Spatio-temporal Distribution of Down-wash Airflow for Multi-rotor Plant Protection UAV Based on Porous Model

Abstract

When the plant protection UAV is used to spray pesticides on orchard, the spatio temporal distribution of down wash airflow inside and around the canopy has a major impact on the adhesion and distribution of the droplets. In order to clarify the spatio temporal distribution of down wash airflow inside and around the canopy of trees when applying multi rotor plant protection UAV to spraying, combining RANS equation, RNG k-ε turbulence model, porous model, sliding mesh technology and SIMPLE algorithm, a three-dimensional CFD model for the spatio-temporal distribution of the down-wash airflow of six-rotor plant protection UAV in hover was established. The results of numerical simulation showed that when without tree, the down wash airflow of the rotor developed downward was approximately in a “cylindrical” shape, and formed the ground spreading after reached the ground, and the “Z-direction (vertically downward) speed stable region” appeared in the region of 0.6~1.7m below the rotor, where the speed range was from 3.0m/s to 4.0m/s. When tree existed, the canopy had an obvious effect on blocking the down wash airflow of the rotor, and it would not appear “Z-direction speed stable zone”. Taking the three trees simulated as an example, the airflow around the canopy of No.Ⅰ tree began to develop downward from the upper part of the canopy in a “conical” shape, and developed to the ground at an inclined angle to form a small area of ground spread. There was a near ground hoisting at the end of the ground spread. The airflow around the canopy of No.Ⅱ and No.Ⅲ trees was heavily hoisted, and it did not have obvious ground spread in the calculation area;the maximum speed in Z-direction was close to 8m/s directly below the rotor center. With the increase of canopy pressure loss coefficient, the speed attenuation in Z-direction was accelerated, while the rotor airflow was spreaded around. Calculating the maximum speed decay ratio in Z-direction inside the canopy, it was found that the maximum speed decay ratio in Z-direction in the canopy of No.Ⅰ, No.Ⅱ and No.Ⅲ trees was increased successively except the No.Ⅲ lower part of the canopy. The relative errors between the test values and the simulated values at 0.3m, 0.8m, 1.3m and 1.8m below the rotor and 2.3m near the ground were less than 10% and not more than 25%, respectively. The overall goodness of fit was 0.9846, and the numerical simulation was accurate. The test results of down-wash airflow speed of trees showed that the airflow speed distribution inside canopy of the experimental tree was in good agreement with that of the simulated tree.

Keywords: porous model, plant protection UAV, numerical simulation, down-wash airflow, spatial-temporal distribution