University of Idaho

 

Community Planning and Design

Home Resources Contact Us About the Faculty

 

     
 

Main Menu

Home

Syllabus

Calendar

Project 1

Lessons
Planning

Urban Design

Form Based Codes

TOD
Open Space
Housing

Density
Culture
Micorclimates 1
Microclimates 2
Ecology
Commerce
Transportation
Space Development

 

Collaboration

Your faculty value the opportunity to collaborate and believe this kind of interdisciplinary effort is necessary for the best community planning and design solutions. Therefore, they ask that you adopt the goal of forming effective interdisciplinary partnerships in this course and in your professional career.

Demonstrate Teamswork!

 

Welcome

Creating Microclimates - Control of Wind

 
Study Questions are provided to help you prepare for your examination over the material presented below.
Introduction
Scientists have studied the interaction of temperature, humidity, wind and solar radiation on human comfort.  Comfort zone charts are available from several authors.  They are of conceptual value for site planning and design but limited in their usefulness because the ability to control some variables outdoors is much more difficult than in a climate controlled building.  For example, changing the humidity outdoors is very difficult because of the effectiveness of atmospheric mixing.  Other elements such as solar radiation are easier to control.
Wind as a Cooling Agent
Interior air flow is generally limited to about 160 feet per minute to avoid drafts and blowing papers but airflow for cooling especially outdoors should be at a higher rate especially as temperature rises.  Rates of 400 feet per minute are comfortable at a temperature of 86 degrees Fahrenheit. 

A problem with wind as a cooling agent is that it is usually the same temperature as still air.  The cooling effect is through evaporation primarily.  This benefit is limited if humidity is high.

At temperatures below 91.5 increased air velocity reduces the sensation of heat on the body.  At temperature between 91.5 and 98.5, there is little impact while at temperatures above 98.5 increased airflow actually increases the sensation of heat.

Understanding Airflow
windBarriers.jpg (49239 bytes)
When the boundary layer of air increases its speed as it flows around an object, a low pressure area is created between the boundary air and the surface of the element. The low pressure area will either cause the element to move, or attempt to pull the boundary layer of air back to its original position. When a barrier is introduced the into an air stream, the wind responds by flowing around or through the barrier, eventually returning to its original flow pattern. The greater the wind velocity, the greater the pressure differential on the leeward side of the barrier, and the quicker the wind returns to its original flow pattern.

Flow Characteristics

laminar.jpg (9746 bytes)

There are three types of air flow 1)  laminar; 2)turbulent and; 3) separated.

Laminar airflow is the common and predictable flowing of layers on top of one another.

Air masses moving in the same direction but in a random pattern are turbulent. Velocity of flow in this type of wind is unpredictable and, therefore, difficult to control. Turbulence can be introduced into laminar airflow by the roughness of the surface over which the air flows. Surfaces of buildings will always produce turbulent airflow.

boundaryAry.jpg (40122 bytes) Separate airflow is defined as layers of air varying in momentum. A separation between layers may cause turbulence. Separation occurs when air flows around or over sharp corners. When air is moving, it exerts a pressure against any surface.  Force tends to inhibit its flow. Force and turbulence are produced between layers of flowing air whenever a blocking element is introduced. If the element is streamlined the air usually flows around with an speed increase in the boundary layer of air next to it. The boundary layer generally follows the contours of the streamlined shape and a minimum of turbulence occurs between layers. If the element is "bluff" (not streamlined) the boundary layer cannot follow the contours of the element. A separation of airflow occurs and the force between the boundary layer and the other air layers becomes greater. Turbulence is likely to occur around a bluff body.

 

landformWindSpeed1.jpg (22367 bytes)

landformWindSpeed2.jpg (19952 bytes)

The weather on the lee slope of a hill is usually quieter than on the weather slope. However, this may be reversed if the weather slope is steeper than the lee slope. When the boundary layer of air is compressed as it passes over a ridge, wind speed is usually 20 percent greater on the top of the ridge than on the slopes.

 

barrierAndWindBehavior.jpg (38765 bytes) When any vertical barrier is introduced into an airflow, a pressure eddy is formed immediately in front of the barrier. A suction eddy is created immediately leeward of the barrier. Beyond the barrier a turbulent wake is created. 

Wind is controlled for a distance of from 2 to 5 times the height of the barrier in front of the wind obstruction, and from 10 to 15 times the height, leeward of such a barrier.

barrierAndWindSpeed.jpg (23716 bytes) windbreakShrubs.jpg (63099 bytes)Deflection of wind over trees or shrubs is another method of wind control. Plants of varying heights, widths, species, and composition, planted either individually or in rows, have varying degrees of the effect on wind deflection.  In this image you can see that deciduous shrub hedges have considerable density even without leaves.  This density of twigs varies with species.

For example, coniferous evergreens that branch to the ground are generally the most effective year round plants for wind control. Deciduous shrubs and trees, when in leaf, are most effective in summer. Wind velocity is 15 to 25 percent of the open field velocity directly leeward of a dense  evergreen screen planting (spruce trees) while a barrier of deciduous Lombardi Poplar trees reduce leeward wind velocity to 60 percent of open field velocity. Wind velocity is cut from 12 to 3 mph for a distance about 40 feet leeward of a 20' tall Austrian pine wind break.

