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Using MRI to Measure 3D Velocity and 3D Concentration in Engineering Flows


This blog was contributed by Filippo Coletti, Postdoctoral Fellow at Stanford University. In January 2014, Coletti will join the faculty at the University of Minnesota as Assistant Professor in the Department of Aerospace Engineering and Mechanics.1

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.

Mixing mechanism of a contaminant

Figure 1. Isosurface at 2.5% of concentration demonstrates 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.

Sections of the flow field

Figure 2. Various sections of the flow field, in the first bifurcation, and at various stations at further generations.

1Filippo Coletti received B.S. and M.S. degrees in Mechanical Engineering from the University of Perugia (Italy) in 2003 and 2005, respectively. He completed the Diploma Course in Fluid Dynamics at the von Karman Institute (Belgium) in 2006, and obtained a Ph.D. in Aerospace Engineering from the University of Stuttgart (Germany) in 2010.
 
After receiving his Ph.D., Coletti worked as Senior Research Engineer at the von Karman Institute, and as Postdoctoral Fellow at Stanford University.
 
He has received several awards, including the Italian Ministry of Education Scholarship from the University of Perugia, the Prize for Excellence in Experimental Research from the von Karman Institute, and the Arthur Charles Main Prize from the Institution of Mechanical Engineers.
 
In January 2014, he will join the faculty at the University of Minnesota as Assistant Professor in the Department of Aerospace Engineering and Mechanics.
 
Coletti is an experimentalist, with expertise in a wide range of flow diagnostics, including particle image velocimetry, infrared thermography, magnetic resonance imaging, and X-ray computed tomography. At the von Karman Institute, he investigated the turbulent flow and heat transfer in internal cooling channels, with focus on solid-fluid thermal coupling and rotational effects. At Stanford he used medical imaging to explore a broad spectrum of transport problems, including the mixing of contaminant injected into a crossflow, the dispersion of fluid and heat through random porous materials, and the air flow inside the human lungs.
 
Coletti’s research interests lie in the area of transport and mixing in turbulent and/or multiphase flows. In Minnesota he will be investigating the aerosol transport in human airways and the interaction of hot inertial particles with turbulence.