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 Lam May 4, 2007 06:10

I am self-taught in mass transport. I check the descriptions of the following terms online. Please ascertain or correct these descriptions:

diffusion - average motion of a molecule (or particle) as a result of its collisions with other molecules (or particle)

convection - vertical motion driven by buoyancy

advection - horizontal motion of the atmosphere and the prevailing winds are known as advective winds.

dispersion - the spreading of mass from highly concentrated areas to less concentrated areas

Many thanks.

 OPS May 4, 2007 13:31

Convection need not be always in vertical direction. Convection can be due to thermal and concentration gradient as well. When there is horizontal gradient of concentration, motion can take the horizontal direction also. You can see the definition of : Double-diffusive convection.

 CFDtoy May 7, 2007 10:16

Here are the definitions (universal no requirement for horizontal, vertical etc)

Convection - movement of particles (most probably driven by some kind of pressure (can be caused by temp, density etc etc)

diffusion - similar. Consider, 1 object with temperature T > other objects surroudning it with a Tneighbor < T

So, object 1 shall diffuse its heat to nearby objects at a rate based on the difference T-Tneighbor

Dispersion - Dispersing mass from a higher accumulation to a lower content region.

Pretty much just by defining which cell or zone has higher information ( pressure, temperature, density..), different phenomena is favored ! Communist way...share it with all !

Difference arise (between ) convection and diffusion in the way the informtion is propagated.

While convection is more like directional front (hyberbolic type), diffusion is like elliptic (requires information of its surrounding neighbors completely to compute its next course of action)

-- CFDtoy

 OPS May 7, 2007 10:48

No !!! Convection and diffusion are not similar. Here are some basic differences between them.

1. Convection is the bulk motion of the fluid and it has the charactericsts of both (motion and diffuion). In diffusion there is no bulk motion of the fluid. It happens on molecular scale while convection occurs on both macroscopic(bulk motion) and microscopic level (diffusion or conduction). In heat transfer, conduction is same as diffusion.

2. Velocity and diffusion coefficient (e.g. thermal conductivity) plays a major role in convection while in diffusion, only diffusion coefficient is necessary. Temperature difference etc. is needed in both the cases.

3. Energy carried by fluid is more in convection than in diffusion (recall Nusselt number)

4. Without diffusion, convection cannot happen. But without convection diffusion can occur. E.g. while heating a beaker of water from below, the first thing that will happen is diffusion (i.e conduction). Later, the fluid starts moving from bottom to top. This phase is called convection.

5. Gravity has no role at all in diffusion where as in convection gravity plays a role (in natural convection). Diffusion occurs along the gradient of the diffusing component irrespective of gravity.

OPS

 CFDtoy May 7, 2007 11:24

Good point. I was moving more towards advection talking about convective stuff. Diffusion does not always require temperatture !..mass diffusion can occur without temperature difference.

When convectin slows down (the rate !) spreading begins ! diffusion. Depends on the velocity and time scale you tell what is thje major phenomena, convection or diffusion !.

You can always send high heat in a particular domain without spreading too much. Diffusion is always there..but how much is the question.

Conduction and convection are different phenomena. Diffusion has to happen due to dissipation!

Gravity role (being a body force and a source in energy equation) is to provide the sufficient pressure drop effect for flow !( Note convection is flow, no pressure (or no temperature) no flow. In that case, it spreads.

Good discussion OPS.

-- CFDtoy

 lavanya May 17, 2007 01:05

is convection and advection the same

 Tom May 17, 2007 04:36

It depends on what you mean by convection.

If you are talking about the material derivative term in the equations (the convective derivative) then they.

If you are talking about buoyancy driven flows (thermal convection) then they are not.

 OPS May 18, 2007 12:04