Ultrasonic flowmeter

A method to determine at least one wave mode generated during flow measurement in an ultrasonic flowmeter, wherein the ultrasonic flowmeter comprises a measuring tube, a first ultrasonic transducer, a second ultrasonic transducer, and a control and evaluation unit, wherein the first ultrasonic transducer and the second ultrasonic transducer are arranged axially offset on the measuring tube, wherein the first ultrasonic transducer is designed at least as an ultrasonic actuator, wherein the second ultrasonic transducer is designed at least as an ultrasonic sensor, wherein the control and evaluation unit controls the ultrasonic actuator in an operating state of the ultrasonic flowmeter such that a guided ultrasonic wave is excited in the measuring tube through which a fluid flows and the guided ultrasonic wave propagates in a combined waveguide comprising the measuring tube and the fluid in the axial direction of extension of the measuring tube in the measuring tube and in the fluid, wherein the ultrasonic sensor receives the guided ultrasonic wave and the control and evaluation unit determines a flow velocity of the fluid by evaluating the received guided ultrasonic wave, wherein at least the ultrasonic transducer designed as an ultrasonic actuator has several excitation structures in the axial direction of the measuring tube, wherein the excitation structures are spaced apart from each other in the axial direction of the measuring tube, wherein ultrasonic waves are fed into the measuring tube through which the fluid flows, so that the wave pattern of a predetermined wave mode of the guided ultrasonic wave is excited in the measuring tube through which the fluid flows, spaced in the axial direction of the measuring tube, the method comprising:

• determining phase velocities and group velocities of sound waves in a frequency range for the geometric and physical boundary conditions of the measuring tube through which the fluid flows;
• and
• performing, for the geometric and physical boundary conditions of the measuring tube through which the fluid flows, a selection of a wave mode that receives the highest rating when evaluating at least one of the following criteria at a specific frequency within the frequency range as the predetermined wave mode:
• a) the less the phase velocity and the group velocity differ from each other, the better,
• b) the closer the phase velocity and/or the group velocity is to the sound velocity in the fluid, the better,
• c) the lower the frequency dependencies of the phase velocity and the group velocity are, the better,
• d) the greater the smallest difference in phase velocity between different modes is, the better,
• e) the greater the smallest distance in the group velocity between different modes, the better,
• f) the more axisymmetric the mode is, the better,
• g) the more similar the relative change in phase velocity and/or group velocity is to a relative change in the sound velocity of the fluid, the better,
• h) the smaller the attenuation of the amplitude of the mode during propagation in the fluid, the better,
• i) the more consistent the amplitude of the mode is across the inner cross-section of the measuring tube, the better.