[Damaja]

I simulated a certain geometry in ansys student once with a power law model and then based on a carreau modell. So I chose the non newtonian fluid and added the paramters once for power law and for carreau. Geometry is both the same. Both boundary conditions are mainly the same. Still, the carreau model simulates a vortex, looks like a Karman vortex street, and the power law hasnt any vortex. So far I checked anything I could think of. Anyone has experience with those two models, and why one creates a vortex street and the other one don't?

[vinerm]

Vortex generation depends on Reynolds number. I am assuming you use same density, hence, if dynamic viscosity is same, so should be kinematic. However, did you plot Re and observe any difference. To go little deeper, try looking at the shear stress on the body.

[Damaja]

Thanks for your answer!


Here are two plots, one of the Cell Re Numbers, pretty much the same to me, or could these differecnes be it? SEcond plot from the shear rate.

How can the difference in outcome be explained using the Ostwald-de-Waele approach and
of the Carreau approach? Is there a connection
between the differences in velocity and viscosity?

[vinerm]

Not the cell Re but Re based on hydraulic diameter of the body. You have to define a CFF. Secondly, stress has to be plotted as an xy plot on the body and not as a contour.

[Damaja]

What is the cff? Can you tell me more on how to do it?

[vinerm]

I apologize to have used abbreviation. CFF is custom field function. It is just an expression defined using available fields in Fluent. Available under User-Defined. It is similar to a calculator where you can select the fields from drop-down menus and define a new equation, in your case, Re number.

For xy plot, you need to go to Plots, almost last in the tree on the left in Fluent. If you are using CFDPost, then you can define an expression instead of CFF and plot is called Chart.

[Damaja]

So here I define the new defintition in that calculator?

[vinerm]

Yes, this is where you define CFF. But do not use cell Re. Define your own function as velocity magnitude x density x hydraulic diameter of the body / dynamic viscosity

You have to choose fields for all and press select, except for hydraulic diameter. For latter, you have to enter a number and it must be in m and not mm or cm or in.

[Damaja]

Thanks! But I only find molecular viscosity!

[vinerm]

Yes, that is dynamic viscosity. It is also called molecular viscosity to differentiate from eddy or turbulent viscosity.

[Damaja]

So then, my chart looks like this, for the carreau case!

[Damaja]

Okay, here we might have a problem.

[vinerm]

For Re, do not plot a chart. Rather, use this CFF in Contour Plot. Just like velocity, you can plot contours of CFF. For the chart, you need to plot wall shear stress.

[Damaja]

Okay, you mean in the post processor? But I didn't see the cff in the drop down list, in the options of the contour plot.

[vinerm]

There are two drop-down menus, top one and bottom one. You have to first choose CFF in top menu and then Reynolds in the bottom one.

[Damaja]

I can't find it. Is it in the menu where I select the contour plot or above?
Edit: okay i saw i have to check it in data quantities first. I try again.

[vinerm]

CFFs are available within Fluent and not in CFDPost. If you want to use Post, you need to define expression within Post.

[Damaja]

I will try it. Never do contour plots in Fluent. And for the wall shear, should I do it in Fluent or post?


Edit: Here are the contour plots.

[vinerm]

Personally, I prefer and recommend Fluent but it is as per user's convenience. You can do both in Fluent and both in CFDPost.

[Damaja]

In both diagrams, the wall shear is zero. I suppose, something is wrong?