Super Deltaflex – Advanced Development of Transit Time Acoustic Flow Measurement
Wolfgang Stedtnitz, Thomas Schott
Thomas Schott,
Berner Fachhochschule, CH‐3400 Burgdorf
Wolfgang Stedtnitz, Gärtnereistrasse 21 D‐79807 Lottstetten
Fig. 1 Aare‐Hagneck, Switzerland flood August 2005
Fig. 2 Principle of the delta function
Above: the transducers TD1 and TD2 creating a path under a certain angle, relative to the flow‐
direction.
Below:
Left: Direct Signal with positive peak
Right: Reflected Signal with negative peak
Abstract
Observation of flooding events over years have shown that all kinds of ultrasonic flow measuring devices – Transit time and Doppler systems alike – fail, when data is most needed:
in case of peak flooding!
This happens because with higher velocity the sediment content is rising dramatically thus leading to catastrophic acoustic attenuation.
Events in Europe, even more so in situ measurements in sediment loaded rivers near the Himalayan mountains have shown that the acoustic attenuation rises with sediment load, but never blocks completely. That means it is possible to obtain reliable data, if the signal to noise ratio can be significantly improved.
Motivation for the further development
Measurements in test tanks have shown that the acoustic attenuation is related to the size of particles in non‐linear manner.
Peak attenuation happens when particles have 4 micrometer diameter. A second peak shows up with particles as large as a centimetre and larger. The problem is that in reality there exists always a wide mix of all sizes. The largest pieces are rolling just over the bottom. Finer ones are moving all over the cross‐section of a river.
Air bubbles from overspills, waterfalls and outlets from Hydro plants cause high attenuation levels too. It takes time to release the bubbles and after a while the acoustic attenuation downstream becomes normal again.
Occasionally the diagnoses of both incidents are mixed, leading to rumours that acoustic systems are in principal unreliable.
‐ Successful flow measurement at 6 kg/m3 of sediments in the yellow River, China. Doppler profiler failed in this situation.
‐ Successful use of passive reflectors (as substitute of river crossing cables) in Mühlheim Ruhr, Germany; Geneva Rhone, Switzerland
‐ the measurement of the velocity of +/‐ 4 mm/s over a path length of 10m in only 5 cm water level using aperture plate to direct the sounding signal in the Ever Glades 1996
‐ the measurement of the velocity over a path length of 450m under a bridge in Ottawa City
‐the measurement of a velocity of 4.6 m/s (Fig. 1) over a path length of 63 m in the Aare river, Switzerland during a flood with high density of sediments.
From Deltaflex to Super Deltaflex
1. Because sediment load is troubling, we had to find solutions to increase the receiver sensi‐
tivity and to boost the transmitter power. More over in order to reduce the noise impact, we found a special software solution to improve the signal to noise ratio further by more than 30 dB using 169 chip Barker pulse compression (cross correlation) code.
2. In some critical ultrasonic sites we observed a sudden drop out of the measurement under the following conditions:
‐ Relative deep installation of ultrasonic path
‐ High velocity at the surface
‐Higher temperature at the surface (gradient) We solved the problem by restoring the shape of the received signal.
3. Real time computing was implemented in modern components like free programmable high speed arrays (FPGA).
The Deltaflex
As basis for our research, we used the Deltaflex that was developed over the years 1990 to 2008 and is still in action in many gauging stations. It is utilising the surface reflection to obtain a second velocity information. The draft below (Fig. 2) illustrates the principle.
Fig. 3 The Deltaflex of year 2000
With this instrument it was possible to discover many special phenomena of nature:
‐ Discovery of the phenomena, that the k‐factor is lower when the river is under a cover of ice.
Measured with the Deltaflex in Canada in many rivers.
‐ Discovery of oscillating rivers, shown thanks to the Deltaflex installed in Burlington, Ontario, Canada; Mühlheim Ruhr, Germany; Sugiez, Switzerland
‐ Discovery of periodically signatures of water reflections in complex tide areas in Florida (presentation in San Diego, Woods hole Inst.
2004)
‐ Improvement in very slow flowing rivers with surface water flowing in opposite direction compared to the lower main stream at the bottom.
‐ Confirmation, that the in situ observed variance of the flow velocity is a result of turbulences in the water. A phenomena that Prandtel postulated 60 years ago.
