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question for specific dissipation rateHi all,
a basic question: What is the mathematical correct definition of omega (specific dissipation rate in the k-omega)? I cannot find a correct answer, anywhere. Also in Wilcox's book it is not mathematically described. Even in all books and publications I got, it is only derived by the turbulent viscosity definition. Is it according to the Baldwin-Lomax as div x c ? Hope, someone has a clue... Thnx, kippo |

Dear Kippo;
omega has a different meaning in k-omega turbulence models. it can be stated as: Epsilon=C*Omega*K in which epsilon is eddy dissipation, K is turbulence kinetic energy and C is the model constant. although in some versions of k-omega C is defined as a function of mean flow strain and rotation rates and omega itself implicitly. regards |

Thnx a lot,
but this is again all the time the same definition I get. What I mean is the following: To derive the equation for k, you can take the trace of the Reynold-shear-stress tensor. epsilon will occure in the equation in this way: eij=2/rho*avg(du'i*du'j)/(dxjdxj) the k-equation is the trace of the Rij-tensor: Meaning in the k-equation, which can be derrived by avg(u'i*Ns(ui))=0 you will get a dissipation-term like the following (factor 2 is not there, because trace): rho*epsilon=avg(µdu'i*du'i)/(dxjdxj) meaning, epsilon is a tensor built out of the second derivation of the turbulent energy k (correct?). the units for k~m²/s², epsilon~m²/s³ If you look then to the specific dissipation rate omega, as also defined as omega=epsilon/(Cµ*k)~1/T (according to the turbulent viscosity definition), then omega should be mathematically seen a tensor built by the tensors k/epsilon. But what is then the correct mathematic definition. If you look further to the k-omega-SST equations. Menter transformed the epsilon in the k-epsilon to suit the omega-equation. So an additional term occurs in the transformed epsilon-equation: +2*rho/(sigma*omega)*dk/dxj*domega/dxj (this is the cross-diffusion modification, see manuals CFX of Fluent, or StarCD). Where can I derive this term from the epsilon and omega-definition? Or again, what is the correct mathematical definition of omega? thnx a lot, kippo |

Quote:
if so where does the solver get the epsilon value from, since its not calculated in the turbulence model? |

Citing cfd-online:
"There is no strict mathematical definition of the specific turbulence dissipation, http://www.cfd-online.com/W/images/m...98d86b4602.png (at least none known by the author, please add one here if you know it). Instead it is most often defined implicitly using the turbulence kinetic energy, http://www.cfd-online.com/W/images/m...21e8759df3.png, and the turbulence dissipation, http://www.cfd-online.com/W/images/m...b060f7daf5.png: http://www.cfd-online.com/W/images/m...0b4f750897.png Where http://www.cfd-online.com/W/images/m...fdd925d922.png is a model constant, most often set to: http://www.cfd-online.com/W/images/m...fa783bccd8.png Please note that some models/codes instead use a different definition without the model constant: http://www.cfd-online.com/W/images/m...f97ee7106e.png" So my question is: which is the definition used by FLUENT for defining omega? I wasn't able to find a precise answer to such a question. According to FLUENT user's guide: "which can also be thought of as the ratio of https://www.sharcnet.ca/Software/Flu...ug/img4040.gif to https://www.sharcnet.ca/Software/Flu...ug/img4041.gif" makes me think there is not C_mu inside the FLUENT definition...but it is not so clear after all. |

Hi, from the paper by Ferrari et al. CFD study of Savonius wint turbine: 3D model validation and parametric analysis. (Elsevier, Renewable Energy)
Omega= [C_mu^(3/4)]*[ (k^(1/2)) / L] Where C_mu= 0.09 L=0.07*HydraulicDiameter k= (3/2)*[(FlowSpeed*Turbulence Intensity)^2] This is the one I am using for my simulations. Hope this helps. |

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