Deposition from Particle-Laden, Round, Turbulent, Horizontal, Buoyant Jets in Stationary and Coflowing Receiving Fluids

Results are presented from a series of laboratory experiments investigating the characteristic features of particle-laden, round, turbulent, buoyant jets discharged horizontally into stationary and coflowing receiving fluids. For the volumetric source concentrations of particles tested ( ～ 0.1%), the presence of the particle load was found to have no significant influence on mean buoyant jet trajectories. Deposition patterns on the bed of the receiving water container indicated the existence of two separate sedimentation processes for discharges into stationary or coflowing ambients, namely (1) a relatively concentrated, narrow band of particle accumulation associated with near-source fallout from the buoyant jet margins; and (2) a broader and more disperse downstream depositional fan associated with particle fallout from the radially-expanding surface gravity current formed by the impingement of the buoyant jet with the free surface of the receiving fluid. Scaling arguments have been developed and applied successfully to deposition length scales associated with these sedimentation patterns, allowing the quantitative characteristics and parametric dependences of the deposition distributions to be established.     

Sedimentation from Buoyant Jets

An integral model is developed to describe sedimentation from a turbulent, buoyant jet injected at an angle into a stationary, uniform ambient fluid. Entrainment is modeled using the standard entrainment assumption and sediment is assumed to fall from the jet where the outward component of the fall velocity normal to the jet boundary exceeds the inward entrainment velocity. When appropriately scaled by source momentum and buoyancy fluxes, turbulent, buoyant jets may be described in terms of a single parameter: the angle θ0 between the flow and the horizontal at the virtual origin (which is close to the actual source for large initial densimetric Froude numbers). Including sedimentation in the model introduces a further parameter ws, which is the ratio of the fall speed of the sedimenting particles to a typical entrainment velocity (and so wS is a nondimensional fall speed). An important result is that this ratio is independent of the source speed, so that the proportion of the sediment load deposited near the source is independent of the flow rate. Sediment remaining in the plume beyond the near-source region is deposited when the plume spreads horizontally once it reaches the free surface. Results for plume shapes, deposition patterns, and the proportion of sediment load deposited in the near-source region (as functions of θ and ws) are given. The results are supported by some preliminary laboratory experiments. The effects of flow in the ambient fluid are discussed briefly and a further parameter uF is introduced, which is the ratio of the ambient flow speed to a typical entrainment velocity (again this ratio is independent of the flow rate).     

Plane Turbulent Wall Jets in Shallow Tailwater

This paper presents a theoretical and laboratory study of plane turbulent wall jets with finite tailwater depth. The main objective was to show that, when the depth of tailwater is finite, the momentum flux of the forward flow in the wall jet decays appreciably with the distance from the nozzle. This decay is due to the entrainment of the return flow, which has negative momentum that requires a depression of the water surface near the gate housing the slot. An extensive set of experiments, with different Froude numbers and tailwater depth ratios, was used to observe and quantify the growth of the wall jet, the decay of the velocity scale and the momentum flux, and the variation of the volume flux. Also, experiments were conducted to measure the length of the surface eddy and the drop in the water surface elevation at the gate. This study contributes to an understanding of the behavior of plane turbulent wall jets when the ambient fluid has a limited extent.     

Turbulent Prandtl Number in Neutrally Buoyant Turbulent Round Jet

A method which combines two nonintrusive imaging techniques, particle tracking velocimetry and laser induced fluorescence, was used to make simultaneous measurements of velocity and concentration in a neutrally buoyant turbulent round jet. The measurements were made at two different Reynolds numbers (R), 360 and 4,210, at a Schmidt number of 1,930. The mean velocity 〈u〉, mean concentration 〈c〉, Reynolds stress ?〈u′v′〉, and turbulent scalar flux 〈v′c′〉 were obtained and the eddy viscosity, eddy diffusivity, and turbulent Prandtl number (Prt) calculated from these measurements. Both the low and high Reynolds number results show self-similar characteristics that are dependent on R with Prt a function of radial position. For the R=4,210 case, it was found that 0.70.12. For the R=360 case, it was found that Prt ≈ 0.4 for 0.06相似文献   

Impact of Vertical, Turbulent, Planar, Negatively Buoyant Jet With Rigid Horizontal Bottom Boundary

The results of a series of laboratory modeling experiments are presented for the case of a vertical, turbulent, plane, negatively buoyant jet impinging on a horizontal solid surface placed a distance H below the jet source. The results show that the impingement results in the generation of a complex two-dimensional disturbance field at the site of the impact and the generation of a buoyancy-driven boundary current carrying away fluid from the impingement zone. The disturbance field is seen to extend vertically along the time-averaged axis of the incident buoyant jet, thereby distorting the vertical velocity and concentration fields over a vertical distance that depends upon the value of the parameter Fd0?4/3, where Fd0 is the source Froude number of the buoyant jet. Transverse velocity and concentration profiles taken at different axial distances from the source reveal systematic departures from the far-field Gaussian similarity profiles as the solid boundary is approached. Such departures are utilized to quantify and parameterize the vertical distance z??? from the boundary at z = 0 beyond which the impingement of the buoyant jet does not affect significantly the incident flow. Measurements indicate that z???/b ～ 0.4Fd04/3. For distances z相似文献   

