A Fast Ultrasonic Imaging System for Measuring Unsteady Velocity Fields in Air

Abstract

Advances in ultrasonic imaging in medicine have prompted the development of ultrasonic imaging systems to be used as diagnostic tools in fluid mechanics. The fact that a sound wave propagating through a medium is convected with the flow velocity can be utilized to probe the velocity field. Such a measurement technique, being nonintrusive, does not require any scattering particles and may be applied in optically opaque media, as pointed out by Johnson et al.¹ In contrast with their work, in which individual time of flight measurements are made, in the present work the entire object wave is measured in both amplitude and phase along a linear transducer array. Hence, Fresnel transforming techniques may be applied to reconstruct the flow field in depth. Furthermore, fast acquisition of the acoustical data offers the possibility of tracking unsteady flow fields. The working medium is air because of its smaller sound speed compared to water, which results in a larger sensitivity to velocity disturbances.