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- - **how to plot Q criterion in fluent
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how to plot Q criterion in fluent
Can anyone suggest how I can plot Q criterion in fluent to visualize the coherent structures?
Thanks for your help. Anindya |

Re: how to plot Q criterion in fluent
hi anindya,
you certainly have the definition of that q criterion which is based on velocity gradient. Fluent do not provide it directly,i mean you have to compose it from computed velocity gradient as a custom field function. for that: define>custom field function and type the name of the criterion "q" for example. you will be then able to visualize the q value on a specified surface on the contour panel... if you want to create an animation of an iso-q surface for example you also have to create an isosurface, hope it helps. best regards Said |

Re: how to plot Q criterion in fluent
Thanks a lot for your message. I have already created that custom field function Q.
So when I create iso-surfaces, do I create iso surfaces of Q ( say specific values of Q) and then plot Q on those Q iso-surfaces? Or do I plot Q on iso-surfaces of low pressure, vorticity-maginitude, etc? Anindya |

Re: how to plot Q criterion in fluent
Fortunately, you don't have to use UDF. Here's a simpler way of visualizing the iso-contours of the second-invariant.
You can use the custom field funtion capbility in FLUENT. First you define the second-invariant of deformation tensor with Q = 0.5(W*W - S*S) where W is the vorticity magnitude (you can find it under the "Velocity" menu) and S is the mean rate-of-strain (you can find it under "Derivatives" menu). Once you defined Q, you can generate iso-surfaces of Q for several positive values. |

Re: how to plot Q criterion in fluent
Thanks a lot for your help.
Anindya |

Q expressed with scalars or tensors...Quote:
Hi Guys, should not it be the following: Q= 0.25*(W*W - S*S)??? As the mean rate-of-strain ( scalar) is defined as:S=(2Sij*Sij)^0.5 and similarly the vorticity magnitude ( scalar):W=(2Wij*Wij)^0.5 with Sij being the mean rate of strain tensor and with Wij the mean vorticity tensor (besides, Sij=symmetric, while Wij=antisymmetric part of mean velocity gradient tensor).Finally the second invariant of velocity grad tensor is defined as: Q=0.5*(Wij*Wij - Sij*Sij). So I think Q should be: Q= 0.25*(W*W - S*S). Is that right?Besides, this formula of Q is only valid for incompressible flows, more precisely for flows with divergence free velocity field so that divUj=0 <=> Sii=0 ! Thanks! |

Q expressed with scalars or tensors |

Q criterion ReferenceHi la7low,
Could you please give the reference, from where you have taken this Q criterion. Thanks |

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