air entrainment in manhole drops-paper_final_reviewed

7/23/2019 Air Entrainment in Manhole Drops-Paper_Final_Reviewed

http://slidepdf.com/reader/full/air-entrainment-in-manhole-drops-paperfinalreviewed 1/8

Numerical modelling of air-water flow in a vertical drop

manhole

V. Sousa*; I. Meireles**, J. Matos* and M. C. Almeida **** Department of Civil Engineering, Architecture and Georesources, Technical

University of Lisbon – IST, Av. Rovisco Pais 1049-001 Lisbon, Portugal

(E-mail: [email protected] ; [email protected] )

** Department of Civil Engineering, University of Aveiro, Campus Universitário de

Santiago, 3810-193 Aveiro, Portugal

(E-mail: [email protected] )

*** National Laboratory for Civil Engineering, Avenida do Brasil, 1700-066 Lisbon,

Portugal

(E-mail: [email protected] )

Abstract This paper presents results of a first step aiming at modelling the

transfer of oxygen in manhole drops. An experimental study was previously

conducted at IST, Lisbon, focusing on the hydraulics and reaeration on a vertical

drop followed by a hydraulic jump. In the present research, a CFD code was used

to replicate the experimental tests. The scope of the present paper is to evaluate

the performance of the CFD model to capture the overall hydraulic features of the

flow. The computed water depths were found to agree with the experimental

measurements, and water velocity profiles were successfully compared with

theoretical velocity laws. At last, an analysis was performed to study the influence

of the air-entrainment model included in the CFD code on the pressure head alongthe invert of the outlet pipe downstream the free overfall. The activation of the

model was found to improve the prediction of the pressures downstream of the

free fall.

Keywords computational fluid dynamics (CFD); drainage system; manhole;

plunge flow; vertical drop

INTRODUCTION

Dissolved oxygen concentration is one of the most relevant parameters used in water quality

assessments, both in natural and artificial streams. Within these dynamic environments,

dissolved oxygen concentration is a result of the balance, on the one hand, of the diverse

chemical and biological processes taking place within the water body that involve oxygen

(i.e., bacterial activity, photosynthesis, etc.) and, on the other hand, of the natural exchange of

gas through the water surface/atmosphere boundary and due to local turbulence (occurring in

singularities such as drops). This is not an easy equation, since the former are also controlled

by dissolved oxygen concentration in the water. Gameson (1957) was the first to report on the

aeration potential of weirs in rivers and, since then, several other studies on local turbulence

aeration in rivers have been presented (e.g., Apted and Novak 1973; Avery and Novak 1978;

Nakasone 1987). Research on the aeration performance of existing hydraulic structures were

reviewed by Wilhelms et al. (1992) and Gulliver et al. (1998), and the studies on air

entrainment by water jets in general, and weirs in particular, is a topic thoroughly reviewed by

mailto:[email protected]














Biń (1993) and Chanson (1996), among others. In sewer drops, the presence, or absence, of

dissolved oxygen is of significance in relation to the build-up or persistence of sulphides. As

long as aerobic conditions are maintained, which have to be sustained by aeration, no sulphide

build-up will occur (Thistlethwayte 1972). Therefore, local turbulence, such as that promoted

by sewer drops, has been recommended for use in places where sulphides are not present as ameasure to prevent their formation (EPA 1985). Despite its health, structural and

environmental relevance, only a limited number of prototype or model studies involved the

use of vertical drops in circular channels, conveying either clean water, polluted river water or

wastewater (e.g., Pomeroy and Lofy 1977; Matos 1991; Almeida et al. 1999).

The use of numerical models to address the process of natural aeration due to local turbulence

pose a significant challenge. When a free nappe plunges into a downstream pool, turbulence

and air entrainment will contribute to the exchange of oxygen between water and air. Avery

and Novak (1978) found that a trade-off exists between bubble residence time, pressure, and

turbulence level. However, the mechanisms of bubble generation are still an issue of debate

and the characteristics of the generated bubbles are yet not totally correlated with the waterflow properties. Up to date, numerical modelling of turbulence and air entrained generated

aeration has been mainly focused on artificial aerators, such as bubble plums (e.g.,

Bombardelli et al. 2007).

