## How can diffusion be measured?

MRI has a unique capability to measure random translational motion of water molecules (diffusion) using a device called a pulsed magnetic field gradient. Using this device, locations of the molecules are tagged within a voxel and translational motion can be detected by the changes in signal intensity.

**What is the heat diffusion coefficient of water?**

For water at 0 °C, a diffusion coefficient of 1.12 μm2/ms is estimated using the default data from Tables 1–5, but after removal of all measurements performed at temperatures > 30 °C.

### How do you calculate average rate of diffusion?

Delta P times A times k over D is the law to use…. Pressure difference, surface area and the constant k are multiplied together. They’re divided by diffusion barrier to determine the exact rate of diffusion.”

**What are the units for diffusion coefficient ( D )?**

The diffusion coefficient (D) describes how long it takes a particular substance (like oxygen or proteins, etc.) to move through a particular medium (like water or molasses). The units of D are length 2 /time, and usually reported in cm 2 /sec. Why length 2 /time?

## How is numerical dispersion minimized in advective transport?

Numerical dispersion is minimized by always having the following relationship between time and distance discretization: where is the time step for advective transport, and is the cell length. Numerical instabilities (oscillations) in the calculation of diffusion/dispersion are eliminated with the constraint:

**How is dispersive transport defined in a central difference scheme?**

Dispersive transport in a central difference scheme is essentially mixing of cells. A mixing factor mixf is defined as where n is a positive integer. The restriction is that never more is mixed out of a cell than stays behind, that is, mixf must be less than 1/3 as follows from equation 109.

### How many dispersion steps are taken for each advection step?

The Advection-Reaction-Dispersion Equation. For the 15-meter cell-size ( mixf = 1/3), one dispersion step is taken for each advection step; for the 5-meter cell size ( mixf = 1), three dispersion steps are taken for each advection step; and for the 1.67-meter cell size ( mixf = 3), nine dispersion steps are taken for each advection step.