Air Work - Microclimate & Flow Studies

Directly observed convection currents seem to often behave more like schools of fish than one thing pressing on another.   What that forces one to consider, at least, is whether we can describe the animated systems of nature using our rules for how inanimate forces work.  We need to leave room in our thinking for a great variety of locally animated behaviors, given the diversity of things we observe having complex organization and dynamic change.   If not apparently displaying external pushes and pulls, they'd implicitly need to be behaving according to their own internal organization, that would naturally be hidden from view within the systems exhibiting the behavior.   Their animation would implicitly be coming from their own emergent organization, as they exploit the gradients giving them energy.   2011 JLH

1999 This is basic empirical physics concerning passive (uncontrolled) fluid flow and the evolving flow structures that can develop in ordinary circumstances, largely done in 1977-79 .    The basic finding it that there are all manner of common flow patterns  you would simply never find in a fluid mechanics lab, that is...., unless you looked around in the doorways, near the floor, walls and ceilings or for the currents that travel in-between.....  P.F. Henshaw

Call them micro weather systems perhaps, but they also raise some macro theoretical problems....

• Air in a Box - by far my most popular page, very curious easy experiments
• House Stories - the 24 hour chart recordings of the more interesting building micro climate studies
• Natural Air Current Networks  - environmental studies
• R & D for a Controlled Convective Wall Jet - patents & experimental devices
• Elementary Structures of Convection - things to observe using a Plexiglas box
• The Development an Individual Air Current - detailed conceptual sketches

Introduction

1999 My introduction to microclimate research came from a curiosity about the wonderful environmental design characters of Brooklyn's Prospect Park, one of the jewels of Frederick Law Olmsted. and Calvert Vaux's invention of American landscape design.   It gave me a subject for a model building and analysis project while studying architecture at the Univ. of Pa. in Philadelphia.  A unique microclimate exists at a forest edge, where the mix of qualities of both exposure and seclusion invite a certain group of plants and animals, and a certain group of human activities.   I also developed a course of study for architects.  There are two tasks in microclimate study, understanding the climatic characters of a place, charting its energy transfer mechanisms and calculating its energy budgets.

One may sit on the porch on a summer evening, unaware of the passing of time until the late stillness of dusk is broken by the first night breeze. This is one of the more reliable microclimate markers of the procession of the day in passive environments.   It may pass unperceived.    It is also as often responded to unaware.  It serves as a reminder of time passing after a period of time suspension, occurring at the end of the evening still.  Markers and characters such as these serve as links between the physical mechanisms of a day, the character of a place, and the quality of personal experience.

In learning how to design passive solar homes it seemed quite natural to consider them as microclimates, made unique because they are enclosed.   I carefully studied a number of both solar and non-solar homes to see where the energy collected and how it traveled.  In the process I stumbling upon a wide field of  undocumented kinds of air currents, some that might materially effect the enjoyment of a home or how it could be heated or cooled.

The homes most carefully studied were in Denver and New Mexico, using a chart recorder to trace temperatures and air velocity in 24 locations.  One of the first observations was that the movement of energy in enclosed spaces is highly eventful, exhibiting complex successions of temporarily stable patterns. An introduction to the detailed study of air current networks follows.  Sometimes a great deal of heat would be found traveling along very narrow pathways, seemingly contrary to the physical laws of convection.   A way to reproduce one of these provided the basis for an experimental device for unusually high efficiency heat transfer.

Air Current Network Research

A description of the behaviors found in two of the building climate studies, Air Current Networks, was originally presented at the 1978 International Solar Energy Society meeting and was published in RAIN magazine of Portland OR, and re written in 1984. It describes some of the air current phenomena which seemed to be responsible for two solar homes working better than expected.

One of the starting points for critical thinking on the subject is to distinguish between the pathways that develop and the currents that travel through them. Many kinds of convective pathways have unusually distinct boundaries, often exhibiting thresholds of stability. These include things like the chimney that develops in a quiet air mass around the plume of warm air continually rising above any stationary person. Initially there is no regular pathway for the rising current, and then some particular pathway develops and stabilizes. It may reach the ceiling, and may not, but the pathway and the current moving through it co-evolve. It may join with paths for rising currents from other sources or not. If the person moves, the pathway that previously developed above them may remain for a while.

