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Improvement of Acoustic Doppler Velocimetry in steady and unsteady turbulent open-channel flows by means of seeding with hydrogen bubbles
Flow Measurement and Instrumentation Flow Measurement and Instrumentation 19 (2008) 215–221 www.elsevier.com/locate/flowmeasinst
Improvement of Acoustic Doppler Velocimetry in steady and unsteady turbulent open-channel flows by means of seeding with hydrogen bubbles T. Meile ∗ , G. De Cesare, K. Blanckaert, A.J. Schleiss Ecole Polytechnique F´ed´erale de Lausanne (EPFL), Laboratory of Hydraulic Constructions (LCH), Station 18, CH-1015 Lausanne, Switzerland Received 2 November 2006; received in revised form 18 August 2007; accepted 27 August 2007
Abstract Velocimetry is used in various fields of research. In hydraulics, Ultrasonic Velocimetry based on the Doppler Shift effect can accurately resolve the quasi-instantaneous flow field [Takeda Y. Velocity profile measurement by Ultrasonic Doppler method. Experimental Thermal and Fluid Science 1995;10:444–53; Takeda Y. Ultrasonic Doppler method for flow measurement. In: International symposium on advanced fluid information. 2002. p. 69–76] if the acoustic scattering level is sufficiently high. But Ultrasonic velocity instruments are known to perform poorly in clear water with low acoustic scattering level, such as often found in laboratory applications. Artificial seeding of the flow can be used to increase the acoustic scattering level. Blanckaert and Lemmin [Blanckaert K, Lemmin U. Means of noise reduction in acoustic turbulence measurements. Journal of Hydraulic Research IAHR 2006;44:3–17] have proposed a technique to seed the flow with micro hydrogen bubbles generated by means of electrolysis. This technique is optimized under steady flow conditions and subsequently applied to measure the flow characteristics around the front of a surge wave. c 2007 Elsevier Ltd. All rights reserved.
Keywords: Ultrasonic Velocimetry; Velocity measurement; Experiments; Hydrogen bubble seeding; Unsteady flow; Surge wave
1. Introduction Velocimetry is used in various fields of research. Openchannel flows have been measured with various techniques, such as propellers, Pitot tubes, electromagnetic velocimeters, hot-wires, hot-films, Laser Doppler Velocimeters, Acoustic Doppler Velocimeters, Particle Image Velocimeters, etc. Acoustic Doppler Velocimeters (ADVs) allow carrying out non-intrusive measurement of the three-dimensional velocity vector. Moreover, they allow measuring entire velocity profiles at one go, whereas most other techniques are limited to pointwise measurements [1,2]. A multitude of Acoustic Doppler velocimeters are available. Although they differ in geometric configuration, transducer characteristics, emitting and measuring frequency, etc., they share the same principle and have similar properties. Hence the results presented in this paper have general validity in Acoustic Doppler Velocimetry. ∗ Corresponding author. Tel.: +41 21 693 28 58; fax: +41 21 693 22 64.
E-mail address: [email protected] (T. Meile). c 2007 Elsevier Ltd. All rights reserved. 0955-5986/$ - see front matter doi:10.1016/j.flowmeasinst.2007.08.009
The uncertainties in mean velocity and turbulence measurements with Acoustic Doppler Velocimeters are typically about 5% and 10%, respectively [4–8]. However, the temporal resolution and hence the uncertainty of Acoustic Doppler Velocimeters are known to depend on the acoustic scattering level of the fluid [9–13,3] which determines the signal-to-noise ratio. Low acoustic scattering levels only allow measuring the mean velocity field in steady flows, whereas measurement of turbulence or unsteady flow characteristics requires high acoustic scattering levels. Blanckaert and Lemmin [3] have developed a simple, lowcost and non-polluting technique to increase the acoustic scattering level in clear water, which consist in seeding the flow with hydrogen bubbles generated by means of electrolysis. Whereas Blanckaert and Lemmin [3] successfully applied it to the measurement of turbulence characteristics in steady flows, the present paper applies it to the measurement of the highly unsteady flow around the front of a surge wave. Surge waves occur when the discharge in a channel changes suddenly, for example due to the release of water from hydropower plants, the operation of valves, or due
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Fig. 1. Experimental set-up and schematic definition of the surge wave characteristics before (index 1) and after (index 2) the surge wave front moving with an absolute celerity c. Q is the discharge and h the flow depth. Velocity profiles are investigated before (1), after (2) and during (3) the surge wave front. The total flume length is 38.3 m.
