|Image from NASA's Juno spacecraft 2018-04-01|
Governance by those who do the work.
Wednesday, August 15, 2018
Friday, July 27, 2018
|Bubbles and parted paints|
While the formation of bubbles could be due to non-Newtonian fluid behavior, it is worth examining the conditions assuming a Newtonian fluid. A 25 mm diameter dowel submerged 12.5 mm will behave more like half of a 25 mm diameter sphere than a cylinder. If the kinematic viscosity (ν=0.001 m2/s) of the liquid is 1000 times that of water, then the Reynolds number is about 5, far less than the 90 needed to spawn vortexes. Re is inversely proportional to viscosity; reducing the kinematic viscosity by a factor of 10 raises Re to 50.
A half sphere of diameter d has buoyant-pressure (restoring force divided by cross-section area) of about 81 N/m2. Surface tension pressure (restoring force divided by cross-section area) of water is roughly 3.7 N/m2.
Drag is the force on the object moving through the tank fluid. There must be an equal and opposite net force on the liquid. Drag D for a sphere is the product of the friction coefficient CD, frontal area (π/4*d2), and dynamic head V2*ρ/2 (for water ρ=997). That force divided by the frontal area of the object is a pressure (suction actually).
A bubble will be formed if this suction behind the moving stylus is larger than the sum of the restoring forces at the liquid surface.
For ν=1000 mm2/s (1000 times that of water) the suction behind our 25 mm diameter dowel is 88 N/m2, which exceeds the restoring pressures 81 N/m2 and 3.7 N/m2, and bubbles can result.
The slower motions and smaller styluses that the St.Johns usually use have Reynolds numbers much smaller than 5. Thus the marblings they create don't evidence inertial effects (versus the mushroom designs of my previous post).
|"Mushroom" flow from straight strokes|
In water, a 5 mm cylinder moving at 2 cm/s would shed vortexes 3 cm apart. A 1 mm diameter stylus moved in a straight path at 5 cm/s would not shed vortexes.
So existing evidence of Karman (shed) vortexes is only likely to be found in marbling produced on a tank filled with water.
Monday, July 9, 2018
This image from the on-line Getty collection is busier, but shows some of the same features.
|Autograph album of Johann Joachim Prack von Asch|
Publication date 1587
In the collection of the Getty Research Institute
"Vortex shedding in Water" from "Harvard Natural Sciences Lecture Demonstrations" shows vortexes being shed from a cylinder at flow speeds in the range of marbling strokes.
My work has focused on laminar and Oseen flows https://arxiv.org/abs/1702.02106 in Newtonian fluids which successfully model most common marbling techniques.
At the lowest Reynolds numbers is Stokes flow, where the passage of the stylus displaces the liquid only temporarily. The next range of Reynolds numbers produces Oseen flow, where viscous forces dominate inertial forces. Straight strokes of finite length result in persistent movement along the stroke and rotation to both sides of the stroke. As the inertial forces grow relative to viscous forces, instabilities such as vortex shedding appear (Re ≥ 90). Much higher Reynolds numbers (≥ 40000) can produce turbulence.
To answer vortex question and to better quantify the fluid dynamics parameters of marbling, Dan and Regina St.John, the Chena River Marblers, recently hosted a session where we performed experiments using their equipment and expertise.
The idea was to increase the Reynolds number of marbling strokes by increasing the stylus size and speed until instabilities such as vortexes appeared. We increased the stylus size to 25 mm, but instabilities did not appear. We increased the speed to the point that it created a tear and bubbles in the paints, but no vortexes appeared. The tear indicates that the assumption that the fluid is Newtonian may not be valid; and the properties of carageenan used to make the "sizing" in the tank are complicated. Note: my later post finds that Newtonian fluids can produce these behaviors.
Reynolds number being the characteristic length times the velocity divided by the kinematic viscosity, the only other thing to try was reducing the viscosity. Diluting the sizing by half with water resulted in a sea change. Instead of fluid motion stopping when the stylus stopped, it would glide for as long as 5 seconds before coming to rest, showing that inertia was in play. Stylus strokes at speeds around 25.cm/s (which is fast for marbling) created the mushroom shapes pictured. Although the St.Johns were able to find an example of this shape in one of their books, it is not a common marbling motif. Looking back at the photo of the 16th century marbling, mushrooms are present.
Are these mushrooms due to flow instabilities? No. The mushrooms appear where the stylus was stopped. In vortex shedding, the vortexes are shed to alternating sides of the ongoing stroke. Even for a fast stroke, the train behind the stylus was smooth and without wiggles.
We know from the video of vortex shedding that it happens in water. Viscosity near that of water may be required in order to see it in marbling.
There is more of interest here. The mushrooms in our marbling have smaller mushrooms inside of them. In the photograph, I have outlined mushrooms at 3 different scales. The mushroom in the smallest box ls less obvious than the others; perhaps because the bands of color comprising it are larger relative to its size.
Pure Oseen flow is reversible; reversing the flow at the origin returns the system to its original state. With its sub-mushrooms, the mushroom flow does not look reversible. Could this mushroom flow be a regime which transfers energy from larger to smaller scales, yet doesn't exhibit instability?
Sunday, July 8, 2018
It's been 7 months since my last post. The process of downsizing to a smaller home last winter put my projects on hold.
Although fractals have stubbornly refused to appear in my investigation of self-similar surface roughness, they have shown up at my day job as a data scientist at Digilant.
Investigating the possibility of combining weekly counts of unique user IDs, I discovered that the L^p-norm does so with surprisingly good accuracy on digital advertising datasets. The L^p-norm implies a scaling law. My son Martin (who also works at Digilant) noticed that the scaling law exponent is a fractal dimension. The L^p-norm and scaling law are implied by the Pareto distribution of lifetimes in a population. This link between the L^p-norm and fractal dimension should have application beyond counting populations.
We wrote a paper about these results at https://arxiv.org/abs/1806.06772
Friday, December 8, 2017
Last week I spoke on The Physics of Marbling at the Form in art, toys and games workshop at the Isaac Newton Institute for Mathematical Sciences in the University of Cambridge!
The four day workshop had many fascinating presentations on a wide variety of topics. Videos for most of the talks are available.
|I am the one in the bright blue jacket.|
Sunday, August 6, 2017
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
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.
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