Vortex shedding flowmeters and ultrasound detection: signal processing and influence of bluff body geometry
Section snippets
Measurement principle
Fig. 1 shows a pipe with a fluid moving at the mean velocity Inserting a vortex body causes vortices to separate periodically from both edges of the body. Because of the linear dependency between the velocity of the vortices and their frequency fυ at a given point behind the vortex body can be used to determine the mean flow velocity bywhere S is the Strouhal number and d the width of the body. Advantages of vortex shedding flowmeters are the validity of Eq. (1)over a wide
Detecting the vortex frequency
The use of an ultrasound wave causes a transformation of the entire measurement process into a high frequency range: the low vortex frequency (usually less than 500 Hz) is shifted to a side-band of the ultrasound frequency (111 kHz in this case). However, the advantages of using a carrier signal are connected to a signal processing algorithm of increased complexity.
The novel signal processing algorithm presented in this paper makes use of the fact that the spectrum of the original signal is
Influence of different vortex body geometries
The bluff body is the part of the measurement system that is responsible for the production of the vortices. Its geometry should be optimized for producing regular vortices to simplify the detection of a defined vortex frequency.
Fig. 7 summarizes the results of this work for two different vortex body geometries. The left column shows the simulated and measured values for a triangular bluff body. The second body, which is described in the right column, differs from the first one only by its
Conclusion
Flow measurement using ultrasound to detect the vortices behind a bluff body is an interesting alternative to other methods. It was demonstrated that handling the measurement process is easy if combined with advanced signal processing methods such as the one that was presented in this paper.
Investigations of the geometry of the vortex generating body and its influence on the form of the recorded signals resulted in an excellent agreement between the experiments and simulations for the two
Acknowledgements
This project is supported by the Deutsche Forschungsgemeinschaft (DFG), ME 484/29-1.
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