J. Colman

Field study of the flow behind single and double row herbaceous windbreaks

Abstract Field experiments in the Argentinean Pampas on strong wind days (pampero) have been carried out to investigate aspects of the wind sheltering ability on single and double-row field shelterbelts of Triticale ) inserted in wheat plantations ( Buck Charrua variety). In the present experiments hot-wire anemometry and continuous wavelet transform analysis were used to describe turbulent features of a relatively small wake region near the shelterbelt 0.5 x / H

Canopy flow and aspects of the response of plants protected by herbaceous shelterbelts and wood fences

Abstract Field experiments have been conducted in the semiarid southwest region of La Pampa, Argentina, in order to investigate the effect of different types of windbreak on wind characteristics and growth parameters of wheat ( Triticum aestivum , Buck Charrua variety) within the sheltered region. Windbreaks decrease wind speed, reducing damage to the crop and improving yield and quality. Sheltering may improve microclimate conditions for plant growth and protects the soil from wind erosion. Aspects of the sheltering ability of four different windbreaks have been tested: single and double row herbaceous ( Tritio secale ) shelterbelts, and two different artificial wooden fences, with the same overall average porosity, but with different porosity distributions. Mean velocity, turbulence intensity, skewness distributions and spectral and wavelet analysis have been performed in order to characterise the turbulent flow downstream of the different windbreaks. Wheat grain yield and harvest index were compared with adjacent unsheltered plantations. The single and double row shelterbelts enhanced the grain yield and the harvest index of the protected plants. An influence of the turbulent structure on plant growth is suggested by the different response of plants sheltered by the two types of artificial fences.

About the penetration of a horizontal axis cylindrical vortex into the nearby downwind region of a vertical porous fence

Abstract Wind tunnel experiments have been conducted on a cylindrical vortex embedded in a low turbulence stationary horizontal stream, running through a two-dimensional narrow vertical woven fence located on the wind tunnel floor. The vortex was continuously generated upwind of the fence by means of a vortex tube located well below the fence top level, with its axis aligned with the mean velocity of the external stream. The fence installed along the entire width of the tunnel had a porosity of 70%. Visualization experiments showed that approaching the fence the vortex moves away from the mean wind direction of the adjacent stream along a rising curved trajectory while the direction of the surrounding mean flow remained nearly horizontal. The results suggest that this deviation could be promoted by the vortex slanting velocity field relative to the fence, which “sees” a fence with much lower optical porosity than the fence perpendicular velocity of the nearby mean flow. The fence top shear layer flow, which dominates the downwind evolution of the mixing layer, appears to be highly sensitive to the presence of this type of vortex. The most energetic changes in the flow due to the presence of the vortex occurred in the mixing layer region. Windbreaks are usually designed in terms of mean velocity, turbulence intensity, geometric dimensions, and porosity. The results presented in this paper suggest that the sheltering ability of a porous fence depend also on the particular flow pattern of the oncoming turbulent structures embedded in the incident wind. The results show the importance for a particular wind sheltering application in knowing a priori at least some aspects of the flow pattern of the most representative turbulent structures of the local wind.

VELOCITY FLUCTUATIONS IN THE WAKE BEHIND WIND BREAKERS AND AT THE EDGE OF THE SHEAR LAYER, AND THEIR CORRELATIONS WITH THE FREE STREAM

Abstract Wind tunnel tests and field experiments have been performed in order to compare, for neutral conditions, aspects of the wake flow in the external mixing layer of the upper region of a porous wire mesh fence, with a porosity of 70%, a tree canopy (cypress macrocarpa) with a porosity of 80%, and a dense wheat field.