Flow Visualization; Lessons in Duct Flow Patterns from my Wood Shop

Sometimes, you learn something about the stuff you do for your
“day job” in unexpected ways. That happened to me the other day
while I was out in my shop cutting some framing to length for my
office remodeling project. I have a little dust collection system
installed in my shop that is made from clear plastic pipe and
powered by my trusty Sears Shop Vac. The picture below is where my
radial arm saw connects to the main.

The top of the tee is the main out to the other inlets on the
system. The bottom of the tee is where my Shop Vac connects to the
system. The branch of the tee with the yellow blast gate is the
connection to a hood on my radial arm saw.

The other inlets on the system also have blast gates and I only
open the one at the connection I am using to maximize the dust flow
and dust collection from what ever tool I am using. As you can see
in the picture, I have the radial arm saw blast gate open since
that is what I was using when I noticed the phenomenon I am about
to tell you about.

If I happened to be teaching a class where I was discussing flow
through the fitting in the picture, I suspect I might illustrate
the flow pattern in a manner similar to the following.

What I noticed out of the corner of my eye the other day as I
was finishing a cut and the last of the sawdust swept into the
elbow was similar what I captured in this little video clip when I
repeated the experiment with a camera mounted on a tripod and
focused on the fitting (in the interest of retaining all my
appendages, I wanted to be able to focus on making the cut rather
than operating the camera).

You can also see this clip, probably with a bit better
resolution on my You Tube site. If
you look closely, the flow pattern, captured by the sawdust that is
entrained in the air as I make a cut, is a bit more complex than my
original sketch would illustrate. From what I can tell, the pattern
is more like this:

I can’t say that I can conclusively explain why the flow and the
sawdust it is carrying wants to swirl up the dead branch of the tee
instead of making a smooth turn down into the shop vac. Here are
some of my thoughts so far.

The other blast gates and piping network are likely
not perfectly leak free
, thus there may be a minor
amount of flow leaking in from that direction that is disturbing
things and setting up what we are seeing. The blast gates to not
lend themselves to making minor adjustments in flow.  If I
tried opening one up, even just a bit, the phenomenon seemed to go
away and the sawdust appeared to be simply swept into the outlet
that connects to the shop vac.

The electro-static forces between the pipe and the
 act to create the flow pattern. 
You can see them to some extent when I reach out and touch touch
the pipe; notice how when I touch the pipe, some of the sawdust
tends to cling to it and how I an sort of move the sawdust around a
but by wiping my gloved hand against the pipe.

As the air flow enters the main from the
, it encounters some disturbances due to
imperfections between the branch joint and the main as in addition
to impinging on the lip of the coupling that is inserted into the
collar of the Y fitting to connect to the Shop Vac. Maybe these
disturbances are setting up the flow pattern we are observing.

Far from any sort of conclusive investigation;  mostly, an
interesting phenomenon that I thought I would share since, if you
read my blog, you probably have at least a passing interest in flow
in ducts.

I’ll probably think of a few more experiments to try and will
share the results as I do. Meanwhile, I suspect I will not be so
casual when I draw little arrows on my duct sketches as I lecture,
having been the benefactor of an impromptu lecture from mother
nature herself, hosted by my dust collection system and enhanced by
her lab assistances in the form of the sawdust from a 2×4.

Senior Engineer –
Facility Dynamics Engineering

here for an index to previous posts

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2 Responses to Flow Visualization; Lessons in Duct Flow Patterns from my Wood Shop

  1. David, Have you considered that the swirling pattern may be from
    the negative pressure built up from sucking air out of the closed
    system beyond the Y joint? If that is the case, either a relief
    vent at the top of the system or an additional valve directly above
    the Y joint should do the trick, although you will most likely
    still have some irregularities do to the nature of the fitting
    opening and its direction into the vertical pipe. Justin Clark, EIT

  2. Tom says:

    David, This is interesting. You wouldn’t have found this if you
    didn’t use clear pipe. What kind of pipe did you use, and is it
    readily available? I would like to use clear pipe to set up some
    things for my kids to play with that would be educational as well.
    Thanks again for your blog.

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