shelter belt

Existing models for predicting air infiltration account for three dominant variables, namely envelope leakage characteristics, indoor-outdoor temperature difference and wind speed. Building shape, wind direction and sheltering, also influence the wind induced component of air infiltration. In this report, these variables are examined analytically and experimentally using wind tunnel data and field infiltration measurements. A sensitivity analysis of a power law infiltration relationship reveals that these factors are most significant at small temperature differences.

Reports daily run-of-wind measurements, made for 3 years at distances of 3.5 h and 7 h to leeward of a 7-row shelterbelt about 6m.high. After elimination of variations in wind direction, the monthly values of relative shelter at these positions showed no evidence of an increase with time. The variations in wind direction were eliminated by regressions of monthly values of relative shelter on the monthly percentage frequency of effective winds, i.e. winds from the normal windward side of the shelterbelt.

The wind pressures on a building can be decreased by a shelter hedge. Gives results of wind tunnel tests which show how this shelter effect depends on the distance between hedge and house, and on the wind direction.

Reports measurements of the wake flows behind solid and porous fences, made with a pulsed wire anemometer (PWA) and a hot-wire anemometer (HWA). Discusses results which show the superiority of PWA in correctly measuring the highly turbulent and sometimes re-circulating wake flows. Gives empirical formula for profile of the velocity defect and shear stress perturbations. Concludes that porosity, and not the form of construction of thefence, determines the structure of the wake flow. States that in general it is difficult to say which value of porosity provides the best shelter.

Reports study of the aerodynamics of wind breaks in a boundary layer wind tunnel. Describes flow patterns and shelter effects in the lee of different fences and discusses efficiency of shelters in relation to pedestrian comfort. Gives results downstream in horizontal planes by nets of isocurves showing mean speed and turbulence. Discusses the influence of permeability, shape, size and wake ventilation and suggests new designs: for example two wind breaks in series.

Reviews work done on the physical and biological effects of wind-breaks and shelter-belts, outlining main results. Discusses reduction in mean speed of wind, turbulence produced by shelter-belt, shading and humidity. Outlines some biological consequences of shelter-belts. Gives bibliography containing forty-seven references.

Discusses the need for shelterbelts over farmland and gives expression for drag force exerted by a barrier in terms of air density, wind speed, barrier height and ratio of wind speed in the shelter to that in the open. Describes field study to determine the effect of a shelterbelt on vertical wind profiles. Presents two-dimensional wind reduction patterns in the lea of the shelterbelt. Calculates drag coefficients for the shelterbelt. Concludes that a shelterbelt can be very effectivein a very short period after planting.

States that porosity is the most important single parameter describing shelterbelts but is very difficult to measure or define. Describes a method for categorizing wind breaks in terms of porosity using only measured minimum leeward-wind velocity. Gives theoretical expressions for the flow through a porous shelterbelt. Describes experiment to measure wind velocities around shelterbelts of low, medium and high porosity. Shows that wind measurements could be made any height without affecting relative reduction in velocity.

Refers to earlier work by Mattingly, Peters, Harrje and Heisler which indicated the possibility of reducing air infiltration by using sheltering devices such as fences, neighbouring buildings and trees. Reports use of wind tunnel air infiltration model to explore the effect of trees in a windbreak on a model home. Presents results of tests determining the effect on wind-induced air infiltration of the variation of various windbreak layout parameters. Introduces concept of turbulence generation as the mechanism of tree wind sheltering.

Describes the ventilation of buildings by analogy with electric circuits and derives expressions for ventilation with and without flow through ducts in the roof. Finds that in general ventilation rate will vary linearly with wind velocity. Considers the effect of shelter belts on wind velocity and derives expression for sheltered ventilation rate. Suggests that eddy motion caused by shelters may be important. Gives measurements made on models in wind tunnels to show the affect on wind pressure of sheltering buildings at various distances.