3D3C & 2D3D Velocity Measurements Using Magnetic Resonance Velocimetry

Transcription

1 3D3C & 2D3D Velocity Measurements Using Magnetic Resonance Velocimetry Sven Grundmann Center of Smart Interfaces Technische Universität Darmstadt Flughafenstrasse Griesheim CFD and Experiment Integration of Simulation April 2011 DLR Göttingen 1

2 Outline MRI working principle State-of-the-art MRV MRV for engineering applications How to do MRV Cyclone 3D Diffuser, well known benchmark case Conclusions 2

3 Magnetic Resonance Velocimetry in Medical Applications Magnetic Resonance Imaging (MRI) is a well known measurment principle - imaging technique - Magnetic Resonance Velocimetry (MRV) is also known in medical sciences -non-invasive measurement procedure -no optical access necessary -no seeding necessary -experiments with water-flow -no post-processing necessary (already integrated) 3

4 MRI Working Principle 4

5 Spin The spin is a quantum-mechanical characteristic of atomic particles It can be aligned by applying a strong external magnetic field The spin magnets do not align steadily. They precess. The spin precession takes place at a fixed frequency depending on the magnetic field strength Source: 5

6 Measuring the Spin The transverse (xy) magnetization could be measured However all spins are out of phase HF pulses at the precession frequency can allign the spins => a decaying signal can be measured while the sins run out of phase again => Measurable signal Source: 6

7 From the Signal to the Image The precession frequency is depending on the magnetic field strength By applying gradients to the field strength slices of the volume can be selected during excitation (slice selection) Phase differences can be obtained by temporal gradients before readout (y-coordinate/phase encoding) Local precession frequencies can be modified during readout (x-coordinate/frequency encoding) A Fourier Transfom can reconstruct the spatial signal intensity distributions Source: 7

8 Measuring Velocity The Fourier Transform reconstructs the original magnitude of the signal in every voxel. That gives the image. Not used so far is the original phase of the the signal in every voxel => Phase Contrast Velocimetry By applying a bipolar gradiend pulse before readout, stationary spins aquire phase and lose it again moving spins: eg. aquire less phase than they loose Source: 8

9 State-of-the-art MRV 9

10 4D MRV by Markl et al.: A Living, Beating Heart linker Herzkammer und Aorta linke Lungenvene rechte Lungenvene Untere Hohlvene Obere Hohlvene Source: Wikipedia 10

11 MRV for engineering applications How to do MRV 11

12 MRV in Engineering Sciences: Cyclone Cooling Flow supply through diffuser swirl generation through tangential jets swirl pipe / test section 12

13 Clinical Magentic Resonance Tomograph Source: 13

14 MRV Experiment 14

15 MRV Procedure Set the parameters FOV (up to 450mm cube size) Resolution (typical: 1mm voxel size) Scan n-times (depending on required SNR and resolution) conduct off-scan (background subtraction) Average scans Subtract off scan done 15

16 MRV for engineering applications Cyclone 16

17 2D 3C MRV Projection of an 8mm thick slice 1500 vectors 1,9s per scan seconds total scan time Axial velocities 17

18 3D 3C MRV 6.5 minutes per scan 3 scans 3D3C velocity measurements 1e6 vectors 25 minutes no post-processing rapid prototyping => quick parameter studies & complex geometries 18

19 MRV for engineering applications 3D Diffuser 19

20 3D Asymmetric Diffuser: Benchmark case for CFD Top view: 2.56 separation Side view: 113 flow high speed flow E.M. Cherry, C.J. Elkins, and J.K. Eaton. Geometric sensitivity of threedimensional separated flows. International Journal of Heat and Fluid Flow, 29/3: ,

21 3D Asymmetric Diffuser: Flow control using Vortex Generators Grundmann, S.; Sayles, E.L.; Elkins, C.J.; Eaton, J.K. (2011) Sensitivity of an Asymmetric 3D Diffuser to Vortex-Generator Induced Inlet Condition Perturbations. Submitted to Experiments in Fluids VG1 Cherry VG2 Isosurfaces axial velocity u=0m/s and u=0.4m/s Isosurfaces vertical velocities v=+/-0.05m/s 21

22 Conclusions Opaque models 3D3C velocity vectors Periodic flows can be measured Indirect procedures for Reynolds stresses concentration, mixing processes Physically possible: 3D temperatur fields requires development Applicable for: - Internal flows - Heat transfer (indirect) - future: heat transfer (direct) - Flows in porous media - Multiphase flows - cavitation 22

23 End 23