1. INTRODUCTION The junction of two open channels is a common occurrence in many hydraulic structures ranging from wastewater treatment facilities to sh passage conveyance fi structures. Conduit outlets into open channels are common hydraulic structures. They are necessarily encountered at the end of urban sewage or agricultural subsurface drainage networks, where they release water. Their construction generally aims at a maximum efficiency discharge and a protection of the channel bottom and banks against erosion processes. Their behavior and design have been considered of interest concerning channel bed erosion that could result from the flow convergence or jet impingement. There are numerous factors that in uence ow characteristics at the fl fl junction of two open channels. One set of variables can be described as geometry variables, such as the size, shape, slope, and angle between the combining channels. Many combinations of these four variables are possible. A second set are ow variables, such as the Froude number in the fl downstream ow, the channel roughness, the ratio of fl discharge between the two tributary channels, and the variation of uid properties. fl Concerning discharge efficiency, pipe outlets have been less thoroughly studied in terms of purely hydraulic processes, or have generally been considered as free ended 1
31
Embed
Experimental Study of a Right- Angled Open Channel Junction
Although open-channel junctions are common in many hydraulic structures, no comprehensive data set has been compiled that describes the 3D flow field within the junction itself. This paper deals with an experimental study of a junction between a closed conduit and an open channel. This study was undertaken to explore hydraulic properties of outlets of subsurface drainage or sewage networks into an open air stream during flood events. Experiments were conducted in a laboratory flume, with a main rectangular channel joined at right angle to a lateral circular pipe. Both branches were supplied with independent flow rates and downstream water level was controlled by an adjustable weir. Several flows patterns were identified, combining free-surface and pressurized flows. Transitions between these flow patterns, as well as changes in water level or energy, were studied. Transitions between free-surface and pressurized pipe flow appeared to be strongly dependent on the whole set of experimental variables and the pipe longitudinal slope. This work contributes to a better knowledge of hydraulic and hydrologic key processes for point source discharging.
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
1. INTRODUCTION
The junction of two open channels is a common occurrence in many hydraulic
structures ranging from wastewater treatment facilities to fish passage conveyance
structures. Conduit outlets into open channels are common hydraulic structures. They are
necessarily encountered at the end of urban sewage or agricultural subsurface drainage
networks, where they release water. Their construction generally aims at a maximum
efficiency discharge and a protection of the channel bottom and banks against erosion
processes. Their behavior and design have been considered of interest concerning channel
bed erosion that could result from the flow convergence or jet impingement. There are
numerous factors that influence flow characteristics at the junction of two open channels.
One set of variables can be described as geometry variables, such as the size, shape,
slope, and angle between the combining channels. Many combinations of these four
variables are possible. A second set are flow variables, such as the Froude number in the
downstream flow, the channel roughness, the ratio of discharge between the two tributary
channels, and the variation of fluid properties.
Concerning discharge efficiency, pipe outlets have been less thoroughly studied in
terms of purely hydraulic processes, or have generally been considered as free ended in
several detailed studies. The hydraulic processes involved in these structures are similar
to those of a junction. It depends on flow coming from upstream (discharge from
subsurface network and concomitant discharge from open channel) and on possible tail
water effects from downstream. Tail water effects from downstream may occur if flood
control structures store water by means of a rise of the free-surface level. Field
experiments by Nedelec (2005) have shown that such a practice in an arterial drainage
ditch could dramatically change subsurface drainage flow hydrographs. Observations
show that this system can behave either as a weir, a classical junction, or a deflected jet.
In all cases, water levels are changed by the flow junction. But the most specific
consequence, in the pipe, is a transition of flow from free surface to pressurized,
controlled by the combination of several geometric parameters and experimental
variables.
1
In this paper a comparison of flow patterns at the junction of two channels and at
the junction of a channel and pipe is done. The experimental study done by Nedelac and
Gay (2008) of a junction between a closed conduit and an open channel and an
experimental study by Weber et al. (2001) of a junction of two channels are reviewed
here.
2. CHANNEL FLOW AT JUNCTIONS
The distinctive characteristics of a junction of closed conduit and channel as well
as that of a sharp-edged, open-channel junction flow, as illustrated in Figs.1 and 2
respectively, are a zone of separation immediately downstream of the junction branch
channel, a contracted flow region in the main channel due to the separation zone, a
stagnation point immediately upstream of the junction, a shear plane developed between
the two combining flows, and an increase in depth from the downstream channel to the
upstream contributing channels. The zone of separation results due to the momentum of
the lateral branch flow causing the main flow to detach at the downstream corner of the
junction
Fig.1 Junction between a pipe and an open channel
2
(Source: Webber et al; 2001)
Fig.2 Flow Characteristics in Open-Channel Junction
3. EXPERIMENTAL STUDY ON A JUNCTION BETWEEN
CLOSED CONDUIT AND OPEN CHANNEL
(Nedelac and Gay. 2008)
In order to study the flow patterns at a junction between a pipe and an open
channel an experimental study published by Nedelac and Gay (2008) is reviewed. The
experimental set up used by them and the important results are discussed in the following
sections.
