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Agitated Tank simulation with ketchup

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Old   September 8, 2019, 14:36
Default Agitated Tank simulation with ketchup
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Mario Sanchez
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Good day
im having some issues interpreting the results of my simulation of an agitating Tank with ketchup.
Ive made a comparisson in a 20cm high and 15 cm diamter agitation tank with a stirrer with four 45 blades. In one case it is filled with water and in the other with ketchup. Both rotating at 334 RPM. Using the VOF method ive been able to prove that the vortex formed in both cases are quite different. Ketchup is significantly more viscous than water, and by agitating in the same RPMs we see a bigger depth of a vortex in the tank with ketchup with this weird behaviour. Why could this relative big deformation in such a high viscous medium occour? Is it logical?
The ketchup viscous model data was generated with a Rheometer so I dont think the problem lies here.


Thanks a lot if you can help me
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File Type: png ketchup.PNG (160.9 KB, 7 views)
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Old   September 8, 2019, 20:03
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Glenn Horrocks
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Before you start inferring things from your results, have you done the standard verification and validation checks? If your result is inaccurate you are wasting your time trying to understand the results. See FAQ: https://www.cfd-online.com/Wiki/Ansy..._inaccurate.3F

If you have confirmed your results are good then you can consider your results. Before we can interpret your results, we need to know what you modelled. Are all the material models Newtonian? If non-Newtonian, what model did you use? Did you use surface tension? What wall surface energy/contact angle did you use? What initial condition did you specify?
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Old   September 9, 2019, 15:01
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Mario Sanchez
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Hello and thanks for your reply.
It has been validated.
The model I used for the one in Ketchup is the Herschel Model. The result with water is with the standard CFX settings.

For the free surface settings I have the ones on the image.
The surf. tens. coeffiecient for water and air is 0.0727Nm-1 and Ketchup 0.032Nm-1.
Both simulations have a timestep of 0,01s with a total time of 3s.
Should I use the Volume fraction smoothing type model?
Both are rotating at 334 RPMs.
The stirrer is in a rotating domain, whereas the Wall and floor in a stationary domain. The openings are considered as openings in each of their domains and the relative pressure is 0.

Thanks for your time
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Old   September 9, 2019, 19:50
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I would think it highly likely that the strange behaviour of the ketchup model is due to the non-Newtonian model you used. The Herschel model has a yield point then a power law, meaning that below yield the fluid does not move much. This could explain the fluid sticking to the central region and the top surface being flat then a big concave section.

If you want to confirm that it is the non-Newtonian model causing these effects I would do a sensitivity analysis on the Herschel Model constants. Do simulations where you increase and decrease the model constants and see what effect they have on results. I suspect you will find the yield stress constant has a major effect on the result.
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