Basics of flow measurements on the basis of the ultrasonic Doppler effect
The term “Doppler effect” refers to the change in the perceived frequency of waves while the transmitter and receiver are in motion in relation to each other. This effect is used to image moving structures. For example, ultrasound can be used to determine the flow velocity and/or the flow rate of a flow of liquid. Here the frequency shift of an ultrasonic wave, which is coupled into the flow of liquid at a particular Doppler angle, is measured with scattering of the wave on small particles, such as impurities. In the experiment, the dependence of the Doppler frequency shift Δf on the flow velocity v (movement speed of the scattered particles) and the Doppler angle a is investigated for different fundamental frequencies f0 by a variation of the pump power, the transmission frequency and the incidence angle. For a pulse-echo system with one ultrasonic probe the following relationship applies, presented in simplified form: Δf ~ f0 v cos(α).
The graphics show the frequency shift and the ratio Δf/cos(a) in dependence on the pump power for different Doppler angles at the transmission frequency 2 MHz. The Doppler frequency shift determined increases as the rotational speed rises and as the Doppler angle becomes smaller. The quotient Δf/cos(a) (the flow velocity v) is constant for even pump powers, i.e. no angle-dependent faulty measurement occurs.