"Comparative study of ADV and LDA measuring techniques"
Top 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: email@example.com). 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  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 33 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 34 Top 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 35 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 . 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  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 36