# Aboout prop efficiency

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 September 1, 2013, 02:51 Aboout prop efficiency #1 New Member   Join Date: May 2013 Posts: 19 Rep Power: 4 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

 September 1, 2013, 05:59 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 11,045 Rep Power: 86 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?

 September 1, 2013, 06:18 #3 New Member   Join Date: May 2013 Posts: 19 Rep Power: 4 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

 September 1, 2013, 06:24 #4 New Member   Join Date: May 2013 Posts: 19 Rep Power: 4 And there is only Z component velocity at the Inlet.

 September 1, 2013, 06:35 #5 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 11,045 Rep Power: 86 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

 September 1, 2013, 22:08 #6 New Member   Join Date: May 2013 Posts: 19 Rep Power: 4 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

September 2, 2013, 04:49
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DB
Join Date: Apr 2011
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Rep Power: 6
Quote:
 Originally Posted by duguyoudi 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 ?
__________________
-D.B

 September 2, 2013, 06:38 #8 New Member   Join Date: May 2013 Posts: 19 Rep Power: 4 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

 September 2, 2013, 07:47 #9 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 11,045 Rep Power: 86 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.

 September 2, 2013, 22:03 #10 New Member   Join Date: May 2013 Posts: 19 Rep Power: 4 Yes, I am now changing boundary conditions and mesh to find out my mistakes. Best, Liu

 September 4, 2013, 02:51 #11 New Member   Join Date: May 2013 Posts: 19 Rep Power: 4 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

 September 4, 2013, 02:55 #12 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 11,045 Rep Power: 86 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.

 September 4, 2013, 03:13 #13 New Member   Join Date: May 2013 Posts: 19 Rep Power: 4 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

 September 4, 2013, 06:03 #14 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 11,045 Rep Power: 86 So now that you are close the FAQ I quoted in post #5 is what to do next.

 September 4, 2013, 09:35 #15 New Member   Join Date: May 2013 Posts: 19 Rep Power: 4 Hi，ghorrocks Do you have example of propellers (better .def)? I think that could be helpful. Thank you very much. Liu

 September 4, 2013, 17:49 #16 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 11,045 Rep Power: 86 No, I do not. ANSYS support probably does. And the CFX tutorials have some which are similar as well.

 Tags effieciency, propeller

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