Learning Forum

[FranckMing]

Hi to all, i have some questions for study a centrifugal water pump. I write because I need to understand if it's right. I have the rotatingDisk between volute and fan.

[YasserSelima]

HellonI am not sure about your first question. But I can answer your second question.nFor the rotating disk, it should be stationary relative to the moving zone.nUnder solution monitor, you will find Residuals .. here you can determine how much residuals you want to save and plot.nAlso you can enable autosave and save case and data files every certain number of time steps ... then load them when you want and start iterations.n

[YasserSelima]

By the way, why don't you try turbogrid ?n

[FranckMing]

Hi YasserSelima thank you very much for third answer, it's fantastic thanks a lot. I don't use turbogrid because i don't have generated the impeller, it is been drawn from my Professor. What do you think is better to analyze with turbogrid? Another question, is better increase the number of cells or to generate inflation on the superface of impeller?n

[Rob]

Turbogrid will do much the same job but is focused on turbo/pumps. For a student project I'd stick to the normal solver interface as it's more widely used. nRe mesh resolution. Yes. You need to resolve the near wall region (blades) to get the drag right, but also the bulk flow field to model the wakes and blade-gap flow features. The former is why you'll see comments on y+ the latter is a bit more of an art as you need to look at the flow gradients after the run and decide if they're well enough resolved. n

[FranckMing]

Thanks Rob, you are very important for this forum, thanks for your job. I have read another your comments for to generate a boolean controls, you can said if my job with design modeller is good? Thanksn

[Rob]

It looks OK from what I can see: the test is whether it runs and gives good results! nI may use that post in my annual review... n

[FranckMing]

Yes when I have finished, I upload the images of my work.n

[FranckMing]

Hi Sr.Rob I have a question, when i analyze the pump, the fluent write me this message nPreparing mesh for display...nNote:zone-surface:cannot create surface from sliding interface zone.nCreating empty surface.nNote:zone-surface:cannot create surface from sliding interface zone.nCreating empty surface.nDone. nthis is a problem?nSecondo question, is if the my inflation is good for to calculate the reynolds number.nthird question I need to know the difference between Transitor e Steady Time.nDifference between Hybrid inizializzation and Standard inizializzation. thanksn

[YasserSelima]

Hybrid initialization start from your inlet and does few iterations to give a good estimate of the pressure and velocity in the flow field. nStandard is the old traditional way where give every region a certain value of velocity and pressure. nHybrid is better if you don't have a specific initial condition in your case n

[YasserSelima]

The mesh are too large!! They are in order of meters!!n

[FranckMing]

Ok thanks for answers, i have a problem with mesh because i use a student version. Number of cells is 420 000.n

[YasserSelima]

I will tell you an advice and I know will not agree.nScale the problem. When you have large geometry, scale it down. Most probably your remember the similarity chapter in Fluid Mechanics. Ensure dynamic similarity and you are able to scale large geometries and get reasonable accuracy.n

[FranckMing]

Yes Sr.YasserSelima I remember but I don't knew this. Ok thanks a lot for answers n

[FranckMing]

Sr.YasserSelima if you can show me this operation I'd appreciate that, or post link. Thanks thanks.n

[YasserSelima]

Hmmm ...nFirst you are going to scale every length .. let's say divide every length by 10 or 20nThen use the Pi theorem and create your dimensionless parameters .. then make sure all dimension less parameters in your model are equal to that in the real pump. You will need to scale velocity and rotational speed and you might need to change the fluid properties , but make this your last option.nFor turbo machines three dimensionless parameters are common for turbo machines Q/(ND^3) , (gH)/(N^2 D^2) and (Power/ rho . N^3 . D^5) ... I am not sure of the last , check it .. Also you can add to this Re number to have effect of viscosity and NPSH/H ... Do a google search and probably you will find morenYou can find the theorem in most Fluid Mechanics textbooks .. Search for similarity ( Fluid Mechanics by White chapter 5, I believe)nAlso most turbo machines textbooks will have chapter on this as it is important to predict custom made pumps, efficiency. n