Atmospheric Boundary Layer Wind Tunnel Simulation Of Field Gusts

Wind tunnel experiment is an important way to study the physics of sand movement.However,there is the evident deviation between the results of wind tunnel experiment and field observation since air flow condition in the wind tunnel is far away from actual field environment.How to approximate the wind gust environment in the atmospheric boundary layer wind tunnel is not only an important basis of the advanced research of aeolian sand transport,but also a hot issue in wind tunnel experimental research.In this paper,power spectral analysis of observed velocity history of field gust has been processed by Fourier transform at first.The result shows that the energy of field gust is mainly concentrated in its low-frequency part.91.15% of the wind energy was concentrated below 0.05 Hz at the top of the sand saltation layer.Three common filtering methods were used to filter out high frequency part of field gust.By comparing the filtering results,it can be found that wavelet transform is the best choice in present study.By means of genetic algorithm,the discrete data of the filtered field gust velocity history have been piecewise curve fitting and transformed into a series of continuous sinusoidal wind velocity curves.Meanwhile,the individual frequency converting control of the fans was performed in the ABL(atmospheric boundary layer)wind tunnel with dual power fans by the control system consisting of the King View software,PLC and two frequency converters.The calibration between the working current frequency of the fans and the wind velocity in the wind tunnel has been measured.Then the sinusoidal unsteady wind and beat waveform unsteady wind have been controllably generated in the tunnel while the elementary wind was set to sinusoidal velocity changing with no lag to the current frequency variety.A series of sinusoidal wind velocity curves resulting from filtered and curve-fitted field gust have been transformed into corresponding current frequency curves based on the measured calibration.The control system would change the dual fans rotation speed in the current frequency varying history.Finally,the unsteady wind with low-frequency characteristic of field gust has been approximately simulated in the wind tunnel.The correlation coefficient between the unsteady wind generated in the tunnel and the filtered field gust equate to 0.8942.The energy loss rate of the simulated wind related to the filtered gust is 3.36%,and the compression ratio is 9.3%.Based on the results and the energy spectrum distribution analysis,it can be concluded that by using the signal process method for wind velocity history and the frequency convert controlling to the fans as mentioned above,the field gust environment can be approximately reproduced in the ABL wind tunnel with the dual power fans.Hence,the wind tunnel experiment can be performed to study the unsteady aeolian sand transport,which would promote the unity of the theory and the practice of aeolian sand transport.