Governance by those who do the work.

Sunday, August 6, 2017

Mixed Convection from an Isothermal Rough Plate

In March 2017 the roughness of the aluminum plate was reduced from 3mm to 1mm.  I installed it in the wind-tunnel and started running experiments.  The forced convection measurements were nearly 30% higher than expected!

I examined nearly every aspect of the physical device and its mathematical model.  The fan-speed calibration was found to be sensitive to the distance between the test surface and the wind-tunnel wall.  Conditioning the rpm-to-speed conversion on the plate's orientation improved the earlier data taken with the plate with 3mm roughness.

When the plate is not parallel to the wind-tunnel, the forced and mixed measurements are affected.  With the 3mm roughness plate, the alignment had been controlled within a couple of millimeters over the plate's 305mm length.  The 1mm roughness plate seemed to require stricter tolerances.  Using a caliper, I am able to control the alignment to better than 1mm.

The primary cause of the measured excess was that, when the height of the posts had been reduced, the size and spacing of the posts had not been reduced.  At high wind-speeds the convection from the flat post tops was exceeding the "fully-rough" mode of convection.  The model incorporating this phenomena is developed in the "Rough to Smooth Turbulence Transition" section of my "Mixed Convection from an Isothermal Rough Plate" paper.

Writing a paper forces one to revisit all the questions and anomalies that occurred during research and experiment.  Understanding, resolving, and testing all of these issues has taken months.  I would appreciate any proof-reading or critiquing that others might provide before I submit it for publication.

Wednesday, February 8, 2017

Oseen Flow in Ink Marbling

Mathematical modeling of ink marbling has long been a fascination of mine.  My Ink Marbling web pages have presented emulations of a number of marbling techniques.  But the raking techniques modeled were either paths across the whole tank or circular paths.

Pictorial ink marbling designs are created using short strokes, where a stylus is inserted into the tank; moved a short distance; then extracted.  There seems to be no way to adapt the line or circle draws to short strokes with endpoints.

Having bought a copy of Boundary-layer theory (Hermann Schlichting et. al.) for my convection project, I started reading from the beginning.  It didn't take long until I found a description of Oseen flow on page 115 (chapter IV, very slow motion).  Its streamline figure looked very promising.  After further research I have written: Oseen Flow in Ink Marbling arXiv:1702.02106 [physics.flu-dyn].

Unlike a cylinder in a 2-dimensional flow, the velocity field induced by an infinitesimally thin stylus can be exactly solved in closed form.  In this sense, marbling is the purest form of Oseen flow.

The partial differential equations solved include conservation of mass (divergence=0), but not conservation of momentum (Navier-Stokes).  It's not clear how much momentum is imparted by the stylus, or how that imparting momentum changes with time.

Liquid marbling is sensitive to the speed of a stylus moving through the tank.  At low speeds the induced flow is laminar; at high speeds the flow becomes turbulent.  Both are used by marblers, but only the laminar flow is possible to solve in closed form.

Sunday, October 2, 2016

Mixed Convection from a Rough Plate

Its been a long time since my last blog entry; there are new developments.

After completing the vertical convection measurements, I returned the Convection Machine to its horizontal orientation and did some runs to make sure everything was as before.  But things were not the same.

With the rough side facing down, the transition where mixed convection dropped below the linear asymptote had disappeared.  Varying parameters did not restore the dip.  Changing inclination of the plate; tilting the wind-tunnel; resealing the cardboard around the fan; nothing I tried restored the dip.  Something has permanently changed in the wind-tunnel or the plate.

So I reran the measurements of horizontal upward and downward facing mixed convection.  The new curves match simple L2 and L4 norms of the forced and natural convection components.  My guess of what changed has to do with the suspension of the plate.  The vertical suspension was a single long wire which, after hooking around the a top corner post, wrapped across the back and hooked around the other top corner post.  Wrapping around the back compressed the back sheet and insulation against the back side of the rough plate.  Perhaps the pressure closed gaps in the glue between aluminum and insulation.