EXPERIMENTAL STUDY

The experiments were conducted in a hydraulic recirculation flume composed of two circular

pipes in acrylic PVC, with an internal diameter of 153.6 mm, assembled at the Laboratory of

Hydraulics and Water Resources of the Technical University of Lisbon, IST (Figure 1), in the

framework of the research work by Sousa and Lopes (2002) and Soares (2003).

Figure 1. Recirculation flume at the Laboratory of Hydraulics and Water Resources at IST

(Soares 2003).

The length of the inlet and outlet pipes was 1.50 m and 3.50 m, respectively. The height of thedrop, between the inlet and the outlet pipes, was adjustable up to 0.60 m and both pipe slopes

were adjustable up to 5%. A vertical lift gate installed at the end of the flume allowed

establishing a fully-developed hydraulic jump in the outlet pipe. Recirculation of the flow was

done through a pressure conduit, connecting the tail water tank (capacity of 0.30 m

3

) and the



feeding reservoir (capacity of 0.07 m3), fed by a small pump (discharges up to 4 l/s). To

control and measure the hydraulic characteristics of the flow, the flume was equipped with: i)

a flow rate sensor (Georg Fischer SIGNET 515), previously calibrated with a triangular weir;

ii) a flow control valve, installed in the pressure conduit; iii) a manually operated point gauge,

both in the inlet and outlet channels; and iii) piezometers installed along the invert centre lineof the outlet channel. The reaeration (not addressed herein) was determined based on the

measurements of two portable dissolved oxygen and temperature meters (YSI 556 MPS), one

installed in the inlet channel and the other installed in the outlet channel. Further details

regarding the experimental study can be found in Sousa and Lopes (2002), Sousa et al.

(2003), Soares (2003) and Soares et al. (2004).

NUMERICAL CODE

The commercial code FLOW-3D®, developed by Flow-Science, Inc, was selected for this

research due to its particularly efficient, robust and accurate method to simulate free-surface

flows. The finite volume/finite differences methods are used to solve the equations of motionin a Cartesian, staggered grid. Single- or multi-block grids can be used to define the domain,

whereas the geometry can be incorporated through (Flow Science 2008): i) a “solid

modeller ”, which allows the use of general quadratic functions; ii) Computer-Aided-Design

(CAD) files; or iii) topographic data. After both the geometry and the grid are defined, the

Fractional Area-Volume Obstacle Representation (FAVOR™) method (Hirt and Sicilian

1985) automatically embeds the obstacles in the computational mesh by computing the

fraction areas and volumes blocked to flow. This feature makes grid generation and geometry

definition separate tasks, allowing for independent modifications in each one. More

information can be found in Flow Science (2008).

General flow model and boundary conditions

The mixture equations for an air-water flow were the base for the theoretical model. For a 3-D

dilute flow, the Reynolds-Averaged Navier-Stokes (RANS) mixture equations are as follows

(Bombardelli et al. 2011):

0 mu (1)

''

00

0 mm

T

mmmm

m uuuu p Buut

u

(2)

wheremu refers to the time – averaged mixture velocity vector;

0 indicates the reference

density (water); B is the vector of body forces; p denotes the time – averaged, modified

pressure; refers to the dynamic viscosity; t is the time coordinate; and'

mu indicates the

fluctuating mixture velocity vector. In turn, refers to the tensor product; T denotes the

transpose of a tensor; and the underline indicates vectors. For this specific problem the body

forces are composed only by gravity. The Reynolds stresses, last term of eq. (2), correlate the

velocity fluctuations and can be interpreted as a mechanism of momentum exchange between

the mean flow and turbulence. This is an additional unknown term to the original Navier-

Stokes equations. To close the problem, the Reynolds stresses are modelled using the eddy



viscosity concept (Rodi 1984), for which the RNG k model was applied to determine the

turbulent kinetic energy ( k ) and its rate of dissipation ( ). The previous equations are valid

in a domain limited by the incoming flow in the inlet pipe, the outgoing flow through the

outlet pipe, the pipe walls and the free surface. Water depth and an uniform velocity profile

were used to define the upstream boundary condition, whereas for the downstream boundary awater depth was specified. An additional symmetry boundary condition was added along the

longitudinal axis of the geometry, allowing simulating only half of the physical model. Null

velocities normal to the pipe walls (impenetrability condition) and the usual wall functions for

the turbulence statistics were employed in the solid surfaces. The free surface is a particular

boundary condition, since it is “a priori” unknown in each time step. Free surface capturing in

