Browsing by Author "Bouratsis, Polydefkis"

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    Effects of wall roughness on turbulent junction flow characteristics
    Apsilidis, Nikolaos; Diplas, Panayiotis; Dancey, Clinton L.; Bouratsis, Polydefkis (Springer, 2015-12-26)
    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.
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    High-resolution 3-D monitoring of evolving sediment beds
    Bouratsis, Polydefkis; Diplas, Panayiotis; Dancey, Clinton L.; Apsilidis, Nikolaos (American Geophysical Union, 2013-02-01)
    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 image-processing 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. Citation: Bouratsis, P., P. Diplas, C. L. Dancey, and N. Apsilidis (2013), High-resolution 3-D monitoring of evolving sediment beds, Water Resour. Res., 49, doi: 10.1002/wrcr.20110.
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    Quantitative Spatio-Temporal Characterization of Scour at the Base of a Cylinder
    Bouratsis, Polydefkis; Diplas, Panayiotis; Dancey, Clinton L.; Apsilidis, Nikolaos (MDPI, 2017-03-20)
    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.
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    Scour at the Base of Hydraulic Structures: Monitoring Instrumentation and Physical Investigations Over a Wide Range of Reynolds Numbers
    Bouratsis, Polydefkis (Virginia Tech, 2015-02-05)
    Hydraulically induced scour of the streambed at the base of bridge piers is the leading cause of bridge failures. Despite the significant scientific efforts towards the solution of this challenging engineering problem, there are still no reliable tools for the prediction and mitigation of bridge scour. This shortcoming is attributed to the lack of understanding of the physics behind this phenomenon. The experimental studies that attempted the physical investigation of bridge scour in the past have faced two main limitations: i) The characterization of the dynamic interaction between the flow and the evolving bed that is known to drive scour, was not possible due to the limitations in the available instrumentation and the significant experimental difficulties; ii) Most of the existing literature studies are based on the findings of laboratory experiments whose scale is orders of magnitudes smaller compared to bridges in the field, while the scale effects on the scour depth have never been quantified. The objective of this research was to enhance the existing understanding of the phenomenon by tackling the aforementioned experimental challenges. To accomplish this, the first part of this work involved the development of a new underwater photogrammetric technique for the monitoring of evolving sediment beds. This technique is able to obtain very high resolution measurements of evolving beds, thus allowing the characterization of their dynamic properties (i.e. evolving topography and scour rates) and overcoming existing experimental limitations. Secondly, the underwater photogrammetric technique was applied on a bridge scour experiment, of simple geometry, and the dynamic morphological characteristics of the phenomenon were measured. The detailed measurements along with reasonable comparisons with descriptions of the flow, from past studies, were used to provide insight on the interaction between the flow and the bed and describe quantitatively the mechanisms of scour. Finally, the scale effects on scour were studied via the performance of two experiments under near-prototype conditions. In these experiments the effects of the Reynolds number on the flow and the scour were quantified and implications concerning existing small-scale studies were discussed.
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    Time-resolved flow dynamics and Reynolds number effects at a wall-cylinder junction
    Apsilidis, Nikolaos; Diplas, Panayiotis; Dancey, Clinton L.; Bouratsis, Polydefkis (Cambridge University Press, 2015-07-13)
    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. In particular, we focused on the effect of Reynolds number based on the diameter of the obstacle and the bulk approach velocity, ReD. Experiments were carried out at ReD: 29000, 47000, and 123000. Data analysis emphasized time-averaged and turbulence quantities, time-resolved flow dynamics, and the statistics of coherent flow patterns. It is demonstrated that two largescale 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 ReD. In addition, the increase of turbulence kinetic energy, momentum, and vorticity content of the flow at higher ReD is documented. The distinctive behaviour of the primary horseshoe vortex for the ReD = 123000 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.