Study Of RANS/LES Hybrid Turbulence Model And Its Application In Flow Around A Cylinder

Large separated flow has been a hotspot of turbulence theoretical research due to its frequent occurrence in industrial applications such as deep-sea platforms,and flow around cylinders can be seen as one of the most representative case of large separated flow.RANS/LES hybrid methods combine the advantages of RANS and LES,which utilize RANS to simulate near-wall laminar flow region while LES is used to simulate far-wall separated flow region.Because RANS/LES hybrid methods can simulate complex turbulent flows with economical computational resources,they have broad application prospects and important strategic purposes and deserve further research.In this thesis,RANS/LES hybrid models are studied and the simulation performance of each turbulence model is compared and analyzed in the test case of flow around a cylinder.Flow around a round cylinder with Reynolds number Re = 3900 and flow around a square cylinder with Reynolds number Re = 22000 are considered as standard cases to test the performance of the turbulence model.All the computational work in this thesis is based on OpenFOAM,an open source CFD platform.Firstly,this thesis introduces the widely used DES method and its problems,then DDES model is introduced,which solves the grid induced separation problem of DES.In order to highlight the advantages of DDES model in simulating large separated flow,both RANS and DDES are used to simulate the flow around a cylinder,and the results show that both the time-averaged and instantaneous characteristics of the flow field calculated by DDES model are better than those of RANS model,which verifies the simulation ability of DDES in large separated flow.Then this thesis introduces the basic principle and concrete form of SAS model,which is the second generation URANS model.And SAS model is applied to simulate the flow around a cylinder and compared with DDES model.The results show that SAS has a certain ability to simulate large separated flow,but its performance is still not as stable as DDES model,and there is still some room for improvement.At the same time,the author finds that the form of von Karman length scale limiter has a considerable impact on the performance of SAS model,which can be seen as one of the starting points for improving SAS model in the future.Finally,the author constructs dynamic DDES model by introducing the dynamic model of LES methods through investigation and reflection,and applies it to the flow around a cylinder.The results of comparative analysis with DDES model show that both the time-averaged and instantaneous characteristics of the flow field calculated by the dynamic DDES model are better than those calculated by DDES model,which proves that the introduction of the dynamic model can indeed improve the simulation performance of DDES model.