to tidal influence in estuaries. Fig. 1 schematically defines the characteristics of the surge wave as investigated in the laboratory. The uniform base flow has a discharge Q b , mean velocity U1 and mean flow depth h 1 . At the upstream boundary of the channel, an additional discharge Q w,upstr is suddenly introduced, resulting in a surge wave that migrates in the downstream direction. The surge wave has a diffusive character, meaning that the height of the wave front decreases and the additional discharge attenuates in downstream direction: h 2 < h upstr and Q 2 < Q b + Q w,upstr . The celerity of the wave front c is considerably higher than the base flow velocity U1 and the average velocity upstream of the front U2 [14]. During the passage of the wave front, the flow characteristics and bed shear stress attain peak values that are relevant for resuspension, erosion and transport of sediment, aquatic species like macro invertebrates, etc. This paper presents the laboratory experimental set-up, including the flume, the Acoustic Doppler Velocimeter and the hydrogen bubble technique to increase the acoustic scattering level. Steady flow experiments are presented that aimed at optimizing the parameters of the hydrogen bubble technique. The technique is subsequently applied with optimal parameters to measure the flow characteristics around the front of surge waves (Fig. 2). 2. Experimental set-up, Acoustic Doppler Velocimeter and hydrogen bubble technique Experiments have been carried out in a 38.3 m long and 0.485 m wide straight laboratory flume, shown in Fig. 1. The bed of the rectangular cross-section consisted of smooth metal, whereas the banks were made out of smooth limestone bricks. The flume had a mean downstream bed slope of S0 = 0.00114. Table 1 summarizes the investigated hydraulic conditions. Steady flow experiments were first carried out in order to optimize the hydrogen bubble technique used to increase the acoustic scattering level. Subsequently the unsteady flow during the passage of a surge wave front was investigated. All experiments concern turbulent flow regimes. Velocity measurements were carried out on the flume axis in the cross-section at 24.3 m from the bend entry, where an Acoustic Doppler Velocimeter was installed as illustrated in Fig. 1. The present study made use of a commercially available
Table 1 Hydraulic test conditions: Q b (steady baseflow discharge), h 1 (flow depth of baseflow), U1,cal = Q/(Bh 1 ) (mean velocity of baseflow), Re1 = U1 Rh1 /ν (Reynolds number of baseflow), Fr1 = U1 /(gh 1 )0.5 (Froude number of baseflow), Q w,upstr (additional discharge introduced at flume entrance 24.3 m upstream of the measurement location) Q b (m3 s−1 ) h 1 (m)
U1,cal (m s−1 )
Re1 (−)
Fr1 (−)
Q w,upstr. (m3 s−1 )
0.0080 0.0092 0.0600 0.0056 0.0057 0.0056
0.37 0.36 0.71 0.33 0.34 0.33
12900 14300 59200 9500 9600 9500
0.55 0.51 0.54 0.56 0.57 0.56
– – – 0.0175 0.0306 0.0535
0.045 0.052 0.175 0.035 0.035 0.035
UVP velocimeter from Met-Flow SA [7]. It was used with one ultrasonic transducer that functions as emitter–receiver and allows measuring the projection of the velocity vector on the profile through the transducer axis. By inclining the transducer, the downstream component of the velocity vector can be derived. Under these conditions the instrument is appropriate for application in one-dimensional flow configurations. The UVP was applied in a non-intrusive way by installing the transducers 0.01 m under the bed level, whereby the space between the transducer and the bed was filled with an acoustically transparent contact gel covered by self-adhesive tape. The ultrasonic transducer was inclined by 30 ◦ C with respect to the vertical. It had an active diameter of 0.01 m. The measured profiles were divided in a string of identical cylindrical measuring bins of height 0.00148 m. The emitted acoustic wave had a frequency of 2 MHz yielding a wavelength of 750 µm. The sampling frequencies were 4.85 Hz and 5.32 Hz in the reported steady and unsteady flow experiments, respectively. The UVP has been successfully applied in similar configurations at EPFL [15,16]. The set-up used for the generation of hydrogen bubbles by means of electrolysis is shown in Fig. 3. It is formed by two arrays of horizontal stainless steel wires with diameter of 100 µm, the anodic one being placed 0.05 m upstream of the cathodic one. The average vertical spacing between the stainless steel wires is 0.008 m, with increased density in the lower part of the arrays. The arrays are supported by vertical stems, which are insulated by a waterproof painting layer in
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Fig. 3. Set-up for hydrogen bubble generation.
Fig. 2. Examples of surge waves. Above: Lateral view of a breaking surge wave with high air concentration at the wave tip. Below: Top view of a surge wave.
order to avoid their participation to the electrolysis. The setup for the hydrogen bubble generation is placed sufficiently far upstream of the measuring volume to prevent flow disturbance. The electrolytic reaction is generated by applying an electric direct current that produces hydrogen at the cathode and oxygen at the anode. The size of the hydrogen bubbles is comparable to the diameter of the electrode wires (100 µm) whereas the quantity of generated bubbles increases with the applied electric current. According to Shen and Lemmin [11], the size of ideal acoustic targets is about half of the wavelength of the emitted acoustic signal, and is thus instrument dependent. The described empirically designed and optimized set-up has been successfully applied, however, with instruments with an emitting frequency/wavelength of 10 MHz/150 µm [5] to 1 MHz/1500 µm [3]. The size distribution and flux of the generated hydrogen bubbles as well as their relation to the acoustic scattering level have been investigated by Birkhofer [17]. It should be noted that the hydrogen bubble generation is inefficient with clean stainless steel wires. However, electrolysis provokes a surface reaction on the stainless steel wires that makes the bubble generation efficient after a short time. 3. Steady flow experiments Steady flow experiments have been carried out for the three hydraulic conditions listed in Table 1, with the aim of analysing and optimizing the hydrogen bubble technique. Investigated parameters included the distance between the hydrogen bubble generation and the measured velocity profile as well as the relation between the hydrogen bubble concentration and the quality of the velocity measurements. Fig. 4 compares velocity profiles measured without and with hydrogen bubble generation for different distances between the hydrogen bubble generation and the measured velocity
Fig. 4. Measured velocity profiles averaged over 10 s in the steady flow experiments without and with hydrogen bubble seeding as a function of the distance between hydrogen bubble generation and measured profile. (a) Discharge of Q b = 0.008 m3 s−1 ; (b) Discharge of Q b = 0.060 m3 s−1 . The dip of the profile at z = 0.005 m probably results from a small bottom irregularity 0.08 m upstream of the measured velocity profile.