FLOW-3D® is accomplished using the TruVOF , a complete version of the Volume-of Fluid

method (Hirt and Nichols 1981). The TruVOF method is centred in the definition, use and

transport of a fraction of fluid function and application of boundary conditions at the free

surface. In this method, unlike others, the cells with gas are neglected and the flow is only

computed in cells with liquid. For this reason, the TruVOF combines the advantages of

minimum memory storage (only one variable has to be recorded), reasonable computationalcost, and satisfactory accuracy. A model of air-entrainment is also included in the FLOW-3D®

, which allows for the simulation of air-water flows. This model is able to simulate self-

aeration due to turbulence. When the disturbing energy due to turbulence exceeds the

stabilizing energy due to gravity and surface tension, a volume of air enters the flow, and is

subsequently transported within the flow and/or released to the atmosphere (see Bombardelli

et al. 2011 for details).

NUMERICAL MODEL IMPLEMENTATION

The geometry was generated using CAD software and then imported into FLOW-3D®. It was

chosen to represent the flume from the feeding reservoir to the furthest downstream known

boundary condition, which is the water depth 3.0 m downstream from the drop. The resulting

geometry represents 4.8 m of the physical model, with an inlet pipe 1.5 m long (LI) and an

outlet pipe 3.0 m long (LO) and part of the feeding reservoir. The pipes are horizontal and

separated by a drop of 0.10 m. The domain was discretized using 4 main blocks covering: i)

the feeding reservoir; ii) the inlet pipe; iii) the drop region, including part of the inlet an outlet

pipes; and iv) the outlet pipe. Mesh refinement was performed until mesh-independent results

were achieved by reducing, progressively, the size of the cells within the defined mesh

blocks, particularly in the drop region. Table 1 details the simulations presented herein for

3.9 l/s discharge.

Table 1. Details of the simulations.

RUN D RUN E RUN F

Turbulence model RNG RNG RNG

Model of air-entrainment activated No No Yes

Number of active cells 9.9E+05 13.1E+05 9.3E+05

Minimum size of cells (mm) 2.5x2.5x5.0 2.5x2.5x2.0 1.9x2.5x2.0

Despite the smaller number of active cells of RUN F when compared with RUN E, the

resolution is not reduced since the domain was shortened using a grid overlay boundary

condition to increase resolution in the drop region. The proximity between velocity profiles,



water depths and pressures of RUNS D and E constitutes a check on the convergence of the

mesh (see Figures 3 to 5). A sensitivity analysis to the turbulence models was also performed

for simulations without the model of air-entrainment activated. The standard and the RNG

k models revealed to return similar results. Figure 2 provides a general overview of the

model configuration, where the mesh blocks limits are outlined. The mesh block of the outletchannel is not completely shown to allow for a better view of the flow in the drop region.

Figure 2. Model configuration and velocity magnitude contours from RUN E (in m/s).

COMPARISON BETWEEN EXPERIMENTAL AND NUMERICAL RESULTS

Velocity profiles

The velocity profiles in the inlet pipe were checked against the theoretical log-law of the wall.

For this comparison, a section 1.3 m downstream from the feeding reservoir was selected, toensure that it was not affected by the inlet boundary condition nor by the brink. The number

of numerical points, in the profile, did not allow for an adjustment of the log-law in inner

variables, but in outer variables the adjustment was found to be good, with the shear velocity

(u*) stabilizing at an average value of 4.25E-02 m/s (Figure 3). In Figure 3, Uc is the critical

velocity, z is the transverse coordinate originating from the inlet pipe invert, and dc is the

critical water depth.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.4 0.6 0.8 1.0 1.2

z / h c

u/Uc

Numerical results

RUN D

RUN E

Log Laws

RUN D u*=4.31E-02

RUN E u*=4.21E-02

Figure 3. Adjustment of numerical velocity profiles in the inlet channel to theoretical log-laws

(at 1.3 m from the upstream reservoir).



