Calculating the inlet lengths of a fluid circulating in a pipe?

[hya_t]

TL;DR Summary: why do we need input length in fluid's dynamics

What is the purpose of calculating the inlet lengths of a fluid circulating in a pipe? and how to calculate it correctly? why do we need it in engineering? This is for a class project where we are asked to calculate it and I think it is better explained by modeling it and using it in a problem or situation that we encounter almost every day.

 

[Lnewqban]

Welcome, @hya_t !

We not always need to exactly calculate the entrance length for full development of flow.
It is important for location of instruments in pipes and to reduce restriction to the flow formed by reduced free cross-section.

These articles may be helpful to you:
https://en.wikipedia.org/wiki/Entrance_length_(fluid_dynamics)

https://en.wikipedia.org/wiki/Velocity_stack

 

[256bits]

This is for a class project where we are asked to calculate it

By what method are you being asked to calculate the input length?
Can you describe what you mean by "input length".
Input length to what?

 

[hya_t]

By what method are you being asked to calculate the input length?
Can you describe what you mean by "input length".
Input length to what?

Here is the formulation of the problem:
Determine the inlet lengths of a fluid circulating in a pipe by numerical resolution (ANSYS Fluent) and compare the result with the analytical solutions seen in class (Turbulent and laminar).
Compare velocity profiles with internal flow for different fluids.

Now,
I need a situation where it is necessary to calculate the entry lengths in everyday life to better explain it to the class and better model it

 

[hya_t]

Welcome, @hya_t !

We not always need to exactly calculate the entrance length for full development of flow.
It is important for location of instruments in pipes and to reduce restriction to the flow formed by reduced free cross-section.

These articles may be helpful to you:
https://en.wikipedia.org/wiki/Entrance_length_(fluid_dynamics)

https://en.wikipedia.org/wiki/Velocity_stack

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Therefore knowing the input length allows us to know the location of the instruments of the circuit and this will affect the performance even of the instrument placed ?

 

[Lnewqban]

Therefore knowing the input length allows us to know the location of the instruments of the circuit and this will affect the performance even of the instrument placed ?

I would say yes to that.
Trying to avoid the locations where turbulence induces a false reading.

 

[hya_t]

I would say yes to that.
Trying to avoid the locations where turbulence induces a false reading.

View attachment 337755

I do not really understand
therefore everything we design in reality takes into account the fact of avoiding turbulence (is it even possible ? ) as best as possible. but I would like to know why turbulence does not suit us in our calculations?
thank you

 

[Chestermiller]

It is necessary to include the effects of the inlet length of fluid in a pipe for the case of short pipes. Otherwise you will get the wrong answer for the relationship between the pressure drop and the flow rate. The inlet flow development effect is more of a factor for laminar flow than turbulent flow.

 

[Lnewqban]

I do not really understand
therefore everything we design in reality takes into account the fact of avoiding turbulence (is it even possible ? ) as best as possible. but I would like to know why turbulence does not suit us in our calculations?
thank you

Turbulence is to be avoided for pressure and velocity measuring instruments only where a “cleaner” flow exist.
For example, velocity sensors in airplanes are located far from the skin of the machine and from vortices.

The problem with measuring turbulent fluids is that there are constant fluctuations of pressure and chages of direction of the velocity vector, which induce a fluctuating measured value.

In other cases, we want to know where internal directional or straightening vanes are required inside a duct, in order to better use the available cross-section and avoid self-restrictions caused by uneven distribution of the velocity gradient.

Please, see:
https://www.modernairliners.com/mod...d-indicator-how-pilots-measure-airplane-speed

https://buildingengineer.wordpress....ary-component-or-efficiency-reduction-device/

There may be other applications of this concept of inlet lengths of a fluid calculations, which I am not familiar with.