[upal_arif]

Hi,

I am looking for a freelancer to simulate supersonic flow through C-D nozzle by fluent 6.2.16

Please contact upal_arif@yahoo.com

Originally Posted by sbaffini

Dear saeedi,

the lambda 2 criterion simply concerns the definition of the scalar lambda2 and how turbulent structures can be visualized by proper isosurfaces of lambda2 (like for the Q criterion). Hence, the real difference with the scalar Q is how you compute the scalar lambda2.

This is defined as the second (in magnitude) eigenvalue of the matrix:



where:





This requires the construction of the characteristic cubic equation and its resolution in order to obtain lambda2 (that is, lambda2 is the second solution of the cubic characteristic equation).

For coherent structures the matrix above can be related to the opposite of the pressure Hessian matrix. As this matrix is real and symmetric, it has two positive eigenvalues when the pressure is at minimum. As a consequence, the matrix above has two negative eigenvalues and lambda2 is certainly negative (for coherent structures). How much negative you have to pick its isosurfaces is (for what i understand) related to the visual appealing of your images (like for the Q criterion)

I still have to seriously use and check it (so it comes without any warranty) but, some time ago, i made a routine for the computation of lambda2 in Fluent (you find it attached). It is very rudimental but i remember it did the job. Also, it is basically C so you can clearly adapt the core part to your needs (Fluent related parts are very understandable also for non-Fluent users)

Once you got lambda2, you just need to pick some negative isosurface and you're done

Hope it helps