Hua Ming1, Tang Hongwu2 and Wang Huimin1
1College of Water Resources and Environment, HoHai University
2College of Water Conservancy and Hydropower Engineering, HoHai University
College of Water Conservancy and Hydropower Engineering
HoHai University. 1,Xikang Road, Nanjing 210098, China
Tel:
+86-25-3713777 ext.50515，Fax: +86-25-3731332
E-mail:
hhuhm@163.net
Abstract: The preliminary results of an experimental investigation into the behavior of a round jet in stagnant ambient are presented here. Velocity fields are measured using Acoustic Doppler Velocimeter (ADV) system that is described in this paper. It is found that ADV could be used in velocity measurements of buoyant jet in stable surroundings. For verification purpose, the ADV system is used to measure the characteristics of a single jet discharging into stagnant surroundings. The velocity fields in the various cross-sections are measured. The mean and turbulent characteristics of velocity, including the distribution of axial velocity in section, the spreading rate and variations of maximum velocity along the jet axis, the turbulence intensity in cross-section and that along the jet axis, the shear stress in cross-section, and the variations of instantaneous streamwise velocity, are acquired through the special analysis WinADV software. The measurement results, as comparing with the existing data, are well satisfactory. Detailed experimental investigations in the turbulent jet in different ambient are continuing with ADV.
Keywords: ADV, velocity fields, mean and turbulent characteristics of velocity
The mechanics of turbulent buoyant jets, although
studied for several decades, has still been paid much attention today.
Especially in recent two decades, more advances were made in studies of
turbulent buoyant jets with the focus of environment. The turbulent
characteristics of buoyant jets are relevant to the dilution and mixing of
pollutants in water bodies. Studies of the behavior of a round jet are the
important basis for understanding more complex turbulent buoyant jet.
Although the investigation of a round jet has been done for many years, there still some important problems unsolved because of the limits of measurement techniques. The advances in behavior of a round jet depend much on the development of measurement techniques that play a crucial role in study of turbulence characteristics of the jet. Advanced and high precise measurement techniques make it possible to understand the behavior of turbulent buoyant jets more deep and detailed. It is an important approach of future study to acquire the turbulence characteristics of velocity field through the advanced and high precise measurement techniques. With the help of advanced measurements techniques, many researchers had achieved in exposing the interior turbulence structure and nature of turbulent buoyant jets further (e.g. Lau et al, 1979; Chen and Yu, 1984; Cheng, 1995). Furthermore, some researchers still devote themselves to the study with different measurements techniques.
In this paper, the results of an experimental investigation into the behavior of a round jet with Acoustic Doppler Velocimeter (ADV) are presented. The ADV is a precise instrument for measuring 3-D fluid flow and has an excellent performance in measuring velocity vector (Snyder and Castro, 1999; Nikora and Goring, 1996; Kraus et al, 1994). The main aim of this article is to introduce the application of ADV technology in the study on turbulent buoyant jets. The data presented here is a verification experiment of this application of a round jet in stagnant ambient.
Nortek ADV is a versatile, high-precision new current instrument that measures all three components of the flow velocity vector in laboratory and field environments (NORTEK AS ADV specifications, 1996). ADV technology is based on the Doppler effect velocity-measurement principle. The versatility and consistent data quality make it possible to replace a variety of traditional sensors, ranging from mechanical propellers and electromagnetic current meters to expensive Laser Doppler systems. An important advantage of the ADV is that it measures the flow in a small sampling volume ,which is 3-9 mm long and approximately 6 mm in diameter, and is 5 (or 10) cm away from the sensing elements. Thus enables measurements to be taken without interfering with the flow. The instrument has the excellent performance characteristics, such as wide velocity range, high velocity resolution, no limitations of velocity direction and others. Also, the range of accuracy is quite acceptable in three-dimensional components (e.g. Snyder and Castro, 1999; NORTEK AS ADV specifications, 1996). The acquisition data can be easily converted to ASCII format with the data conversion programs supplied with the ADV system. Then, Excel, Matlab, Grapher and some other software can easily analyze the converted data.
The experiment is being conducted in a towing tank that is 0.4m deep, 6m long and 0.2m wide. Acoustic Doppler Velocimeter (ADV) measurement technique is used to accurately measure velocity profiles in the turbulent round jet. The experiment system is shown schematically in Fig.1. The jet is discharged from a horizontally positioned port that has round outer profiles and inner diameter 4mm. The ADV probe is mounted on a carriage that could be moved along the tank when it is necessary. When the measurements need, the ADV also can be moved vertically by electrical motor. Then, ADV can measure the whole flow field freely. During the experiments, the jet flow fluid comes from a stabilized storage tank, which keeps the discharge velocity stable.
Several experiments are conducted in the towing tank with ADV system. As verification experiments and also due to the space limitation, only results of one of the experiments are briefly described here. The discharge velocity is 1.69m/s in the experiment. A large amount of data of the velocity is collected with ADV, including the results of the mean and turbulent characteristics.
