Predicting total force on a rotor blade

Muntasir · March 21, 2009, 12:24

In a 2D simulation for flow around rotor blade i've predicted the force on the rotor, the result deviated 20% from theoratical caculations.

1. How does fluent predict the force, equation wise.
2. What are the factors that effecting the result.

Best regards

O.D.Y. · March 21, 2009, 13:20

Hi,

what do you mean with "theoretical calculation"...?

As far as I know, the calculation of the force depends on severals parameters.

first of course your mesh-density. You need to be sure to have a AS FAR AS POSSIBLE "mesh-independent" solution. I mean 20% is a lot. You can find out about your quality by successively increasing the number of mesh cells and watch the force change its value.

Than you need to be sure about the resolution of the boundary layer close to the wall although the viscous force will be of minor interest compared to the order of magnitude of the pressure force.

FLUENT calculates the force by integrating the pressure over the blades surface around the blade which is described in the User Guide. And as far as i remember the force depends on the depth and some more values (like the pressure) in the reference value menu.

regards

Muntasir · March 22, 2009, 12:26

Hi,
I mean by theoratical calculations; the estimation of the force on the blade from derived equations for an axial flow turbine. this because i've not experimental results to compare with.

For this simulation i've used ske turbulence model with standard wall function. is the wall function have a great deal in the results? if we assume that the mesh is fine enough.

Thanks for your reply and advice.

paka · March 22, 2009, 22:49

The details how forces are computed could be found in Fluent User Manual:
29.3.1 Computing Forces, Moments, and the Center of Pressure

I cannot say about wall functions, I know boundary layer makes a big deal. For example, I was simulating pretty fast flow in one direction. For the majority of the flow no turbulence was involved, water traveled really fast (btw. it was two-phase flow model). I was told and similarly expected there was no need for boundary layer refined mesh. Results showed something else, with y+ value jumping up to 1000. Even mesh refinements somehow did not help too much. Just to be sure I would keep that area refined from beginning.

If you could provide some details about your model that would be great!

1. How long was your simulation? I mean how many seconds of real flow were your simulating?

2. How much time did all computation take for ske model? and on how many processors did you calculate it.

Best!

March 21, 2009, 12:24	Predicting total force on a rotor blade	#1
Muntasir New Member Muntasir Join Date: Mar 2009 Posts: 2 Rep Power: 0	In a 2D simulation for flow around rotor blade i've predicted the force on the rotor, the result deviated 20% from theoratical caculations. 1. How does fluent predict the force, equation wise. 2. What are the factors that effecting the result. Best regards

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March 21, 2009, 13:20		#2
O.D.Y. Member Join Date: Mar 2009 Posts: 49 Rep Power: 17	Hi, what do you mean with "theoretical calculation"...? As far as I know, the calculation of the force depends on severals parameters. first of course your mesh-density. You need to be sure to have a AS FAR AS POSSIBLE "mesh-independent" solution. I mean 20% is a lot. You can find out about your quality by successively increasing the number of mesh cells and watch the force change its value. Than you need to be sure about the resolution of the boundary layer close to the wall although the viscous force will be of minor interest compared to the order of magnitude of the pressure force. FLUENT calculates the force by integrating the pressure over the blades surface around the blade which is described in the User Guide. And as far as i remember the force depends on the depth and some more values (like the pressure) in the reference value menu. regards

March 22, 2009, 12:26		#3
Muntasir New Member Muntasir Join Date: Mar 2009 Posts: 2 Rep Power: 0	Hi, I mean by theoratical calculations; the estimation of the force on the blade from derived equations for an axial flow turbine. this because i've not experimental results to compare with. For this simulation i've used ske turbulence model with standard wall function. is the wall function have a great deal in the results? if we assume that the mesh is fine enough. Thanks for your reply and advice.

March 22, 2009, 22:49		#4
paka Senior Member Join Date: Mar 2009 Posts: 225 Rep Power: 18	The details how forces are computed could be found in Fluent User Manual: 29.3.1 Computing Forces, Moments, and the Center of Pressure I cannot say about wall functions, I know boundary layer makes a big deal. For example, I was simulating pretty fast flow in one direction. For the majority of the flow no turbulence was involved, water traveled really fast (btw. it was two-phase flow model). I was told and similarly expected there was no need for boundary layer refined mesh. Results showed something else, with y+ value jumping up to 1000. Even mesh refinements somehow did not help too much. Just to be sure I would keep that area refined from beginning. If you could provide some details about your model that would be great! 1. How long was your simulation? I mean how many seconds of real flow were your simulating? 2. How much time did all computation take for ske model? and on how many processors did you calculate it. Best!