Numerical Simulation Of Fluid Flow And Heat Transfer Based On PL-DDES Turbulence Model

With the improvement of the computational ability,numerical simulation becomes an important method for studying turbulent flows.As a turbulence model,the hybrid RANS/LES method is focused increasingly for combining the advantages of the LES and the RANS.In this thesis,an improved hybrid RANS/LES turbulence model is proposed.This improved model is aimed to improve the issues of the previous hybrid RANS/LES model simultaneously.The issue include grid induced separation?GIS?,log-layer mismatch?LLM?and weak ability of resolving Kelvin-Helmholtz?KH?unstability.Since the delayed detached-eddy simulation?DDES?is one of the widely used hybrid RANS/LES model,the DDES model is chosen as the study object.Firstly,the production-limited DDES model is developed by combining the new resolution control function,the modified shielding function and the new cut off length scale.The PL-DDES model is validated though simulating several benchmark cases.Then,the PL-DDES model with Pr_t=0.9 is tested for predicting turbulent force and mixed convection.The detailed research contents and conclusions are as follows:?1?For improving the GIS issue,the constant Cd1 in the shielding functions is modified according the shielding principle of the shielding function.The zero pressure gradient boundary layer computation reveals that the GIS issue is alleviated when C_d1 is set to be 14.For obtaining the principle of alleviating the LLM issue,the previous SST DDES and IDDES models are chose as the comparative models.The simulation results of the plane channel flows reveals that the PL-DDES model has good performance in improving the LLM issue due to the reasonable region of the RANS model.?2?The cut off length scale is modified for resolving the KH unstability.The simulation results of the backward facing flow prove that the PL-DDES model has strong ability of resolving the KH unstability.On the same grid,comparing with the SST DDES model,the PL-DDES model reasonably depletes the turbulent viscosity due to the used the cut off length scale0.2??_x?_y?_z?^1/3,resulting in resolving the KH unstability strongly.In addition,the prediction of the periodic hill flow testified that the PL-DDES model also do well in simulating complex flow separation and reattachment.?3?The undelaying affects on simulating the flow structure of the flow past a circular cylinder?FPCC?and the flow past a wavy cylinder?FPWC?at subcritical Re from the diffident cut off length scales are studied in detail in this thesis.The simulation results of the FPCC and the FPWC at subcritical Re conclude that the PL-DDES model obtains small turbulent viscosity and computes the reasonable location of the shear layer unstability and the length of the recirculation zone resulting from the smaller cut off length scale comparing with the SST DDES model.What is more,the PL-DDES model could resolve reasonable flow structures that conform well to the experimental and the LES results.?4?The performance of the PL-DDES model with Pr_t=0.9 in predicting the convective heat transfer is investigated finally.The predicted results of the convective heat transfer in the plane and the wavy channels prove that the PL-DDES model not only alleviates the LLM issue of the temperature but also accurately simulate complex turbulent heat transfer.Finally,the PL-DDES model is applied to study the influence of Bo on the flow and the thermal fields in a vertical concentric annulus.The results of the opposed mixed convection flow reveal that the flow field is affected by the Bo more remarkably than the thermal field.It is also noted that the flow and thermal fields are influenced by the Bo more greatly in the outer wall region comparing with those in the inner wall region.