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High Re K Epsilon vs Low Re K EpsilonHi folks,
I am somewhat new to CFD (only been doing for some months :o), and I have a doubt which I hope some can clear... I am trying to implement a Low Re K-Epsilon turbulence model to a backstep and compare it to a High Re K-Epsilon turbulence model, to show that, in this particular case, the Low Re K-Epsilon turbulence model represents the situation better. However, I get a pressure drop that is higher for the Low Re K-Epsilon turbulence model than the pressure drop for the High Re K-Epsilon turbulence model... Is this correct? shouldn't it be the opposite? :confused: Or perhaps I made a mistake in the modelling step? Hope someone can help! Best Regards, Rui Silva |

Anyone?
I'm really stuck in this matter... |

Hi Rui,
In this specific case, you will need to ensure that the mesh is appropriate to the model employed (and ensure that the wall treatment is correct). Low-Reynolds k-epsilon turbulence model: - The first cell y+ should be around 1 (or less); - the wall treatment is based on first cell being the viscous sub-layer. High-Reynolds k-epsilon turbulence model: - the first cell y+ should be around 15-30; - A high-Reynolds wall treatment is to be used. From your post, I am unsure that you are using two different mesh that are consistent with the employed model. Kind regards, Julien |

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Hello Julien,
thank you for your reply. What I am doing is modifying the standard high Reynolds k-epsilon (according to Wilcox) that comes with a cfd software (comsol) so that it becomes the Launder-Sharma Low Re k-epsilon Turbulence model. So I modified the equations to incorporate the new set of damping functions and boundary conditions near the wall. After reading some posts and papers I have refined my mesh. But since I am using a modified standard k-epsilon model, I cannot see y+! I have attached pics of my mesh near the wall and some size info! Hope the pics help... Is there a way to evaluate y+ for a low re k-epsilon model?? Best Regards, Rui |

Have a look at: http://www.cfd-online.com/Wiki/Dimen..._wall_distance for the definition of y+ - which can be evaluated for a low Reynold k-epsilon model.
There is also a simple y+ estimator on the web page. Also, you could use the other calculations to estimate the friction velocity (under the assumption that the other calculations friction velocity is accurate). The rule y+ around 1 is a guidelines - do not get stuck with it. |

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