Turbulent Velocity Fluctuations and Vertical Flow as Affected by Windbreak Porosity

HE ultimate goal of research on T shelter from the wind is to delineate the most effective windbreaks and to predict their effects on crop yields, soil stabilization, and evaporation. Baltaxe (1)’ suggested that the first step toward this goal should be to link the windbreak characteristics with the nature of the leeward airflow. Many researchers (6, 14, 15, 16) have studied windbreak windspeed reduction and made many differing assertions on how far leeward windspeed reduction extends. Most of them noted, however, that porosity is the major factor determining a windbreak‘s ability to reduce windspeed. Mean vertical flow and turbulent fluctuations have not been studied extensively in relation to windbreak porosity (5, 15). Assuming that airflow over a windbreak is incompressible (Le., Reynolds number less than lOG), the continuity equation for steady flow indicates that a mean vertical flow exists in the lee of windbreaks. One consequence of a mean vertical flow can be demonstrated by considering a property, the measure per unit mass of which is s and the vertical transport of which can be described by thk equation: pws = (pw)s + (pw)’s’ ........ where p is air density, w is vertical velocity (the primes denoting instantaneous departures from the mean and the bar indicating a time average (13)). At low heights above uniform terrain, pw is usually zero and equation [ l ] represents the flux due to eddy motion alone. Leeward of a windbreak pW is not likely to be zero, however; and if it is large, PW should influence the microclimate. Turbulent velocity fluctuations are important in the vertical transport process, as shown in equation [ 11. Both the magnitude and spectral distribution 6f turbulent velocity fluctuations have been studied extensively in the open field (2, 4, 10, 11, 12). Knowledge is limited on how windbreak L. J. Hagen and E. L. Skidmore Assoc. MEMRFR ASAE