Studies on the Windbreak Nets.

Abstract

A micrometeorological measurement was carried out by the use of an infrared thermometer, a set of three-cup anemometer and an ultrasonic anemometer for cheese cloth net (No. 110, net density 50.2%) of 50cm height and 50m length at low grassland in National Institute of Agricultural Sciences. The warming effect of leaf-soil temperature, decreasing effect of wind speed and aerodynamic change of micro-structure of wind, i. e., turbulent characteristics.The obtained results were mainly as follows:(1) Warming effect of leaf-soil temperature by windbreak net was recognized on both fine and cloudy-rainy days. It is more than 1.0°C at -2 to 10 H for the former and 0.4°C at 0.5 to 4 H for the latter, however, a rather cooling at immediate windward and leeward of the net and a slight cooling at the leeward in an early morning were obtained.(2) There is 45% minimum value on the horizontal pattern of wind speed at the half level of the net height. The region that a relative wind speed is lower than 50% is 0.5 to 4 H. A recovery of wind speed is accelerated by the increase of wind speed, roughness and unstability. Wind speed returns to the standard value around 30 H (27 to 35 H). The decrease of wind speed at the lower region of the grassland is relatively large, because that the height is 1/4 comparing with the 2m high net for paddy rice field. At the immediate leeward, the vertical profile of wind speed is changed significantly and the stronger region of wind speed is found at a 60 to 70cm height at 0.5 H.(3) The upward wind is recognized at the immediate windward. Maximum of upward wind is also obtained at 1 to 2 H leeward and downward wind of minimum value around 8 H. The wind recovers to the standard characteristic one up to 30 H with an oscillation of upwind and downwind.(4) Turbulent intensity increases a little at the immediate windward. The relative magnitude is an order of v, u and w components even if the wind passes through the net. Minimum and maximum are represented respectively at 0.5 to 1 H and 8 H and the value returns until 30 H.(5) Skewnesses of u and v components were almost constant in a windward and leeward. On the other hand for w component, a skewness increased and a kurtosis decreased sharply around 4 H. It was shown that these frequency distributions were significantly shifted from the Gaussian distribution. It was clarified that a windbreak net particularly affected on the w component of wind velocity fluctuation.(6) Changing patterns of characteristic time scale (T) of the Eulerian autocorrelation coefficient and their ratios are opposite to those of turbulent intensity. The values for u and v components have a maximum at an immediate leeward and a minimum between 5 and 10 H. The variation for w component is a little small and its phase is faster than those for u and v components, i, e., a minimum appeared at 3 to 4 H.(7) The slopes of the Eulerian autocorrelation and of spectral density approximately expressed a power law for u and v components, however, did not for w component. The changing range is mainly from -5 to 10 H.(8) The horizontal variations of the scales of the largest and smallest eddies or turbulons are similar to characteristic time scales. The difference of 3 components for the smallest turbulon is small, but for the largest one large. The scale order of the smallest and largest turbulons is contrary. The scale of the smallest turbulon is directly effected by a mesh of net.(9) The tendency of energy dissipation rate is similar to turbulent intensity. The changing tendency is significantly because of the proportion to 3 powers of wind speed.(10) There are two maxima of normalized peak frequency of spectral density for