"Unusual" result in 3D mixing

[Pádraig]

Hello,

 

I'm currently doing 3D mixing in a computer programme called Ansys. I have a cylinder full of water, featuring a square cuboid of calcium (the rectaungular cuboid is a stirrer bar, and the smaller cylinder is a mixing zone. I have gravity defined as the standard 9.81 m/s² downwards) :

 

 

 

I have the mixing set at 60 rpm at the moment.

I find that, after 2.5 minutes, my calcium is down around the bottom (this is a volume fraction graphic, with more blue meaning higher fraction of water, and as the colour leans into red, you have more calcium) :

 

 

Then, as the mixing continues, the homogeneity of the entire cylinder stabilises much more (This is about ten minutes in. I've calculated that 3.4671e-04 throughout the cylinder is perfectly homogenous, so it's very close now) :

 

 

My question (a basic mechanical/chemical one, I hope) is, why does the calcium initially "stick" to the bottom, despite being set as a cuboid a bit above the bottom at zero seconds, and then is "happy" to mix as time increases?

[sethoflagos]

 

33 minutes ago, Pádraig said:

My question (a basic mechanical/chemical one, I hope) is, why does the calcium initially "stick" to the bottom, despite being set as a cuboid a bit above the bottom at zero seconds, and then is "happy" to mix as time increases?

Different forces are in play with different orders of magnitude through the start up process.

When you start rotating the mixing bar, it acts like the impeller of a centrifugal pump and creates a pressure low spot at the 'eye of the pump'. This draws water down past the calcium block while the vortex is developing its parabolic profile above.

The calcium rich water is then propelled radially outward until the flow regime is fully established.

Now entropy takes over and calcium slowly diffuses up through the water column until it's evenly distributed. 

Edited January 23, 2023 by sethoflagos
typo

[Pádraig]

22 hours ago, sethoflagos said:

 

Different forces are in play with different orders of magnitude through the start up process.

When you start rotating the mixing bar, it acts like the impeller of a centrifugal pump and creates a pressure low spot at the 'eye of the pump'. This draws water down past the calcium block while the vortex is developing its parabolic profile above.

The calcium rich water is then propelled radially outward until the flow regime is fully established.

Now entropy takes over and calcium slowly diffuses up through the water column until it's evenly distributed. 

 

That's really interesting, thank you. I've never really looked at fluid flow in that "detail" (may not be detailed to ye!), I'll look into what you've talked about now.

 

I'm assuming I can't give you reputation as I'm a new member?

[studiot]

33 minutes ago, Pádraig said:

 

That's really interesting, thank you. I've never really looked at fluid flow in that "detail" (may not be detailed to ye!), I'll look into what you've talked about now.

 

I'm assuming I can't give you reputation as I'm a new member?

You surely can and I agree Seth deserves one.  See below to make your choice of like (blue) or upvote (green) .

The only restrictions are that new members are restricted to 5 posts in their first 24 hours to prevent spambombers.

 

[Pádraig]

37 minutes ago, studiot said:

You surely can and I agree Seth deserves one.  See below to make your choice of like (blue) or upvote (green) .

The only restrictions are that new members are restricted to 5 posts in their first 24 hours to prevent spambombers.

 

 

I'm getting the issue that there's no heart for Seth's OP, so when I went looking around it I couldn't give it karma; I have one for your post (both attached).

 

Edit: reloaded page and I was able to give him the karma, heart appeared.

 

 

Edited January 24, 2023 by Pádraig
update