A dimensional analysis of supersaturated total dissolved gas dissipation

Elevated levels of total dissolved gas (TDG) may occur downstream of dam discharges, leading to increased incidence of gas bubble disease in fish. Accelerating the dissipation of supersaturated TDG in the downstream river can mitigate this negative problem. However, developing effective mitigation techniques is hampered by limitations in present models of TDG dissipation processes. Furthermore, data useful for modelling the dissipation of supersaturated TDG through the free surface in natural rivers are limited. Past studies indicated that the TDG dissipation process is quantitatively different from the reaeration process, and TDG behavior is quantitatively different from dissolved oxygen. However, a correct parameterization of the TDG dissipation process is still missing. The paper presents a novel dimensional analysis of the dissipation of supersaturated TDG. This approach can provide a relationship between the TDG dissipation coefficient and some classical fluid mechanics index-numbers. This dimensional analysis considers some key parameters for the dissipation process both water and TDG properties as well as flow characteristics, including turbulence. These parameters are water kinematic viscosity, TDG molecular diffusivity and vertical turbulent diffusivity, and channel width. The application of dimensional analysis pointed out that the TDG dissipation coefficient is a function of the Schmidt number, the aspect ratio of the channel, and the shear Reynolds number. The dimensional analysis was then verified using both field data collected in some large natural rivers and reservoirs in Sichuan and experimental data in laboratory flume at State Key Laboratory of Hydraulics and Mountain River Engineering of Sichuan University. The analysis revealed the key role of turbulence in controlling the TDG dissipation while the importance of gas/water characteristics remains still unclear and needs further investigations

A numerical study of the lateral hyporheic flow about a channels confluence

Stream and pore waters continuously interact and mix within streambeds due to spatial and temporal variations in channel characteristics. This mixing is termed as hyporheic exchange and the zone where groundwater and stream water are mixing is called hyporheic zone, which can greatly affect water quality is both surface and subsurface water systems. Typical examples of hyporheic fluxes are those under bedforms, intra-meander or across point bar deposits, while the lateral and vertical hyporheic exchanges about a riverine confluence was almost never investigated. The paper presents some results of a numerical study carried out to investigate the basic features of the lateral hyporheic exchanges about the confluence of two channels. A 2D triangular geometry was used. Laminar flow in the channels and Darcian flow in the porous medium between them were considered. The simulations highlighted the role of the confluence planform and velocity ratio between the channels in controlling both the direction and the magnitude of the lateral hyporheic exchange

Effect of Froude number on bubble clustering in a hydraulic jump

The study of bubble clustering processes may provide a significant insight into turbulent air–water flows. Previous studies investigated these processes in plunging jets, dropshafts and hydraulic jumps. This research investigates the bubble clustering process in hydraulic jumps using experimental data collected in a rectangular horizontal flume with partially developed inflow conditions for inflow Froude numbers in the range 6.5–14.3. Two criteria for cluster identification were applied: one criterion was based upon a comparison of the local instantaneous water chord time with the median water chord time, whereas the second identified a cluster if the water chord time was smaller than the air chord time of the preceding bubble, i.e. a bubble was in the near-wake of the leading bubble. The results highlight significant patterns in clusters production both over the flow depth and the distance from the jump toe. The effect of the inflow flow Froude number on the clustering process is also discussed. © 2010 International Association for Hydro-Environment Engineering and Researc

Numerical Simulation of Turbulent Flow Past a Cylinder Placed Downstream of a Step

This study investigates the effect on the flow structure in a backward-facing step (BFSF) due to a cylinder placed downstream of the step. Numerical simulations were carried out using OpenFOAM with several turbulence models (standard k-ɛ, RNG k-ɛ, standard k-ω, and SST k-ω). The recirculating flow, the skin friction coefficient (Cf), and the pressure coefficient (Cp) of the bottom wall were comparatively analyzed. The added cylinder modified the structure of flow and increased the skin friction coefficient (Cf) in the recirculation zone. Also, the pressure coefficient of the bottom wall increased immediately downstream of the cylinder and farther downstream of the reattachment point remained stable in the flow recovery process

Influence of a rigid cylinder on flow structure over a backward-facing step

In the present study, laminar and turbulent flow over a backward-facing step (BFSF) where a cylinder was placed immediately downstream of the step was investigated through numerical simulation using OpenFOAM. In laminar flow mean errors between numerical and literature experimental data for velocity profiles and reattachment lengths were lower than 8.1% and 18%, respectively. The cylinder significantly modified the structure of recirculating flow over the BFSF. In addition, the cylinder increased the skewness of the velocity profiles, and the location of the maximum velocity shifted towards the upper wall. In turbulent flow, the results from several RANS models (standard k-ɛ, RNG k-ɛ, standard k-ω, SST k-ω, and RSM (SSG)) were compared with literature experimental data. The average error in predicting reattachment length and velocity profiles ranged from 2.2% to 28.5% and from 7.8% to 14.5%, respectively. The most accurate model in predicting reattachment length and velocity profiles was the standard k-ɛ and SST k-ω models respectively. The cylinder modified flow structure and the distribution of turbulent kinetic energy, whose largest value was found downstream of a cylinder in the separated shear laye

Clustering Process Analysis in a Large-Size Dropshaft and in a Hydraulic Jump

Recent efforts in the characterization of air-water flows properties have included some clustering process analysis. A cluster of bubbles is defined as a group of two or more bubbles, with a distinct separation from other bubbles before and after the cluster. The present paper compares the results of clustering processes two hydraulic structures. That is, a large-size dropshaft and a hydraulic jump in a rectangular horizontal channel. The comparison highlighted some significant differences in clustering production and structures. Both dropshaft and hydraulic jump flows are complex turbulent shear flows, and some clustering index may provide some measure of the bubble-turbulence interactions and associated energy dissipation

Clustering Process and Interfacial Area Analysis in a Large-Size Dropshaft

Dropshafts are commonly used in sewers and stormwater channels as energy dissipator systems. Since recent effort has been devoted to characterize dropshaft hydraulics and air-water flow properties, and the present paper develops an analysis of the bubble clustering process using new experimental data collected in a large-size facility. The results highlight some significant patterns in clusters production. Finally, interfacial areas for mass-transfer were measured

Analysis of Air Bubble Probability Distribution Functions in a Large-Size Dropshaft

Dropshafts are commonly used in sewers and stormwater channels as energy dissipator systems. Since recent effort has beendevoted to characterize dropshaft hydraulics and air-water flow properties, and the present paper develops an analysis of thebubbles probability distribution functions (PDF) in a shaft pool using new experiments conducted in a large-size facility. First,theoretical trajectory calculations of the free-falling nappe were compared with the maximum void fraction data observed withinthe pool. Then, statistical analysis was performed on both air and water chord length data. PDF was derived as well as mean,median, mode, standard deviation, skewness and kurtosis values. Further, these results were compared with some earlier work.The results obtained over the pool length at various vertical elevation highlighted some new insights on the interactions betweenturbulence, bubble coalescence and detrainment of the air bubbles