for the understanding of the physical processes that drive scour. Although there has been significant progress towards the experimental characterization of the flow field in setups with complex geometries, little has been done with respect to the quantitative investigation of dynamic sediment bed geometry, mainly due to the limited capabilities of conventional instrumentation. Here, a recently developed computer vision technique is applied to obtain high-resolution topographic measurements of the evolving bed at the base of a cylinder during clear water scour, without interrupting the experiment. The topographic data is processed to derive the morphologic characteristics of the bed such as the excavated volume and the slopes of the bed. Subsequently, the rates of scour and the bathymetry at multiple locations are statistically investigated. The results of this investigation are compared with existing flow measurements from previous studies to describe the physical processes that take place inside a developing scour hole. Two distinct temporal phases (initial and development) as well as three spatial regions (front, side and wake) are defined and expressions for the statistical modelling of the bed features are derived.

This paper reports a comprehensive study of the major scour agent around bridge piers: the turbulent horseshoe vortex . The intricate and inherently unsteady characteristics of the junction flow are captured within a series of scaled laboratory experiments. We applied the state-of-the art Time-Resolved Digital Particle Image Velocimetry (TR-DPIV) technique to measure the velocity field at the centerline plane of symmetry upstream of a cylindrical model. Three levels of Reynolds numbers (ReD) based on the obstacle diameter were studied: 26,000, 48,000 and 117,500. We evaluated the effect of this factor based on the time-averaged analyses of velocity and vorticity. Basic statistical analysis of the fluctuating velocity components provided insight to the physical mechanism that governs the behavior of the horseshoe vortex at the aforementioned levels of ReD.

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