Dilution characteristics of dual buoyant jets in wavy cross-flow environment

Due to the difference in density between the discharge effluent and coastal water,partially treated wastewater is often discharged into the marine environment as a buoyant jet via submarine outfalls with multiport diffusers.The dilution characteristics of effluent discharge(dual buoyant jets)in a wavy cross-flow environment were studied in a laboratory.The planar laser-induced fluorescence technique was used to obtain the concentration data of the jets.The effects of different environmental variables on the diffusion and dilution characteristics of the jets were examined through physical experiments,dimensional analysis,and empirical formulations.It was found that the dilution process of the dual jets could be divided into two components:the original jet component and the effluent cloud component.The jet-to-current velocity ratio was the main parameter affecting the concentration levels of the effluent cloud.The merging of the two jets increased the jet concentration in the flow field.When the jets traveled further downstream,the axial dilution increased gradually and then increased significantly along the axis.Under the effects of strong waves,the concentration contours branched into two peaks,and the mean dilution became more significant than under the effects of weak waves.Therefore,the dilution of the effluent discharge was expected to be significant under strong wave effects because the hydrodynamic force increased.A dilution equation was derived to improve our understanding of the dilution process of buoyant jets in a wavy cross-flow environment.This equation was used to determine the influences of the jet-to-current velocity ratio,wave-to-current velocity ratio,and Strouhal number on the minimum jet dilution.It revealed that the wave and buoyancy effects in effluent discharges were significant.

MEAN BEHAVIOR OF THREE DIMENSIONAL LINE BUOYANT JETS IN CROSS FLOWS

This paper presents the results of a numerical calculation on the meanbehavior of finite length line buoyant jets from slot with width B , discharged perpendicularly intorelatively deep cross-flows in the mixing region. The length of diffuser was varied from 4 to 20times the width of diffuser. The calculations were performed with the standard K-ε model and HybridFinite Analytic Method (HFAM) with staggered grid. The phenomenon and development of vortex pairsare simulated successfully and the influence of diffuser length and buoyant on turbulent buoyantjets are analyzed.

STABILITY AND MIXING CHARACTER FOR BUOYANT JETS IN QUIESCENT SHALLOW WATER

The near field stahility and mixing characteristics of buoyant jets produced by thermal diffuse in quiescent shallow water are inw.stigated numerically to predict under what combinations of discharge and ambient characteristics the near field will be stable or unstable, Analyses for different discharging types show that the discharge stability is purely dependent on the near-field hehavior of the jets, or the dynam ic interaction of the buoyant jet region, the surface impinge ment region and the internal hydraulic jump region, and is independent of the far-field geometry of the receiving water. The stability criterion is a function of the relative submerged depth, and source densimetrie Froude number.

BEHAVIOR OF OBSTRUCTED SQUARE BUOYANT VERTICAL JETS IN STATIC AMBIENT (Ⅰ)-VERIFICATION OF MATHEMATICAL MODEL AND NUMERICAL METHOD

Some experiments were made for the buoyant jet from a square orifice with a square disc placed on it in static ambient and concentration along the axis in self-similar area behind disc was measured. And at the same time a three-dimensional mathematical model was established to simulate the whole flowing under different conditions. All the results predicted by the numerical calculation were substantiated by the experiments. The results were compared with experiential formula for obstructed round buoyant ver- tical jets in static ambient and it was found that the two concentration distributions had good accordance. Star shape of temperature isolines on cross-sections in the near areas from the disc was found and it was a very special figure for obstructed square buoyant vertical jets with a square disc. The shape will transform to concentric circles gradually alike to the round buoyant vertical jet in self-similar area with increasing of the distance from the disc.

BEHAVIOR OF OBSTRUCTED SQUARE BUOYANT VERTICAL JETS IN STATIC AMBIENT (Ⅱ)-ANALYSIS ON BEHAVIOR OF FLOW FIELD

Based on a series of numerical calculations, the behavior of flow field in obstructed square buoyant vertical jet is summarized and analyzed. Based on the axial line velocity distribution, the flow after the disc can be divided into three regions, i.e., recirculation region, transitional region and self-similar region The characteristic of selfsimilarity of upright velocity was validated. The three regions can also be distinguished based on the axial velocity. The axial velocity in self-similar region was found to obey the same law and the formula presented by introducing the velocity expression used by Chen and Rodi. The isolines of pressure on cross-sections of different heights were displayed and the production, expansion, breaking and disappearing of negative pressure regions were found.

