[AS_Aero]

Hi All

I am trying to simulate a rotor simulation (image attached) here there is a back ward facing step where the flow is recirculating at the outflow region, my doubt is should I increase the domain at the outflow region inorder to completly resolve the recirculation.
Also there is a recirculation near the rotor domain as well. I dont want to have these effects on my rotor. So kindly give me some suggestions regarding the same.
Then at the upper side I have placed a no slip wall, so now its like a channel flow , from the streamlines it shows no effect on the rotor domain, so thinking of reducing the domain. Is there any rule of thumb or some suggestions ? Kindly let me know.

[ghorrocks]

Should I move my downstream boundary further away? Do a sensitivity check and work it out for yourself. Run another model with the boundary double the distance away. Then check the simulation for key output parameters of interest to you. If the key output parameters changed significantly then you need to keep moving the outlet boundary further downstream until it converges.

Recirculation near rotor? This is a physical design thing. Assuming the simulation is correct, then you need to try different designs and rotor configurations to stop or reduce this.

Top wall position? See first answer. Do a sensitivity study and determine for yourself how far away the top boundary needs to be. This is on the assumption that you are trying to simulate this device operating in free atmosphere.

[AS_Aero]

Dear Glenn

Thanks a lot for your reply. Ok eventually I will do this, but my doubt was is it a good idea if we cut through the recirculation region our domain ?

And I have kept the bounday conditon at outflow as outlet and the top as no slip walls. Is it better to keep no slip walls or should I change to free slip walls ?

Then I am doing this as a quasi 2D simulation with one cell thickness along the Z direction (5mm) as of now. But I also want to know if there is any rule of thumb that my domain in the Z direction should be x thickness based on my geometry dimension ? I have attached the geometry with dimensions. Kindly let me know if there is any rule of thumb for this

[ghorrocks]

The short answer is: It is not a good idea to put a boundary in or near a recirculation.

Top wall slip or no slip? Slip would be better but it still constricts the flow. You really need to do the sensitivity analysis to determine this.

The thickness of the 2D mesh should be approximately the smallest element edge length.

[AS_Aero]

Hi Glenn

Some people say it should be 1/100th of the biggest length, that means here in my case the length in X direction* 1/100.

And by your suggestion : The smallest element edge length means, does this comes from the mesh element edge or you mean the smallest edge length in the 2D geometry ?

If its the mesh element edge length then I will have smallest element near to the wall right ? As I will have some inflation with first layer thickness of 1e-6m to have a y+ of 1.
Correct me if I am wrong.

[ghorrocks]

And some people say other fractions - that is why you should do a sensitivity study to work out what you need in your case.

I do not understand your question about the smallest element edge length. I am simply saying the Z direction extrusion length should equal the smallest element edge length in the model. Yes, it is likely this occurs near a wall.

[AS_Aero]

Hi Glenn

Thanks for your suggestion, but still not clear about this '' smallest element edge '' do you mean the smallest mesh element edge length or the smallest edge length in my geometry ? (Like if its a channel of length 10m and height 5m, then the smallest edge is 5m)

[ghorrocks]

I am referring to mesh element edge length, not geometry edge length.

[AS_Aero]

How can I measure the minimum and maximum edge length of a surface geometry ?

How can I get the information about it ? And what is the minimum and maximum Aspect Ratio whic is allowed or considered to give good results.

And my minimum edge length will be my first layer thickness of the boundary layer right ?
And maximum edge length will be the length of the element at the far field as I coarsen my mesh at the outflow region.

Kindly help me with this.

[ghorrocks]

You are taking that a little too literally. An estimate of the smallest edge length is fine, there is no need for precision. So just use the height of your first inflation layer.

[AS_Aero]

Ok Perfect....!! Thanks a lot Glenn