Buoyant Jets of Elliptic Shape: Approximation for Duckbill Valves

The intrusion of seawater into a pipeline servicing an ocean outfall can significantly reduce its operational efficiency. Duckbill valves are sometimes installed on sewage outlet ports to help prevent such intrusions. While there is growing literature associated with the hydraulics of duckbill valves, there appears to be little published information on the trajectory and dilution achieved by the buoyant jets when the outlet ports are fitted with duckbill valves. Further, no models presently exist that incorporate the effects on the rise and dilution of buoyant jets discharged through orifices fitted with duckbill valves for which the size and shape of the opening varies with the effluent flow. Solutions to the asymptotic equations for jets and plumes are generated for ports fitted with duckbill valves by assuming that the shape of the duckbill is elliptical. This allows asymptotic expressions to be generated for the trajectory and dilution of the jet/plume. In the limiting case when the ellipse becomes circular, these expressions reduce to those for discharges from round outlets and are consistent with expressions for round ports found in literature.     

Numerical Simulation of Obstructed Round Buoyant Jets in a Static Uniform Ambient

The k-ε turbulence closure model is used to simulate obstructed round buoyant jets in a static uniform ambient, and the results compare well with available experimental data. On the basis of the axial line velocity distribution, three regions in the flow behind the disk are identified: the wake region, the transitional region, and the self-similarity region. The length of the wake region, which varies with flow and geometrical parameters, and the existence of self-similarity are also addressed.     

Combined Particle Image Velocimetry/Planar Laser Induced Fluorescence for Integral Modeling of Buoyant Jets

Combined particle image velocimetry and planar laser induced fluorescence is an efficient measurement approach for laboratory studies in environmental hydraulics. The coupling of the two well-known techniques enables synchronized planar measurements of flow velocity and concentration in an area yielding both their mean distribution as well as turbulence covariance. In this paper, the merits and limitations of the combination are first discussed. An example of experimental setup is then briefly described. Finally, an application is demonstrated for the integral modeling of an inclined round buoyant jet that takes full advantage of the capability of the combined approach.     

Buoyant Surface Discharges into Water Bodies. I: Flow Classification and Prediction Methodology

Buoyant surface discharges into ambient water bodies can exhibit multiple complex flow processes, which cover the spatial range from the near field with initial jet mixing to the far field with passive ambient diffusion. Multiple flow phenomena can occur, such as buoyant collapse motions, bottom attachment, deflection by the ambient current, and dynamic shoreline interaction, in the near field, and lateral and/or upstream spreading motions and turbulent diffusion processes, in the far field. Efficient and reliable predictive techniques covering the whole range of these processes are needed for the design and prediction of wastewater effluents that are subject to water quality regulations that can apply in either near and/or far field. A new comprehensive classification framework distinguishes among ten hydrodynamically distinct flow classes within four major flow categories: free jets, shoreline-attached jets, wall jets, and upstream intruding plumes. A prediction methodology for these discharges has been presented that covers the entire spatial domain from the near to the far field. It is based on the linkage of separate predictive modules in form of the expert system CORMIX3. These hydrodynamic modules are implemented by specific flow protocols and transition criteria determine their spatial extent. The methodology, corroborated by numerous detailed laboratory and some field data sources, constitutes a simple and efficient, yet accurate and robust, tool with few data requirements for surface discharge design and mixing analysis.     

Buoyant Surface Discharges into Water Bodies. II: Jet Integral Model

The near-field region of a buoyant surface discharge into water bodies often displays significant jet-like motions in form of free jets, shoreline-attached jets, and wall jets, respectively, as classified by the CORMIX3 expert system [see Jones et al., (2007, Paper I)]. A new jet integral model CorSurf has been developed that addresses in a single formulation this entire spectrum of jet motions in both deep or shallow environments. The model employs an entrainment closure approach for the separate contributions of entrainment resulting from transverse shear, buoyant damping, advected puff motions, frontal mixing, and interfacial mixing due to lateral spreading. It also contains a quadratic law turbulent drag force mechanism. An alternative model formulation applies to the two-dimensional bottom-attached form of the jet. This formulation contains a deflecting pressure force mechanism as well as the bottom shear force. Specific criteria describe bottom attachment and detachment processes. Finally, a number of confinement effects on the jet dynamics due to shallow water and/or lateral boundaries are included. The model has been validated under a wide range of geometric and dynamic conditions using, in particular, hitherto unavailable high-resolution laboratory data.     

