How does dispersion work in fluid dynamics?

Question

this is what wiki gives:

"dispersion of water waves generally refers to frequency dispersion. Frequency dispersion means that waves of different wavelengths travel at different phase speeds. Water waves, in this context, are waves propagating on the water surface, and forced by gravity and surface tension. As a result, water with a free surface is generally considered to be a dispersive medium"

Can someone please translate that in everyday language. I have trouble understanding that or links to other sites that give a clearer explanation.

I was wondering whats the difference between 'molecular' and 'mechanical' dispersion in soil water in a river stream, lake etc.

Thanks.

Answer 1

If you are talking about dispersion of chemical constituents in the water, then the Wiki answer is not strictly relevant.

Basically, the wiki explanation says that waves come in different speeds and will separate from each other as they travel away from the source, at least at the surface of a body of water.

For me, there are three principal modes for the migration of constituents in water : convection, dispersion, and diffusion.

Convection is the process of the components moving with the bulk of the water.

Dispersion is the process of random movement in all directions that leads to the slow spread of the components in all directions.

Diffusion is the migration of components from areas of high concentration to areas of low concentration under chemical forces.

Lets say you add a drop of food coloring to a glass of water.  You can immediately see the color move part way through the water because of convection caused by the energy of the drop entering the water.  (you see little lobes of color radiating from the spot where the dye entered the water).  If you stir the water, additional convection spreads the color throughout the glass.

If you leave it alone (don't stir), the color will slowly spread throughout the water by dispersion.  Diffusion is also occuring but is small relative to dispersion in this particular example.

Convection is fast, dispersion is slow, and diffusion is very slow, basically.