Laboratory investigation on acoustic agglomeration in turbulence and field exploration on acoustic impact in artificial rainfall technology

Abstract

For securing more water resources, traditional solutions, including seawater desalination and cloud seeding, are limited due to high costs and restricted operating conditions. However, acoustic rainfall stimulation is a promising and potential alternative method due to its low cost and convenient operation. This thesis adopts laboratory techniques to investigate the mechanism of acoustics-induced particle agglomeration and open-air field tests to explore the feasibility of acoustic rainfall enhancement technology. The laboratory explorations focus on microscopic particle motion and variation of particle size spectrum within turbulent particle-laden flow under low-frequency acoustic fields. Moreover, the field studies interrogate the impact of acoustic interference on rainfall distribution and practical evaluation methods for acoustic artificial rainfall. The observation of acoustics-induced particle motion is achieved using the Particle Image Velocimetry (PIV) technique. A new sampling method is developed to determine the velocity variations of fine particles within a low-frequency acoustic field. The study discloses that acoustic frequency and intensity determine the characteristics of fine particle motion. The Brand-Freund-Hiedemann (BFH) equation has considerable accuracy in evaluating the fine particle entrainment factor in the Stokes flow. Further, the particle size spectrum is measured by the Laser Particle Size Analyzer (LSPA) to assess the impact of acoustic agglomeration in a turbulent flow. The results suggest that the turbulent environment is conducive to improving the probability of acoustics-induced particle agglomeration in the low-frequency acoustic field with moderate intensity, which would be attributed to the uneven particle spatial distribution in the turbulent dissipation region. The open-air field acoustic rainfall tests were conducted in Nyingchi City in Tibet Plateau, China, for a period from May to July 2020. Due to the topographic impact, the natural rainfall is more concentrated in the eastern region of the experimental site. In comparison, the rainfall center has shifted to the western region for acoustic rainfall events due to the combined effect of low-frequency acoustic field and near-surface convections. The two-stage rainfall pattern has been identified in the acoustic rainfall process with a duration longer than 2 hours. The first stage has the characteristics of relatively short duration and rainfall intensity decaying with the increase of distance from the central region with the acoustic devices. In comparison, the rainfall in the second stage is randomly distributed and generally lasts for hours. This phenomenon is mainly caused by the trigger effect of acoustic waves, which has accelerated the forming process of large rainwater droplets and improved the rainfall intensity in the central region. To identify the influence factors on the rainfall intensity and quantify the efficiency of acoustic artificial rainfall, the study confirms that acoustic interference is more effective in longer-duration rainfalls, and the average rainfall intensity is improved by 72% with the acoustic operation. The possible reason is that the cloud conditions of long-duration rainfalls have a large amount of water storage in the atmosphere and a broad droplet size spectrum, which are beneficial to the occurrence of acoustic agglomeration.