Comparative study of ADV and LDA measuring techniques

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                       6th International Symposium on Ultrasonic Doppler Methods for Fluid Mechanics and Fluid Engineering




Comparative study of ADV and LDA measuring techniques
       Zdenek Chara1* and Vaclav Matousek2
       1
         Institute of Hydrodynamics ASCR, v. v. i., Pod Patankou 5, 166 12 Prague 6, Czech Republic
       (*Corresponding author, e-mail: chara@ih.cas.cz).
       2
        Czech Technical University, Faculty of Civil Engineering, Prague, Czech Republic, Thakurova 7,
       166 29 Prague, Czech Republic

       The contribution compares measurements of local velocities for water flows of different configurations
       carried out with the help of the ADV (Acoustic Doppler Velocimeter) and LDA (Laser Doppler
       Anemometry) measuring techniques. Three different geometrical configurations were tested – open
       channel flow, flow behind a rectangular cylinder and flow behind a hole in a plate.

       Keywords: Flow measurements, turbulent flow, LDA, ADV.


                                                                 second one was the free surface flow behind a
1 INTRODUCTION                                                   rectangular cylinder of the cross section 30x30 mm
Nowadays a lot of experimental techniques exist to               symmetrically tilted (see Fig. 1, left) where lower
measure flow characteristics of complex free                     edge was 30 mm above the channel bottom. Finally,
surface flows as HWA (Hot-Wire Anemometry),                      the third configuration was the free surface flow
LDA, PIV (Particle Image Velocimetry) , UVP                      behind the plate of 80 mm height with a rectangular
(Ultrasonic Velocity Profiler), ADV. However for 3D              hole 30x30 mm (see Fig. 1, right). The lower rim of
measurements in a non purely transparent flow a                  the hole was 20 mm above the bottom. Working
number of suitable experimental methods is limited.              frequency of the ADV system was 25 Hz.
An acoustic doppler method represents one of such
techniques. This contribution presents a comparison              3 RESULTS AND DISCUSSION
of two measuring techniques - LDA and ADV.                       Fig. 2 depicts velocity profiles of the longitudinal
Similar study was published in [1] for near-bottom               velocity component measured by the both
comparison of LDA and ADV; study the mean                        experimental techniques in the open channel. The
velocities measured by the ADV techniques were                   measuring volume of the ADV system was set to 6
always lower compared to the LDA data. On the                    mm and the velocity range was 30 cm/s. As can be
other hand the ADV velocity fluctuations were higher             seen in Fig, 2 the ADV data compared with the LDA
than those measured by the LDA.                                  data underestimated the velocity values. The
                                                                 deviation seems to be more or less constant and if
2 EXPERIMENTAL DETAILS                                           the ADV data are multiplied by a factor 1.07 the
The measurements were carried out in a hydraulic                 results fit very well the LDA data. RMS (Root-Mean-
flume length of which attains 6 m and rectangular                Square) values of the horizontal as well as vertical
cross section 0.25 x 0.25 m. The LDA system                      velocity fluctuations are shown in Fig. 3. The RMS
consisted of a Dantec two-component system with                  values of the horizontal velocity component
two BSA processors. The ADV system used was a                    measured by the ADV method are about 12% lower
Nortek 3D side-looking probe (two sensors were                   compared to the LDA data.
oriented parallel with the channel bottom).                                  120


                                                                             100


                                                                              80
                                                                    h [mm]




                                                                              60
                                                                                             LDA
                                                                                             ADV
a)                    b)                                                      40
                                                                                             ADV*1.07
Figure 1: Schematic view of the geometrical
arrangements, a) flow behind a rectangular cylinder, b)                       20
flow behind a hole in a plate.
                                                                              0
Three different configurations were tested, the first                         0.00   0.05   0.10              0.15   0.20   0.25
one was a pure flow in the hydraulic flume, the flow                                               ux [m/s]
depth was 110 mm and flow discharge 4.8 l/s. The
                                                                 Figure 2: Profiles of the longitudinal mean velocities



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6th International Symposium on Ultrasonic Doppler Methods for Fluid Mechanics and Fluid Engineering



         120                                                                                     100


         100
                                                                                                  80

         80                         LDA
                                    ADV
                                                                                                  60
h [mm]




                                                                                       h [mm]
         60


         40                                            LDA                                        40
                                                       ADV

         20
                                                                                                  20                           LDA
                                                                                                                               ADV
          0
          0.00    0.01       0.02       0.03 0.00     0.01          0.02   0.03
                                                                                                       0
                     u'x [m/s]                              u'y [m/s]                                  -0.10                  -0.05           0.00                 0.05                      0.10

