An Investigation of Turbulence within a Crop Canopy

Abstract

Measurements of turbulence were carried out by use of a sonic-anemometer at several heights above and within a corn canopy with a mean height of 300cm. Turbulent intensity, autocorrelation coefficients, mixing length, transfer coefficient and energy spectra were computed for vertical and horizontal components of wind. (1) Both √w′² and √u′² are in proportion to the mean wind velocity at each height. Above and in the upper part of the canopy, √w′² is smaller than √u′². In the middle part, √w′² is nearly equal to√u′² which may indicate the existence of isotropic turbulence. (2) The autocorrelation coefficients above and within the canopy are calculated for vertical and horizontal components of wind fluctuation. Using the calculated values, the vertical mixing length and the transfer coefficient at each level are estimated respectively by the following equations l_w=√w′²∫^T₀R_wdt (1) K=√w′²·l_w (2) where R_w is the autcorrelation coefficient for the vertical component of wind fluctuation, and T is the smallest time at which R_w becoms zero. The values of both l_w and K above the canopy agreed well with those calculated from the log-profile of wind velocity above the canopy. The horizontal mixing length, l_u is also calculated by use of the autocorrelation coefficient for the holizontal component of wind fluctuation. Above and in the upper part of the canopy, l_w increases with wind velocity, but it seems independent of wind velocity in the middle and lowest part. l_u increases with wind velocity at any height above and within the canopy. The transfer coefficient is, above and in the upper part of the canopy, in proportion to the square of the wind velocity, but, in the middle and lower part, is proportional to the wind velocity. (3) With the values of τ and du/dz, l_w which is defined by τ=ρl²_w(du/dz)² (3) is obtained, τ is the flux of momentum calculated from K and du/dz. The values of l_w, from eq. (3) becomes in the canopy as large as three times of that at the top. It is therefore considered that equation (3) is not applicable in the canopy. (4) Energy spectra of turbulence are estimated of vertical and horizontal components of wind fluctuation above and within the canopy. By expressing the spectra at high frequenciy end as F(n)-n^p, (4) the value of p increases with the gradient of wind velocity. There are some developed peaks of n·F(n), which is probably due to local eddies created by plant. A following equation is obtained for the vertical component of wind fluctuation at any height above and within the canopy l_w·n_max/u=0.08 where n_max is frequency at which n·F(n) is maximum.