[Miss Anna]

Hi everyone,

I'm very new to CFD and Ansys CFX.

I am doing research regarding the undershot water wheel. I want to simulate the rotation of undershot water wheel when the water strikes it. But, i dont know how to make the setup so that there will be reaction on the water wheel.

For the beginning, I set the different domain between water wheel and water zone. I select the water wheel as a submerged solid and water zone to be rotating. But there are still no rotation of my water wheel..

Is there any tutorial or video that I can referred to?

Thank you very much.

[ghorrocks]

You appear to be modelling this as submerged solid, so as a rigid body simulation. You are much likely to have success if you use rotating frames of reference. Have a look at the CFX tutorial examples for how to set up rotating frame of reference simulations. Also note that the RFR approach is easiest done by assuming a speed and doing a model at that condition - and then doing a series of model to develop the system performance curve. This is much easier than a rigid body model.

[Miss Anna]

Thank you for your reply. I got very useful information from it and obtain clearer view to do the simulation.

But, can you explain more details about how to set up rotating frame of reference?

Is there any related tutorials which you would suggest for a really beginner as me.

Thank you.

[ghorrocks]

There are several examples of rotor/stator simulations which comes in the CFX tutorials. These are available under the software documentation. There are more examples available at the ANSYS customer webpage.

[Valentin Maniu]

Hi everyone,

I'm also a beginner in ansys cfx. I need to simulate a rotating circular saw so that I can evaluate the turbulences that form around the teeth for future geometry optimizations.

After watching tutorials and reading some material I'm still not sure what method I should adopt. Should I define the saw as a rigid body immersed in a fluid (air) or should I just modell the fluid around the saw and define it as a rotating domain? Do I Need to define an inlet for the fluid? Ideally the flow should stem from the rotation of the saw.

Thanks guys!

[Thomas MADELEINE]

if you can always prefer using a rotating domain than immersed solid (you will have better precision on the near wall area).

if you are working in atmosphere with no clear idea where the inlet/outlet is, I would recommend an opening (fluid can both enter or go out your domain).

[Valentin Maniu]

Thank you very mush for the answer.

What about defining the circular saw walls of the fluid as a rigid Body ?

[Thomas MADELEINE]

rigid body is useful is you want to model the interaction between the fluid and the solid (a moving ship for example). so you have to define mass, inertia, etc...
why not simply define them as walls ? maybe I have not understand your problem correctly but if you only want to see the turbulence generated by the saw you should only model the air...

[Valentin Maniu]

I would like the saw to rotate at 3000 rev/min. So I set the fluid (air) domain as rotating and I set this angular velocity. Do I have to activate the mesh deformation? I defined the circular saw walls as no slip walls but I don't know what Kind of mesh motion they need or if i need to define a wall velocity

[Thomas MADELEINE]

there is no mesh deformation here you have created and whole domain who rotate with the saw the tooth wall.

the problem I see here is that your domain is not big enough. As far as I understood, you want to model the non moving walls of your saw too (the camber).
In fact I suspect that the stronger effect will occur between the blade and the walls of the camber.
There was a topic about a fan inside a room few times ago. Have a look on it I think it is quite close of what your model should be (geometry will be different of course).

[Valentin Maniu]

Thank you so much for helping me out

I'll increase the size of the fluid as well to encompass the lateral walls of the saw, but in order to get this Simulation going do I need to fix the saw walls in place by activating the Counter rotating walls Feature so they don't rotate with the whole fluid.

I did a few simulations on just a few teeth profiles by defining an inlet where the fluid velocity was 150m/s (I assumed this was the cutting speed of the saw) and got some turbulences (photo). But I'd like to get this turbulences by rotating the fluid around the saw geometry (or the saw-geometry in the fluid). Is that possible?

[Thomas MADELEINE]

you will need a counter rotating wall or another domain (stationary) that will circle the first one, like the ceiling fan

[Valentin Maniu]

Should the side walls of the fluid be counter rotating with the saw wall as well?

[Miss Anna]

I am currently trying to make tutorials regarding Rotating Frame of Reference. But, now I am confused when to set water wheel as which type of domain. Is it the solid domain? Or fluid domain? Because in the tutorials about impeller, it set the impeller as fluid domain.

[ghorrocks]

You only model the impeller as a solid domain if you want to model something (probably temperature) inside the impeller.

Most people just model the fluid flow, so then you just model the cavity around the impeller as a fluid domain.

[Miss Anna]

Thank you Ghorrocks.

How about the immersed solid? because the research is about the undershot water wheel, where the flow of water is from below the wheel axis. Besides, I want model the flow of water when it pass the water wheel (velocity and torque produced).

So, do the immersed solid are applicable for the analysis?

Thank you very much.

[ghorrocks]

No, do not use immersed solids. Rotating machinery should be modelled as a rotating frame of reference.

[Miss Anna]

This is setup for the project, Please correct me if I'm using wrong setting.

There are two domains which the water wheel as the solid domain and the box as the river and fluid domains. I want to use the rotating frame of reference. But the problem is I'm confused how to setup the RFR. Is it available in the coordinate frame? One more thing, do I need to use domain interface since there are two domain here.

[Chris_321]

You need a mesh for the rotating domain and a mesh for the stationary domain.

Then you can specify a rotating velocity to the domain that you want to rotate. Between the two meshes you need a coupling interface.

Do you really have two meshes?

There is a tutorial in ansys available where you can get a good look into the rrf modeling. I think it is some kind of blender.

[Miss Anna]

I mesh both of the domains together/simultaneously.

So, Do I need to mesh the domain separately?
Thank you.