Understanding the Moody Chart and R Value

[foo9008]

Homework Statement

I was told that the moody chart can be divided into 4 sections , i don't really understand the 'transition part' . it is said to be a function of R and roughness . what is R ? radius of the pipe ?

Homework Equations

The Attempt at a Solution

 

[SteamKing]

Homework Statement

I was told that the moody chart can be divided into 4 sections , i don't really understand the 'transition part' . it is said to be a function of R and roughness . what is R ? radius of the pipe ?

Homework Equations

The Attempt at a Solution

R is the Reynolds No. for the flow. Some sources use Re for this number.

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In the transition zone, there is some turbulence in the flow, but complete turbulence has not yet been established. This is why the friction factor is dependent partly on the relative roughness of the pipe and partly on the Reynolds No.

 

[foo9008]

What's

R is the Reynolds No. for the flow. Some sources use Re for this number.

​

In the transition zone, there is some turbulence in the flow, but complete turbulence has not yet been established. This is why the friction factor is dependent partly on the relative roughness of the pipe and partly on the Reynolds No.

What's the differences between critical zone and transition zone??

 

[SteamKing]

The critical zone is where the flow stops being wholly laminar and begins to develop turbulence. The friction factor can show large changes with a very small change in Reynolds No.

 

[foo9008]

how

The critical zone is where the flow stops being wholly laminar and begins to develop turbulence. The friction factor can show large changes with a very small change in Reynolds No.

about transition zone ?

 

[SteamKing]

how

about transition zone ?

See Post #2, below the Moody Chart.

 

[foo9008]

See Post #2, below the Moody Chart.

can you explain what is transition zone ? what's the difference between it and critical zone ?

 

[SteamKing]

can you explain what is transition zone ? what's the difference between it and critical zone ?

In the transition zone, enough turbulence is present in the flow so that the friction factor is only slightly dependent on Reynolds No., but is highly dependent on the relative roughness of the inside of the pipe. This is what the Moody Diagram shows, with the different friction factors for different values of relative roughness.

When the flow becomes fully turbulent, the friction factor will not depend on Reynolds No. at all.

 

[foo9008]

so , for the critical zone and transition zone shows the water begin to change from laminar to turbulent ? where the transition zone is 'closer' to turbulent than critical zone ?

 

[foo9008]

In the transition zone, enough turbulence is present in the flow so that the friction factor is only slightly dependent on Reynolds No., but is highly dependent on the relative roughness of the inside of the pipe. This is what the Moody Diagram shows, with the different friction factors for different values of relative roughness.

When the flow becomes fully turbulent, the friction factor will not depend on Reynolds No. at all.

in the transition zone , there is no laminar flow at all ? or there's some laminar ?

 

[SteamKing]

in the transition zone , there is no laminar flow at all ? or there's some laminar ?

Laminar flow has ended when the Reynolds No. reaches about 2000-2300. After that, characteristics of turbulent flow appear rapidly with increasing Re, and by Re = 4000, the friction factor has stratified according to Relative Roughness completely.

 

[foo9008]

Laminar flow has ended when the Reynolds No. reaches about 2000-2300. After that, characteristics of turbulent flow appear rapidly with increasing Re, and by Re = 4000, the friction factor has stratified according to Relative Roughness completely.

but , as i see from the chart , when the water become complete turbulent when Re = 7000 , not 4000 , why ?

 

[SteamKing]

but , as i see from the chart , when the water become complete turbulent when Re = 7000 , not 4000 , why ?

Fluid flow is not like flipping a light switch on and off. There are periods when one type of flow dominates and transition periods in between where a mixture of flow characteristics is present.

It's not clear what you mean by "but , as i see from the chart , when the water become complete turbulent when Re = 7000 , not 4000 , why ?"

The Moody diagram can be used for other fluids besides water. That's why it's developed using Reynolds No. as the independent parameter describing the fluid flow.
If you look at the long, sweeping dashed line which separates the transition zone from the fully turbulent flow zone, you will see that fully turbulent flow does not develop all of a sudden at Re = 7000, but is dependent on the relative roughness of the pipe to determine the Re at which turbulent flow does develop. The scale of Re at the bottom of the diagram is logarithmic. For a rough pipe, fully turbulent flow can develop at Re = 20,000; for a really smooth pipe, fully turbulent flow may not occur until Re = 10,000,000 or higher. That's a difference in Re of three orders of magnitude.

 

[foo9008]

Fluid flow is not like flipping a light switch on and off. There are periods when one type of flow dominates and transition periods in between where a mixture of flow characteristics is present.

It's not clear what you mean by "but , as i see from the chart , when the water become complete turbulent when Re = 7000 , not 4000 , why ?"

