In this work the phase-resolved internal flow of a bi-stable fluidic oscillator was measured using phase-locked 3D3C-MRV, also termed as 4D-MRV. A bi-stable fluidic oscillator converts a continuous inlet-mass flow into a jet alternating between two outlet channels and, as a consequence provides an unsteady, periodic flow. Since data acquisition of a single time step in a 3D volume can take up to several minutes, only a small portion of data is acquired per period cycle, i.e. the acquisition of the entire data set is segmented over many cycles of the periodic process. By this technique a phase-resolved velocity field is created. However, the procedure requires triggering to the periodic nature of the flow. Therefore, triggering the MR scanner precisely on each flow cycle is one of the key issues discussed in this manuscript. A triggering system was developed, based on a piezo-element pressure transducer, providing a reliable signal despite the harsh magnetic and electro-magnetic environment inside an MR scanner. The obtained 4D-MRV data are compared to data measured using Laser-Doppler anemometry and good agreement between the results is found. Subsequently, the validated 4D-MRV data is analyzed and the fluid-mechanic features and processes inside the fluidic oscillator are investigated and described and a detailed description of the internal jet-switching mechanism is provided.
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