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Scheme of horseshoe vortex upstream the cylinder in the scour hole (Adapted from Muzzammil and Gangadhariah 2003)
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Laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV) are indirect techniques to measure flow velocities. Both
require small particles in the moving fluid, both measure specific statistics of the velocity of such particles, and both
are among the least intrusive techniques to measure flow velocities. LDA techniques have been used in t...
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Citations
... The output laser beam is green with wavelength of 532 nm. This system is described in Ferreira [16]; Ricardo et al. [34]; Brito et al. [7]. A charged coupled device (CCD) Flowsense 2M camera, commercialized by Dantec, with a resolution of 1600 × 1200 pix 2 was positioned perpendicularly to the laser sheet in both layouts. ...
We address the capability of large eddy simulation (LES) to predict the physics of density currents interacting with bluff obstacles. Most density currents of interest in engineering and geophysical applications interact with obstacles or topographic features. Validating LES solutions in these contexts is crucial to establish it as a trusted tool. We thus propose a validation effort based on simple geometries that nonetheless pose challenges common to more complex systems, including boundary layer separation and convective instabilities. We focus on lock-exchange gravity currents in the slumping phase interacting with an emergent vertical circular cylinder. Our main investment was in ensuring that the comparison of experimental data and numerical results include, at least, the velocity and the density fields , and derived quantities (e.g., second order moments). Measurements of both density and velocity fields were performed in the side and plan views for cylinder Reynolds numbers, \(Re_d\), in the range 1300 to 3475. It was found that the LES accurately predicts the temporal evolution of the current front position. The computed front velocity exhibits a maximum relative error less than 8%. A good agreement between the LES and the experimental size and shape of the current head, and billows was found. The overall features upstream the cylinder, including a reflected wave, adverse pressure gradient and backflow, and downstream the cylinder, including the backflow, wake and the formation of a new head are well reproduced by LES. The agreement between the LES and the experimental time-space evolution of current spanwise- and depth-averaged density contours and the instantaneous velocity fields are not affected by \(Re_d\).
... The flow was illuminated by a led panel from the rear and from the under in the side and plan layouts, respectively. The two components of the velocity field in the side view u and w, and plan view u and v were measured by a 2D particle image velocimetry (PIV) system with a double-cavity neodymium-doped yttrium aluminum garnet (Nd:YAG) laser source able to generate a pulse of 30 mJ (Ferreira, 2011;Ricardo et al., 2014). The charged coupled device (CCD) camera was positioned perpendicularly to the laser sheet in both layouts. ...
... One of the waking points of the PIV system is the large time required for the first step of raw data analysis: this process consists in converting the acquired images into instantaneous velocity fields calibrated in metric units and referenced to the channel coordinate system. The typical components of a PIV system are (Ferreira, 2011): ...
En condiciones de flujo uniforme y no uniforme, la escala de velocidad más empleada en canales abiertos es la velocidad de fricción. En condiciones de flujo no uniforme con transporte de sedimentos, la distribución de velocidad de la ley logarítmica podría estar ausente, por lo que no es posible aplicar métodos convencionales para calcular la velocidad de fricción. El objetivo de este trabajo es definir un método para calcular la escala de velocidad en flujos no uniformes con diferentes condiciones de transporte de sedimentos en ausencia de la distribución de velocidad de la ley logarítmica. Las pruebas de laboratorio se realizaron utilizando un sistema PIV para medir los vectores de velocidad en tiempo y espacio, los perfiles de esfuerzo cortante total se calcularon a través del análisis de equilibrio de momento, y el coespectro de la fluctuación de velocidad para definir diferentes valores del factor de escala de velocidad. Para definir cuál de los diferentes valores proporciona la mejor escala de los perfiles de velocidad se hizo una comparación de los perfiles de velocidad adimensionales. Esta comparación muestra que el valor del esfuerzo cortante al nivel de las crestas del fondo móvil ―obtenido a través de una regresión lineal del perfil de esfuerzo cortante calculado a partir del análisis de equilibrio de momento― es el mejor valor de escala de velocidad.
... At both submergence ratios, S f of sweeps in the topboulder region decreased in comparison with the near-bed region. For an unobstructed rough bed, Ferreira (2011) similarly reported attenuation of sweeps as z=H increased. Fig. 5 shows the distribution of S f of each quadrant for all the experiments in the near-bed region upstream and downstream of the boulder. ...
In-stream boulders significantly affect local flow hydrodynamics, sediment transport, and aquatic habitats. This study experimentally investigates the Reynolds shear stress (RSS) profiles and turbulent events around an intermediately submerged boulder under a wake interference flow regime. The frequency of turbulent events and their contribution to the RSS are quantified at two specific submergence ratios of 1.56 and 1.90 around the boulder in the near-bed and top-boulder regions. Ejection and sweep events were generally dominant in the near-bed and top-boulder regions. The dominance of turbulent events around the boulder varied as the submergence level changed. Relationships are proposed to predict the contribution of the near-bed turbulent events to the RSS around the boulder.
