2,255 research outputs found
Closure to "new Approach to Predicting Local Scour Downstream of Grade-Control Structure" by M. Ben Meftah and M. Mossa
No full textFlow Hydrodynamic in Open Channels: A Constantly Evolving Topic
No full textStreams and riverbeds are subject to considerable hydromorphological alterations due to the interaction of their flow with natural or man-made structures found throughout them, i [...
New Approach to Predicting Local Scour Downstream of Grade-Control Structure
No full textDespite the numerous studies on scouring processes, the prediction of scour-hole dimensions downstream of hydraulic structures remains challenging because of the complexity of the phenomenon and its dynamic sensitivity to structure and sediment properties. This study experimentally focuses on scour-hole development downstream of a sloped grade-control structure (GCS) in alluvial channels. A large series of laboratory experiments were carried out in a rectangular channel with a noncohesive sediment bed. Based on the data from this study and data collected from previous studies, the effect of the downstream face slope of a GCS on scour morphology was analyzed. In this regard, it was found that the face slope has an effect only if it is smaller than the slope of the upstream equilibrium scour side obtained with a GCS of a vertical downstream face. Before reaching an equilibrium state, the scour process evolves into three distinct phases, a very rapid initial phase, an intermediate gradual phase, and an equilibrium phase. A general empirical expression for predicting temporal scour evolution is proposed and extended to different types of GCS. Moreover, a new scaling approach is proposed that leads to the derivation of new equations predicting equilibrium scour profiles with different entering jet-flow typologies. To make these equations operational, a series of estimating expressions for the characteristic lengths of equilibrium scour holes is also proposed
Experimental investigation on dispersion mechanisms in rigid and flexible vegetated beds
No full textVegetation in channels strongly affects flow structure and turbulence, with consequences on the hydrological storage of nutrients and chemical tracers, the shelter of stream biota as well as the trapping or transport of sediments. At the same time, all these phenomena are inevitably subjected to alteration of hydrological conditions in fluvial systems due to climate change. The present study intends to provide a thorough investigation into the processes of transport and dispersion induced by flow turbulence within the vegetation structure. Specifically, velocity measurements in vegetated channels were intensively conducted and analyzed in the case of both flexible submerged and rigid emergent canopies. The experiments aimed to: (i) highlight the differences in the hydrodynamic structures induced by different plant types and configurations; (ii) study the spatially varying dispersive properties and turbulent behavior of the current, attributable to different plant density, submergence and stiffness. Further, the spatial variability of velocity and turbulence distributions was taken into due consideration. The experimental results showed how longitudinal and transversal hydrodynamics and transport differ, depending on vegetation configuration and stem properties
Undular jump formations in very large channels
No full textIn this paper, undular hydraulic jumps in very large channel (channel width equal to 4.0 m) for low Rey-nolds number have been investigated. Jumps with very high aspect ratios are very rare in literature, and, therefore, experimental works are necessary. The main aims are (i) analyzing the lateral shock wave in order to verify the experimental validity of the shock wave theory in very large channel, (ii) analyzing the flow conditions of undular jumps in very large channels. The main results are the following: (i) the pres-ence of well developed lateral shock wave similar to those of oblique jumps were observed; (ii) the com-parison of the experimental results and the theoretical ones show that the classical shock wave theory is confirmed, taking into account the experimental errors; (iii) the literature law of the wave height of first wave crest was confirmed also in the case of very large channel
Vegetation effects on vertical jet structures
No full textThis paper deals with measurements of the three-velocity components of a vertical, round, turbulent jet discharged into a vegetated cross flow. Over the last years, a large number of experimental studies and numerical models on turbulent jets discharged into a cross flow have been carried out, as well as several studies on vegetated channels. However, these studies show a lack of data regarding the combination between the vegetated channels and jets. The present study aimed at obtaining a more thorough understanding of the vegetation effects on the jet behaviors. To simulate the vegetation, arrays of emergent, rigid, circular steel cyl-inders were used. The jet source was placed at the centre of the experimental vegetated area. The time-averaged velocity field was investigated in the longitudinal, cross and horizontal planes of the channel. The results show that vegetation has significant effects on the jet structure as compared with the case of non-vegetated channel. Above all, the rigid stems reduce streamwise velocities, giving rise to an increase of the jet penetration height within the ambient flow. Moreover, the familiar pair of counter-rotating vortices and kid-ney shape observed in the cross section of the jet discharged into the non-vegetated channel disappears and transforms under the effects of stems into a complex flow motion structure for the jet discharged into the vegetated flume
