Steady Flow-Does That Mean ALL Properties of Flow are Steady?

[ltkach2015]

QUESTION:

In general: when a flow is considered steady does that mean all properties of the flow are steady?

Or: can a flow have steady and unsteady properties?

 

[Geofleur]

I can think of one way this can happen, sort of. Different physical processes sometimes act over very different time scales. For example, one can imagine a situation where the pressures in a fluid are approximately constant in time, while the temperatures are still evolving. I suppose you could call it quasi-steady state.

 

[Chestermiller]

QUESTION:

In general: when a flow is considered steady does that mean all properties of the flow are steady?

Or: can a flow have steady and unsteady properties?

In a steady flow, nothing is changing with time at each spatial location.

Chet

 

[insightful]

I would say turbulence in a vessel or pipe could be considered unsteady-state in a steady-state process.

 

[Chestermiller]

I would say turbulence in a vessel or pipe could be considered unsteady-state in a steady-state process.

Is that your professional judgement in addressing practical problems in turbulent flow? In my judgement, it depends on the time- and spatial scale of the averaging. After all, we don't treat individual molecular collisions and vibrations as part of the unsteadiness, even though that can certainly be regarded as unsteadiness on a very short time and spatial scale. In my experience, the time- and spatial scales in typical practical problems involving turbulent flow does not necessitate specifically treating the system as an unsteady flow. Even when studying how turbulence plays out in determining the stresses in a fluid in turbulent flow, the objective is to average out the statistical variations so that the results can be applied at larger time- and spatial scales. This is called the hierarchy approach.

Chet

 

[insightful]

Is that your professional judgement in addressing practical problems in turbulent flow?

Of course not. The straw man lives!

 

[Chestermiller]

Of course not. The straw man lives!

Sometimes a simple answer, on the level of the question asked by the member, is a better choice than a more complicated intricate response (even if the more complicated response fleshes out the answer in greater detail), in terms of giving the member what he needs. It is often less confusing that way. The member will have plenty of opportunity to study the situation in greater detail as his studies progress in the future. In other words, sometimes less is better.

Chet