The Moody diagram can be used for other fluids besides water. That's why it's developed using Reynolds No. as the independent parameter describing the fluid flow.
If you look at the long, sweeping dashed line which separates the transition zone from the fully turbulent flow zone, you will see that fully turbulent flow does not develop all of a sudden at Re = 7000, but is dependent on the relative roughness of the pipe to determine the Re at which turbulent flow does develop. The scale of Re at the bottom of the diagram is logarithmic. For a rough pipe, fully turbulent flow can develop at Re = 20,000; for a really smooth pipe, fully turbulent flow may not occur until Re = 10,000,000 or higher. That's a difference in Re of three orders of magnitude.

what do you mean is when the Re is larger than 4000 , the fluid flow is not necessarily complete turbulent because it's also depend on the relative roughness?

 

[SteamKing]

what do you mean is when the Re is larger than 4000 , the fluid flow is not necessarily complete turbulent because it's also depend on the relative roughness?

Yes, partly. When the flow is fully turbulent, the friction factor will depend entirely on the relative roughness and not on the Reynolds No.

That's why the Moody diagram is so marked with the long curved dashed line which separates the zone of complete turbulence from the transition zone.

 

[foo9008]

Yes, partly. When the flow is fully turbulent, the friction factor will depend entirely on the relative roughness and not on the Reynolds No.

That's why the Moody diagram is so marked with the long curved dashed line which separates the zone of complete turbulence from the transition zone.

I was told that when Re > 4000 , it's said to be turbulent flow , so the more accurate is the flow start to become turbulent , and not laminar at all ? am i right ?

 

[SteamKing]

I was told that when Re > 4000 , it's said to be turbulent flow , so the more accurate is the flow start to become turbulent , and not laminar at all ? am i right ?

The flow starts to take on the characteristics of turbulent flow for Re > 4000. Laminar flow has ceased by the time Re ≈ 2300.

 

[foo9008]

The flow starts to take on the characteristics of turbulent flow for Re > 4000. Laminar flow has ceased by the time Re ≈ 2300.

how about at the region between 2300 and 4000 ?

 

[SteamKing]

how about at the region between 2300 and 4000 ?

Essentially the critical zone. The characteristics of the flow are rapidly changing in this region. It's hard to describe what is going on, which is why the Moody diagram is mostly blank in this region.

 

[foo9008]

Essentially the critical zone. The characteristics of the flow are rapidly changing in this region. It's hard to describe what is going on, which is why the Moody diagram is mostly blank in this region.

transition zone is for Re >4000 ?

 

[foo9008]

Essentially the critical zone. The characteristics of the flow are rapidly changing in this region. It's hard to describe what is going on, which is why the Moody diagram is mostly blank in this region.

do you mean there are times that the flow is laminar and also there are time that the flow is turbulent ?

 

[SteamKing]

do you mean there are times that the flow is laminar and also there are time that the flow is turbulent ?

No, I mean that the flow is somewhere in between laminar and turbulent, being neither one or the other, and it is difficult to describe physically and mathematically.

 

[foo9008]

No, I mean that the flow is somewhere in between laminar and turbulent, being neither one or the other, and it is difficult to describe physically and mathematically.

transition zone occur when Re > 4000 ??

 

[SteamKing]

transition zone occur when Re > 4000 ??

Yes. We seem to be going round and round with this discussion.

Study the Moody diagram carefully. You can Google other source material on the internet besides the slide show you have been provided.

 

[foo9008]

Yes. We seem to be going round and round with this discussion.

Study the Moody diagram carefully. You can Google other source material on the internet besides the slide show you have been provided.

in the link , http://www.engineeringtoolbox.com/laminar-transitional-turbulent-flow-d_577.html
i was told that the Transitional flow is a mixture of laminar and turbulent flow, with turbulence in the center of the pipe, and laminar flow near the edges. but , you said when Re = 2300 , laminar flow stopped ... transitional flow occur at Re > 4000 ? this is confusing

 

[SteamKing]

in the link , http://www.engineeringtoolbox.com/laminar-transitional-turbulent-flow-d_577.html
i was told that the Transitional flow is a mixture of laminar and turbulent flow, with turbulence in the center of the pipe, and laminar flow near the edges. but , you said when Re = 2300 , laminar flow stopped ... transitional flow occur at Re > 4000 ? this is confusing

The Reynolds No. is calculated based on the average flow velocity inside the pipe. The formula Re = v ρ D / μ does not distinguish what happens at the pipe wall versus what happens at the center of the pipe with regards to fluid flow characteristics.

Now, if you want to analyze the flow at the wall versus the flow in the center of the pipe, you will need to do a more sophisticated analysis than that provided by consulting the Moody Chart.