... Understanding of the interaction between the sediment motion and the open-channel rough-bed hydrodynamics requires enhanced techniques which in general enable insight into the flow and to quantify turbulent flow characteristics (Ferreira 2011). In this paper velocity profiles, shear stress and turbulence distributions over a rough nonporous bed and the effect of the presence of rolling sediment grains is examined based on Laser Doppler Anemometry and Particle Image Velocimetry (LDA and PIV) in a hydraulic flume. ...
The current work focuses on locally resolving velocities, turbulence, and shear stresses over a rough bed with locally non-uniform character. A nonporous subsurface layer and fixed interfacial sublayer of gravel and sand were water-worked to a nature-like bed form and additionally sealed in a hydraulic flume. Two-dimensional Particle Image Velocimetry (2D-PIV) was applied in the vertical plane of the experimental flume axis. Runs with clear water and weak sediment transport were done under slightly supercritical flow to ensure sediment transport conditions without formation of considerable sediment deposits or dunes. The study design included analyzing the double-averaged flow parameters of the entire measurement domain and investigating the flow development at 14 consecutive vertical subsections. Local geometrical variabilities as well the presence of sediment were mainly reflected in the vertical velocity component. Whereas the vertical velocity decreased over the entire depth in presence of sediment transport, the streamwise velocity profile was reduced only within the interfacial sublayer. In the region with decelerating flow conditions, however, the streamwise velocity profile systematically increased along the entire depth extent. The increase in the main velocity (reduction of flow resistance) correlated with a decrease of the turbulent shear and main normal stresses. Therefore, effects of rough bed smoothening and drag force reduction were experimentally documented within the interfacial sublayer due to mobile sediment. Moreover, the current study leads to the conclusion that in non-uniform flows the maximum Reynolds stress values are a better predictor for the bed shear stress than the linearly extrapolated Reynolds stress profile. This is an important finding because, in natural flows, uniform conditions are rare.
... The seeding particles were fine enough to become ambient fluid particles even though their mass density was greater than that of water. 29,30 Furthermore, care was taken to feed the seeding particles into the flow in a manner that satisfied the general criteria for an accurate PIV measurement. 31 The laser sheet illuminated the test section in order to aid in visualizing the seeding particle motion captured by using the camera placed parallel to the laser sheet. ...
The turbulence characteristics within flows over water-worked gravel beds (WGBs) and screeded gravel beds (SGBs) were examined by measuring the instantaneous flow velocity field using a two-dimensional particle image velocimetry system. To compare the responses of a WGB and an SGB to velocity and various turbulence characteristics, the flow Froude number was kept identical for both the beds that remained immobile. The roughness structures of both the beds were measured using a laser scanner. The results showed that the bed surface roughness was higher in the WGB than in the SGB. However, the longest axis of the gravels of WGBs was oriented streamwise owing to the action of water work, but the gravels of SGBs were randomly poised. The distribution of bed roughness fluctuations was negatively skewed in the WGB and positively skewed in the SGB. Double averaging methodology was applied to analyze the flow parameters. In this paper, the vertical profiles of the double-averaged streamwise velocity and the turbulence parameters, specifically the spatially averaged (SA) Reynolds shear and normal stresses, form-induced shear and normal stresses, turbulent kinetic energy (TKE) and form-induced TKE fluxes, quadrant analysis of SA Reynolds shear stress, etc., are presented and analyzed critically by focusing on comparisons between a WGB and an SGB. A comparative study reveals that in the near-bed flow zone, the SGB underestimates the turbulence parameters compared to the WGB. Therefore, in order to represent the prototypical flow in laboratory, the experiments should be performed in a WGB.
... PIV is a nearly nonintrusive technique that allows to obtain quasi-instantaneous velocity fields by measuring the displacement of seeding particles in the flow (Raffel et al. 2007;Ferreira 2011). The seeding particles used in this work were made of polystyrene with nominal diameter of 15 μm and density close to the water density. ...
... These features ensure that the particles follow the water motion, not affecting the flow characteristics (Raffel et al. 2007). The seeding particles are illuminated twice in a short period of time by means of thin light sheet from a pulsed laser and the light scattered by the particles is captured by a CCD camera (Ferreira 2011). A cylindrical lens was used to produce the laser sheet and placed in a small cylindrical box attached to the laser head, allowing the orientation of the light sheet. ...
... A double-cavity neodymium-doped yttrium aluminum garnet (Nd:YAG) laser source able to generate a pulse of 30 mJ is used. The output laser beam is green with 532 nm of wavelength (Ferreira 2011). ...
