Visualization Of Flow Separation Induced By Abrupt Change Of Flow Passage Area

Because of its simple shape and geometry,the fixed separation point can be observed by using the corresponding means,and the flow separation and attachment flow phenomenon is generated in the burst expansion and return flow area,so the burst expansion step structure is regarded as an ideal experimental object for studying the separation flow.Flow separation phenomenon is common in our practical engineering applications,such as the separation flow of the aircraft during flight,the sudden expansion or reduction of the pipeline in the oil pipeline,the flow of the turbine and blades,the flow in the combustion chamber,the flow of the air flow through high-rise buildings or bridges and other buildings.Postponing the emergence of flow separation phenomena and reducing the return separation area are of positive significance for improving the dynamic efficiency of fluid machinery,making the aircraft work more smoothly and stably during flight,reducing drag,reducing the vibration of the system structure,and reducing or suppressing noise.The main work and conclusions of this article are as follows:1.Design related equipment and build an experimental platform,including the independent design of water tank,rectifier box,pipeline connection structure design,dye injection system and the design of each connection part.2.Before the experiment,calibrate the experimental device and pipeline respectively,including the calibration of flowmeter,the calibration of pipeline diameter,the test of measuring the damping coefficient of surfactant solution to judge the property of solution,etc.these calibrations are prepared for accurate observation and measurement of reattachment length.3.To explore the dependence of Reynolds number,step height,inhibitor solution concentration and reattachment length.At the same step height,the reattachment length first increases with the increase of the Reynolds number,and after reaching a peak,it begins to gradually decrease with the increase of the Reynolds number,and after decreasing to a certain dynamic stable value,it no longer changes with the increase of the Reynolds numberAt the same Reynolds number and step height,the addition of surfactant makes the reattachment curve as a whole "delayed" than that of water,and the peak also appears earlier than 10-20,and the peak value of the reattachment length of the two is similar.There are differences in the dynamic and stable reattachment length in the turbulent flow area,and the value of 10-20 step pipelines is larger than that of 20-50.In the same Reynolds number,step height state,the addition of surfactant so that the reattachment as a whole is "delayed" compared with water,and different concentrations of reattachment at the same step height are different,500 ppm "postponement" effect is better at 10-20 step height,and 300 ppm effect is better in 20-50.