Biomedical subjects

J D Seymour

Publications and source records attributed to J D Seymour.

4 recordsLinked to original sources

PubMed2000

Correlation time and diffusion coefficient imaging: application to a granular flow system.

A parametric method for spatially resolved measurements for velocity autocorrelation functions, R(u)(tau) = , expressed as a sum of exponentials, is presented. The method is applied to a granular flow system of 2-mm oil-filled spheres rotated in a half-filled horizontal cylinder, which is an Ornstein-Uhlenbeck process with velocity autocorrelation function R(u)(tau) = e(- ||tau ||/tau(c)), where tau(c) is the correlation time and D = tau(c) is the diffusion coefficient. The pulsed-field-gradient NMR method consists of applying three different gradient pulse sequences of varying motion sensitivity to distinguish the range of correlation times present for particle motion. Time-dependent apparent diffusion coefficients are measured for these three sequences and tau(c) and D are then calculated from the apparent diffusion coefficient images. For the cylinder rotation rate of 2.3 rad/s, the axial diffusion coefficient at the top center of the free surface was 5.5 x 10(-6) m(2)/s, the correlation time was 3 ms, and the velocity fluctuation or granular temperature was 1.8 x 10(-3) m(2)/s(2). This method is also applicable to study transport in systems involving turbulence and porous media flows.

Diffusion↗

PubMed1999

Taylor dispersion and molecular displacements in Poiseuille flow.

We have used pulsed gradient spin echo (PGSE) NMR to measure longitudinal displacements of octane molecules undergoing Poiseuille flow in a 150 microm diameter pipe, accessing time scales which approach the Taylor dispersion limit. We monitor the change in displacement distribution which occurs as molecules undergoing Brownian motion sample an increasing proportion of the ensemble of streamlines, observing the effects of wall collisions and the gradual transition of the propagator from Poiseuille to Taylor-Aris behavior. The further use of a double PGSE sequence allows the direct measurement of the stochastic part of the motion alone.

Journal Article↗

PubMed1997

Sonochemical reactions at 640 kHz using an efficient reactor. Oxidation of potassium iodide.

Ultrasound can be used to oxidize aqueous pollutants. However, due to economic reasons higher oxidation/destruction rates and higher energy efficiency are needed. Recent studies suggest that the higher ultrasound frequencies provide better oxidation rates than the conventional 20 kHz. Another area for improvement is reactor configuration. We have tested two new reactor configurations with proper focusing and reflection of ultrasound for maximum utilization. Reactor configuration plays an important role in the overall efficiency. In the new reactors, transducers and reaction mixture are separated by a polymer acoustic window which allows efficient transfer of ultrasound energy and not the heat from the transducer to the reaction mixture. One reactor at 640 kHz provides a 100% enhancement over the best reported rate for the oxidation of potassium iodide, on a per-Watt basis. Experiments conducted at varying initial KI concentrations show interesting behavior. Increasing the KI concentration by over eight fold merely increases the iodine production rate by two fold. This suggests that in the oxidation region surrounding the bubble, the KI concentration is much different than in the bulk. It is proposed that the hydrophobic bubble region has lower and near saturation KI concentration.

Journal Article↗

PubMed1994

Turbulent pipe flow studied by time-averaged NMR imaging: measurements of velocity profile and turbulent intensity.

A time-averaged method to obtain quantitative measurements in turbulent flow by phase flow encoding NMR imaging is introduced. With this method time-averaged velocity profiles and turbulence intensities can be determined. Time-averaged velocity profiles for pipe flow of water driven by a constant pressure gradient at Reynolds numbers from 1200 to 9400 were visualized. A precise correlation between the pixel intensity of the time-averaged NMR flow image and the local turbulence intensity of the flow is derived. The measured turbulence intensities are in agreement with published data obtained using other experimental methods.

Magnetic Resonance Spectroscopy↗