Permaculture Designers Manual
CHAPTER 4 – PATTERN UNDERSTANDING
Section 4.10 –
Flow Over Landscape and Objects in Permaculture
The simple involuted mushroom, called an “Overbeck Jet” by D’Arcy Thompson (1942), is also shown in its “apple core” model form in Figure 4.13.
While we can produce these patterns by jetting smoke, fluid, gases or oils into other media, they occur as a part of the natural streaming of fluids and gases past fixed objects such as bluff bodies (e.g. posts) in streams, islands in tides, and trees in wind.
Jet streams at altitude can generate such vortices by pushing into different air masses, as can muddy water entering the sea.
Whirlpools or VORTICES are shed alternately from a fixed bluff body located in flow, each side generating its own vortex, each with a different rotation. Beautiful and complex forms are thus generated (Figure 4.14) and these are the basis of the work at the Virbela Institute on flow forms.
The sets of vortices shed or generated downstream from fixed bodies in now are called Von Karman trails.
Tile trails are stable at the 1:3.6 ratio shown in Figure 4.14.
In many streams, and on foreshores, the day-beds, silts and underlying rock may develop such patterns, and posts fixed in streams commonly produce them in water.
Trees and windbreaks produce similar effects in wind, as do waves at sea.
In wind, they are called EKMAN SPIRALS, and in air the spiral lift effect compresses air streamlines to a height 20-40 times the height of the tree or fence fixed in the air flow.
It is obvious that the stable spirals of the Von Karman trails will produce successive pulses downstream and this is in fact how we observe most flow phenomenon to behave.
Thus the pulsing of wind, water, and flow in general may rely on the elastic or deformation properties of the medium itself rather than on electro-chemical “timers” as found in organisms.
In nature, there are many fixed impediments to perfect streamlined flow.
It is typical of Von Karman trails generated from a fixed body that the effect persists as 4-5 repeats (Figure 4.14), and then the stream of water gradually resumes stream lined flow.
At higher velocities of stream flow, chaotic turbulence occurs, and at slower velocities, simple streaming persists around objects.
Thus we see that the Von Karman trail is just one form of pattern generated by specific flow conditions. It is nevertheless a common form in nature.
The spiraling of wind over tree lines produces a secondary effect, analogous to the streaming of tides around atolls; the wind changes direction past the obstacle (about 15 degrees). Such effects may occur within media of different densities (temperatures) as when warm high-pressure wind cells ride over colder low-pressure fronts.
The temperature, pressure, and velocity of wind or gas systems are often related:
Low Pressure – High Velocity – Cool Temperature (Expansion).
High Pressure – Low Velocity – Warmer Temperature – (Contraction).
Velocity in gases and fluids is strictly governed by contact with stationary surfaces, so that the velocity is effectively nil very close to static surfaces, increasing as a series of (imaginary) laminar sheet flows above that surface (Figure 4.17).
This is the effect that is observed in viscous flow in small canals or vessels and that governs the shapes and strategies of organisms such as limpets and starfish.
Thus, we see that media in now ran produce pulsers, vortices, and spirals as a result of irregular or obstructing objects or resistant media, and that these phenomena are interconnected.
The relationships between fluid flow, boundary conditions, and the form these impose on organisms is clear from our pattern models and their deflection states.
Life as evolved by its internal and external patterns and flows is very well discussed by Vogel (1981) in a lively and scholarly book entitled Life in Moving Fluids.
Carried in the flow or media, as a thistle down in air, are many events looking for a place to happen.
A “net“, resting surface or detonator is needed for these potential events to express themselves. We can provide many such receptors or triggers in our design systems, catching nutrients in flow and ensuring events for future growth in our system.
Some such nets form starting-places for events, while others are resting or death places for those entities dependent on flow, or stranded out of their nutrient media.
Just as a series of corks floating on the sea have a predictable path to shore perpendicular to the wave fronts, so does matter flow in a wave-tank model.
Similarly, drift-lines form at sea (STREAMLINES in the core model), and as these end on shorelines, they deposit or remove material.
It is along these streamlines that energy acts, by medium of the waves of growth or surface waves in motion. This is how the event and its material expand: streamlines diverge as wave fronts and disseminate into open media, but are strong, concentrated, and visible at constrictions, near origins, or in powerful or refracted flows.
A small restricted orifice, in the time-front or wave-front, acts as a secondary origin. Just as a grub encircling a tree causes it to branch out at that point, so constrictions in the flow of events generate secondary origins.
I once watched this happen with a eucalypt that I planted in 1953 and saw it twin and twin again as swift-moth grubs encircled its stem.
It is now a mighty tree, much branched, but the evidence of these minute constrictive events is still buried in its stem.
As designers, we can impose small constricting events or place fixed objects in flow to produce such specific results.
We can then be the external shapers of patterned events.