3.1 EXPERIMENTAL SETUP
The experimental apparatus was built at the Cemagref Research Center in
Antony, near Paris. Fig. 3 shows a simplified view of the model. A detailed description of
the experiment, and the instrument used is as follows:
3
(Source: Nedelac et al; 2008)
Fig. 3. Experimental setup overview
The main part of the model was a rectangular glass flume, 6 m long, 0.3 m wide,
and 0.3 m in depth. Its downstream end was closed by a gate sliding vertically, acting as a
horizontally crested weir. The channel bed slope was adjustable and the bottom wall was
covered with rough PVC lining in order to increase the friction slope. One of the flume’s
sidewalls was equipped with a 0.2 m wide opening in its middle, so that a rectangular
plate with a circular hole could be inserted, and ensure wall continuation. This plate
connected a lateral circular pipe positioned at a right angle. The pipe connection to the
channel allowed small vertical angular deviations. It was designed so that the pipe slope
could be adjusted. The pipe length was 1.20 m and its interior diameter was 0.08 m. The
pipe invert was concordant with the channel bed. After the setup of the installation, the
4
channel slope was initially measured as 0.324%, and the pipe slope as 0.23%. They were
kept unchanged because of their being typical of values in actual subsurface drainage
systems.
Supply water came into the channel and pipe from a constant head tank through
circular PVC conduits. Both input flow rates could be independently controlled by
butterfly gate valves.
Both approach flows were improved using transition structures within stilling
zones, 0.8 m long for the channel, and 0.2 m long for the pipe. The channel’s input flow
fell from the pipe through a vertical perforated cylinder before being guided and stilled
by plastic grids. The pipe’s flow was introduced upstream using a T-junction and guided
by a deflecting grid followed by a bundle of 0.2 m long and 0.025 m diameter parallel
perforated tubes. The length left for flow development before the test section was
equivalent to 12 diameters for the pipe. It was at least 25 times the hydraulic radius of the
channel.
Both discharge flow rates were measured by electromagnetic flow meters with an
accuracy of 0.66L/s for the maximum input of 12 L/s. Flow depths were measured in
the channel by a point gauge and a spanning bridge. Flow depths in the pipe were
estimated from pressure values, when a free-surface could be observed. Twenty
capillaries connected to a single pressure transducer were used to measure the static
pressure at five cross sections. Each cross section comprised the four ends of one vertical
and one horizontal diameter.
3.2 Experimental Variables and Test Procedures
The results presented here concern steady-state flows, in response to experimental
variables. The limits were chosen to encompass real observations made in an arterial
drainage ditch and discharging drainage collector drain, by an application of Froude
similitude. The Reynolds similitude could not be simultaneously satisfied because of a
limited space for the apparatus. The Froude similitude was preferred because of its
applicability to open channel flow. Its application at the scale of 1:6 resulted in prototype
values of maximum flow rates up to 1,060 L/s for a 1.8 m wide channel, and 280 L/s for a
5
0.48 m diameter pipe. These limits, combined with a downstream gate level ranging from
0 to a little more than one pipe diameter, allowed the observation of various flow patterns
and of the transitions between them. Two realistic combinations between upstream flow
rates were chosen:
(1) Sets of upstream flow linked by a given constant sum downstream
(2) Sets of upstream flow rates linked by a given constant ratio.
The first kind of combination simulated the influence on junction behavior of one
given downstream control structure or channel flow capacity. The sum of the two flow
rates together with one gate level became a single and invariable downstream constraint
condition.
The second kind simulated the junction’s typical behavior in case of concomitant floods
coming from similar catchments. In this case, at any time during one flood event, both
upstream flows (yet considered here as constant) were set proportional to a common
specific flow rate. The ratio between flow rates represented the ratio between watershed
areas. Because of a head loss due to the flow guides at the pipe’s inlet, an undesirable
pressurized flow could be introduced upstream. The installation of a small air vent
upstream of the conduit could change the depth-discharge curve beyond a given
nondimensional discharge. In order to check for possible air presence influence on flow
patterns, a tiny hole (0.005 m in diameter) was drilled through the pipe upstream wall 1 m
away from the junction point. Along with visual flow observations, quantitative data sets
for each test comprise results from variables listed in Table 1.
The dashed line denotes the horizontal free surface.