                                                                                                                                             uy [m/s]
Figure 3: RMS data of horizontal (left) and vertical (right)
velocity component                                                                 Figure 5: Flow behind the cylinder - mean velocity profiles
                                                                                   in the vertical direction
In the upper part of the flow the RMS data of the                                  values measured by the ADV are always lower, in
vertical velocity component measured by the both                                   the vertical direction the values are lower behind the
methods are practically the same, close to the                                     cylinder, close to the cylinder edges the values are
channel bottom the ADV data systematically                                         higher.
increased.
                                                                                                 100
The second flow configuration was represented by
flow behind the rectangular cylinder. In this case the
                                                                                                  80
flow measurements were performed at a distance
40 mm behind the cylinder. ADV velocity range was
set to 100 cm/s and a size of the measuring area                                                  60
attained 6 mm. Profiles of the mean velocities in the
                                                                                       h [mm]




                                                                                                                                                               LDA
                                                                                                                                                               ADV
longitudinal direction are presented in Fig. 4 where a
                                                                                                  40
vertical axis shows distances from the channel
bottom. A similar tendency as in the previous case
was observed - the ADV velocity data are lower                                                    20

compared to the LDA data, but the differences are                                                                  LDA
                                                                                                                   ADV
higher - about 20%.                                                                                0
                                                                                                   0.00            0.05          0.10       0.15 0.00       0.05       0.10       0.15       0.20
         100
                                                                                                                          '
                                                                                                                      u x [m/s]                                    u'y   [m/s]


          80                                                                       Figure 6: Flow behind the cylinder - RMS data of
                                                                                   horizontal (left) and vertical (right) velocity component

          60
h [mm]




                                                                                                0.4                                                                                      ADV

          40
                                                                                                0.2


                                                                                                  0
          20              LDA
                          ADV                                                                   -0.2
                                                                                                       0       1      2           3     4       5       6          7          8          9      10

           0
           0.00    0.05          0.10      0.15      0.20         0.25     0.30
                                                                                                0.4                                                                                      LDA
                                          ux [m/s]
                                                                                   ux [m/s]




Figure 4: Flow behind the cylinder - mean velocity profiles                                     0.2
in the longitudinal direction
                                                                                                  0
Mean velocity profiles in the vertical direction are
                                                                                                -0.2
shown in Fig. 5. Also in this case the ADV data are                                                    0       1      2           3     4        5      6          7          8          9      10
lower but the discrepancy between the both                                                                                                   t [sec]

methods is higher. The RMS values of longitudinal                                  Figure 7: Flow behind the cylinder - time series of the
as well as vertical directions are shown in Fig. 6. In                             longitudinal velocity component, h = 70 mm
the     longitudinal       direction   the      RMS


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                               6th International Symposium on Ultrasonic Doppler Methods for Fluid Mechanics and Fluid Engineering


Fig. 7 depicts the parts of the time series of the                       Figure 9: Flow behind the hole - mean velocity profiles in
longitudinal     velocity  component      measured                       the longitudinal direction
simultaneously at a distance 70 mm above the
channel bottom where a periodical vortex shedding                        plotted in Fig. 10. In the central part of the hole the
was observed. As can be expected due to the                              velocity ratio is about 1.07 which coincides with the
relatively large measuring volume and low data rate                      free surface flow but close to the hole rim the ratio is
(25 Hz) the ADV method is able to follow quite well                      continuously increasing. An example of the time
only low velocity frequencies. This expectation was                      series    of     longitudinal  velocities    measured
confirmed by our measurements. From the time                             simultaneously at the level of lower rim is shown in
series a FFT analysis was performed and the results                      Fig. 11. Due to the low data rate of the ADV probe
are plotted in Fig. 8. The both methods practically                      and large measuring volume the ADV technique is
indicate the same dominant peak frequency                                unable to measure correctly in an area where a
corresponding with the vortex shedding, f=1.378                          large turbulence level occurs. In such area the ADV
(ADV), f=1.366 (LDA).                                                    has a tendency to smooth the velocity fluctuations
                                                                         as can be seen in Fig. 11. Such inaccuracy was also
         50
                                                                ADV
                                                                         observed on RMS profiles of the longitudinal velocity
                                                                         component depicted in Fig. 12. At the centre of the
 E




           0
             -1                 0                     1
                                                                         hole the RMS values of the ADV probe correspond
           10                  10                    10
                                                                         to the LDA data, but at a distance 7 mm above the
         100
                                                                LDA
                                                                         hole rim (u’x ~ 0.20 m/s) the ADV data are suddenly
         50                                                              decreasing.
E




           0
             -1                 0                     1
           10                  10                    10
                                      f [Hz]                                         40

Figure 8: Flow behind the cylinder - FFT analysis of the
longitudinal velocity component, h = 70 mm
                                                                                     30

