Abstract: Wind tunnel experiments were conducted to study the variation of the drag coefficient of particle in turbulent layer. A ping-pong ball was selected to simulate particle. The horizontal drag force on the ping-pong ball was measured by three-component accelerometer. The wind velocity around particle was measured synchronously and the sample frequency was set to 125 Hz. Advanced devices were used to measure both the horizontal drag force exerted by wind on a particle and the wind velocity with different wind velocities and different heights. A new relation between horizontal drag force(F_x) and the horizontal wind velocity(U_x) is obtained. The calculations by the formula have good agreement with the observations. The formula provides a good way to predict the drag coefficient of particle in turbulent layer. The results show as follows:(1) The horizontal drag force has the same tendency with particle surface wind velocity and increases with the increasing wind velocity and height of particle position.(2) When the Reynolds number ranges from 9 000 to 40 000,the relation between horizontal drag force(F_x) and the horizontal wind velocity(U_x) can be written as F_x=0.514(1/2AρU_x²)+0.019,where C_D=0.514 is the horizontal drag coefficient. It can be seen from the equation that the drag coefficient C_D=0.514 is larger than that in water flow field(0.44). The constant term 0.019 in the equation indicates that the effect of other forces,for example,added mass force,history force,cannot be ignored. Overall,the variation of the drag coefficient in the water flow cannot be applied directly to that in the air flow. Further studies on the variation of the drag coefficient in the air flow are required. Although,the velocities measured in this paper could cover most real situations,the Reynolds number should be extended in order to understand the rule of the drag coefficient C_D. In addition,the sand particle is represented by a ping-pong and the differences between them are apparent,for example, size,shape and surface roughness. However,the laws of the force exerted by wind on particle and ping-pong are similar. The method used in this paper is similar to experiments conducted to study the drag coefficient in water flow. Therefore,the results in this paper can be used to predict the drag force enacted on particle in air flow.

Key words: drag coefficient, drag force, air flow, wind tunnel experiment