some simple quires
hi.. i have some simple quires for CFD gurus here.
1. Is the assumption of "continumm" valid for all types of flows? if not, what type of assumption we should make in such cases? 2. How does a fluid flow in a uniform pressure field? what makes it to flow? Does it violate the laws of nature ? Thanks b4 hand. ram 
Re: some simple quires
I think that your question is not simple. But I will do my best.
1. Continumm mechanics is valid for the flow where smallest characteristic length scale is enough large to compare with 'molecular mean free length. But invaild for rarefied flow, such as the flow under very low pressure(near vacumm). 2. If the flow velocity is uniform everywhere(u=v=w=const) and every time, fluid can flow in the uniform pressure field. But this situation is meaningless because, at least, one solid surface(or volume), where velosity gradient is essential, should be considered in the real world. Even rocket application in the space, rocket surface (solid) should be considered. And flow velocity can't be maintained as constant because of turbulent fluctuation. Sincerely, Jinwook Sincerely, Jinwook 
Re: some simple quires
(1). The best place to start is to read books on kinetic theory of gases, and the statistical thermodynamics. (2). In general, flow in vacuum tubes, or outer space will be treated as individual moleculle instead of continuum. But still, it is relative. It is based on the characteristic length of the object and the mean free path of the molecule in the flow. (3). In the NavierStokes equations, one can set the pressure gradient terms to zero, that is the uniform pressure field. This is how we obtain the solution of the flow over a flat plate. I think, in the real world, it can exist. But it is probably very difficult to achieve. (4). That is why we are dealing with analysis. Analysis deals with models of the real world problem.

Re: some simple quires
Thank you very much
regards ram 
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