Numerical Simulation And PIV Experimental Research Over A Backward-facing Step And A Square Cylinder

The separation and reattachment of turbulent flow is a common phenomenon inengineering practice. The flow across a backward-facing step and square cylinderare typical case studies that demonstrate this phenomenon. In engineering practice,flow separation and reattachment is also evident in hydraulic valves placed in pipelines, the wind flow fie ld around bridges and high-rise buildings and in heatexchangers. Therefore, the study of flow across a backward-facing step and squarecylinder has significance. In this study, numerical and experimental simulationswere conducted on a theoretical basis.First, particle image velocimetry (PIV) was used to measure the cross-sectionvelocity distribution, the characteristics of the speed pulse and the boundary layerparameters. From the PIV measurements, it was found that it was the average of thedistribution of velocity in three different heights of the flow field, and theturbulence intensity was basically less than6%in the central flow line.These resultsverify the feasibility of using PIV for measuring the flow field across abackward-facing step and square cylinder at a cross-section.Subsequently the flow field was measured under six flow conditions. For eachflow condition, the time averaged flow field parameters and flow reattachment pointwere calculated. The distances from the backward facing step or square cylinder tothe flow reattachment point are consistent across all six flow-conditions. Thereforethe distance is insensitive to the Reynolds number of the flow. This result arisesbecause all six flow-conditions were in the turbulent state. In addition, the distancesfound in this study were consistent with those found by foreign researchers.The vortices field component, pulse field and Reynolds stress distribution werecalculated from the time-averaged results. This thesis analyzed the process ofenergy transfer and the dissipation by the quantitative point. From the results, the shear layer in the X/H>2.0region was well established, indicating that a strongnon-steady-state mixing phenomenon occurred in this region. Areas of high flowintensity occurred mainly the leading edge of the backward-facing step.and thisregion is also the most intense part of the entire flow field.Two numerical models were developed: one using the standard k-ε model of theReynolds average method, the other using the Large Eddy Simulation method. Thenumerical model was used to simulate both steady and unsteady flow conditions.From the numerical simulation results, a number of analyses were performed. Bycomparing the results from the experimental data and the numerical simulationunder six flow conditions, it was found that both the standard k-model and theLarge Eddy Simulation model were able to simulate the flow field. However, thestandard k-model was not able to describe small vortices as satisfactorily as theLarge Eddy Simulation. The location of the reattachment point calculated from theLarge Eddy Simulation was more consistent with experiment data then that obtainedfrom the standard k-model. Therefore, the Large Eddy Simulation appears to bemore accurate. In addition, the Reynolds-averaged-u’v ’ and the turbulent kineticenergy k under different cross-sections were compared and analyzed. The resultsshowed that both the experimental results and Large Eddy Simulation results agreewell in most regions. Therefore, this thesis shows that the Large Eddy Simulationwas able to adequately calculate the turbulent flow field. Consequently, this thesisdeepens understanding regarding the characteristics of two-dimensional separationof stream flow, and provides a standard model from which turbulence numericalsimulations can be verified.