The third experimental run represented flow behind
the rectangular hole in the plate. This arrangement                                  20
was chosen for a comparison of both methods in a
                                                                          h [mm]




region characterized by higher velocities and higher
turbulent intensities. The measurements were                                         10
performed at a distance 45 mm behind and along
the vertical axis of the hole. The setting of the ADV
measurements was as follows: size of the                                                 0

measuring volume - 6 mm, velocity range - 250
mm/s.                                                                               -10
Mean velocity profiles in the longitudinal direction                                   1.0        1.2        1.4              1.6        1.8        2.0

are plotted in Fig. 9. The origin of the vertical axis is                                                         ux LDA/ux ADV
placed at the lower rim of the hole. The velocities                      Figure 10: Flow behind the hole - ratio of the LDA mean
measured by the ADV are again lower but the ratio                        longitudinal velocities to ADV mean velocities
of the values of LDA and ADV velocities is no more
constant as it was observed in the free surface flow.
The ratio is
                                                                                                                                                ADV
          40                                                                       0.8
                                                                                   0.6
                                                                                   0.4
          30                                                                       0.2
                                                                                     0
                         LDA                                                       -0.2
          20             ADV                                                           10    11   12    13   14        15     16    17    18   19     20
h [mm]




          10                                                                       0.8
                                                                                   0.6
                                                                        ux [m/s]




                                                                                   0.4
            0
                                                                                   0.2
                                                                                     0                                                         LDA
          -10                                                                      -0.2
            -0.2   0.0    0.2         0.4      0.6        0.8   1.0                    10    11   12    13   14        15     16    17    18   19     20
                                                                                                                    t [sec]
                                    ux [m/s]
                                                                         Figure 11: Flow behind the hole - part of the time series of
                                                                         the longitudinal velocities, h=0 mm



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6th International Symposium on Ultrasonic Doppler Methods for Fluid Mechanics and Fluid Engineering



         40



         30


                                              LDA
         20                                   ADV                                40
h [mm]




         10                                                                      30


                                                                                                                      LDA
          0                                                                      20                                   ADV




                                                                        h [mm]
         -10                                                                     10
           0.00   0.05   0.10    0.15       0.20     0.25   0.30

                                u'x [m/s]
                                                                                   0
Figure 12: Flow behind the hole - RMS data of the
longitudinal velocities
                                                                                 -10
Mean velocity profiles in the vertical direction are                                0.0   0.1   0.2   0.3       0.4   0.5   0.6   0.7

shown in Fig. 13. While in the centre of the hole the                                                   u'y [m/s]
ADV velocities correspond to the LDA data, close to                 Figure 14: Flow behind the hole - RMS data of the vertical
the hole rim the deviation is strongly increasing. At               velocities
the lower hole rim the ADV velocities are several
times higher than the LDA data. Similar behavior                    is consistent with the results published in [1]. But for
can be seen in Fig. 14 where RMS profiles of the                    increasing turbulence level is increasing the
vertical velocity component are plotted.                            deviation between both techniques is also
                                                                    increasing. Mean velocity profiles in the vertical
                                                                    direction were measured for flows behind the
         40
                                                                    rectangular cylinder and behind the hole. In both
                                                                    cases the deviation was larger in comparison to the
         30
                                                                    longitudinal direction and for flow behind the hole
                                                                    the ADV method measured even higher mean
                                                    LDA
                                                    ADV
                                                                    velocities. We suppose that the measurement in the
         20                                                         vertical direction is affected by one of the ADV
                                                                    probe which seems to be more sensitive.
h [mm]




         10
                                                                    ADV technique tested is suitable for flow conditions
                                                                    with relatively low turbulence level. In the case of
                                                                    higher turbulence the results have to be carefully
          0                                                         analyzed.

         -10
           -0.1   0.0    0.1      0.2       0.3      0.4    0.5

                                uy [m/s]
                                                                    ACKNOWLEDGEMENT
                                                                    The paper was written with the support of the grant
Figure 13: Flow behind the hole - mean velocity profiles in         GA AS CR No. A200600802 and the Institutional
the vertical direction                                              Research Plan No. AV0Z20600510.

                                                                    REFERENCES
5 CONCLUSIONS                                                       [1] Precht E, Jansen F, Huettel M: Near-bottom
                                                                    performance of the Acoustic Doppler Velocimeter (ADV) -
In this study the comparative study of the ADV and                  a comparative study. Aquatic Ecology 40, (2006), 481-492
LDA measuring techniques was performed for three
different geometrical arrangements of free surface
flows. In all tested cases the ADV method
underestimated the mean velocities in the
longitudinal direction. The deviation attains about
7% for relative simple flow geometry without large
disturbances as open channel flow. This conclusion



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