Water depths

The comparison between measured and computed water depths (d) showed reasonable

agreement (Figure 4). In Figure 4, the water depths are normalized by the critical depth (dc)

and xI and xO represent, respectively, the stream-wise coordinates originating at the upstreamend of the inlet and outlet pipes.

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

d / d c

xI/L I

0.00

0.50

1.00

1.50

2.00

2.50

0.5 0.6 0.7 0.8 0.9 1.0

d / d c

xO/LO

Exp.

RUN D

RUN E

Figure 4. Water depth in the inlet (left) and in the outlet (right) pipes.

Pressure heads

The largest differences between experimental and numerical results for RUNS D and E are in

the pressure head (p), since the measured values were taken in the self-aerated flow region at

the impact zone downstream of the drop and the model of air-entrainment was not activated.

Nevertheless, the qualitative evolution of the pressure along the pipe is captured (RUNS Dand E of Figure 5). In Figure 5, the pressure head is normalized by the pressure head at the

critical section in the upstream pipe (pc). A significant improvement is observed with the

activation of the model of air-entrainment (RUN F of Figure 5).

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

p / p c

xO/LO

Exp.

RUN D

RUN E

RUN F

Figure 5. Pressure along the invert of the outlet pipe.



CONCLUSION

This paper provides a first step on the numerical assessment of the hydraulic performance of

vertical drops in sewer manholes. The simulations performed in this study show that the use

of a RNG k model combined with the TruVOF method allows for an adequate overallrepresentation of the flow features in terms of water depth and pressure head. The activation

of the model of air-entrainment was found crucial for the accuracy of the simulation of the

pressure head of the air-water flow along the invert of the outlet channel. It is worth noting

that no significant difference was observed among the results of water depth and water

velocity with and without the activation of the model of air-entrainment, as expected, since

these respected to a water flow region. Further, the multi-block feature embedded in FLOW-3D®, which helps optimizing the mesh, was decisive for computational time savings.

Since the main objective of the ongoing research is to address numerically the topic of gas

transfer downstream of vertical sewer drops, the results presented in this paper are very

encouraging. They show that the code seems simulates air-water flows down vertical dropscorrectly, in terms of pressure head. Future work will require a more extended research on the

features of the model of air-entrainment, in particular the air release through the free surface,

to address the issue of the volume of air entrained before studying oxygen transfer.

The model chosen for this stage of the research is a simplified representation of many

drainage systems where free drops exist at the manholes, particularly in older systems. The

experimental program includes, also, models with guided drops present in more recent

systems that will be studied in the future. Considering that many drainage systems in Portugal

and other countries are comprised of 200 mm (wastewater) and 300 mm (stormwater) pipes,

the scale of the physical model is a close representation of both the water and air flows and

the Froude similitude provide a good approximation for extrapolating the results. The main

simplification comparing to the prototypes was the assumption of zero slope for the inlet and

outlet pipes.

ACKNOWLEDGMENTS

Vitor Sousa and Inês Meireles were Visiting Scholars at the University of California, Davis,

to develop numerical research under the supervision of Prof. Fabián Bombardelli, and were

financially supported by Fulbright Research Grants. This support is gratefully acknowledged,

as well as the support of the ICIST Research Institute from IST, Technical University of

Lisbon. The research is being carried out under the scope of the projectPTDC/ECM/108128/2008 and the grant SFRH/BD/39923/2007 from FCT, Foundation for

Science and Technology, Portugal.

REFERENCES

Almeida M. C., Butler D. and Matos J. S. 1999 Reaeration by sewer drops. 8th Int. Conf. onUrban Storm Drainage, Sydney, Australia, pp. 413-420.

Apted R. W., and Novak P. 1973 Some studies of oxygen uptake at weirs. Proc. XV IAHRCongress, IAHR, Paper B23, Istanbul, Turkey, pp. 177-186.

