The laser Doppler vibrometer (LDV) is an instrument based on a heterodyne interferometer that can be used to measure vibrations at a location on a structure. By moving the laser beam using scanning mirrors a full field vibration measurement can be performed. While the main purpose of the scanning LDV is to measure vibrations, recently several authors proposed to use the LDV for flow and sound measurements. The idea is to aim the laser vibrometer at a rigid block. When a pressure field is present between the LDV head and the rigid block this will result in changes of the refraction index that are related to the measured Doppler shift. Because no seeding particles are needed, the method opens new opportunities to measure phenomena that were previously difficult to measure. Unfortunately, the current LDV measurement techniques are only able to measure dynamic flow fields (both turbulent and coherent).
In this paper we present a novel method that is able to measure a steady‐state velocity flow field. The technique is based on the use of an ultrasound field that is generated in addition to the flow field. At locations where a non‐zero flow velocity is present, the ultrasonic wave fronts will be phase shifted. The ultrasound fields can be measured using a scanning LDV. By calculating the phase shift from the measurements the flow velocity can be obtained. The proposed method will be validated on measurements of a jet.
