Vortex-shedding flow metering using ultrasound

Abstract

The coincidence of vortices generated by a bluff body in a gaseous flow, which is well-known as the Krmn vortex street, with an ultrasonic beam crossing these vortices raises many questions concerning physics and signal processing. Usually the vortices are detected by pressure sensors in the pipe wall or inside the bluff body. Using ultrasound for vortex detection is a powerful advantageous alternative combination. The ultrasonic beam is modulated by the velocity components of the vortices and varying fluid density. The kind of modulation depends on the structure of the vortices that are influenced by the bluff-body geometry and size. Measurements and simulations show that conventional triangular bluff bodies with the large size used for pressure measurements are unsuitable for ultrasonic measurements because of the lack of knowledge about secondary vortices. They can be avoided by turning the bluff body so as to face the tip to the inflow. As ultrasound is very sensitive to the smallest influences, the bluff-body size can be reduced considerably. Even the shape can be adapted. The best results are obtained with a 3-mm threaded rod as the bluff body in a pipe of 100mm diameter. Pressure losses behind such small bluff bodies can be neglected. Independent of the bluff-bodies shapes and sizes vortex meters are sensitive to disturbed profiles and pulsation. © Springer-Verlag Berlin Heidelberg 2005.