Wind infiltration

Wind infiltration Infiltration of outdoor air into a building caused by the pressure difference across the faces of the building. 

Outdoor air is generally less polluted than the system return air. However, problems with reentry of previously exhausted air occur as a result of improperly located exhaust and intake vents or periodic changes in wind conditions. Other outdoor contamination problems include contaminants from other industrial sources, power plants, motor vehicle exhaust, and dust, asphalt vapors, and solvents from construction or renovation. Also, heat gains and losses through the building envelope due to heat conduction through exterior walls, floor, and roof, and due to solar radiation and infiltration, can be attributed to effects from external sources. 

This section will describe general features of airflow patterns and then present information on the dimensions and locations of recirculating (stagnant) zones around the building envelope, which determine wind pressures and contaminant dilution. This knowledge allows one to select the locations of stacks and air intakes and to calculate infiltration and natural ventilation rates. 

One of the effects of airflow or wind around buildings is the exertion of wind pressure forces on rhe surface of the building, which contributes to natural ventilation of the building and infiltration of outside air into the building. As discussed above, pressures tend to be positive (into rhe building) on upwind surfaces and negative (suction) on lateral, downwind, and roof surfaces. 

A detailed method of determining pressure coefficients is to perform experiments with a wind tunnel facility. Cochran and Cermak compared wind tunnel pressure coefficient measurements with field measures on a test building and found excellent results, with the exception of small areas beneath the vortices near the upwind roof corner for winds approaching at 45 . For infiltration and natural ventilation designs, wind tunnel results should be sufficiently accurate. 

The airflow rate infiltrating and exfiltrating through each air leakage pass, Q , due to the combined effect of wind, stack, and mechanical ventilation system perfotmance can be calculated ftom the mass balance equation... 

Air infiltration The uncontrolled air interchange through structural imperfections and other openings into a space, due to natural convection, rising currents, or wind forces over a building. 

Infiltration The leakage of air through the imperfections in a building structure, due to thermal or wind forces. 

Chemical Vapor Infiltration (CVI). Recall from Section 3.4.2 that CVI is primarily nsed to create ceramic matrix composites, CMCs. Fabrication of CMCs by CVI involves a sequence of steps, the first of which is to prepare a preform of the desired shape and fiber architecture. This is commonly accomplished by layup onto a shaped form of layers from multifilament fibers using some of the techniques previously described, such as filament winding. 

The cycle is more complex when precipitation falls on land. As with the direct route, the cycle begins with ocean water evaporating into the atmo-sph ere. Instead of forming clouds over the water, however, the moist air is blown by winds until it is over land. Now there are four possibilities for what happens to the water once it precipitates. It may (1) evaporate from the land back into the atmosphere, (2) infiltrate the ground, (3) become part of a snow pack or glacier, or (4) drain to a river and then back to the ocean. 

Figures 6.7 and 6.8 are taken from Jann (1989) and were prepared to assess quickly the impact of various outside challenges on safe time by selecting the appropriate infiltration rate or air changes per hour (ACH) for the haven, and safe inside concentration. It must be noted that wind speed has not been accounted for in these curves. Higher wind speeds will cause high ACH for most buildings, thereby reducing the safe time for the haven.

The main benefits from leaving crop residues on the surface are in the reduction of wind and water erosion their effects on evaporation are usually minor. When rain falls at comparatively short intervals, mulches of plant residues retard loss by evaporation for a time, but after the surface moisture has disappeared the stubble-mulched soils lose moisture about as fast as a bare soil. This is true chiefly because there is little movement of water by capillarity from the deeper horizons. The extra retention of surface moisture, even if temporary, is often beneficial to seed germination and also to increased infiltration and downward movement of rainfall. 

Bondy (1968) reported that some farmers in central Kansas are successfully growing continuous wheat, grain sorghum and soybeans according to the stubble-mulch system even without the use of herbicides. The main purpose is to prevent wind and water erosion, but better infiltration also leads to more efficient use of the rainfall. 

Some of the minimum tillage methods that are becoming popular in the United States are highly effective in erosion control. For row crops that are suited to this type of culture, the soil is disturbed very little and the crop residues remain in place. This system favors good aggregation, high infiltration, minimum runoff, and little loss via winds. 

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