JETS,PLUMES AND BUOYANT JETS IN STATIC ENVIRONMENT

Turbulence represents an essential aspect in atmospheric and oceanic circulations.In particu- lar,it is a preponderant factor in the dispersion of pollutants of artificial or natural origin.The vertical turbulent jets,plumes and buoyant jets discharging into static environment is one of the most important and basic flow patterns related with the environment pollution.These flows have numerous important applications,for example,thrust augmentors,waste disposal plumes from stacks and combustion systems,and significant problems of turbulent diffusion.

ON MUSHROOM-CLOUD AND SPREADING LAYER REGIONS OF PLANE BUOYANT JETS

The whole field of buoyant jets in stratified environment is divided into buoyant- jet region,mushroom-cloud region and spreading layer region.In this paper a modelling method is used to predict the behaviors of mushroom-cloud and spreading layer regions.The demarca- tions of these regions are given also.

Behavior of near-field dilution of thermal buoyant jet discharged horizontally in compound open-channel

The RNG κ-ε model considering the buoyancy effect, which is solved by the hybrid finite analytic method, is used to simulate the mixture of the horizontal round thermal buoyant jet in compound open channel flow. The mixing features near the spout and flowing characteristic of the secondary currents are studied by numerical simulation. Meanwhile, （1） the distribution of the measured isovels for stream-wise velocity, （2） secondary currents, （3） the distribution of the measured isovels for temperature of typical cross-section near the spout, were obtained by the three-dimensional Micro ADV and the temperature measuring device. Compared with experimental data, the RNG κ-ε model based on buoyancy effect can preferably simulate the jet which performs the bifurcation phenomenon, jet reattachment （Conada effect） and beach secondary currents phenomenon with the effect of ambient flow, buoyancy, and secondary currents of compound section and so on.

ROUNDED FLOWING STATES OF OBSTRUCTED BUOYANT JET

The mutual relationships of three effective factors, the diameter D/d （d is the diameter of exit） of obstructed plate, exit densimetric Froude number and the distance Hid of the plate from jet orifice for obstructed buoyant jet in static ambient, are analyzed to explain normal and abnormal rounded flowing （reverberated and bifurcated flowing）. The critical Froude numbers for obstructed buoyant jets with H/d=2, 4, 6, 8 which distinguished normal and abnormal flowing pattern are obtained. Normal rounded flowing is found only for a plate under a special value of H/d. A fitted formula of critical Froude numbers with H/d and D/d is presented to distinguish rounded flowing types. The occurring of reverberated or bifurcated flowing in abnormal rounded flow is analyzed. Based on the results of obstructed buoyant jets with D/d=1, normal rounded flowing occurred only for all conditions and axial dilution behind the plate under different H/D is obtained.

Theoretical Analysis of A Buoyant Jet Interacting with Small Amplitude Waves

A new theoretical solution is presented here for the dynamic characteristics of a buoyant jet due to opposing small amplitude waves. The conservation equations of mass, tangential moment^n and vertical momentum are solved by the integral method which encompasses the Gaussian profiles of velocity and density. The action of waves is incorporated into the equations of motion as an external force and a new exact solution is obtained to predict the trajectory, velocity distribution and boundary thickness of the buoyant jet over an arbitrary lateral cross section. It is found that the velocity along the centerline is inversely proportional to the ratio of the momentum of the wave to the buoyant jet. The averaged bound- ary width varies with the fluctuation of the boundary width, the distance from the orifice and the velocity correction function. Owing to the motion of waves, the fluctuation of the boundary width is proportional to the wave steepness.

NUMERICAL STUDY ON THE STABILITY AND MIXING OFVERTICAL ROUND BUOYANT JET IN SHALLOW WATER

The k-epsilon model was applied to establish the mathematical model of vertical round buoyant jet discharging into confined depth, and it was solved using the Hybrid Finite Analytic Method (HFAM). The numerical predictions demonstrate two generic flow patterns for different jet discharge and environmental parameters: (ⅰ)a stable buoyant flow discharge with the mixed fluid leaving the near-field warm in a surface warm water layer; (ⅱ)an unstable buoyant flow discharge with recirculation and re-entrainment of warm water in the near field. Furthermore, the mixing characters of vertical round buoyant jet were numerically predicted. Both the stability criterion and numerical predictions of bulk dilutions are in excellent agreement with Lee and Jirka's experiments and theory.