The water level difference tended to align to that of a nearly horizontal free
surface at high gate levels. The difference increased as the gate was lowered, and at an
increased rate as the pipe’s flow rate was higher. Thus the influence of a conduit outlet
may not be negligible in case of a high discharge into a channel with a high discharge
capacity. Conversely, a channel management policy of keeping water at high levels will
reduce the pipe discharge influence on upstream channel levels compared to an
improvement of channel discharge capacity.
12
3.3.3 INFLUENCE OF JUNCTION ON ENERGY LOSS
The energy loss resulting from the junction of two flows of the same kind was
thoroughly studied and modeled and considering that the portions of the channel free
surface in both the upstream and downstream directions were nearly horizontal (as in Fig.
4), we may define the energy loss coefficient Ke as
= Ke ----------------------------- (1)
where A and α = correction coefficients for energy, considered from hereon as equal to 1;
Z= vertical free-surface or piezometric level;
Zu in the channel and in the pipe are supposed to be nearly equal, for open channels;
θ= angular variable representative of free-surface elevation above the pipe invert,
defined as the angle between a radius extending from the pipe’s bottom to the central
axis, and a radius extending from the central axis to the point of contact between the free
surface and pipe:
θ= Π- 2 Arccos (√ h/d) --------------------- (2)
Serre et al. (1997) introduced 2 coefficients defined by
------- (3)
13
Neglecting the friction energy loss in Eq (3), the combination of Eqs. (1) and (3) yields
Ke= ----------- (4)
Fig.9 Comparison of energy loss coefficients Ke, computed from experimental data
versus those computed from Serre. formulas. (x) Computation with observed
upstream and downstream water levels. (Δ) Computation considering only
downstream (Zu=Zd) water level. The dashed line is the line of perfect agreement
The values of Ke calculated from experimental data and those computed from
Serre et al. formulas are shown in fig 9.
14
Considering that friction energy loss was neglected in the application of Eq. (3),
the comparison shows a rather good correspondence between a model developed for
circular conduit junction and the system studied here.
4. EXPERIMENTAL STUDY ON JUNCTION BETWEEN
TWO OPEN CHANELS
[WEBER et al. (2001)]
4.1 EXPERIMENTAL DETAILS
The experiments were performed in a 900 combining flow flume Head tanks on
both the main and branch channels supplied the discharge. To ensure properly developed
flow entering into the junction branches, perforated plates and 100 mm thick honeycomb
were placed at the main and branch channel inlets. Volumetric measurements were made
with manometer readings. The tailwater depth in the downstream channel was controlled
by an adjustable tailgate.
The coordinate system defined for this testing had the positive x-axis oriented in
the upstream direction of the main channel. The positive y-direction points to the main
channel wall opposite of the channel junction. Thus the positive z-axis is upward in the
vertical direction. The origin from which all points are measured was the bed at the
upstream corner of the channel junction. All distances were normalized by the channel
width. All test sections in this study were denoted by the distance in channel widths
measured positive in the x-direction for upstream main channel measurements, negative
in the x-direction for combined tailwater flow measurements, or negative in the y-
direction for measurements in the branch channel.
4.2 RESULS AND DISCUSSIONS
4.2.1 VELOCITY PATTERN
15
The longitudinal velocity u* is the dimensionless velocity in the x-axis direction.
Plate 1 displays u*-velocity contours near the water surface for q* = 0.250 {q* or flow
ratio is defined as the ratio of the upstream main channel flow (Qm) to the total flow (Qt)}.
The separation zone can be seen as the area of low velocity along the junction adjacent
wall immediately downstream of the channel junction. Recirculation inside the separation
zone is shown as the region of positive velocity, indicating upstream motion. The largest
velocities occur just downstream of the junction, in the main channel flow region
contracted by the zone of separation.
All longitudinal velocity contours near the bed are distinctly different from the
near surface velocity patterns. The separation zone is larger near the surface (both in
length and width) its size varies from top to bottom because of the angle of entrance of
the branch channel flow. There is also more recirculation inside the separation zone near
the surface. In the constricted reach immediately downstream of the junction, higher
velocities occur near the bed. This effect is also attributed to the entrance angle of the
lateral flow. Once the contracted region is passed, velocities readjust to the typical open-
channel condition of higher velocities near the surface.
.
16
(Source: Weber et al; 2001)
Fig.10. Schematic of Flow Structure
As more discharge enters from the main channel the separation zone decreases in
width and length. This can be justified by considering the limiting condition of all flow
entering the junction from the main channel which would likely exhibit no separation at
the downstream corner of the junction. Lower velocities in the junction region result from
the reduced contraction of the main channel flow due to a smaller zone of separation
17
Fig.11 Plate1. Surface Velocity Pattern
Fig. 12 Plate2. Cross-Sectional Velocity Pattern.