profile. The velocity profiles are measured at a sampling rate of 4.85 Hz and averaged over a period of 10 s. Figs. 4(a), (b) refer to the steady flow experiments with discharges of Q b = 0.008 m3 s−1 and 0.060 m3 s−1 , respectively. Without hydrogen bubble seeding, velocity profiles are characterized by important scatter. Long measuring times would allow obtaining accurately the time-averaged velocity profile, but not the quasi-instantaneous velocity field which is
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Fig. 6. Velocity profiles as a function of the depth-averaged DMEA. Discharge of Q b = 0.009 m3 s−1 . Table 2 Estimation of uncertainty in measurements by comparing experimental and theoretical values of the dept-averaged velocity, U , and the bed shear velocity, u∗ Experiment Qb (m3 s−1 ) 0.0080 0.0092 0.0600 Fig. 5. Relation between the electric current applied to the hydrogen bubble generating set-up and the measured demodulated echo amplitude (DMEA), which parameterizes the acoustic scattering level of the fluid. (a) Measured profiles of DMEA; (b) Depth-averaged DMEA.
required for investigating characteristics of turbulence or highly unsteady flow [3]. The influence of the distance between the hydrogen bubble generation and the measured profile has been tested in the range between 0.75 m and 4 m, corresponding to about 10–100 times the flow depth. Within this range, the distance did not significantly affect the quality of the measured velocity profiles. Figs. 5 and 6 investigate the relation between the hydrogen bubble concentration and the quality of the velocity measurements in the experiment with steady uniform flow discharge of Q b = 0.009 m3 s−1 . Fig. 5(a) shows the relation between the applied electric current and the measured demodulated echo amplitude (DMEA), which parameterizes the acoustic scattering level of the fluid, and hence the hydrogen bubble concentration [17]. Figs. 5(a), (b) show the measured profiles of the DMEA and the depth-averaged DMEA for different applied electric currents, respectively. The depth-averaged value of the DMEA for “clear” water conditions (no current applied) is 69. The depthaveraged DMEA increases rapidly when hydrogen bubbles are generated. Applying a current of 0.1 A increases the depthaveraged DMEA by a factor of about 5. From 0.15 A on, the depth-averaged DMEA increases about linearly with the current applied to the electrolysis, without signs of saturation within the investigated range. Recent analysis on the number
Bed shear velocity u ∗ (m s−1 )
Dept-averaged velocity, U (m s−1 ) Experimental Theoretical
Experimental
Semi-theoretical
0.38 0.38 0.71
0.0203 0.0204 0.0308
0.0206 0.0219 0.0337
0.37 0.36 0.71
of reflectors [17] reveals that an increase of the applied voltage mainly increases the number of generated bubbles whereas the bubble size is not significantly affected. Fig. 6 shows the relation between the depth-averaged DMEA on the one hand and the measured velocity profiles on the other. The quality of the velocity profiles does not seem to improve gradually with increasing acoustic scattering level, but rather in a stepwise way. An insufficient acoustic scattering level yields erroneous or extremely noise velocity profiles. From a threshold value on (around DMEA = 300), the velocity profiles are of a good quality that hardly improves when further increasing the acoustic scattering level. The uncertainty in the velocity measurements can be estimated by comparing measured flow quantities to theoretical values for steady uniform flow. Table 2 compares the depthaveraged downstream velocity, UUVP , as computed from the measured velocity profile, to the theoretical cross-sectional averaged velocity, Uth = Q/ (Bh). Agreement is within 5%. Table 2 also compares the experimental and theoretical values of the bed shear velocity, u ∗ , which is by definition related to the bed shear stress as τb = ρu 2∗ . This flow quantity parameterizes the turbulence quantities, friction losses, sediment transport, etc. The experimental estimation is based on the hypothesis of a logarithmic velocity profile in steady straight uniform flow in the flow region 0.05 < z/ h < 0.8 [18], which seems to be satisfied according to Fig. 7. The semi-theoretical value is obtained by expressing the equilibrium between the driving gravitational force and the resisting friction force in steady straight uniform flow as u ∗ = (g Rh S0 )0.5 . Agreement is better
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Fig. 7. Logarithmic-type velocity profiles measured in the three steady uniform flow experiments (cf. Table 1).
than 10%. Differences may be partially attributed to the short measuring time, the slight non-uniformity of the flow, sidewall effects, etc. In general, the accuracy obtained is in agreement with that obtained in other experimental studies by means of Acoustic Doppler Velocimetry [4–8]. 4. Unsteady flow experiments The passage of the wave front has been measured for three different surge waves superposing at the upstream boundary of the channel an additional discharge of Q w,upst. = 0.0175 m3 s−1 , 0.0306 m3 s−1 and 0.0535 m3 s−1 to the same baseflow of Q b = 0.0056 m3 s−1 (cf. Table 1). Measurements have been carried out with optimal hydrogen bubble seeding. The measuring frequency of 5.32 Hz allowed obtaining one quasi-instantaneous velocity profile every 0.188 s. Fig. 8 shows quasi-instantaneous velocity profiles measured before, during and after the passage of the surge wave front for the three experimental conditions. In all three experiments, velocity profiles just before and after the passage of the wave have a logarithmic shape, indicating a quasi-uniform flow distribution. It is amazing to note how fast (within less than 0.2 s) quasi-uniform flow conditions are re-established after passage of the surge wave front. Table 3 indicates the measured depth-averaged velocities, U1,UVP and U2,UVP , flow depths, h 1 and h 2 , and discharges, Q 1 = U1,UVP Bh 1 and Q 2 = U2,UVP Bh 2 just before (index 1) and after (index 2) the passage of the wave front. During the passage of the wave front, the flow shows a kind of “two-layer-type” velocity profile. Flow in the lower layer accelerates strongly, to comply with the different mean velocities in the quasi-uniform flow just before and after the wave front. Flow in the upper layer represents the surge wave front that moves faster than the flow bodies before and after the surge wave front, in agreement with elementary translatory wave theory [14]. As a result, the baseflow is overtaken by the surge wave front that migrates on top of it. The positive surge wave diffuses when travelling in the downstream direction leading to discharges after the surge wave front, Q 2 , that are smaller than the discharge introduced at the upstream boundary of the channel, Q b + Q w,upstr (see Table 3).