Considerations on shock wave/boundary layer interaction in undular hydraulic jumps in horizontal channels with a very high aspect ratio
No full textIt can be seen in the literature that the fundamental factors governing oblique shock wave development,
typically in very large channels with straight sidewalls, have not yet been completely understood and
remain at the level of indicating its presence and formation. In this study, in addition to an analysis
of various properties of hydraulic jump behaviour in very large channels, some aspects of boundary
layer development and its detachment from the channel lateral sidewall are also investigated. At the
detachment point of the lateral shock waves, it was noted that the displacement thickness experiences
a significant increase; this is accompanied by a significantly reduced gradient normal to the channel
sidewalls of the flow velocity as well as the occurrence of a strong, sudden adverse pressure gradient. An
analysis of the flow velocity distribution and the background turbulence intensity of both the streamwise
and spanwise velocity components was also carried out. Furthermore, it is argued that the supersonic
flow separation analogy with a supercritical free surface flow can be applied to this case study and that
the behaviour of the supercritical flow during separation can be interpreted by the free interaction theory
typically used in aerodynamics
Physical modelling of buoyant effluents discharged into a cross flow
No full textThis study focuses on physical modelling of turbulent vertical buoyant jets, discharged into a transversal current and interacting with localized background turbulence. The physical model was developed in the Coastal Engineering Laboratory of the Technical University of Bari. The physical model consists of a sophisticated system that allows to monitor and adjust all the characteristic parameters of both the channel flow (e.g. discharge, flow depth) and the buoyant jets (e.g. flow rate, temperature, salinity). Positively and negatively buoyant
jets are realized by discharging water respectively at a temperature and salinity higher than that of the receiving environment. Due to the complexity of the jet-current hydrodynamic phenomena, a set of sophisticated instruments to measure the jet spreading within the cross flow is used. The average jet dilution is measured by (i) four different Resistance Temperature Detectors (RTD) for the positively buoyant jet, and (ii) a MicroScale Conductivity Temperature Instrument (MSCTI) of high resolution for the negatively buoyant jet. Whereas, a Nortek Acoustic Doppler Velocimeter (ADV) system is used to measure the field flow velocities, together with CollectV software for data acquisition and ExploreV software for data analysis. The measured scalar and vector fields will be illustrated in this paper, with the aim to emphasize that a well- set physical model is able to explain the behavior of buoyant jets in an open channel with ambient factors, such as cross flow and vegetation
Wave and turbulent Reynolds stresses in irregular shoaling waves
No full textThe dynamics of regular breaking waves has been both widely and successfully
investigated. In any case, many natural coastal processes are commonly due to
irregular breaking waves, the behaviour of which requires thorough study. The
present research aims to investigate the distributions of the wave and turbulent
Reynolds shear stresses in a laboratory irregular wave, characterized by a narrow
banded spectrum, which develops on a sloping sand bottom, in intermediate waters.
Experiments focused on the wave shoaling region, in order to analyze the effects of
breaking induced turbulence outside the surf zone, taking into account that
turbulence is not limited to the breaking region but it spreads also outside the surf
zone. The phase-averaging technique was used to separate the turbulent components
from the steady ones. All the analysed values derive directly from real
measurements and are not interpolated. Moreover, a 3D Acoustic Doppler
Velocimeter was adopted to measure the wave velocity, consequently the longshore
component of the velocity is also available to estimate the shear stresses. These
experimental data were also used to test some literary numerical models and
relevant results have been obtained, which confirm the outputs of the
abovementioned models in the cases of non dissipative waves propagating above a
flat bottom and dissipative waves propagating over a sloping bottom
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