High-speed echographic particle image velocimetry (echo-PIV)

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1) AT A GLANCE Echo-PIV (echographic particle image velocimetry) is limited to low frame rates, which reduces flow measurement capability. We will develop and validate high-speed echo-PIV offering up to a 100-fold increase in sampling rate. 2) CONTEXT: OPTICAL vs. ECHOGRAPHIC PIV Particle image velocimetry (PIV) is a non-intrusive optical technique of flow visualization for experimental fluid dynamics. PIV takes two closely timed images and estimates the distance that tracer particles travelled within this time. From the known time interval and the measured displacements, the instantaneous velocity fields are calculated, from which several flow properties can be deduced. In optical PIV, the seeding particles are illuminated with short-pulse-laser light to freeze the motion. Optical PIV is thus only applicable with materials transparent to light. Echo-PIV (echographic PIV) is based on the same principles as optical PIV. Instead of laser light illumination, the seeding particles are insonified with ultrasound waves. Ultrasound backscattering generates speckle patterns which can be tracked to deduce the displacements from one image to the next one. Echo-PIV has been of high interest in blood flow investigation. Echo-PIV, however, is hampered by conventional ultrasound imaging which offers a frame rate much smaller than optical PIV. Indeed, conventional ultrasound imaging emits successive focused beams to fully cover the region of interest. Typically, 128 emissions are required to obtain one single image. The objective of this research project is to use ultrafast ultrasound to produce high-speed echo-PIV working at frame rates up to 10,000 fps. High-speed echo-PIV has the potential to enlarge the family of fluid measurement systems. 3) METHODS To test high-speed echo-PIV, we will carry out velocity measurements in well-controlled Womersley and vortical flows, as well as in a pulse heart duplicator (ViVitro). The ViVitro system is compatible with both ultrasound and optical PIV. Pulsatile Womersley flows will be generated in a cylindrical pipe with a piston pump. From the known flow rate (given by an electromagnetic flowmeter) and pipe radius, the theoretical velocity profiles will be calculated using the Womersley equation. Three-dimensional vortical flows will be also created in a Plexiglas square box, transparent both to light and ultrasound, with a magnetic stirrer. In addition, to test high-speed echo-PIV with complex flow patterns, we will investigate intraventricular filling in the heart duplicator. The velocities will be measured by optical three-dimensional volumetric PIV (V3V-9800, TSI Inc.). In the different experimental setups, the velocities determined by high-speed echo-PIV will be compared with the ground-truth velocities, given by theory and/or optical PIV. Pixelwise comparisons will be performed on several planes and for different flow conditions.