So, this blog is my attempt to do that.
I wrote an email to a colleague I hold in high regard to somewhat give a direction for my research:
Friction factors for large diameter pipe change with
velocity. Research for large pipe has been limited to 48” in the lab setting.
I would like to understand how the friction factor changes
with change in velocity, hydraulic roughness, density of water, and kinematic
viscosity in large diameter pipe. The data I collected in my Master’s work shows that large pipe does not follow the traditional Moody diagram relationship. I found when plotting the calculated friction factor (from
observed data) against the calculated Reynolds number, the ratio of the
hydraulic roughness and the diameter of the pipe is smaller than what is shown
on a traditional Moody diagram.
I suspect the hydraulic roughness calculation isn’t a good
approximation for large diameter pipe because the boundary layer of the
water/pipe interface may be smaller due to quick dissipation of shear stresses.
In other words, the boundary layer in smaller pipe may be much larger due to
the shear velocity being higher. I believe the larger diameter pipe has smaller
boundary layers due the fact that the water has the ability to “normalize” in
the large column of water.
To test this, I’d like to do computational fluid dynamic
model of large diameter pipe and then calibrate it in the field with observed
data points.
Since I have a number of different surface conditions, from
new smooth pipe to highly pitted. I propose to identify locations where
existing petrographic analysis reports give physical measurements of the depth
of roughness. I can use these numbers to help identify a physical roughness
value and use field measurements to be able to determine an average roughness.
I can use kriging to help develop a hypsographic chart similar to what has been
done by the Bureau of Reclamation by using a robotic 3D imaging system (http://gigamacro.com).
I got the following revisions from him:
Energy loss (i.e. Resulting from Friction
at the wall) factors for large diameter
pipe is dynamic and the magnitude of resulting
resistance to fluid flow changes with not only
the characteristics of the pipe wall, but the characteristics of the fluid and velocity.
Empirical Research on
the development of friction coefficient to estimate energy loss for
large pipe has been limited to 48” in the lab setting. I
would do a little work here to explain Darcy and Weisbach’s empirical work and
the fundamental assumptions they made at the boundary and how it was decided to
use the average velocity for turbulent flow estimates of head loss. I would be
a little careful here because this is fundamental stuff and you would need to
do tremendous data collection to truly evolve the established friction
coefficients.
Suggest you read some of the early
work (c. Late 1930s) and understand their basic assumptions. It is know that
the cross sectional velocity profile in a full flowing pipe is not properly
considered in the DW equation. Look at “internal flow systems” by D.S. Miller
it’s a great reference with a significant amount of research performed by BHRA.
I would like to understand how the friction factor changes
with change in velocity, hydraulic roughness, density of water, and kinematic
viscosity in large diameter pipe. The data I collected in my Master’s work,
show that the large pipe does not follow the traditional Moody diagram
relationship. I found when plotting the calculated friction Factor (from
observed data) against the calculated Reynolds number, the ratio of the
hydraulic roughness (this is a measured value and as a
result the ratio is a known, a given) and the diameter of the pipe is
smaller than what is shown on a traditional Moody diagram. I believe what you are seeing or more fundamentally is the
lack of consideration of the cross sectional velocity profile in large pipe.
You may even see a transition from fully turbulent flow to more laminar flow
from center to wall. This would be very difficult (not feasible) to put into an
empirical equation, much less a coefficient. Take a look at the empirical work
that has been done on sledges. This is a non-Newtonian fluid that viscosity and
Reynolds No. are very very important to understand. Sludge, because it’s easier
to move once it is moving has much higher resistance at low flow. Again this is
a characteristic of a non-Newtonian fluid and may be happening at your pipe
wall. There has been some very good work done on this on the Oil Sands in Canada.
Worth a look.
I suspect the hydraulic roughness calculation isn’t a good
approximation for large diameter pipe because the boundary layer of the
water/pipe interface may be smaller due to quick dissipation of shear stresses.
In other words, the boundary layer in smaller pipe may be much larger due to
the shear velocity being higher. I believe the larger diameter pipe has smaller
boundary layers due the fact that the water has the ability to “normalize” in
the large column of water.
To test this, I’d like to do computational fluid dynamic
model (this is where I think your approach is suspect,
you should be working on improving the boundary condition formulation in the
CFD model because it is flawed and steer away from trying to calibrate it for
large diameter pipe) of large diameter pipe and then calibrate it in the
field with observed data points. (Use your data to
prove the new boundary condition in the CFD) Going back to the sludge
comparison…..in the engineering world the friction of sludge is estimated using
a sliding scale with relation to it solids content and the average flow
velocity. This is simply a curve fit (empirical). The complexity of the fluid
at the wall is not easily solved. I believe this would be where you could make
a true contribution in relating this type of curve to large diameter pipe and
various wall conditions (bio film, mussels, lining types, etc.).
Since I have a number of different surface conditions, from
new smooth pipe to highly pitted. I propose to identify locations where
existing petrographic analysis reports give physical measurements of the depth
of roughness. I can use these numbers to help identify a physical roughness
value and use field measurements to be able to determine an average roughness.
I can use kriging to help develop a hypsographic chart similar to what has been
done by the Bureau of Reclamation by using a robotic 3 d imaging system (http://gigamacro.com).
I just really wanted to capture this dialogue here.
My next goal is to go back and look over the topics I vivisected from my Master's Thesis work.
I didn't do a good job freewriting. Let me try this again.
I like to work with large diameter pipe. What I am finding is that the tried and true equations for smaller pipe doesn't necessarily apply to larger pipe - i.e., it doesn't scale well.
So what kind of "significant" work could I contribute? Crud, I don't know. There are two different directions I can go with this work: Fluid Mechanics and Environmental. I'm struggling with where I should focus my work. I'm stronger with Fluid mechanics but find the environmental characteristics of the water absolutely impacts fluid mechanics. That's one of the reason's why I want to take some fluid mechanics courses with the ME department. Their classes address all sorts of constituent fluids, not just "incompressible" water.
It looks like the roughness coefficient really doesn't hold up with larger pipe. The original development of the roughness coefficient, if I recall correctly - I have to verify it - is based on running water across sandpaper of varying sand grain roughness.
Could there be a way to really quantify roughness now that technology has gotten better? Furthermore, could I use kriging methods I learned in Stocastic methods to better help quantify a true hydraulic roughness coefficient? Is there a true relationship between the hydraulic roughness with energy loss? (Of course there is - just freewriting). What exactly is it? I need to go back and review some fluid fundamentals. It's been a while since I looked at shear stress along a boundary (in fact, it was for an open channel flow project I had). How does pressure figure in all this? How does it affect the shear stress? Seems like the larger the pipe diameter, the less shear stress along the pipe will impacts energy loss. Has someone quantified that yet? I have research to do.
So, Action Items:
Review old Thesis work
Go back and review fluid fundamentals
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