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چكيده انگليسي :

A hybrid model for the calculation of Reynolds stresses in turbulent flows Mohsen Fatehi M Fatehi@me iut ac ir Date of Submission 2011 12 11 Department of Mechanical Engineering Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc Language FarsiSupervisor Mohsen Davazdah Emami Mohsen@cc iut ac irAbstract Turbulence modeling is among the most important challenges of researchers Over the past decades differentmodels have been proposed to explain the behavior of turbulent flows An important feature of turbulent flows isthe chaotic characteristics of the flow field Such a feature has led researchers to adopt statistical approaches intackling turbulence modeling challenge The averaging method in which the instantaneous quantities aredecomposed into the mean and fluctuating components is a well known approach in turbulence modeling As aresult of this scheme new unknowns are introduced into the governing equations that are subject to furthermodeling Two of the most common sub models in this approach are the two equation model namely the k model and the algebraic stress model The former has the advantage of computational economy and relativestability of the numerical solution procedure but suffers from inaccuracy in predicting the normal stresses Thelatter model is able to deliver a better estimation of normal stresses but weaker stability and inaccurate shear stresspredictions The present work aims to combine the strong features of both models while avoiding their flaws Anew blend model for the turbulent stresses is introduced The model adopts two well known turbulence models thek model and the explicit algebraic stress model of Speziale SSG and blends them The model may beimplemented in a binary method in which the k model for shear stresses and the explicit algebraic stress modelfor normal stresses is used or a fuzzy method which is a combination of both the aforementioned models Theimplementation is done by writing the momentum equations based on the k model and adding the extra terms dueto the blend model as momentum sources The new model benefits from favorable features of k and algebraicstress models while avoiding their deficiencies in terms of accuracy and computational economy Results of an in house CFD code based on the blend model for two benchmark cases namely a backward facing step and a shallowcavity show that both the proposed binary and fuzzy models are capable of correctly predicting the turbulentnormal and shear stresses The simulation results for both cases reveal that the blend model performs better thanthe k model and the explicit algebraic model in terms of the accuracy The results of the fuzzy model in bothcases are superior to those of k SSG and the binary model The CPU time consumed for the blend model binary or fuzzy is less than the SSG model but more than the k model A more complex geometry in 3D hasbeen considered to verify the ability of the proposed models in complex flows An industrial case namely theStairmand cyclone has been simulated using k SSG and the proposed hybrid models Results show that theproposed models are capable of correctly predicting the tangential velocity but yielding wrong predictions for theaxial velocity Overall the blend models proved to be able to predict the flow field quantities on average 32 4percent and at worst 8 34 percent improvement compared to other models Keywords turbulence models hybrid model algebraic stress two equation backward facing step shallow cavity stairmand cyclone

فاتحي، محسن

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