Numerical Study of A Round Buoyant Jet Under the Effect of JONSWAP Random Waves

This paper presents a numerical study on the hydrodynamic behaviours of a round buoyant jet under the effect of JONSWAP random waves. A three-dimensional large eddy simulation （LES） model is developed to simulate the buoyant jet in a stagnant ambient and JONSWAP random waves. By comparison of velocity and concentration fields, it is found that the buoyant jet exhibits faster decay of centerline velocity, wider lateral spreading and larger initial dilution under the wave effect, indicating that wave dynamics improves the jet entrainment and mixing in the near field, and subsequently mitigate the jet impacts in the far field. The effect of buoyancy force on the jet behaviours in the random waves is also numerically investigated. The results show that the wave effect on the jet entrainment and mixing is considerably weakened under the existence of buoyancy force, resulting in a slower decay rate of centerline velocity and a narrower jet width for the jet with initial buoyancy.

VIDEO RECORDING AND DIGITAL IMAGE PROCESSING APPLIED TO EXPERIMENTAL STUDY OF BUOYANT JETS

Video recording and digital image processing methods have been used in the experimental study of buoyant jets in a towing tank. The laws of jet trajectory and jet width have been obtained as empirical relations by data fitting.

EXPERI MENTAL INVESTIGATION INTO HOT WATER SLOT JETS WITH NEGATIVELY BUOYANCYIN CROSS FLOW

An experiment was conducted to examine the near-field behavior of negatively buoyant planar jets in flowing environment. Hot water jet was projected downwards at dif fercnt angles from a slot into a uniform cross flow. Micro Acoustic Doppler Velocimeter （Micro ADV） system is used to measure the velocity and turbulent fluxes of Reynolds stresses. The whole field temperatures were measured with fast response thermocouples. Pure jets experiments were made also to study the effect of buoyancy in negatively buoyant jets. It is found that the influenced area of hot jets is larger than which of pure jets when the jet angle is 90° and the influenced area of hot jets is smaller than which of pure jets when the jet angle is 45°. The difference is not obvious at 60° angle jets. This means that the rising of temperature has effect not only on negatively buoyancy, but also on the intensity of turbulence. The contrast of these two influences dominates the trend of jet flow.

Numerical study of the flow and dilution behaviors of round buoyant jet in counterflow

Pollutants are usually discharged into the receiving water bodies in the form of a turbulent jet or plume, and the presence of a counterflow enhances the initial dilution of the jet effluent. To understand the behaviors of jets in actual situations, a round buoyant jet issued horizontally into a uniform counterflow is simulated for different combinations of densimetric Froude number and jet-to-current velocity ratio. A two-phase mixture model is used to simulate this flow, and the renormalization group k - ε model is used to address the flow turbulence. The inter-phase interactions are described in terms of the relative slip velocity between phases. The jet features, including the trajectory of the jet centerline and the decay of the centedine velocity and the concentration, are investigated. The length scale analysis reveals the relationships between the distance and the centerline dilutions, and different flow mechanisms are revealed before and after the penetration point.

Performance of novel overhead crane fume-collecting hood for pollutant removal

In industrial buildings,the presence of overhead cranes severely affects roof exhaust ventilation systems when capturing and discharging fumes,resulting in severe deterioration of the indoor plant environment.In this study,an overhead crane-based ventilation auxiliary device,called overhead crane fume-collecting hood(CFCH),is proposed to guide pollutants blocked by the overhead crane back to the roof exhaust hood.The airflow characteristics and pollutant distribution under the three modes of no overhead crane,overhead crane,and overhead crane+CFCH were compared using numerical simulations.Subsequently,the effects of the CFCH length(a),width(b),and height(h)on the pollutant capture performance were determined through orthogonal experiments and computational fluid dynamics.Finally,the pollutant capture efficiency(PCE)of the optimal CFCH was investigated considering different exhaust airflow rates.The results showed that the pollutants captured by the CFCH can be classified into directly and secondary captured pollutants,with the directly captured pollutants dominating.In addition,with the introduction of different sizes of CFCH around the overhead crane girders,the PCE significantly improved by 49.9%–74.6%.The length,width,and height of the CFCH on the PCE were statistically significant,and the priority of the three factors was as follows:h>b>a.The PCE decreased with increasing a,initially increased and then decreased with increasing b,and increased with h.Subsequently,when the optimal CFCH was used,the excessive exhaust air rate had no evident PCE improvement.This provides a new concept for the control of pollutants in industrial buildings and provides a theoretical basis for the design of CFCHs.