Fig. 8. Quasi-instantaneous velocity profiles measured before (1), during (3) and after (2) passage of the surge wave front for three different hydraulic conditions (cf. Table 1). The dashed line indicates the theoretical absolute celerity cth .
The observed flow field is characterized by pronounced spatial and temporal velocity gradients. Note that the fastmoving surge wave front is not detected in the first experiment (Fig. 8(a)), which we attribute tentatively to the relatively low measuring frequency (5.32 Hz). Elementary translatory wave theory estimated the surge wave front celerity as [14] c=
1 Q2 − Q1 . B h2 − h1
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Table 3 Base flow discharge Q b and additional surge wave discharge Q w,upstr. , measured flow depths h 1 and h 2 , depth-averaged velocities U1,UVP and U2,UVP , and discharges Q 1 = U1,UVP Bh 1 ≈ Q b and Q 2 = U2,UVP Bh 2 , just before (index 1) and after (index 2) the passage of the wave front, as well as measured and theoretical values of surge wave front celerity, c Q b (m3 s−1 )
Q w,upstr. (m3 s−1 )
h 1 (m)
U1,UVP (m s−1 )
Q 1 (m3 s−1 )
h 2 (m)
U2,UVP (m s−1 )
Q 2 (m3 s−1 )
cexp (m s−1 )
cth (m s−1 )
0.0056 0.0057 0.0056
0.0175 0.0306 0.0535
0.035 0.035 0.035
0.32 0.32 0.31
0.0056 0.0057 0.0056
0.057 0.065 0.088
0.62 0.79 0.96
0.0170 0.0249 0.0410
(?) ≈1.10 ≈1.30
1.07 1.32 1.38
Estimated values of the absolute surge wave front celerity cth are in acceptable agreement with the measured velocities in the upper layer of the velocity profiles (Table 3, Fig. 8). Discrepancies can be attributed to the relatively low measuring frequency, which did not allow us to measure exactly during the passage of the surge wave front. The presented measurements nevertheless allow gaining insight into the physics of the highly unsteady surge waves.
optimized in an empirical way. It has been successfully applied to highly steady and unsteady flows (present paper) and turbulence measurements [3] with Acoustic Doppler Velocimeters having different emitting frequencies/wavelengths of 10 MHz/150 µm [5] to 1 MHz/1500 µm [3]. Further research could highlight the relation between the acoustic scattering level and the size, concentration and acoustic properties of the seeding material.
5. Conclusions
Acknowledgments
Ultrasonic Doppler Velocimetry can accurately resolve the quasi-instantaneous flow field if the acoustic scattering level is sufficiently high. The low-cost and non-polluting technique to increase the acoustic scattering level [3], which consists in seeding the flow with micro-hydrogen bubbles generated by means of electrolysis, has been applied with success. This study first investigated and optimized the hydrogen bubble technique under steady uniform flow conditions. The distance between the hydrogen bubble generation and the measured velocity profile was not found to influence significantly the data quality within the range of investigated distances (10–100 times the flow depth). The acoustic scattering level of the fluid was found to increase linearly with current applied to the electrolysis. Data quality does not improve gradually with increasing acoustic scattering level, but rather in a stepwise way. Velocity measurements are noisy or even erroneous with low acoustic scattering level, but of good quality when the acoustic scattering level is above a certain threshold value. The uncertainty in time-averaged flow quantities was found to be about 5%, which is in agreement with literature on Acoustic Doppler Velocimetry [4–8]. The hydrogen bubble seeding technique also allowed measuring the highly unsteady and highly non-uniform flow in the vicinity of the front of three different positive surge waves. Quasi-uniform flow with a depth-averaged velocity U2 re-established very quickly after passage of the surge wave front. Measurements revealed “two-layer-type” velocity profiles in the vicinity of the surge wave front. Flow in the lower layer, with the height of about the uniform base flow, shows a uniform-flow-type accelerating velocity profile. Its depthaveraged velocity is between that of the base flow, U1 , and that after the wave front, U2 . Flow in the upper layer represents the surge wave front that moves faster than the flow bodies before and after the surge wave front, c > U2 > U1 , in agreement with elementary translatory wave theory [14]. The described hydrogen bubble generation technique to increase the acoustic scattering level has been designed and
The measurement equipment was supported by Met-Flow SA, Lausanne, Switzerland. The Ph.D. research project is granted by the Swiss Federal Office for the Environment (FOEN). References [1] Takeda Y. Velocity profile measurement by ultrasonic Doppler method. Experimental Thermal and Fluid Science 1995;10:444–53. [2] Takeda Y. Ultrasonic Doppler method for flow measurement. In: International symposium on advanced fluid information. 2002. p. 69–76. [3] Blanckaert K, Lemmin U. Means of noise reduction in acoustic turbulence measurements. Journal of Hydraulic Research IAHR 2006;44:3–17. [4] Lemmin U, Rolland T. Acoustic velocity profiler for laboratory and field studies. Journal of Hydraulic Engineering 1997;123:1089–98. [5] Nortek. ADV operation manual. Norway: Nortek AS; 1997. [6] Hurther D, Lemmin U. A correction method for turbulence measurements with a 3-D acoustic Doppler velocity profiler. Journal of Atmospheric and Oceanic Technology 2001;18:446–58. [7] Met-Flow SA. UVP Monitor—Model UVP-DUO users guide. Switzerland: Metflow SA; 2002 [access 12.07.2007] http://www.met-flow.com. [8] Garcia CM, Cantero MI, Ni˜no Y, Garcia MH. Turbulence measurements with acoustic Doppler velocimeters. Journal of Hydraulic Engineering 2005;131:1062–73. [9] Garbini JL, Forster FK, Jorgensen JE. Measurement of fluid turbulence based on pulsed ultrasound techniques. Part I. Analysis. Journal of Fluid Mechanics 1982;118:445–70. [10] Lhermitte R, Lemmin U. Open-channel flow and turbulence measurement by high-resolution Doppler sonar. Journal of Atmospheric and Oceanic Technology 1994;11:1295–308. [11] Shen C, Lemmin U. Ultrasonic scattering in highly turbulent clearwater flow. Ultrasonics 1997;35:57–64. [12] Voulgaris G, Trowbridge JH. Evaluation of the acoustic Doppler velocimeter (ADV) for turbulence measurements. Journal of Atmospheric and Oceanic Technology 1998;15:272–89. [13] McLelland S, Nicholas A. A new method for evaluating errors in highfrequency ADV measurements. Hydrological Processes 2000;14:351–66. [14] Favre H. Ondes de Translation. Paris: Dunod; 1935. [15] De Cesare G, Schleiss AJ. Turbidity current monitoring in a physical model flume using ultrasonic Doppler method. In: Proc. 2nd international symposium on ultrasonic doppler methods for fluid mechanics and fluid engineering. Switzerland: Villigen PSI; 1999. p. 61–64.