With the increasing of distance from the nozzle, the velocity along the centreline decreases, the curve of velocity distribution tends to be flatness, and the spread width of jet increases too. The distribution of axial velocity in section can be determined by regression analysis of non-dimensional velocity ux/um with no-dimensional radius r/re. The relations of them can be written as:
(1)
where
is the x-direction mean velocity where
the radius is r,
is the centerline mean velocity,
is the radius,
is
the radius where
.
It is clear that the mean velocity profiles across the jet is consistent with Gaussian distribution and has good self-similarity.
Also, the spread function can be acquired as the form:
(2 )
where
is spread width where
,
is constant and has a value
of 0.109 here,
is the distance from the
orifice along the jet axis.
Rodi (1982) investigated a great deal of experiments and gave k an average value of 0.107. It is very close to the value of this paper.
Analyzing the non-dimensional
parameter
and
, the relationship can be determined as:
(3)
where
is the mean velocity at jet
orifice,
is constant and has a value
of here,
is the diameter of jet nozzle.
The k-value of 6.104 is much consistent with that of 6.2 given by Albertson (1950).
It is shown that the comparison of the results of mean characteristics with the large body of existing experimental results is in good satisfactory. It demonstrates that the ADV measurement technique is an appropriate and convenient tool for measurements of turbulent jets.
A large amount of data of the turbulent characteristics
is obtained with the ADV technology. Fig.2 illustrates the radial distribution
of relative streamwise turbulent velocity intensity scaled by the mean centerline
streamwise velocity at different profile with variable relative distance
. It can be seen that there is a very distinct maximum in the value of the relative
r.m.s velocity at
. The results have a satisfactory agreement with that by Wygnanski and Fiedler
(1969), but Papanicolan and List (1988) and Cheng
(1995) thought that the maximum value
of the relative r.m.s velocity lies on the jet axis with LDV measurements. The
figure also shows that the profiles of r.m.s streamwise turbulent velocity intensity
across the jet are clearly self-similar. The variation of relative turbulent
velocity intensity along the jet axis is shown in Fig.3. All the relative turbulent
velocity intensities on three dimensions increase to a top value in the transition
region firstly, and then rapidly drop to steady state for
. It indicates that the self-similarity of turbulence characteristics should
be remained for a long distance from the jet orifice. Fig.3 also shows that
the pulsation and the mean velocity fall into the same order of magnitude, and
the turbulence intensity of three-velocity components also belong to the same
magnitude. The results agree well with those of Chen (1988), but are different
with those of Cheng (1995) who suggested that
the relative turbulence intensity of centerline streamwise velocity increase
along the jet axis and reach a steady value for
.
Measurements of the maximum values of the shear
stress show relative values of the order of 0.021 at about
in the fully developed flow regime.
The distribution of Reynolds shear stress across the jet axis is clearly self-similar
for
when scaled by the mean centerline
streamwise velocity, as shown in Fig.4. The results are coherent to those of
Rodi (1982) and Wygnanski and Fiedler (1969).
For comparison, the results of Rodi are also shown in Fig.4. The relative maximum
value of Reynolds shear stress measured by Rodi (1982) is of the order of 0.017~0.018
at the same position.
When the jet issuing into the stagnant ambient,
the discontinuous interfaces are generated at the jet boundary and are perturbed
inevitably. Then, the vortices are produced, as is clearly the key element in
the initial diluting ability of turbulent jets. Fig.5 shows the variations of
instantaneous streamwise velocity at different radius on profile of
. The variations of these velocities near the boundary are shown in Fig.5 (a)
and Fig.5 (b). The figures appear that there are the intermittences and the
alternations of negative flow and positive flow in the neighborhood of the jet
boundary. These velocity characteristics indicate that large amount vortices
are produced at the jet boundary, and also verify the phenomenon of complicated
vortices measured by flow visualization technology. As shown in Fig.5 (c) and
Fig.5 (d), the intermittences are not clear again, but the positive velocities,
which can be said to be “flow reversals”, still exist in the inside of jet flow.
All the results of variation of instantaneous velocity agree well with the existing
conclusions (e.g. Kotsovinos, 1975; Lau et al, 1979). The results reveal that
in the inside of the jet flow there are large-scale turbulent structures which
cause a majority of the turbulent mass transport. The studies of real time record
of instantaneous velocity at different profiles also indicate that the “flow
reveals” are not obvious or even disappear with the increase of distance from
the jet orifice.
An ADV system is successfully applied in the study of the experimental measurements of the velocity field of a round jet in stagnant environment. Comparing with the existing information, the results show good agreement. With the capacity of capturing the velocity characteristics both qualitatively and quantitatively, the ADV system is very useful for experimental studies of the turbulent jet. Detailed experimental investigations in the turbulent jet in different ambient are continuing with ADV system.
Acknowledgements
This work is supported by National Natural Science Foundation of China (NNSFC) on projects of No.59779021.
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