Avery S. and Novak P. 1978 Oxygen transfer at hydraulic structures. J. Hydraul. Eng ., ASCE

104 (HY11), 1521-1540.

http://www.fct.mctes.pt/projectos/pub/2006/Painel_Result/vglobal_projecto.asp?idProjecto=108128&idElemConcurso=2713

http://www.fct.mctes.pt/projectos/pub/2006/Painel_Result/vglobal_projecto.asp?idProjecto=108128&idElemConcurso=2713



Biń A. 1993 Gas entrainment by plunging liquid jets. Chemical Engineering Science, 48 (21),

3585-3630.

Bombardelli F. A., Buscaglia G. C., Rehmann C. R., Rincón L. E. and García M. H. 2007

Modeling and scaling of aeration bubble plumes: a two-phase flow analysis. J. Hyd. Res.,

IAHR, 45 (5), 617-630.Bombardelli F. A., Meireles I. and Matos J. 2011 Laboratory measurements and multi-block

numerical simulations of the mean flow and turbulence in the non-aerated skimming flow

region of steep stepped spillways. Environ. Fluid Mech. 11(3), 263-288.

Chanson H. 1996 Air bubble entrainment in free-surface turbulent shear flows. Academic

Press, London.

EPA 1985 Odor and Corrosion Control in Sanitary Sewerage Systems and Treatment Plants.

US EPA Report 625/1-85/018.

Flow Science 2008 FLOW-3D User’s Manuals Version 9.3. Flow Science, Inc., Los Alamos,

New Mexico, USA.

Gameson A. L. H. 1957 Weirs and aeration of rivers. J. Inst. Water Eng . 11(5), 477-490.

Gulliver J. S., Wilhelms S. C. and Parkhill K. L. 1998 Predictive Capabilities in OxygenTransfer at Hydraulics Structures. J. Hydr. Eng., ASCE, 124 (7), 664 – 671.

Hirt C. W. and Sicilian J. M. 1985 A Porosity Technique for the Definition of Obstacles in

Rectangular Cell Meshes. Proc. 4th Int. Conf. Ship Hydro., National Academy of Science,

Washington, DC, USA.

Hirt C. and Nichols B. (1981) Volume of Fluid (VoF) Method for the Dynamics of Free

Boundaries. J. of Comp. Physics, 39, 201-225.

Matos J. S. 1991 Aerobiose e Septicidade em Sistemas de Drenagem de Águas Residuais.Ph.D. Thesis, IST, Lisbon (in Portuguese).

Nakasone H. 1987 Study of aeration at weirs and cascades. J. Environ. Eng., ASCE 113 (1),

64-81.

Pomeroy R. D. and Lofy R. J. 1972 Feasibility Study on In-Sewer Treatment Methods. NTIS

Nr.271445, US EPA, Cincinnati, OH.

Rodi W. 1984 Turbulence Models and their Application in Hydraulics. State-of-the-Art Paper,

IAHR.

Soares A. A. 2003 Rearejamento em Quedas em Colectores de Águas Residuais. M.Sc. thesis,

FCTUC, Coimbra (in Portuguese).

Soares A., Almeida M. C. and Matos, J. 2004 Measuring oxygen transfer in sewer drops.

Water and Environmental Management Series, IWA Publishing, pp. 86-93 (ISBN

1843395061), pp. 86-93.

Sousa C. M. and Lopes R. R. 2002 Hidráulica e Rearejamento em Quedas Verticais em

Colectores. Research report, IST, Lisbon (in Portuguese).Sousa C., Lopes R., Matos J. and Almeida M.C. 2003 Reaeration by Vertical Free-fall Drops

in Circular Channels: A Model Study. Proc. XXX IAHR Congress, IAHR, Theme B,

Thessaloniki, Greece, pp. 361-367.

Thistlethwayte D. K. B. (Ed.) 1972 The Control of Sulphides in Sewerage Systems. Ann

Arbor: Ann Arbor Science.

Wilhelms S. C., Gulliver J. S. and Parkhill K. 1992 Reaeration at Low-head HydraulicStructures. Tech. Rep. HL-91, US Army Engineer Waterways Experiment Station, Vicksburg,

Mississipi, USA.

air entrainment in manhole drops-paper_final_reviewed

Documents