Deflection of wind, passing under or through plants, is a method of control. There are instances where it may be desirable to speed up wind. The speed of air directly beneath a tree is measurably increased over open field speeds.

Wind Reduction
windScreeb.jpg (43287 bytes) Designer can provide protection by placing use areas in the lee of a building or designing windbreaks from berms, plantings and architectural elements.

Plants and the Control of Wind
channelingBreeze.jpg (26376 bytes)

airFlowNoVeg.jpg (20642 bytes)

 

Plants control wind by obstruction, guidance, deflections, and filtration.  The plants chosen as well as their arrangement determine the effectiveness of the wind control scheme.

As with all other barriers, obstruction by trees reduces wind speed by increasing the resistance to wind flow.  Coniferous and deciduous trees and shrubs, used individually or a combination, affect air movement.

airFlowLowHedge.jpg (38871 bytes) The three sets of sections here, illustrate the impact of placing hedges, of various heights, differing distances from the windward wall of a building with window openings (see th plan of building above).  

 

airFlowMedHedge.jpg (30524 bytes)

Both the low and medium-height shrubs at the building cause airflow to drop inside the building and generate secondary eddies.  These conditions provide positive natural ventilation and air mixing characteristics inside buildings.  

juniperHedge383.jpg (145880 bytes) In addition, when hedges are placed some distance from the windward side of the building, lee eddies of relatively calm air are created.  These areas are useful outdoor spaces in the spring, fall and winter wind protection from wind permits outdoor activity.  In this image a evergreen hedge is located on a rock faced berm to protect an outdoor space from wind.

airFlowHighHedge.jpg (33666 bytes) Except when placed some distance from the building, tall hedges direct most of the airflow over the building rather than through it.
airFlowShrubs.jpg (52253 bytes)

This plan, and four sections illustrate the impact of planting shrubs and spaced five feet on center at various distances from a building.  

airFlowHedgeTreeCombo.jpg (68174 bytes) The plans and sections provided here, illustrate the impacts of combining tree and hedge plantings on the windward side of the building.  Notice that in the plan and top section that the direction of airflow is reversed in one section of the building as a result of the location of plants.
airFlowHedgeTreeSect.jpg (36221 bytes) This plan view and sections A, B and C illustrate airflow patterns of the combination of a hedge and tree.
airFlowHedgeBldgCombo.jpg (39583 bytes) These plans view diagrams illustrate airflow conditions and building hedge combinations oriented 90 degrees to the wind.
airFlowTreeBldgCombo.jpg (37339 bytes)

 

This set of plan view diagrams illustrates airflow patterns under varying tree and tree hedge combinations oriented 90 degrees to the wind.

Windbreak Protection  windbreak1.jpg (41670 bytes)
Microclimate creation is possible by controlling wind at a larger scale that illustrated in the design interventions above.  Large scale windbreaks (shelterbelts) with multiple rows of trees and shrubs can significantly reduce wind over large distances. Data is available on the performance of various types of windbreaks due to extensive application to reduce soil erosion and snow drifts. 

The image at left illustrates the dramatic aesthetic impact of large windbreaks.  Color, texture, distinctive shape and monumental scale offer the designer the opportunity to structure a large landscape.

velWindbreaks.jpg (83223 bytes)

Wind breaks placed perpendicular to the prevailing wind may reduce velocity is by 50 percent for a distance of 10 to 20 times the tree height down wind. The decree of protection and wind reduction is dependent on the height, width, and permeability of the plants used.  

These sections show the zone of wind speed reduction.  Notice that a thirty foot tall windbreak reduces a 22 mile per hour wind to just 6 mph on the lee side of a dense windbreak.

 

windVelChart.jpg (49774 bytes)

barrierHeights.jpg (50709 bytes)

Wind speed is also affected on the windward side of a wind break. For example, the wind speed is reduced for a distance of 100 yards on the windward side of a 30 ft. high wind break. It is reduced for a distance of 300 yards on the lee side of the windbreak.  The chart at top left shows the performance of three windbreaks composed of different tree species.  The lower chart is a plan view diagram showing the percentage of wind speed reduction related to distance from the windbreak, which is the dark shape at 0.

windbreak2.jpg (29596 bytes)

windbreakDensityProtection.jpg (61869 bytes)

These images address the permeability of the windbreak.  The trees in the upper image are deciduous cottonwoods arranged in a single row.  They are more permeable to wind than an evergreen windrow especially near the ground.  In the chart at bottom left, they would be represented by the "medium density above, open below" line.  The trees in the image are 30' tall.  According to the chart what is the percentage of original wind velocity 300 feet from the windbreak?