One of the rare examples of the kind that is commonly observed can be seen in the patterns of smoke rising from a recently extinguished candle, or other smoldering ash. Sometimes the smoke stream can be seen rising in a straight line, to then suddenly tumble in meandering turbulence, perhaps a foot above the source. If the room is calm the point at which the tumbling develops will rise higher and higher. Then a wobble will develop somewhere lower in the column, and that becomes the point at which the tumbling starts. The length of the fluid chimney, and the streamlined column rising within it, varies. Sometimes the structure grows, becomes unstable and collapses repeatedly. Without the formation of a columnar pathway the stream would billow about immediately from the bottom, another pattern that is frequently observable.

On a much larger scale there are things like cumulus clouds. If a chimney in the air below the cloud hadn't developed, providing a central pathway for warm air generated over a large area of the ground, there wouldn't be these enormous thunderheads billowing in the sky. It's the invisible chimney below the cloud and its central column of rising air that produces them. What we usually see of it is only the condensate at the top, where the virtual chimney structure ends and the rising column of air flattens out. There are also more unusual convection chimney phenomena, like cumulus cloud down drafts, where massive invisible currents of cold air plummet toward the ground. These have sharp invisible boundaries that aircraft may cross without any warning, and often find fatal. There is also weather that occurs in fires. The term "back draft" is probably familiar, referring to explosive changes in the convective pathways. The term "super-heated air" refers to fire that spreads through air passages, jumping multiple floors at a time without setting fire to materials in-between.

The usual explanation for why these behaviors remain difficult to describe and predict from the scientific laws is that evolving systems display a critical sensitivity to initial conditions. Both turbulence itself, and the more discretely patterned flows of both large and small scale weather systems, remain largely intractable to mathematical modeling because they concern spontaneously evolving unique individual events.

What one might call my 'moment of discovery' is worth retelling. It was not when I first recognized the unusual woven sheet current sliding up an adobe wall that I have called 'structured convection'. It was when I was watching two very gentle convection currents (moving at most 4" per second) which were making alternate use of the same upward pathway. They were gathering from sunlit patches on a floor and both rising near the corner of the room. One would flow briefly (~1 liter volume), until it depleted its local reserves and then the a flow from the other source would take over, back and forth using the same pathway in fairly regular alternation. Individual air currents frequently cross pathways and gently 'gurgle' through openings, somewhat like water pouring from a jug.

There was a time when the right hand current, for some reason, missed its turn at the opening and a second pulse in a row came in from the left. Then a stronger than normal pulse developed from the right hand current, but blocked by the momentum of the flow from the left, it broke out in a new direction as if to travel upward on a new path. Then it stalled a foot or so above the floor. Just as the smoke trace that enabled me to visualize this movement was fading beyond recognition I saw the wayward pulse dip back toward its original normal pathway, apparently filling the gap in the larger system of flows its absence had left behind. This was self-correcting systematized flow! Having found what to look for I then began to find it in many places.

Devices Using Structured Convection ..... (a bounded convective wall jet)

1999 Perhaps it should have been difficult to restrict otherwise turbulent wall convection to a thin boundary layer stream 1/8th as thick and 8 times as fast as expected for normal turbulent boundary layer flow.  It was actually fairly easy.   You do it by clearing the pathway for ejections to follow, drawing air through a narrow slit along the streamwise edge of the surface,  not by somehow restricting the turbulent  motion.   It's been the documentation that has been difficult.

I wholly miscalculated the level of disbelief that the claim would generate.   Air flow patterns are quite hard to document without specialized laboratory equipment and cooperating engineering expertise.   All I have for proof are thermocouple chart recordings which display otherwise impossible behavior.   What I was able to learn  by way of painstaking observation with smoke tracers taught me much more, but is hard to document.

I've called it "structured convection", a name of convenience.  The advantage gained is that the thermally charged portion of the working fluid can be removed from the reservoir without the turbulent mixing.   Neither the reservoir nor  the current become thermally polluted (by mixing with each other).   It  greatly improves the heat transfer efficiency.   This might have diverse useful applications, including things like controlled chemical deposition as well as heating and cooling applications.

Only some basic facts are known about the detailed structure of the bounded convection layer.   Rhythmic patterns that allow working fluid to pass through the wall layer without interrupting it can be directly observed, along with its very narrow thickness and high velocity.    What causes it is drawing fluid at the right rate from a slit along the streamward edge of a heat transfer surface.   Given the right conditions a layer of organized turbulence propagates over the whole surface.

