Aerodynamic parameters of windbreak based on its optical porosity

Abstract

The paper summarizes the results of wind velocity measurement in different
distances from windbreak in combination with optical porosity (OP) determination in
different periods. Experimental data were obtained by direct measurement of wind speed
and OP analyses of images of four windbreaks in Southern Moravia (part of Czech Republic).
Wind speed at 2 m above the soil surface in a defined distance from windbreak
had been measured since 2006. Thirty images of different phenological stages of selected
windbreaks from 2006 to 2010 were analyzed. Windward or leeward images were converted
into black and white spectrum. All available measurements were used for a map
creation. The highest values of OP (up to 50%) are achieved in non-vegetation period.
Due to the high OP variability in height of terminal tree branches “OP reduced” was
also assessed (just squares to 2/3 height of windbreaks were evaluated). The wind speed
reduction on the leeward side relatively strongly correlated with OP value. The highest
correlation was found out when the wind speed measurement at 50 m on leeward side was
used. The dependence decreases with increasing distance. Full foliage in summer (10%
OP) reduces a wind speed about 60% at 50 m and about 30% at 150 m on the leeward.
These values for non-foliaged windbreaks decrease to 80 and 90%. Maximum distance
of windbreak effect on wind speed reduction was found out by the extrapolation of the
curves constructed using the regression equation of wind speed reduction in dependence
on OP and different distances from the windbreak. Regardless of OP value, the reduction
effect disappears at a distance of 250 m. The quietest zone of evaluated windbreak with
an average height of windbreaks 15–18 m was detected in the area about four times the
height (4H). The effect of windbreaks decreases with increasing porosity.