T. Meile et al. / Flow Measurement and Instrumentation 19 (2008) 215–221 [16] De Cesare G, Boillat J-L. Flow velocity measurements using ultrasound Doppler method—10 years experience in hydraulic modeling. In: Proc. 5th international symposium on ultrasonic doppler methods for fluid mechanics and fluid engineering ISUD. Switzerland: ETH Zurich;
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2006. p. 113–116. [17] Birkhofer B H. Ultrasonic in-line characterization of suspensions. Ph.D. thesis. ETH Zurich; 2007. [18] Graf WH, Altinaker M. Fluvial hydraulics. Chichester: Wiley; 1998.
Improvement of Acoustic Doppler Velocimetry in steady and unsteady turbulent open-channel flows by means of seeding with hydrogen bubbles T. Meile ∗ , G. De Cesare, K. Blanckaert, A.J. Schleiss Ecole Polytechnique F´ed´erale de Lausanne (EPFL), Laboratory of Hydraulic Constructions (LCH), Station 18, CH-1015 Lausanne, Switzerland Received 2 November 2006; received in revised form 18 August 2007; accepted 27 August 2007
Abstract Velocimetry is used in various fields of research. In hydraulics, Ultrasonic Velocimetry based on the Doppler Shift effect can accurately resolve the quasi-instantaneous flow field [Takeda Y. Velocity profile measurement by Ultrasonic Doppler method. Experimental Thermal and Fluid Science 1995;10:444–53; Takeda Y. Ultrasonic Doppler method for flow measurement. In: International symposium on advanced fluid information. 2002. p. 69–76] if the acoustic scattering level is sufficiently high. But Ultrasonic velocity instruments are known to perform poorly in clear water with low acoustic scattering level, such as often found in laboratory applications. Artificial seeding of the flow can be used to increase the acoustic scattering level. Blanckaert and Lemmin [Blanckaert K, Lemmin U. Means of noise reduction in acoustic turbulence measurements. Journal of Hydraulic Research IAHR 2006;44:3–17] have proposed a technique to seed the flow with micro hydrogen bubbles generated by means of electrolysis. This technique is optimized under steady flow conditions and subsequently applied to measure the flow characteristics around the front of a surge wave. c 2007 Elsevier Ltd. All rights reserved.
Keywords: Ultrasonic Velocimetry; Velocity measurement; Experiments; Hydrogen bubble seeding; Unsteady flow; Surge wave
1. Introduction Velocimetry is used in various fields of research. Openchannel flows have been measured with various techniques, such as propellers, Pitot tubes, electromagnetic velocimeters, hot-wires, hot-films, Laser Doppler Velocimeters, Acoustic Doppler Velocimeters, Particle Image Velocimeters, etc. Acoustic Doppler Velocimeters (ADVs) allow carrying out non-intrusive measurement of the three-dimensional velocity vector. Moreover, they allow measuring entire velocity profiles at one go, whereas most other techniques are limited to pointwise measurements [1,2]. A multitude of Acoustic Doppler velocimeters are available. Although they differ in geometric configuration, transducer characteristics, emitting and measuring frequency, etc., they share the same principle and have similar properties. Hence the results presented in this paper have general validity in Acoustic Doppler Velocimetry. ∗ Corresponding author. Tel.: +41 21 693 28 58; fax: +41 21 693 22 64.
E-mail address: [email protected] (T. Meile). c 2007 Elsevier Ltd. All rights reserved. 0955-5986/$ - see front matter doi:10.1016/j.flowmeasinst.2007.08.009
The uncertainties in mean velocity and turbulence measurements with Acoustic Doppler Velocimeters are typically about 5% and 10%, respectively [4–8]. However, the temporal resolution and hence the uncertainty of Acoustic Doppler Velocimeters are known to depend on the acoustic scattering level of the fluid [9–13,3] which determines the signal-to-noise ratio. Low acoustic scattering levels only allow measuring the mean velocity field in steady flows, whereas measurement of turbulence or unsteady flow characteristics requires high acoustic scattering levels. Blanckaert and Lemmin [3] have developed a simple, lowcost and non-polluting technique to increase the acoustic scattering level in clear water, which consist in seeding the flow with hydrogen bubbles generated by means of electrolysis. Whereas Blanckaert and Lemmin [3] successfully applied it to the measurement of turbulence characteristics in steady flows, the present paper applies it to the measurement of the highly unsteady flow around the front of a surge wave. Surge waves occur when the discharge in a channel changes suddenly, for example due to the release of water from hydropower plants, the operation of valves, or due
216
T. Meile et al. / Flow Measurement and Instrumentation 19 (2008) 215–221
Fig. 1. Experimental set-up and schematic definition of the surge wave characteristics before (index 1) and after (index 2) the surge wave front moving with an absolute celerity c. Q is the discharge and h the flow depth. Velocity profiles are investigated before (1), after (2) and during (3) the surge wave front. The total flume length is 38.3 m.