Having data for horizontal forced flow with 3 plate orientations, it was time to measure downward forced flow with a vertical plate. Because vibration of the plate had caused excess convection with the single wire suspension, I added two wrap-around wires pulling in opposite directions to the plate suspension.  This new suspension is quite rigid and works with the wind-tunnel in any orientation.

I added four legs to support the wind-tunnel upright with the fan drawing downward.  I was expecting either L2-norm mixing or for convection to drop below the natural level when the natural and forced components were equal.  But it was neither!  I devised a model which transitions between L2 and L4 norm that matched the measurements well; it is detailed in my paper.

Because the opposed mixing was unexpected, aided mixing had to be tried.  It also turned out to involve a transition between L2 and L4 norms, but with a gentler transition.

I have finished writing the article and put it and the supplementary data on http://people.csail.mit.edu/jaffer/convect

As described in the paper, the next step is to shave 4.mm off the rough side of the plate and repeat the measurements.

Monday, June 13, 2016

Mixed Convection from a Vertical Rough Surface

I turned the wind-tunnel on its side and hung the plate vertically as shown in the photograph.

These graphs show that my mixed convection model is successful from natural through forced convection for horizontal and vertical rough plates with forced flow perpendicular to the natural flow.  The leftmost red dot in each graph is the natural convection (Re=0) for that orientation; it is placed at Re=1000 so that it can appear on the graph.

The L4-norm for downward convection indicates that the interaction between the downward mode and forced convection is more competitive than the fairly cooperative L2-norm of the vertical and upward modes.

Horizontal downward and vertical natural convection from the rough plate match that expected from a smooth plate.  Horizontal upward convection matches assuming that the upper (rough) surface convection is reduced by 93% of the non-forced convection from the four adjoining sides.  In order to test if horizontal upward convection is the same for rough and smooth, I will cover the rough surface with a flat sheet of aluminum and repeat the test.

I have started writing a paper titled "Mixed Convection from a Rough Plate".  Which journal should I submit it to?

Saturday, June 4, 2016

Fan Windspeed

With the wind-tunnel fan being phase-locked now, the speed variability which plagued earlier speed measurements should be reduced or eliminated.

The process of conducting the measurements for these graphs finds this to be the case.  Although some variability remains above 3 m/s (1000 r/min), at slower speeds the anemometer readings are steady after the phase-lock-loop settles.  The measured traces are in blue; the black curve is that used in convection calculations.  This first graph is for the wind tunnel with horizontal plate.


This second graph is with the wind tunnel laying on its side with the plate vertical.

An earlier post found that the kink just above 2 m/s is due to the anemometer; it remains.  Friction in the anemometer makes the measurements below fan speeds of 700 r/min unreliable; below fan speeds of 200 r/min the anemometer reads 0.0.

Saturday, May 7, 2016

How to Phase-Lock a Fan

Using an auto-transformer to reduce the voltage to the wind-tunnel fan
in order to reduce its speed didn't work below 45 r/min (it ran for a
while and stopped).  So I modified The Convection Machine to toggle
the fan power with a solid-state relay controlled by micro-processor.

Consider the shaft of the wind-tunnel fan.  Every full rotation of the
fan results in 3 micro-processor interrupts.  A phase-accumulator
register is incremented by the desired rotation rate (in r/min) 1200
times a second and decreased by 24000 every time a fan blade crosses a
light beam.  If the fan rotates at the desired rotation rate, then the
average phase-accumulator value is constant.  If the fan is too slow,
then the phase-accumulator value increases with time; if it is too
fast, then the phase-accumulator value decreases with time.

Phase-locking is the process of controlling the fan-speed so that the
average phase-accumulator value is 0.  Such feedback systems are
tricky to stabilize.  My fan controller operates in one mode when the
desired speed is less than 400 r/min and a different one otherwise.

