Time resolved velocity measurements of unsteady systems using spiral imaging.

Spiral imaging has been assessed as a tool for the measurement of spatially and temporally resolved velocity information for unsteady flow systems. Using experiments and simulated acquisitions, we have quantified the flow artefacts associated with spiral imaging. In particular, we found that despite the adverse effect of in-plane flow on the point spread function, for many physical systems the extent of blurring associated with spiral imaging is marginal because flows represented by high spatial Fourier coefficients, which would be those most affected by the distortion of the point spread function, exist at the physical boundaries of the flow and are therefore associated with much smaller velocities than are characteristic of the bulk flow. The necessity for a flow imaging technique which is robust to the accrual of velocity proportionate phase during imaging was demonstrated in an experimental comparison of spiral imaging and echo-planar imaging (EPI) applied to turbulent flow in a pipe. While the measurements acquired using EPI accrued substantial velocity proportionate phase, those acquired using spiral imaging were not significantly affected. High temporal velocity measurements using spiral imaging were demonstrated on turbulent flow in a pipe (image acquisition time 5.4 ms; 91 frames per second), which enabled the transient behaviour of wall instabilities to be captured. Additionally, the technique was applied to a multiphase flow system, where the wakes behind single rising bubbles were characterised. Spiral imaging thus seems an auspicious basis for the measurement of velocity fields for unsteady flow systems.