Ultrasonic Doppler Effect

Introduction to the basics of flow measurement based on the acoustic or ultrasonic Doppler effect

The experiment introduces the basics of flow measurement based on the acoustic or ultrasonic Doppler effect and investigates its dependence on flow velocity and Doppler angle.

Keywords: frequency shift, scattering, Doppler effect, Doppler sonography, flow measurement

The Doppler effect is the change in the perceived frequency of waves while the transmitter and receiver are moving relative to each other. This effect is used to image moving structures. For example, the flow velocity or flow rate of a liquid stream can be determined using ultrasound. The frequency shift of an ultrasonic wave that is coupled into the liquid stream at a certain Doppler angle is measured when the wave is scattered by small particles, such as contaminants. 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 α for different fundamental frequencies f ₀ is investigated by varying the pump power, the transmission frequency and the angle of incidence. The following simplified relationship applies to a pulse-echo system with an ultrasonic transducer: Δf ~ f ₀ v cos(α).

The graphics show the frequency shift and the ratio Δf / cos(α) as a function of the pump power for different Doppler angles at the transmission frequency of 2 MHz. The determined Doppler frequency shift increases with increasing speed and decreasing Doppler angle. The quotient Δf / cos(α) (the flow velocity v) is constant for the same pump power, ie no angle-dependent measurement errors occur.

Screenshot of the FlowView measurement software

Deoppler frequency shift depending on pump speed and Doppler angle

Flow velocity depending on pump speed

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