Plate 2(a) displays a cross-sectional view of the u*-velocity downstream of the
junction for q* = 0.250. The separation zone is clearly depicted along the junction
adjacent wall, y*= 0.00. It is apparent that the constricted flow has been deflected to the
outer half of the main channel.
Plate 2(b) contrasts the flow downstream of the channel junction with q* = 0.750.
The separation zone, at the same cross section, immediately downstream of the channel
junction, extends much further into the main channel for q* = 0.250 [Plate 2(a)] than for
q* = 0.750 [Plate 2(b)].
This is caused by the decreased momentum of lateral channel flow as q*
increases. The higher momentum for the low q* condition allows the branch channel flow
18
to extend further into the main channel before being deflected downstream, therefore
causing a wider zone of separation.
4.2.2 FLOW PATTERN
Water surface mappings allow visualization of the dynamics of the water surface
through the channel junction region. A depth decrease from the flow upstream of the
junction to the tailwater flow is generally observed.
Fig 13. Water Surface Mapping- Flow Level
Fig.14 Flow Pattern Graph at Junction of Two Channels (Along Wall Side)
19
For all flow conditions the water surface generally displays a drawdown
longitudinal profile, Fig.14, as the flow enters the contracted region and then exhibits a
depth increase as the flow expands to the entire channel width downstream of the
separation zone. This pattern is more distinctive for lower q* flow conditions where the
water surface depression within the separation, adjacent and downstream of the lateral
branch, is deeper and more extensive.
5. COMPARISON BETWEEN FLOW PATTERNS OF JUNCTION
OF TWO CHANNELS AND JUNCTION OF A CHANNEL AND
PIPE
The experiment indicates that the maximum difference in the average upstream
depths is very small and decreases with increasing q*. The main channel upstream cross-
section profile is horizontal; however, the branch channel depth decreases as x*
approaches 1.00. For low q* flow conditions this decrease is substantial but for high q*
the cross-section profiles show smaller cross-channel variations. This trough exhibited at
low q* is attributed to the drawdown feature evident in the junction flow . From fig 11, it
can be seen that the flow has not completely recovered from junction effect until beyond
x*=-6.00. This complicates the design of successive junctions. Figure 13 indicates that as
main channel flow reaches the junction more decrease in surface elevation is caused and
only after reasonable flow, does the main channel regains its original characteristics.
While in the case of a junction between a channel and pipe, the decrease in surface
elevation is less compared to former. The effect of flow patterns in pipe vary from free-
surface flow to pressurized pipe flow depending on the discharge and carrying capacity
of pipe.
20
6. CONCLUSIONS
The results raised the following important effects resulting from particular
combinations of experimental variables or geometrical parameters:
(1) The conduit outlet may cause non-negligible tail water effects upstream in case of
high pipe flow combined with high channel discharge capacity.
(2) The channel tail water effect upstream due to pipe outlet flow may increase with
flow rates in both the pipe and channel, which could occur through the simultaneous flow
from both the upstream catchments of the channel and pipe.
(3) The influence of combined pipe slope and downstream tail water effects may
cause a change in pipe flow from free surface to pressurized. The present observations
21
extend available knowledge on free-pipe outlet flow. This influence may lead to two
forms of transitional cavity flow, with possible different behaviors in transient conditions.
(4) The velocity dip in open channel flow is responsible for the following:
4.1 A reduction in main channel longitudinal velocities as flow passes through the
junction.
4.2 No main-channel recirculation at the free surface.
4.3 Lower through flow and recirculating velocities in the branch channel.
(5) The surface elevation of main channel flow decreases more in the case of junction
of two channels rather than a junction between a channel and closed conduit. This
theory should be considered in the construction of drainage ditch or underground
drainage systems.
REFERENCES
1. Barkdoll, B. D., Hagen, B. L., and Odgaard, A. J. (1998). “Experimental comparison of dividing open-channel with duct flow in a T-junction.” Journal of Hydraulic Engineering, ASCE Vol-124-1, pp. 92–95.
2. Ramamurthy, A. S., and Zhu, W. M. (1997). “Combining flows in 90° junctions of rectangular closed conduits.” Journal of Hydraulic Engineering, ASCE Vol-123-11, pp. 1012–1019.
3. Weber, L. J., Schumate, E. D., and Mawer, N. (2001). “Experiments on flow at a 90° open-channel junction.” Journal of Hydraulic Engineering, ASCE Vol-127-5, pp 340–350.
22
4. Y.Nedelec., and B.Gay. (2008). “Experimental study of a right-angled end junction between a pipe and an open channel.” Journal of Hydraulic Engineering, ASCE Vol-134-5-, pp 616–625.