The Conditions And Characteristics Of Internal Waves Induced By Mechanical Mixing In Stratified Fluids

The prominent eutrophication of lakes and reservoirs directly influences the sustainable utilization of urban water resources,and threatens the safety of urban water supply.In this study,the periodic outflow from a mechanical mixing device was used as a disturbance source to induce internal waves,a serious of pilot experiments on internal waves were performed under various temperature gradients,outlet opening ratios and the motor speeds,the disturbance of periodic circulated water flow was studied,the mechanisms of generating internal waves were analyzed based on the disturbance source,the formation conditions,characteristics and the mixing time required for the stratified water were explored.The main conclusions are as follows:(1)The characteristic equation between the flow rate(Q)and motor speed(Nc)for the pilot device was developed as Q=1.022 Nc,the measurement of outlet velocity was thus simplifyed,the disturbance period of the circulated water flow,motor speed and the outlet opening degree are important parameters influencing the character of disturbance source.(2)Periodic circulated water flow with low temperature and high density interacted with the stratified fluid in the thermocline,which caused the vertical vibration of stratified water and then induced the stable regular internal waves.The induced internal waves gradually propagated to the epilimnion and the hypolimnion,thus weakening the temperature gradient of the thermocline and finally completely mixing the stratified water.Fixing the operation cycle of 55s(running time of 10 s,stopping time of 45s),the operation period of the pilot mechanical mixing device determines the average period of induced internal waves,and the operation period of the pilot device was similar that of the average wave period.Stable regular internal waves could only be induced by continuous disturbance by the circulated water,when the operation period of mechanical mixing device was longer than 150 s,the internal waves could not be induced and transferred.(3)From the view of disturbance source,the Fr numbers of the stratified water were within 0.46~3.96,which were less than 4,the internal waves induced by the periodic circulated water flow belongs to the internal waves by volume effects.(4)At the same temperature gradient,with the increasing of motor speed,the amplitude of internal waves increased first and then decreased,which reached the maximum value at the motor speed of N=1750r/min.As the outlet opening degree increased,the amplitude of internal waves also increased first and then decreased.Under same motor speeds and the outlet opening degrees,when the temperature gradient increased,the amplitude of internal waves also increased first and then decreased,which peaked at the temperature gradient of GradT4=0.42?/cm.(5)Under same temperature gradient conditions,the mixing time decreased with the motor speed,but increased with the outlet opening degree.Under same motor speeds and outlet opening degrees,the mixing time decreased with the temperature gradient.(6)In the experimental range,the optimal conditions of inducing internal waves for the mechanical mixing device were as followed: the operation cycle of 55s(running time of 10 s,stopping time of 45s),temperature gradient of 0.42?/cm,motor speed of1750r/min and outlet opening area of 0.01m2.