Magnetic Resonance Imaging (MRI) is a well‐established technique in the medical community, able to produce volumetric reconstructions of the body by applying a combination of magnetic field gradients and radiofrequency pulses. MRI can also be used to perform velocimetry in fluid flows, thanks to the phase‐sensitivity of its signal to motion.
Traditionally, MRI has been used to measure flow properties in “in vivo” applications. At Stanford, the group led by John Eaton has pioneered the use of MRI to measure 3D velocity (MRV) and 3D concentration (MRC) in engineering flows. Coletti, a postdoctoral fellow in Eaton’s lab, has been using and advancing this method in various applications.
Coletti is investigating transport and mixing in complex flows and turbulent flows. Because he deals with millions of experimental data points, visualizing large 3D data sets is critical to understanding the dynamics of the flows he studies. Coletti uses Tecplot 360 to visualize his results and to communicate his findings to others.
One example is the dispersion of a contaminant injected into a crossflow. In this case, Coletti collaborated with Honeywell to understand the flow physics of film cooling for gas turbine airfoils.
Video 1 shows isosurfaces of time-averaged concentration of a contaminant injected into the turbulent cross-flow. The animation displays decreasing concentration levels, which extend further downstream as the contaminant gets diluted by the crossflow.
Video 2 shows progressive slices of the 3D volume as they move downstream from the injection. Both concentration contours and in-plane velocity vectors are plotted.
A second example is the flow through a stack of porous fins. The random pore distribution produces a meandering of the flow through the solid matrix, leading to significant transverse mixing. In Video 3, isosurfaces of positive (red) and negative (blue) streamwise vorticity are shown, highlighting elongated structures that swirl in the direction of the flow.
To illustrate the mixing mechanism, a plume of contaminant was injected upstream of the fin stack. Figure 1 depicts an isosurface at 2.5% of concentration, demonstrating how the random structure of the fin contributes to the spreading of the contaminant.
A third example is the inspiratory flow in human airways. The X-ray scan of a subject was used to fabricate a 3D model by stereolithography which replicates the patient anatomy from the mouth to the eighth generation of bronchial branching. Figure 2 shows various sections of the flow field, at the first bifurcation, and at further generations. In-plane velocity vectors (superimposed onto color contours of flow speed) demonstrate that strong recirculation of the inspiratory flow persists deep down into the bronchial tree.