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Old   September 1, 2013, 02:51
Default Aboout prop efficiency
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Hello,

I try to simulate a propeller NACA 6623-A published in NACA 650, 1939. But my results are different from the data. The efficiency is only 0.42 at advance radio=0.65 as well as that from the data is 0.8.

The following is how I calculated the efficiency:
force_z()@BladeSurface
torque_z()@BladeSurface*25.24*2*3.1415926
ForceZ*massFlowAve(Velocity in Stn Frame w)@Inlet/Power
(the rotation speed of the propeller is 1514 rpm)

If CFX-pre(.def) setting is need, please tell me your e-mail.

Thank you very much.

Liu
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Old   September 1, 2013, 05:59
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Where does the 25.24 come from in the torque equation? But you do not use the torque equation to calculate anything anyway.

Is there any components in the Velocity in Stn Frame at the inlet?

Are you calculating this in CFD-Post or in the solver as a monitor point?
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Old   September 1, 2013, 06:18
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Hi,

Thank you for your reply

Sorry, I forget list the left of the equation.

ForceZ=force_z()@BladeSurface
Power=torque_z()@BladeSurface*25.24*2*3.1415926
Efficiency=ForceZ*massFlowAve(Velocity in Stn Frame w)@Inlet/Power

25.24 is the rotation speed (r/s) = 1514/60

Every parameter is calculated in both CFX-Solver and CFX-Post

Liu
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Old   September 1, 2013, 06:24
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And there is only Z component velocity at the Inlet.
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Old   September 1, 2013, 06:35
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No need to put 25.24 in. In fact it is bad practise to do this - it obscures where this number comes from and does not contain the correct units (does not resolve to power).

Rather just put in 1514 [rpm], and let CFX handle all the unit conversions. Then you can drop the 2*pi as well.

Your general question is a FAQ: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F
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Old   September 1, 2013, 22:08
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Thank you, ghorrocks

But thrust which is ForceZ in the expression is only 1/3 of that from the data. ForceZ was obtained directly from the expression CFX provided. So I'm not sure why the results are so different.

Best,

Liu
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Old   September 2, 2013, 04:49
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Quote:
Originally Posted by duguyoudi View Post
Thank you, ghorrocks

But thrust which is ForceZ in the expression is only 1/3 of that from the data. ForceZ was obtained directly from the expression CFX provided. So I'm not sure why the results are so different.

Best,

Liu

I know its a simple and u might have checked it, but are the propeller sizes same ? and if yes, how many propellers were simulated in the paper and how many are u simulating ? Also are you sure about the formula ?
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Old   September 2, 2013, 06:38
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Thank you D.B.

The propellers sizes are same. I only simulated situations when blade angle at 0.75R is 15 degree.

Wind tunnel tests of six propellers were reported in the paper and I only simulated the NACA 6623-A propeller.

I am not sure what the formula means? If you indicated the expression of ForceZ, I look up in the help document of CFX: force_x()@wall1 returns the total force in the x-direction acting on wall1. I think it could be used to calculate the thrust. By the way, fluid model is ideal air in all domains.

Best,

Liu
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Old   September 2, 2013, 07:47
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Have you check the FAQ link I posted?

Your question is "why is my result inaccurate?" and this is a VERY frequently asked question so we wrote an FAQ to cover the key things to check.
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Old   September 2, 2013, 22:03
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Yes, I am now changing boundary conditions and mesh to find out my mistakes.

Best,

Liu
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Old   September 4, 2013, 02:51
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Hi,ghorrocks

I look up the help document of CFX-Pre. I found that the torque function may raise error when calculating the torque of 3D surface. So how to obtain the value of torque or the power?

Thank you

Liu
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Old   September 4, 2013, 02:55
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No, the documentation says "If the location specified is not 2D, an error is raised." This is trying to say any surface (including with 3D curvature) is fine, but a 3D region like a domain will cause an error.

So you can use the torque function.
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Old   September 4, 2013, 03:13
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Thank you.

The thrust of the blade fits the data, but the power obtained from torque is about 30% higher. If I use mass flow average total temperature to calculate the power, the result is 10 times compared to the date. It confused me since I could not find obvious mistakes.

Best,

Liu
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Old   September 4, 2013, 06:03
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So now that you are close the FAQ I quoted in post #5 is what to do next.
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Old   September 4, 2013, 09:35
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Hi,ghorrocks

Do you have example of propellers (better .def)? I think that could be helpful.

Thank you very much.

Liu
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Old   September 4, 2013, 17:49
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No, I do not. ANSYS support probably does. And the CFX tutorials have some which are similar as well.
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