For those who carefully think about it, reasonably good proof  is provided by an anomaly found in thermal chart recordings of the solar collector test cell.   When the test cell was operating as a passive convection loop, at around 2:00 PM, the temperature of the chamber shot up 40 degrees F.   The question is why the temperature went up so rapidly at the point where the solar gain was  in decline.   Something had to occur to suddenly reduce the efficiency of heat transfer to storage.   Only a sudden change in the convective pattern is available to explain it.

As seen in the chart (fluew1.gif click to enlarge), the shape of the temperature spike,  in the middle of a bright and clear afternoon, is entirely inconsistent with any change of energy input.   A second clue is that at the same time as the temperature on the absorber side shot up, the temperature of the heat storage mass began to decline.  The rate of heat transfer between the two sides of the cell evidently collapsed, leaving the incoming energy to build up in the collector chamber.   What explains it is that as the sun began to wane the heat storage mass gradually approached saturation.  At a point where the draw along the top edge fell below a critical level, the currents formerly rising in tight formation near the absorber surface became disorganized and spilled out into the absorber chamber, causing its temperature to suddenly rise.

In the test cell there was a fan available to drive the flow, and the record shows that when the fan was then turned on the temperatures of the cell returned to the previous levels, causing the temperature of the storage to begin rising again.   This verifies that the collapse in heat transfer rate was due to a decline in the total air flow, and that the more efficient form of air flow was restored when themass flow rate was restored. (for a longer discussion see stcevid.pdf )

The working test cell Original indicators of behavior and efficiency

Test Cell Thermal anomaly, analysis of sudden rise in temp as incoming energy waned

Technical Abstract:

Otherwise turbulent and disorganized convection currents in a fluid medium of larger dimension become confined in an organized accretion flow structure hugging a heat transfer surface as a consequence of appropriately aspirating the natural convective flow.. In the conditions investigated the resulting structured ‘sheet current’ had nominally eight times the velocity and 1/8 the thickness of the theoretically normal turbulent boundary layer and entirely inhibited the normal convective movement of surface boundary currents into the surrounding medium. Applications include reducing the size and raising the efficiency of fluid heat transfer devices and heat assisted control of mass fluid flow pathways.

Surprising Elementary Structures

In exploring how the paths of air flow develop, a number of small test cells were made.   One of the more successful was a simple 4" square by 3/4" deep hollow plexiglas box  (gluing together pieces of 1/8" plex).  Openings were provided on the top or bottom as needed and smoke from a cigarette or incense stick was introduced.   The image at the left shows smoke filling the bottom half of the container, and then a pulse of clear air rising through the smoke pool from a momentary opening at the bottom.

These and a other examples are displayed and discussed in more detail.   The series of sketches was once called 'Mostly Nonsense' because of the strange conundrums that they pose.   In the first example one might ask why smoke, that rises in open air, would lie in the bottom of a container at room temperature.   It just doesn't seem to make sense...

The Development an Individual Air Current
There are two primary  kinds of natural convective motion.  One is motion directly due to the change of density due to temperature, and the pressures of buoyancy that result.  The other, is the following development of organized patterns of flow and counter-flow.  These include intricately layered shapes and small scale 'weather' systems of various kinds.

The most common and most compact form of fluid flow system is a 'vortex ring',  something like a smoke ring.  Vortex rings are sometimes complex, but orderly systems of coiling laminar flows, one of the main constituents of the highly complex flows of 'turbulence'.   It is usually thought that well ordered air flow is laminar (layers sliding by each other smoothly).  With large differences in speed between adjacent layers, laminar flow becomes unstable and systems of interaction develop, filling the region with vortex rings and other complex twisting motions.

Isolated individual vortex rings can develop spontaneously at static inversion instabilities and elsewhere.   There they develop as a mechanism of creating a path through which warm air can rise, acting as air 'tunneling bodies', a design for opening a temporary channel of flow through an otherwise static barrier.  A simple diagram of one,  is depicted in 6 stages of development and passing.

Please note, the shapes shown do not closely match what you will most normally observe, but are idealized to clearly show the main parts of the succession that you should look for.   (click figures to enlarge - From "An Unhidden Pattern of Events" 1979)

• first disturbance in static, but unstable inversion
•  development of vortex ring propagation
•  travel and remains

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