to tidal influence in estuaries. Fig. 1 schematically defines the characteristics of the surge wave as investigated in the laboratory. The uniform base flow has a discharge Q b , mean velocity U1 and mean flow depth h 1 . At the upstream boundary of the channel, an additional discharge Q w,upstr is suddenly introduced, resulting in a surge wave that migrates in the downstream direction. The surge wave has a diffusive character, meaning that the height of the wave front decreases and the additional discharge attenuates in downstream direction: h 2 < h upstr and Q 2 < Q b + Q w,upstr . The celerity of the wave front c is considerably higher than the base flow velocity U1 and the average velocity upstream of the front U2 [14]. During the passage of the wave front, the flow characteristics and bed shear stress attain peak values that are relevant for resuspension, erosion and transport of sediment, aquatic species like macro invertebrates, etc. This paper presents the laboratory experimental set-up, including the flume, the Acoustic Doppler Velocimeter and the hydrogen bubble technique to increase the acoustic scattering level. Steady flow experiments are presented that aimed at optimizing the parameters of the hydrogen bubble technique. The technique is subsequently applied with optimal parameters to measure the flow characteristics around the front of surge waves (Fig. 2). 2. Experimental set-up, Acoustic Doppler Velocimeter and hydrogen bubble technique Experiments have been carried out in a 38.3 m long and 0.485 m wide straight laboratory flume, shown in Fig. 1. The bed of the rectangular cross-section consisted of smooth metal, whereas the banks were made out of smooth limestone bricks. The flume had a mean downstream bed slope of S0 = 0.00114. Table 1 summarizes the investigated hydraulic conditions. Steady flow experiments were first carried out in order to optimize the hydrogen bubble technique used to increase the acoustic scattering level. Subsequently the unsteady flow during the passage of a surge wave front was investigated. All experiments concern turbulent flow regimes. Velocity measurements were carried out on the flume axis in the cross-section at 24.3 m from the bend entry, where an Acoustic Doppler Velocimeter was installed as illustrated in Fig. 1. The present study made use of a commercially available
Table 1 Hydraulic test conditions: Q b (steady baseflow discharge), h 1 (flow depth of baseflow), U1,cal = Q/(Bh 1 ) (mean velocity of baseflow), Re1 = U1 Rh1 /ν (Reynolds number of baseflow), Fr1 = U1 /(gh 1 )0.5 (Froude number of baseflow), Q w,upstr (additional discharge introduced at flume entrance 24.3 m upstream of the measurement location) Q b (m3 s−1 ) h 1 (m)
U1,cal (m s−1 )
Re1 (−)
Fr1 (−)
Q w,upstr. (m3 s−1 )
0.0080 0.0092 0.0600 0.0056 0.0057 0.0056
0.37 0.36 0.71 0.33 0.34 0.33
12900 14300 59200 9500 9600 9500
0.55 0.51 0.54 0.56 0.57 0.56
– – – 0.0175 0.0306 0.0535
0.045 0.052 0.175 0.035 0.035 0.035
UVP velocimeter from Met-Flow SA [7]. It was used with one ultrasonic transducer that functions as emitter–receiver and allows measuring the projection of the velocity vector on the profile through the transducer axis. By inclining the transducer, the downstream component of the velocity vector can be derived. Under these conditions the instrument is appropriate for application in one-dimensional flow configurations. The UVP was applied in a non-intrusive way by installing the transducers 0.01 m under the bed level, whereby the space between the transducer and the bed was filled with an acoustically transparent contact gel covered by self-adhesive tape. The ultrasonic transducer was inclined by 30 ◦ C with respect to the vertical. It had an active diameter of 0.01 m. The measured profiles were divided in a string of identical cylindrical measuring bins of height 0.00148 m. The emitted acoustic wave had a frequency of 2 MHz yielding a wavelength of 750 µm. The sampling frequencies were 4.85 Hz and 5.32 Hz in the reported steady and unsteady flow experiments, respectively. The UVP has been successfully applied in similar configurations at EPFL [15,16]. The set-up used for the generation of hydrogen bubbles by means of electrolysis is shown in Fig. 3. It is formed by two arrays of horizontal stainless steel wires with diameter of 100 µm, the anodic one being placed 0.05 m upstream of the cathodic one. The average vertical spacing between the stainless steel wires is 0.008 m, with increased density in the lower part of the arrays. The arrays are supported by vertical stems, which are insulated by a waterproof painting layer in
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Fig. 3. Set-up for hydrogen bubble generation.
Fig. 2. Examples of surge waves. Above: Lateral view of a breaking surge wave with high air concentration at the wave tip. Below: Top view of a surge wave.