At high speeds, the fan speed is roughly proportional to the average
voltage applied, which is proportional to the duty cycle of applied
voltage.  The phase accumulator operates as described above.  Its
instantaneous value is compared with a variable which decrements from
the upper phase range bound to 0 ten times a second.  If greater, the
fan is turned on, otherwise it is turned off.  Some of the
instabilities of the fan speed may be due to a centripetal switch
disconnecting the starter capacitor and hooking in the running
capacitor, which increases the loop gain of the system.  The change in
gain causes the system to overshoot and undershoot the desired r/min
with long settling times.

At low speeds each pulse of power incrementally increases the fan
speed while friction continually slows it.  The solid-state relay has
"zero-crossing" control, so only complete half-cycles of 60 Hz power
are applied to the fan motor.  The combination of the motor windings
and phase capacitor stores energy, so the acceleration of the rotor is
delayed from the application of power.  At low speeds the rotational
inertia of the rotor introduces 90 degrees of phase shift.  The
microprocessor clock is not synchronized to the line voltage, so the
minimum pulse width varies with the relative phase, another source of
loop gain variation.

This photo shows the new fan-speed control.  The number on the
7-segment displays is the rotation rate in r/min measured every
second.  The right 3 dial switches set the desired rotation rate.


This video which shows the phased-locked fan in
operation at a variety of speeds.  The low light level was necessary
so that the stroboscopic interaction of camera shutter with the
scanned 7-segment display didn't render the numbers unreadable.  If
you turn up the audio volume you can hear the fan chugging as its
power is switched on and off.

Saturday, March 19, 2016

Make Square Opening by Drilling Five Round Holes

In making a speed control for an off-the-shelf electric fan, I needed to install a square power receptacle in the phenolic box I am using for the speed dial switches and 7-segment displays.  Phenolic is brittle and does not machine well with the woodworking tools that I have, chipping instead of cutting.

A straightforward way to cut the hole would be to drill a hole, disassemble a coping saw and reassemble it with the blade through the hole, and sawing.  But the small size of the box would limit the saw strokes to a few centimetres.

A reciprocating "Sabre" saw might do the job but is hard to control.

Twist drill bits seemed to work the best of my tools on the material, and I have a good selection of sizes.  How close to a square opening can one create by drilling a small number of round holes?  It turns out that I can come fairly close.  There are two ways the problem can be posed, the largest opening bounded within the square or the smallest opening just larger than the square.  I am interested in the latter; the other solution can be had by scaling.

The idea is to drill holes on the diagonals near the 4 corners such that the corner touches the rim of the hole.  One then drills a hole in the center which is enough larger than the square so that its points of contact with the square are points of contact with the 4 smaller holes.  If the diameter of the 4 corner holes is reduced, then the diameter of the center hole must be increased in order for it to intersect the smaller holes and the square.

Let L be the length of one side of the square, R be the radius of the center hole, and r be the radius of the corner holes.  The center of each corner hole is L/sqrt(2)-r from the center of the square so that the rim lies on the corner point.  The furthest that the corner holes exceed the desired square is

  r+(L/sqrt(2)-r)/sqrt(2)-L/2 = r-r/sqrt(2) = r*(1-sqrt(1/2))

The furthest that the large center hole exceeds the desired square is R-L/2.  Desired is:

  R-L/2 = r*(1-sqrt(1/2))

The other constraint is that the rim of the center hole and the rim of the corner hole intersect on the side of the square.  The distance from the middle of the side to the intersection of the side with the large hole is x

  R^2 = (L/2)^2 + x^2

The distance from the corner to the intersection is r*sqrt(2).  So:

  L/2 = r*sqrt(2) + x

R, r, and x scale with L.  Let L = 1.

  R^2 = 1/4 + x^2

  1/2 = r*sqrt(2) + x

  R-1/2 = r*(1-sqrt(1/2))

Solving this system:

  r = 1/(4+sqrt(2))

  r = L * 184.69903125906464e-3

  R = L * 554.097093777194e-3

The diameters when L = 1.125 are:

  d = 415.57282033289544e-3

  D = 1.2467184609986863

I made the holes with a 3/8 inch twist drill and a 1.25 inch hole saw.  A 7/16 bit  would have been closer in size.

The flange on the outlet covers the non-square parts of opening.