Talk:Fluid parcel

message: Large parts of this article, on "Buoyancy" and "Applications" digress very far from the topic of this article: air parcels, which are small material elements retaining their identity as they move with the flow. Their use is far wider than suggested by the editor of this material (Darrenvc), and covers the whole field of fluid dynamics, not just a simplistic approach to buoyancy. I intent to remove this material, and rewrite the article as a general article on fluid parcels. -- Crowsnest (talk) 18:22, 9 March 2009 (UTC)
replies: text: Meteorology uses air parcel the same way as the rest of fluid dynamics, see e.g. , page 1. So this just seems to be a way of this editor to write some essay, but which is off-topic. I will remove it when I change the article. -- Crowsnest (talk) 10:21, 10 March 2009 (UTC)

text: I have several references. It is indeed a concept used in continuum mechanics: the parcel needs to be large with respect to the mean free path length, but small with respect to the phenomenon you want to model (Knudsen number). The problematic thing, conceptually, is diffusion, since the assumption is that you follow the same material as it moves. A fluid parcel is also not a rigid cube. Other names are material element (Batchelor, 1967) or fluid element.  But the article as it is now, is mainly a coatrack. -- Crowsnest (talk) 18:19, 10 March 2009 (UTC)

text: I rewrote the article to one about fluid parcels as commonly used in fluid dynamics, meteorology, astrophysics. Anyone who thinks she/he can reshape the buoyancy material from the old version into something useful (hopefully not in this article), can find it here. -- Crowsnest (talk) 20:11, 26 March 2009 (UTC)
thread_title: Off topic

message: It seems to me that an air parcel (or fluid parcel) is really just a way of discretizing a continuum.  Discretizing a continuum is useful for: 
1) Conceptualizing (grasping!) movement of the flow in the mind.
2) Numerical analysis of the flow.
However, when it comes down to it, a continuum is an approximation and an imperfect model.  When we make the discrete steps (the "parcels" or the wee-little fixed control volumes) small to reduce error due to discretizing, the "continuum approximation errors" become larger.  (Because molecules become large compared to parcels/CVs.)
Heck, now that I think about it, a fluid "parcel" isn't even a real thing!  It's not physics! :-)  Sure, it's used in the math that supports physics, and for conceptualizing (1 and 2 above), but it just ain't real.  Actually, a fluid parcel is a merely a way of approximating a fluid continuum, which itself is an approximation for "more correct" molecular interactions.
Fluid parcel is just so trivial.  Hmmm... maybe "air parcel" or "fluid parcel" could be written up in the context of "continuum" or "continuum approximation".  Only a short paragraph or two if it has its own article.  Maybe, as well, application to meteorology could be mentioned in that the especially large parcels used there are "interesting".  Maybe the whole current article could also be kept and renamed "Air Parcels Applied To Weather Modeling" or something like that.
Gummer85 (talk) 18:08, 10 March 2009 (UTC)
replies: text: I have several references. It is indeed a concept used in continuum mechanics: the parcel needs to be large with respect to the mean free path length, but small with respect to the phenomenon you want to model (Knudsen number). The problematic thing, conceptually, is diffusion, since the assumption is that you follow the same material as it moves. A fluid parcel is also not a rigid cube. Other names are material element (Batchelor, 1967) or fluid element.  But the article as it is now, is mainly a coatrack. -- Crowsnest (talk) 18:19, 10 March 2009 (UTC)

text: I rewrote the article to one about fluid parcels as commonly used in fluid dynamics, meteorology, astrophysics. Anyone who thinks she/he can reshape the buoyancy material from the old version into something useful (hopefully not in this article), can find it here. -- Crowsnest (talk) 20:11, 26 March 2009 (UTC)
thread_title:  Fluid parcel in context of a fluid continuum