Plane Turbulent Wall Jets on Rough Boundaries with Limited Tailwater

Combining the results of a laboratory study of plane turbulent wall jets on rough boundaries with shallow tailwater, with the results of an earlier work of Rajaratnam on wall jets on rough boundaries with deep tailwater, this paper attempts to describe the effects of boundary roughness and tailwater depth on the characteristics of plane turbulent wall jets on rough beds, which are important in the field of hydraulic engineering. The time-averaged axial velocity profiles at different sections in the wall jet were found to be similar, with some difference from the profile of the classical plane wall jet. The normalized boundary layer thickness δ/b, where b is the length scale of the velocity profile, was equal to 0.35 for wall jets on rough boundaries compared to 0.16 for the classic wall jet. Two stages were seen to exist in the decay of the maximum velocity um as well as in the growth of the length scale, with the first stage corresponding to that of deep tailwater and the second stage to shallow tailwater. In the first stage, the decay of the maximum velocity um at any section in terms of the velocity u0 at the slot, with the longitudinal distance x in terms of L which is the distance where um = 0.5U0, was described by one general function, for smooth as well as rough boundaries. The length scale L in terms of slot width decreases as the relative roughness of the boundary increases. The onset of the second stage was not affected significantly by the bed roughness. The growth rate of the length scale b of the wall jet increased from 0.076 for a smooth boundary to about 0.125 for a relative roughness ks/b0 in the range of 0.25 to 0.50, where ks is the equivalent sand roughness and b0 is the thickness of the jet at the slot.     

Comparison of Far-Field Turbulent Structure of a Rectangular Surface Jet to Three-Dimensional Free and Wall Jets

The turbulence structure of a rectangular surface jet is compared to that of the three-dimensional free and wall jets. The surface jet turbulence quantities are mapped using laser Doppler velocimetry. In general, the turbulence structure of these three jets is found to be significantly different. For the surface jet, the free surface kinematic condition has a predominant effect on the whole structure, while for the wall jet, the influence of wall kinematic constraint is contained in the wall layer. A surface current with a higher lateral spreading rate than the submerged portion of the jet is developed, which does not exist for the wall jet because of the no-slip boundary condition. Unlike free jets, the submerged portion of the rectangular surface jet is characterized by two length scales. The Prandtl hypothesis with constant eddy viscosity provides a good estimate for the shear stresses in the lateral direction, but fails in the vertical direction, where the velocity profiles are much flatter, due to the free surface condition, than those for the free and wall jets.     

Characteristics of Submerged Jets in Evolving Scour Hole Downstream of an Apron

This paper reports an experimental investigation on the velocity and turbulence characteristics in an evolving scour hole downstream of an apron due to submerged jets issuing from a sluice opening detected by an acoustic Doppler velocimeter. Experiments were carried out for the conditions of submerged jets, having submergence factors from 0.96 to 1.85 and jet Froude numbers from 2.58 to 4.87, over sediment beds downstream of a rigid apron. The distributions of time-averaged velocity vectors, turbulence intensities, and Reynolds stress at different streamwise distances are plotted for the conditions of initial flat bed, intermediate scour holes, and equilibrium scour hole downstream of an apron. Vector plots of the flow field show that the rate of decay of the submerged jet velocity increases with an increase in scour hole dimension. The bed-shear stresses are determined from the Reynolds stress distributions. The flow characteristics in evolving scour holes are analyzed in the context of self-preservation, growth of the length scale, and decay of the velocity and turbulence characteristics scales. The most significant observation is that the flow in the scour holes (intermediate and equilibrium) is found to be plausibly self-preserving.     

Velocity Pulse Model for Turbulent Diffusion from Flowing Water into a Sediment Bed

The “velocity pulse model” simulates the transfer of turbulence from flowing water into a sediment bed, and its effect on the diffusional mass transfer of a solute (e.g., oxygen, sulfate, or nitrate) in the sediment bed. In the “pulse model,” turbulence above the sediment surface is described by sinusoidal variations of vertical velocity in time. It is shown that vertical velocity components dampen quickly inside the sediment when the frequency of velocity fluctuations is high and viscous dissipation is strong. Viscous dissipation (ν) inside the sediment is related to the apparent viscosity depending on the structure of the sediment pore space, i.e., the porosity and grain diameter, as well as inertial effects when the flow is turbulent. A value ν/ν0 between 1 and 20 (ν0 is kinematic viscosity of water) has been considered. Turbulence penetration into the sediment is parametrized by the Reynolds number Re = UL/ν and the relative penetration velocity W/U, where U=amplitude of the velocity pulse; and W=penetration velocity; L = WT=wave length of the velocity pulse; and T is its period. Amplitudes of vertical velocity components inside the sediment and their autocorrelation functions are computed, and the results are used to estimate eddy viscosity inside the sediment pore system as a function of depth. Diffusivity in the sediment pore system is inferred by using turbulent or molecular Schmidt numbers. Turbulence penetration from flowing water can enhance the vertical diffusion coefficient in a sediment bed by an order of magnitude or more. Penetration depth of turbulence is higher for low frequency velocity pulses. Vertical diffusivity inside the pore system is shown to decrease more or less exponentially with depth below the sediment/water interface. Vertical diffusivities in a sediment bed estimated by the “velocity pulse model” can be used in pore water quality models to describe vertical transport from or into flowing surface water. The analysis has been conducted for a conservative material, but source and sink terms can be added to the vertical transport equation.