Uncovering the Complexities of Streamlined and Steady Flow in Fluid Dynamics

[Kaushik]

TL;DR

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What is the difference between streamlined and steady flow? Is unsteady streamlined flow possible? If yes, could you please explain what are the characteristics of unsteady streamlined flow?

 

[Cutter Ketch]

I think these are both language rather than precisely defined states, and can be used with different meanings in different contexts. However, to me there is a difference in connotation. To me “steady” suggests a lack of temporal variation. This could be the lack of local turbulent variation and so could be used to mean streamlined, but it also could mean a lack of variation in the total flux.

“Streamlined” on the other hand indicates a lack of turbulence. (well, it is also an adjective that means designed to minimize resistance, but here I assume you mean “streamlined flow”) and seems synonymous with laminar. To me it doesn’t say anything about temporal variations in the total flux.

So, I would have no problem if somebody said “a steady turbulent flow” or “streamlined flow with an unsteady varying rate”. On the other hand, I also wouldn’t be surprised if people used steady to mean streamlined.

 

[boneh3ad]

"Streamlined flow" has no technical definition in fluid dynamics literature. It doesn't imply laminar or turbulent or anything since it has no technical definition. It's a popular terminology that usually means "low drag," but could mean slightly different things to different people.

Steady flow means no time-varying component. In the strictest sense, this means absolutely not time variation. You could also have flows that are quasi-steady, i.e. they are steady only in a certain way such as temporarily or in an average sense. Quasi-steady flows allow a lot of similar assumptions to be made as an true steady flow.

“a steady turbulent flow”

A turbulent flow always has a time-varying quality to it. That's part of the definition of turbulence. If you want to treat a turbulent flow as steady, you have to do it in an average sense to average over those fluctations. See, for example, the concept of Reynolds averaging.

“streamlined flow with an unsteady varying rate”

What is "an unsteady varying rate?"

 

[sophiecentaur]

Perfect streamlined flow is a fiction but it can be used successfully below appropriate speeds where it gives good answers with acceptable errors. Imo, it describes a situation in which turbulence is low enough to ignore and where it is damped by viscosity.
I'm not sure that Steady flow is as well defined except that it could indicate a situation in which turbulence happens to be relatively constant as opposed to a situation in which there are large variations over time. A 'Steady' situation would perhaps need to be defined in the context of a particular situation.
As in most cases, it's not a good idea to get too hung up on verbal definitions. They can lead you into confusion.

 

[boneh3ad]

Why is everyone obsessed with turbulence? First, it's not inevitable and there Re plenty of flows with zero turbulence involved, even in the real world. Second, it is only tangentially related to what we are discussing here, yet it seems to be the first thing everyone types.

 

[sophiecentaur]

Why is everyone obsesses with turbulence?

It all started with Henry the second and his Archbishop Becket.