order to avoid their participation to the electrolysis. The setup for the hydrogen bubble generation is placed sufficiently far upstream of the measuring volume to prevent flow disturbance. The electrolytic reaction is generated by applying an electric direct current that produces hydrogen at the cathode and oxygen at the anode. The size of the hydrogen bubbles is comparable to the diameter of the electrode wires (100 µm) whereas the quantity of generated bubbles increases with the applied electric current. According to Shen and Lemmin [11], the size of ideal acoustic targets is about half of the wavelength of the emitted acoustic signal, and is thus instrument dependent. The described empirically designed and optimized set-up has been successfully applied, however, with instruments with an emitting frequency/wavelength of 10 MHz/150 µm [5] to 1 MHz/1500 µm [3]. The size distribution and flux of the generated hydrogen bubbles as well as their relation to the acoustic scattering level have been investigated by Birkhofer [17]. It should be noted that the hydrogen bubble generation is inefficient with clean stainless steel wires. However, electrolysis provokes a surface reaction on the stainless steel wires that makes the bubble generation efficient after a short time. 3. Steady flow experiments Steady flow experiments have been carried out for the three hydraulic conditions listed in Table 1, with the aim of analysing and optimizing the hydrogen bubble technique. Investigated parameters included the distance between the hydrogen bubble generation and the measured velocity profile as well as the relation between the hydrogen bubble concentration and the quality of the velocity measurements. Fig. 4 compares velocity profiles measured without and with hydrogen bubble generation for different distances between the hydrogen bubble generation and the measured velocity
Fig. 4. Measured velocity profiles averaged over 10 s in the steady flow experiments without and with hydrogen bubble seeding as a function of the distance between hydrogen bubble generation and measured profile. (a) Discharge of Q b = 0.008 m3 s−1 ; (b) Discharge of Q b = 0.060 m3 s−1 . The dip of the profile at z = 0.005 m probably results from a small bottom irregularity 0.08 m upstream of the measured velocity profile.
profile. The velocity profiles are measured at a sampling rate of 4.85 Hz and averaged over a period of 10 s. Figs. 4(a), (b) refer to the steady flow experiments with discharges of Q b = 0.008 m3 s−1 and 0.060 m3 s−1 , respectively. Without hydrogen bubble seeding, velocity profiles are characterized by important scatter. Long measuring times would allow obtaining accurately the time-averaged velocity profile, but not the quasi-instantaneous velocity field which is
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Fig. 6. Velocity profiles as a function of the depth-averaged DMEA. Discharge of Q b = 0.009 m3 s−1 . Table 2 Estimation of uncertainty in measurements by comparing experimental and theoretical values of the dept-averaged velocity, U , and the bed shear velocity, u∗ Experiment Qb (m3 s−1 ) 0.0080 0.0092 0.0600 Fig. 5. Relation between the electric current applied to the hydrogen bubble generating set-up and the measured demodulated echo amplitude (DMEA), which parameterizes the acoustic scattering level of the fluid. (a) Measured profiles of DMEA; (b) Depth-averaged DMEA.
required for investigating characteristics of turbulence or highly unsteady flow [3]. The influence of the distance between the hydrogen bubble generation and the measured profile has been tested in the range between 0.75 m and 4 m, corresponding to about 10–100 times the flow depth. Within this range, the distance did not significantly affect the quality of the measured velocity profiles. Figs. 5 and 6 investigate the relation between the hydrogen bubble concentration and the quality of the velocity measurements in the experiment with steady uniform flow discharge of Q b = 0.009 m3 s−1 . Fig. 5(a) shows the relation between the applied electric current and the measured demodulated echo amplitude (DMEA), which parameterizes the acoustic scattering level of the fluid, and hence the hydrogen bubble concentration [17]. Figs. 5(a), (b) show the measured profiles of the DMEA and the depth-averaged DMEA for different applied electric currents, respectively. The depth-averaged value of the DMEA for “clear” water conditions (no current applied) is 69. The depthaveraged DMEA increases rapidly when hydrogen bubbles are generated. Applying a current of 0.1 A increases the depthaveraged DMEA by a factor of about 5. From 0.15 A on, the depth-averaged DMEA increases about linearly with the current applied to the electrolysis, without signs of saturation within the investigated range. Recent analysis on the number
Bed shear velocity u ∗ (m s−1 )
Dept-averaged velocity, U (m s−1 ) Experimental Theoretical
Experimental
Semi-theoretical
0.38 0.38 0.71
0.0203 0.0204 0.0308
0.0206 0.0219 0.0337
0.37 0.36 0.71
of reflectors [17] reveals that an increase of the applied voltage mainly increases the number of generated bubbles whereas the bubble size is not significantly affected. Fig. 6 shows the relation between the depth-averaged DMEA on the one hand and the measured velocity profiles on the other. The quality of the velocity profiles does not seem to improve gradually with increasing acoustic scattering level, but rather in a stepwise way. An insufficient acoustic scattering level yields erroneous or extremely noise velocity profiles. From a threshold value on (around DMEA = 300), the velocity profiles are of a good quality that hardly improves when further increasing the acoustic scattering level. The uncertainty in the velocity measurements can be estimated by comparing measured flow quantities to theoretical values for steady uniform flow. Table 2 compares the depthaveraged downstream velocity, UUVP , as computed from the measured velocity profile, to the theoretical cross-sectional averaged velocity, Uth = Q/ (Bh). Agreement is within 5%. Table 2 also compares the experimental and theoretical values of the bed shear velocity, u ∗ , which is by definition related to the bed shear stress as τb = ρu 2∗ . This flow quantity parameterizes the turbulence quantities, friction losses, sediment transport, etc. The experimental estimation is based on the hypothesis of a logarithmic velocity profile in steady straight uniform flow in the flow region 0.05 < z/ h < 0.8 [18], which seems to be satisfied according to Fig. 7. The semi-theoretical value is obtained by expressing the equilibrium between the driving gravitational force and the resisting friction force in steady straight uniform flow as u ∗ = (g Rh S0 )0.5 . Agreement is better
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Fig. 7. Logarithmic-type velocity profiles measured in the three steady uniform flow experiments (cf. Table 1).