A comprehensive knowledge of energy dissipation during the accelerations and decelerations in transient flow is essential to develop robust and accurate hydraulic transient solvers. Currently, most transient solvers are not accurate enough to describe the physical phenomenon, tending to underestimate observed energy dissipation. The current paper aims to contribute to a better understanding of transient flow by measuring instantaneous velocities using 2D particle image velocimetry (PIV) and discussing key features of the mean and of the turbulent flow. Measurements were carried out at the middle section of a coiled copper pipe for Reynolds and Dean numbers of 7,000 and 1,040, respectively. In steady-state flow, the time-averaged velocity profiles have shown a strong asymmetry, with maximum axial velocity in the outer bend region of the pipe, caused by the centrifugal force. During transient flow, the ensemble-averaged velocity profiles have clearly shown regions with strong gradients near the pipe wall and reverse flow. The direction of flow is firstly reversed in the inner bend region of the pipe. The unsteady wall shear stress is different for the outer and inner wall, and is lower than in the steady state, confirming that energy dissipation cannot be estimated based on steady-state formulae for the wall shear stress.
... [2] Understanding the role of coherent flow structures in the initiation of sediment motion remains a key challenge for contemporary Earth scientists who seek new insights into the processes of sediment transport and riverbed evolution [Ferreira, 2011;Hardy et al., 2011]. Early studies based on the flow visualization techniques have revealed the occurrence of near-bed bursting events [Kline et al., 1967;Grass, 1971]. ...
We conduct an experimental study to test the hypothesis that particle
entrainment is associated with a passing retrograde vortex (spanwise
vortex rotating counter to the mean shear). The pre- and post-entrainment quadrant structures are probed with the laser Doppler velocimetry (LDV) at a near-particle measurement spot, using the
shifted-time cumulative quadrant fraction approach. The results are
characterized by a pre-entrainment spiky rise of the Q1 fraction
(outward interactions) and a mild increase of the Q4 fraction (sweeps), followed by the post-entrainment dominance of the Q4 fraction and a drastic drop of the Q1 fraction. Such results suggest that it is highly probable that particle entrainment is a result of the interactions with a passing retrograde-vortex-type coherent structure. The time series of 2-D velocities at the near-particle spot consistently exhibit the short-term peaks present at the time of entrainment. The quadrant signature at an alternative spot one grain diameter upstream of the target particle exhibits a sequence in which the pre-entrainment
dominant Q1 fraction is replaced by the Q4 fraction, followed by a
post-entrainment peak of the Q4 fraction. The results obtained from
these two locations confirm the theoretical predictions, showing that
different quadrant signatures would be detected at different spots
during the passage of a retrograde vortex. We also perform an extra set of experiments, in which the target particle is set in an alternative
pocket geometry with diagonal downstream valleys. The similar
pre-entrainment quadrant signatures detected in all the experiments
performed with different types of pocket geometry and the unique
post-entrainment quadrant signature detected in those performed with the alternative pocket geometry imply that an obliquely oriented retrograde vortex may have passed, entraining the particle in diagonal directions. The results point to the potential discrepancy in the observed signatures that arises from the misalignment of the velocity measurement plane with the direction of particle entrainment.
... Antes de proceder à aplicação do algoritmo de correlação cruzada é possível aplicar uma sub-área junto às fronteiras de interrogação para prevenir e limitar a ocorrência de loss of pairs dentro do plano. Esta acção não tem influência para loss of pairs fora do plano (Ferreira 2011). ...
... Para a medição de campos de velocidades instantâneas, recorreu-se a um sistema PIV (Particle Image Velocimetry). O PIV é um método pouco intrusivo, utilizado em medições de escoamentos com grande heterogeneidade espacial(Ferreira 2011). O equipamento é constituído pelos seguintes componentes: i) cabeça do laser de dupla cavidade (Figura 3.8 a));ii) gerador do feixe de laser (Figura 3.8 b)); ...
Remote sensing enables us to measure fluvial systems without disrupting their dynamics. Small-scale physical models of rivers allow us to observe their geomorphic evolution, but we need remote sensing methods to monitor these laboratory landscapes without altering their flow or topography, just as with field-scale rivers. In this paper, we review how experimental geomorphologists have adapted remote sensing for the laboratory. We consider how remote methods to monitor model topography, flow depth, velocity and planform have been employed, enabling uninterrupted experimental evolution. We also explore the transfer of techniques between field-scale and experimental remote sensing; the controlled conditions in the lab aided the development of some methods, while others benefited from airborne deployment. We consider recent developments offered by laboratory remote sensing, including through-water laser scanning and adaptations of structure-from-motion photogrammetry; we also consider new challenges associated with these developments, such as computational power. Finally, we discuss new research problems that laboratory remote sensing is opening up to geomorphology. We hope this review will be useful for experimentalists seeking to collect data remotely, continuously and/or cost-effectively. © 2023 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.
