[jins9158]

When I researched for material of simulation for steady & transient, I saw a result about a guide book.

It offered that steady simulation is unphysical flow and transient analysis is physical flow about crossflow fan below picture.

I was confused because many cfd expert have used steady simulation to improve fan blower.

In research ceter of company. usually they have used steady simulation because transient analysis is expensive

Also CFD professional company have used steady analysis and It is same also in university research institute.

In korea, the trend is to use steady simulation except for noise analysis.

So I am wondering that it is wrong to simulate steady simulation istead of unsteady simulation.
(transient anaysis is not realistic due to time expensive)

and why is unphysical to simulate steady analysis about fan?

Why are the positions shown in the picture unphysical in case of steady simulation(FRM analysis)?

[Opaque]

Be careful how you interpret the information, and vocabulary being used.

A Steady-state is an approximation, i.e. a compromise, to reduce the computational expense you highlighted is unrealistic. Then, the approximation must be clearly stated. From your pictures, it seems the steady-state approximation named "frozen rotor" was used; therefore, the solutions will show some degree of unrealism if used incorrectly.

The next level of a steady-state approximation for the case you showed is to use the "mixing-plane" which better accounts for the fact there is a sliding interface.

Most turbomachinery design teams use the mixing-plane as their work-horse approximation and refine their design using transient when needed.

Saying steady-state is bad vs transient is good is meaningless because of the lack of the correct context.

[jins9158]

Originally Posted by Opaque

Be careful how you interpret the information, and vocabulary being used.

A Steady-state is an approximation, i.e. a compromise, to reduce the computational expense you highlighted is unrealistic. Then, the approximation must be clearly stated. From your pictures, it seems the steady-state approximation named "frozen rotor" was used; therefore, the solutions will show some degree of unrealism if used incorrectly.

The next level of a steady-state approximation for the case you showed is to use the "mixing-plane" which better accounts for the fact there is a sliding interface.

Most turbomachinery design teams use the mixing-plane as their work-horse approximation and refine their design using transient when needed.

Saying steady-state is bad vs transient is good is meaningless because of the lack of the correct context.

Thanks for your answer.

Any advice was helpful.

However, the question was not resolved.

It does not make sense that it is a non-physical flow characteristic under steady-state conditions.

A vortex occurs at the location shown in the figure below (cut off peripheral velocity approaches zero).

I think this is possible enough with the flow generated when the fan rotates.

The flow discharged between the blades will have a vector as shown below.

This flow will hit the cut off, causing flow instability. This eventually shows the possibility of a vortex occurring around the cut off.

There is a paper that conducted an experiment on a fan, and there, too, a vortex or countercurrent flow occurred around the cut off.

So I think the flow can become unstable enough around the cut off.