Purely Phase-Encoded Magnetic Resonance Mapping of Turbulence Anisotropy

The measurement of flow velocities much greater than 1 m/s can be a challenge for conventional magnetic resonance imaging (MRI) methods. Motion during the spatial encoding interval can lead to a variety of geometric and anemometric distortions. Purely phase-encoded MRI methods, such as SPRITE (single-point ramped imaging with T$_1$ enhancement) can employ a short encoding interval (hundreds of microseconds) for the time-averaged measurement of fast flows. The interval is not only short, but also constant, which saves SPRITE from artefacts caused by interfaces in multi-phase flow. Most recently, we have been using SPRITE to measure mean-squared displacements in turbulent flow in order to quantify the anisotropy of velocity fluctuations. By analogy with diffusion tensor imaging, we measure the components of an eddy self-diffusivity tensor downstream of a Venturi constriction at Reynolds numbers on the order of 10$^5$.