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 vijesh joshi March 15, 2006 07:21

Hello friends, I am a new to the Fluid dynamics subject. PLZ help me in understanding the terms below. convection, diffusion, dispertion and advection. I am learning CFX software. In one of my problems I got a shock wave at the front of the nose of the body. Can you plz explain why it gets birth?. (up till now I know that it is due to pressure difference)Also that, whether temperature, pressure and density are more at the tip of the nose when the blunt cone body flies at supersonic or hypersonic speeds?

 Jim March 15, 2006 08:13

Re: convection, diffusion, dispertion, advection ?

Convection refers to the fluid motion that results from forces acting upon/within it (pressure, viscosity, gravity etc). Diffusion refers to the transport of mass due to the presence of concentration gradients. NUMERICAL diffusion is a numerical error resulting from the descritisation of the Navier-Stokes equations, which results in a higher apparent viscosity. Dispersion is the combination of convective, diffusive and body forces (e.g. electric fields) acting upon a scalar concentration. For instance we talk about smoke dispersion from a chimney stack, which is the result of convective (the wind blowing it), diffusive (smoke diffuses in air) and buoyancy forces (hot air rises). Advection refers to the convection of a scalar concentration. Hunt around in your books/internet for the Generalised Tranport Equation. That should cover all the above.

The shock wave is formed at the front of your body because it is travelling faster than the speed of sound. Any good book on thermo or aerodynamics will have a section on this. Or search for "compressible flows" on the internet.

Not quite sure what you're asking by your last sentence, but the stagnation of the fluid on the leading edge will lead to a significant temperature rise, especially when M>3.

 vijesh joshi March 15, 2006 08:24

Re: convection, diffusion, dispertion, advection ?

Thanks a lot my dear.

 Harry Fulmer March 15, 2006 10:35

Re: convection, diffusion, dispertion, advection ?

Although its difficult to translate directy from the Latin derivation, the difference between advection and convection can be studied.

Advection - 'carry to' Convection - 'carry with'

We're always arguing over the distinction. The point is that there's no accepted formal distinction between the two in the wonderful world of CFD/fluid dynamics etc. Or is there? Argue away! :)

 Mohamed March 15, 2006 22:55

Re: convection, diffusion, dispertion, advection ?

Dear Vijesh,

If our semi object cone angle is less than max. Oblique shock angle, then we get a attached oblique shockwave(shock certain angle to flow dirn). if our object angle is greater than 45.28 ( The detail derivation and discussions are avilable in any compressible flow book) then we will get detached shock wave. (you have to make a clear view between Normal and Oblique shockwave). For ex. in airfoil case the instantaneous nose semi cone angle is 90 degree. So that we are getting datached shock.

Then for Hypersonic or Supersonic mach numbers flow, we have to avoid attached shockwave because it will rise huge temp and pr on the nose tip. For that we are designing as a blunt body nose instead of sharp nose.

I think it will help

regards Mohamed

 Jim March 16, 2006 05:44

Re: convection, diffusion, dispertion, advection ?

The way I have always understood it is that the resultant motion of the whole fluid (from the conservation of momentum) is the convection. So the transport of momentum is convection.

When we talk about mass transport of a scalar concentration, the resultant motion is advection.

So momentum transport is convection, and mass fraction transport is advection. This is only how I have used it, and how I have interpreted other peoples usage, but there is some confusion as to what the "correct" usage is.

Of course this is just the generalised transport equation:

Convection or advection = diffusive fluxes + sources

where sources are pressure gradients, body forces, chemical reactions, boundary conditions etc etc. In the case of momentum transport, the diffusive fluxes are your viscous terms (since viscosity acts to "diffusive" momentum).

 Tom March 16, 2006 07:08

Re: convection, diffusion, dispertion, advection ?

In this terminology what is meant by advection and convection is the same. The word advection was introduced in geophysical fluid dynamics so that convection would not be confused with buoyancy driven motions (i.e. thermal convection).

 Harry Fulmer March 16, 2006 07:12

Re: convection, diffusion, dispertion, advection ?

Ahh, that explains all the dictionary references to geophysics! I wonder why "advection" has crept into the more general CFD/fluid dynamics lexicon?

 Tom March 16, 2006 08:09

Re: convection, diffusion, dispertion, advection ?

Probably because a lot of the early pioneering papers on finite differences & grid staggering arrangements etc were for Meteorological problems (some of the earliest CFD calculations were in weather prediction; e.g. Richardson & Charny and Neumann). This carries through even today with the use of LES (Smagorinsky was a meteorologist).

 vijesh joshi March 16, 2006 09:24

Re: convection, diffusion, dispertion, advection ?

Thanks for replying my friends. but I am not able to conclude. I am not getting the difference between advection and diffusion. any one plz convince me.

 Tom March 16, 2006 09:41

Re: convection, diffusion, dispertion, advection ?

Advection/Convection refers to the tendacy to be moved along by the fluid (the convective terms arise from the change from Lagrangian to Eulerian frames). Diffusion refers to the dissipation/loss of a particles property (such as momentum) due to internal frictional forces.

 vijesh joshi March 17, 2006 06:44

Re: convection, diffusion, dispertion, advection ?

But why the two names advection and convection???

 Tom March 17, 2006 07:51

Re: convection, diffusion, dispertion, advection ?

I explained this in an earlier post

 Renato. March 17, 2006 18:59

Re: convection, diffusion, dispertion, advection ?

Some time ago I tried to explain a similar question...

hope it helps

Regards

Renato.

 vijesh joshi March 18, 2006 00:20

Re: convection, diffusion, dispertion, advection ?

Thank you friends.

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