than 10%. Differences may be partially attributed to the short measuring time, the slight non-uniformity of the flow, sidewall effects, etc. In general, the accuracy obtained is in agreement with that obtained in other experimental studies by means of Acoustic Doppler Velocimetry [4–8]. 4. Unsteady flow experiments The passage of the wave front has been measured for three different surge waves superposing at the upstream boundary of the channel an additional discharge of Q w,upst. = 0.0175 m3 s−1 , 0.0306 m3 s−1 and 0.0535 m3 s−1 to the same baseflow of Q b = 0.0056 m3 s−1 (cf. Table 1). Measurements have been carried out with optimal hydrogen bubble seeding. The measuring frequency of 5.32 Hz allowed obtaining one quasi-instantaneous velocity profile every 0.188 s. Fig. 8 shows quasi-instantaneous velocity profiles measured before, during and after the passage of the surge wave front for the three experimental conditions. In all three experiments, velocity profiles just before and after the passage of the wave have a logarithmic shape, indicating a quasi-uniform flow distribution. It is amazing to note how fast (within less than 0.2 s) quasi-uniform flow conditions are re-established after passage of the surge wave front. Table 3 indicates the measured depth-averaged velocities, U1,UVP and U2,UVP , flow depths, h 1 and h 2 , and discharges, Q 1 = U1,UVP Bh 1 and Q 2 = U2,UVP Bh 2 just before (index 1) and after (index 2) the passage of the wave front. During the passage of the wave front, the flow shows a kind of “two-layer-type” velocity profile. Flow in the lower layer accelerates strongly, to comply with the different mean velocities in the quasi-uniform flow just before and after the wave front. Flow in the upper layer represents the surge wave front that moves faster than the flow bodies before and after the surge wave front, in agreement with elementary translatory wave theory [14]. As a result, the baseflow is overtaken by the surge wave front that migrates on top of it. The positive surge wave diffuses when travelling in the downstream direction leading to discharges after the surge wave front, Q 2 , that are smaller than the discharge introduced at the upstream boundary of the channel, Q b + Q w,upstr (see Table 3).
Fig. 8. Quasi-instantaneous velocity profiles measured before (1), during (3) and after (2) passage of the surge wave front for three different hydraulic conditions (cf. Table 1). The dashed line indicates the theoretical absolute celerity cth .
The observed flow field is characterized by pronounced spatial and temporal velocity gradients. Note that the fastmoving surge wave front is not detected in the first experiment (Fig. 8(a)), which we attribute tentatively to the relatively low measuring frequency (5.32 Hz). Elementary translatory wave theory estimated the surge wave front celerity as [14] c=
1 Q2 − Q1 . B h2 − h1
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Table 3 Base flow discharge Q b and additional surge wave discharge Q w,upstr. , measured flow depths h 1 and h 2 , depth-averaged velocities U1,UVP and U2,UVP , and discharges Q 1 = U1,UVP Bh 1 ≈ Q b and Q 2 = U2,UVP Bh 2 , just before (index 1) and after (index 2) the passage of the wave front, as well as measured and theoretical values of surge wave front celerity, c Q b (m3 s−1 )
Q w,upstr. (m3 s−1 )
h 1 (m)
U1,UVP (m s−1 )
Q 1 (m3 s−1 )
h 2 (m)
U2,UVP (m s−1 )
Q 2 (m3 s−1 )
cexp (m s−1 )
cth (m s−1 )
0.0056 0.0057 0.0056
0.0175 0.0306 0.0535
0.035 0.035 0.035
0.32 0.32 0.31
0.0056 0.0057 0.0056
0.057 0.065 0.088
0.62 0.79 0.96
0.0170 0.0249 0.0410
(?) ≈1.10 ≈1.30
1.07 1.32 1.38
Estimated values of the absolute surge wave front celerity cth are in acceptable agreement with the measured velocities in the upper layer of the velocity profiles (Table 3, Fig. 8). Discrepancies can be attributed to the relatively low measuring frequency, which did not allow us to measure exactly during the passage of the surge wave front. The presented measurements nevertheless allow gaining insight into the physics of the highly unsteady surge waves.
optimized in an empirical way. It has been successfully applied to highly steady and unsteady flows (present paper) and turbulence measurements [3] with Acoustic Doppler Velocimeters having different emitting frequencies/wavelengths of 10 MHz/150 µm [5] to 1 MHz/1500 µm [3]. Further research could highlight the relation between the acoustic scattering level and the size, concentration and acoustic properties of the seeding material.
5. Conclusions
Acknowledgments
Ultrasonic Doppler Velocimetry can accurately resolve the quasi-instantaneous flow field if the acoustic scattering level is sufficiently high. The low-cost and non-polluting technique to increase the acoustic scattering level [3], which consists in seeding the flow with micro-hydrogen bubbles generated by means of electrolysis, has been applied with success. This study first investigated and optimized the hydrogen bubble technique under steady uniform flow conditions. The distance between the hydrogen bubble generation and the measured velocity profile was not found to influence significantly the data quality within the range of investigated distances (10–100 times the flow depth). The acoustic scattering level of the fluid was found to increase linearly with current applied to the electrolysis. Data quality does not improve gradually with increasing acoustic scattering level, but rather in a stepwise way. Velocity measurements are noisy or even erroneous with low acoustic scattering level, but of good quality when the acoustic scattering level is above a certain threshold value. The uncertainty in time-averaged flow quantities was found to be about 5%, which is in agreement with literature on Acoustic Doppler Velocimetry [4–8]. The hydrogen bubble seeding technique also allowed measuring the highly unsteady and highly non-uniform flow in the vicinity of the front of three different positive surge waves. Quasi-uniform flow with a depth-averaged velocity U2 re-established very quickly after passage of the surge wave front. Measurements revealed “two-layer-type” velocity profiles in the vicinity of the surge wave front. Flow in the lower layer, with the height of about the uniform base flow, shows a uniform-flow-type accelerating velocity profile. Its depthaveraged velocity is between that of the base flow, U1 , and that after the wave front, U2 . Flow in the upper layer represents the surge wave front that moves faster than the flow bodies before and after the surge wave front, c > U2 > U1 , in agreement with elementary translatory wave theory [14]. The described hydrogen bubble generation technique to increase the acoustic scattering level has been designed and
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