Ducted propeller MRF issue
 

So, basically, i'm working on a project that involves the usage of ducted propellers and since the tip clearance of the propeller to the duct surface is extremely small, problems occur between the interface of the rotating domain and the bottom surface of the duct.

My question is, if i include the duct into the rotating domain instead of just trying to capture the hub and blades, would the results be reliable?

Cheers,

Cagri.

Hi Cagri,

I am trying to simulate something similar, and I am also having the same idea of including the Duct inside the rotating domain. Can you please share your experience, it would be very much valuable. I am actually using OpenFOAM to simulate the propeller with duct. But the principles should be the same.

Thanks,

K. Rao

Cagri,

I don't know if it's possible to have interfaces that don't line-up on the same planar face, but I am interested to find out whether it worked or your final solution.

I had a similar problem 2 weeks ago on a pump analysis where the impeller outer diameter is almost touching the pump casing. There was not enough space to have a wall/interface between the two edges i.e. rotating domain (impeller) and the static domain (volute). The elements here were extremely small or skewed.
So, what I did was to have the rotating frame outer diameter just touch/penetrate the tip of the impeller i.e. the impeller edge 'broke through' just the domain. I think this is acceptable because I came across web images showing similar concepts.

Curious to hear how you resolved it though.

Thanks.

Hi Arnie,

Can you please explain, how did you resolve the above issuey? It would be interesting to know, as I am stuck with this problem from long time.

Thanky ou

Sometimes a picture is worth a thousand words.

I have attached a screenshot of a similar concept I found on the internet.

Note how the tip and flanks of the pump vanes break the surface of the rotating domain !

Hi Arnie,

Thank you, for the explanation. I will see how I can implement it in OpenFOAM, for a MRF simulation involving AMI interface.

Hi Arnie,

I have got the results, thank you. But the results are 7% lower than what I have expected to be. To study this I have varied the dimensions of MRF domain, and found out if the size of MRF domainis less than 1.3D x 0.3D the simulation results keep on decreasing. Is it same in ANSYS too?

I am curious to know, thank you.

What CFD software are you using ?

I'm not sure I understand the MRF dimensions (in reference to impeller diameter) you are talking about ... can you post some screenshots ?

PS. A difference of 7% to experiments does not seem like a big amount. Rarely does one get such good results on first attempt with all the variables/assumptions with boundary conditions, operating conditions, equipment variability, etc.

Hi Arnie,

Thank you very much for your kind reply, and I have missed my notification and sorry for the late reply. I am using OpenFOAM. The diameter of MRF region I mean. For example I have a propeller of 450 mm. If I chose the rotor region(MRF) to be around 460 mm, the simulation takes too long to converge and the results are around 20 % away from my experimental results. But if I chose any diameter of rotor region greater than 580 mm. The simulation converge fast, and the results will not change, if I increase my diameter too.

Is it same in ANSYS also? Are the results are so much dependent on MRF region size?

Waiting for your comments eagerly.

Thank you.