1,721,096 research outputs found
On the oscillating characteristics of hydraulic jumps
No full textThis paper investigates oscillating characteristics and cyclic mechanisms in several hydraulic jumps, with experimental study made of the hydraulic jumps under two periodically repeated flow conditions. The analysis shows that the vortex roll-up process is linked to fluctuations of the longitudinal location of the jump toe. The paper includes comments on those few works in literature which deal with the subject. Measurements were made of the time interval during which each type of hydraulic jump was present along with the surface profile elevations downstream of the roller. Results, oscillating characteristics and cyclic mechanisms are in agreement with the new concept of turbulence. Indeed, it has become increasingly evident that the organized periodic motion is superimposed on a background of random turbulence, with many flows containing eddies, vortices whose description is more influential than previously thought
Flow visualization in bubbly two-phase hydraulic jump
No full textThe present study investigates bubbly two-phase flow in a hydraulic jump using a flow visualization technique. Bubbly two-phase flow is encountered in many engineering problems; however, mainly because of experimental difficulties, little is known opt the internal structure of these flows, although such knowledge is clearly essential to a thorough understanding of the mass transfer between the two component phases. In the past, same authors measured the distribution of void ratio in a hydraulic jump using hot-film anemometry. Nowadays this interesting technique may be improved using a flow visualization technique, which enables one to obtain the percentage of air across each vertical section of the jump. This is possible by evaluating the gray levels of the first principal axes of transformed images starting from RCB images. The experiments considered the phenomenon of air concentration in a hydraulic jump, which was studied and analyzed using image processing techniques, aimed at obtaining reliable quantitative measurements. To achieve this, the processing system was planned and tested at the hardware level and a procedure for managing rifle processing was set up, The calibration curve was obtained using the McCorquodale and Khalifa law (1983). The results permit the visualization of flow structures and the estimation of air concentration of the flow along all the jump and to show the position in which the air concentration reaches the maximum value versus time
Flow visualization in bubbly two-phase hydraulic jump
No full textThe present study investigated the bubbly two-phase flow in hydraulic jump, using flow visualization technique. Bubbly two phase flow is encountered in many engineering problems; however, mainly because of experimental difficulties, little information is at present available on the internal structure of these flows although such knowledge is clearly essential to a thorough understanding of the mass transfer between the two component phases. In the past some authors measured the distribution of void ratio in hydraulic jump using hot-film anemometry. Now this interesting technique may be improved using flow visualization technique, which enable to obtain the percentage of air in each pixel and across each vertical section of the jump. This is possible by evaluating the gray levels of the first principal axes transformed images starting from RGB images. The experiments considered the phenomenon of air concentration in hydraulic jump, which was studied and analysed using image processing techniques, with the aim of obtaining reliable quantitative measurements. To achieve this, the processing system was planned and tested at hardware level and a procedure for managing the processing on software was set up. The calibration curve was obtained using McCorquodale J.A. and Khalifa A. (1993) law. The results enable to evaluate air concentration of the flow along all the jump and to show the position in which the air concentration reaches the maximum value against time
Regular and Irregular Waves on a Sloping Beach
No full textTaking into account that many coastal processes, such as undertow currents, sedi-ment transport and action on maritime structures, are greatly affected by wave behav-iour, the aim of this paper is to investigate the hydrodynamics of both regular and irreg-ular waves. Hence, it illustrates the results of an experimental research carried out in two different laboratory wave flumes of the Water Engineering and Chemistry Depart-ment of Bari Technical University (Italy).
The first laboratory experiment examines the hydrodynamics of three different regular waves breaking on a fixed sloping bottom. The principal interest is therefore fo-cused on the wave behaviour in the surf zone.
The second experiment refers to an irregular wave characterized by a narrow banded spectrum and developing on a sloping sand bottom, in intermediate waters up to the surf zone. In this case, the shoaling region is thoroughly investigated in order to understand how the wave is affected by breaking induced turbulence.
In both experiments, the phase-averaging technique is used to extract the turbulent component from the time series of the acquired signal. This procedure is routinely used in laboratory studies when regular waves are studied. On the contrary, for irregular waves, it is not always assumed, being feasible only when some specific rules are re-spected, as occurred in the present study.
The analysis focused on the vertical profiles of time-averaged orbital velocities, wave and turbulent cross-correlations, turbulent kinetic energy, turbulent intensities. At the same time also the phase-averaged velocity components and turbulent kinetic energy were considered. Finally some considerations were derived for the transport of both turbulent kinetic energy and sediments, at various depths of each investigated location
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
Closure to "new Approach to Predicting Local Scour Downstream of Grade-Control Structure" by M. Ben Meftah and M. Mossa
No full textMonitoring Coastal Waters Close to a Sea Outfall
No full textThe design of a water treatment plant and its outfall should respect receptors’ environmental quality standards as well as physicochemical parameters of the discharge itself. Moreover, in order to avoid polluting the outfall, because of the natural processes of dilution and self-depuration, coastal outfall pipes are used to issue the depurated water at a sufficiently long distance from the preserved shoreline. Once recognising that sea currents play a pivotal role on physical and microbiological effects, the following study aims at describing the results of some monitoring surveys carried out in the Italy’s Southern coastal area of Bari. The target area is close to the Eastern wastewater sea outfall system of the city, thus, its analysis is necessary for investigating the process of diffusion and transport of the wastewater outfall and to validate predictive hydrodynamic models.
Firstly, monitoring surveys were carried out during the period from summer to autumn 2001 and successively, they were repeated in the following winter 2003 and spring 2003. A Vessel Mounted Acoustic Doppler Profiler was used to measure the components’ speed at different depths. Also, salinity and water temperature data were collected by means of a CTD recorder system while wind intensity and air temperature were recorded with an anemometer and a thermometer respectively, for each station’s measurement
How obstructed jets experience detrainment
No full textA comprehensive understanding of turbulent jets discharged into obstructed environments remains a critical gap in the current literature. This issue holds significant importance for applications ranging from environmental fluid dynamics to industrial processes. The primary goal of this study is to theoretically investigate the dynamics of planar turbulent non-buoyant jets interacting with arrays of rigid obstacles, supported by a comparison between theoretical predictions and experimental data. Specifically, our analysis focuses on the entrainment process, revealing that obstructions in non-stratified flows impede entrainment, reversing it into detrainment. This finding is novel because (i) detrainment in natural settings is typically associated with buoyancy-driven flows, such as plumes or density currents in stratified environments, and (ii) to the best of the authors' knowledge, this is the first validation of theoretical entrainment coefficients with experimental data for obstructed non-buoyant jets. Experiments were conducted with turbulent non-buoyant jets using particle image velocimetry, providing detailed insights into flow structure and entrainment dynamics. Furthermore, the study explores jet particle dispersion and diffusivity through a Lagrangian framework. The results demonstrate significant differences in dispersion behavior between unobstructed and obstructed jets, showing that obstacle-induced blockage profoundly influences flow characteristics and jet detrainment. In particular, obstructions play a fundamental role, initially affecting the dispersion mechanism through obstacle diameter and later through the free spacing between obstacles. These findings provide valuable contributions to understanding flow physics in complex environments and have implications for engineering and environmental applications
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