The Processes And Mechanisms Of Inducing And Breaking Of Aeration-induced Internal Waves In Stratified Waters

The water pollution of lakes and reservoirs is becoming more and more prominent,especially for deep lakes and reservoirs Destratification is one of the effective methods to control such water quality problems.Using self-designed stratified reservoir physical model,under the air flowrate of75L/h and thermocline temperature gradients of 0.20-0.50?/cm,pilot experiments on internal waves were performed at water depths of 80?100?115cm,the process of destratification by aeration-induced internal waves was analyzed,the influence of thickness of thermocline on characteristic parameters of internal waves and on the mixing efficiency was investigated,the destratification mechanism by internal waves was revealed.Main conclusions were as followed:With the thickness of thermocline increasing,the biggest buoyancy frequency gradually increased.When the thickness of thermocline were respective 15?20?24 cm,during the diving 10 cm from the initial position,the falling rates were 0.83?0.84?1.10m/s,the stability factors were 0.91?1.73?2.19 g.cm/cm2.Under the same condition of the air flowrate,when the thickness of thermocline increased,the amplitude of internal waves increased,but the period of internal waves decreased.The thickness of thermocline influenced the formation of internal waves and water mixing,Fixing air flowrate of 75L/h and the initial temperature gradients of 0.375?/cm,the pilot study was carried out under no disturbance,with and without flow reduction devices.The process of destratification by aeration-induced internal waves was analyzed together with the breaking of internal waves,the relationship between the mixing efficiency and the characteristic parameters of internal waves was explored,the effects of air-water two-phase flow formed by aeration and the wave breaking during the wave evolution on the destratification were revealed.Compared with the case withflow reduction device,the mixing time for the case without flow reduction device was decreased by 56.8%,the wave amplitude,wave period and the falling rate of thermocline during the destratification gradually increased about 80%,52% and 80%respectively.Under the same conditions,during the propagation of internal waves formed by continuous disturbances without flow reduction device,the area with the Richardson numbers less than 1/4 was rare and was maintained for a very short time,the breaking of internal waves didn't occur,and the water mixing was mainly dependent on the vertical oscillation of fluid particles in the thermocline by the wake-generated internal waves.For the case with flow reduction device,if the stratified water was disturbed for 30 mins,during one releasing period of air piston of60 seconds,the average Richardson number was less than 1/4 within the initial start of32 seconds,the local rotation frequency was greater than the buoyancy frequency,the shear instability of fluid micelle played a leading role,the instability mechanism of water was gradually enhanced,which resulted in the breaking of internal waves and so water flow transferred to a turbulence mode.