Generally the taller the trees in the windbreak, the more rows of trees required for good protection.  An increase in tree height causes windbreaks to become more open, especially near the ground.  Instead of reducing the wind, avenues of trees open at the bottom increase wind speed, as the air stream is forced beneath the tree canopy and between the trunks.

forestBlock.jpg (75883 bytes)

mapleForestWind.jpg (75328 bytes)

The zone of wind reduction on the lee side and windward side of a barrier is largely dependent on the height of the barrier.  The width of the barrier is important only as it relates to the permeability of the windbreak.  More rows of trees and shrubs create a more impermeable barrier.  In addition, wide windbreaks create a particular microclimate within.

The chart at left shows wind movement inside a deciduous forest. Significant reductions in wind were measured both when the trees had leaves and when deciduous. Shelter belt wind protection reduces evaporation at ground level, increases relative humidity, lowers the temperature in summer and reduces heat loss in winter, and reduces blowing dust and drifting snow. 

heightVelocity.jpg (86536 bytes)

As you might expect from our previous consideration of air flow over streamlined and bluff barriers, air speed increases with height.  This could have a detrimental effect on wind speed for rooftop outdoor spaces.
windbreakShape.jpg (33019 bytes) windbreakpine.jpg (58886 bytes)The shape of the windrow in section influence the effectiveness of the windbreak.  This young windbreak is made of two rows of evergreen trees presenting a dense barrier to wind.

wideWindbreaks.jpg (205684 bytes) Detailed studies of the number of rows of trees and the cross-sectional shape show that great width does not improve lee side protection.
windDownhill.jpg (32064 bytes)

These sections illustrate the principle that cool air flows downhill at night.  Dense plantings can capture this cold air to produce a microclimate effect.  This characteristic might be pleasant in summer but disagreeable in winter.
Wind and Snow
snowtrap.jpg (130620 bytes) windbreakEvergreen.jpg (91937 bytes)Windbreaks can be arranged to trap snow so that snow does not drift onto road ways or against buildings.

Airflow Patterns Through Building Openings
airflow1.gif (9189 bytes)

 

As wind approaches the face of a building the airflow is slowed, creating positive pressure and a cushion of air on the building's windward face. This cushion of air, in turn, diverts the wind toward the building sides. Airflow as it passes along the sidewalls separates from building wall surfaces and, coupled with high-speed airflow, creates suction (negative pressure) along these wall surfaces. On the building leeward side a big slow-moving eddy is created. Suction on the leeward side of the building is less than on the sidewalls (Figure 1).

If windows are placed in both windward and leeward faces, the building would be cross ventilated and eddies will develop against the main airflow direction (Figure 2).

airflow2.gif (7940 bytes)

 Ventilation can be enhanced by placing windows in sidewalls due to the increased suction at this location; also, greater air recirculation within the building will occur due to air inertia (Figure 3). Winds often shift direction, and for oblique winds, ventilation is best for rooms with windows on three adjacent walls (Figure 4) than on two opposite walls (Figure 5). 

airflow3.gif (7392 bytes)

However, if wind is from the one windowless side, then ventilation is poor, since all openings are in suction (Figure 6).

airflow4.gif (6361 bytes) If the building configuration only allows for windows in one wall, then negligible ventilation will occur with the use of a single window, because there is not a distinct inlet and outlet. Ventilation can be improved slightly with two widely spaced windows. Airflow can be enhanced in these situations by creating positive and negative pressure zones by use of architectural features such as wing walls (Figure 7). Care must be exercised in developing these features to avoid counteracting the natural airflow, thereby weakening ventilation (Figure 8).

airJetsDiag.gif (6040 bytes)

Air Jets

As airflow passes through a well-ventilated room, it forms an "air jet." If the windows are centered in a room, it forms a free jet (Figure 9). If, however, the openings are near the room walls, ceiling, or floor, the air stream attaches itself to the surface, forming a wall jet (Figure 10). Since heat removal from building surfaces is enhanced with increased airflow, the formation of wall jets is important in effecting rapid structure cooling. To improve the overall airflow within a room, offsetting the inlet and outlet will promote greater mixing of room air (Figure 11).

Trees and Air Filtration

airFiltrationTrees1.jpg (44115 bytes)

airFiltrationTrees2.jpg (25854 bytes)

Large masses of plants physically and chemically filter and deodorize the air, reducing air pollution. (See top graphic in "Air Filtration" drawing.) Particulate matter trapped on the leaves is washed to the ground during rainfall. Gaseous pollutants are assimilated by the leaves. (See bottom graphic on "Air Filtration" drawing.) Fragrant plants can mechanically mask fumes and odors. As well, these pollutants are chemically metabolized in the photosynthetic process.

 

Past Projects of This Studio


We have contributed to the positive planning and design efforts of many communities during the ten years that this combined studio has been doing outreach work. We have worked in these Idaho communities: Orofino, Riggins, Grangeville, Harrison, Hayden, Sandpoint, Lewiston, Star, Rupert, New Meadows, McCall; these Washington communities - Clarkston ; and these Montana communities - Seeley Lake.

 

Copyright Gary Austin. All Rights Reserved. Dreamweaver-Templates.org

oscommerce and ecommerce templates store