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Old   May 23, 2013, 10:20
Default Problems about propeller simulation
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I want to analyze a ten-bladed thruster propeller. I found that thrust calculated is just 50% of which I predicted.

I used rotating reference frame for the propeller-fluid domain where I gave the rpm (1698) with which the propeller is rotating.

-The global domain entrance is defined as inlet (velocity component W=237 ms^-1 (0.8Ma at 10000m) ) and turbulent intensity 5%.

-The reference pressure is defined as 26500Pa (at 10000 m)

-The opening is defined as opening with static pressure = 0.

-And the outlet also is defined as opening with static pressure = 0.

-I defined fluid-fluid interfaces between the stationary default domain and the rotating propeller domain.

-The frame change option is set on both fluid-fluid interfaces to frozen rotor.

I alter the rpm from 500 rpm to 1698 rpm ( at step about 500rpm) in order to ensure it would convergence.

I am not sure what led to the result far from what I predicted. I use force_z()@BladeSurface to calculate the thrust.

And the he 3D streamlines from blade suface confused me.

covergence.jpg

boundary condition.jpg

streamline.jpg

Blade streamline.jpg

Thanks

Liu
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Old   May 23, 2013, 13:13
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Benny
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Hi,

try not static pressure at outlet. A more "realistic" description is "opening pressure =0".
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Old   May 23, 2013, 17:20
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Hi,

very interesting, I made some prop analysis to.

in my design prop open or outlet boundary conditions give no sense.

I just want to ask is it rotation domain should not to be a bit longer? here considered no effect of propeller end tip along over flow?

in my design prop end tip turned up and rotation domain can not be same
length as prop
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Old   May 24, 2013, 06:25
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Glenn Horrocks
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Sounds like a FAQ to me: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F
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Old   May 24, 2013, 09:26
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Hi, Benfa

Thank you for your answer. I changed the boundary condition as you said, but I got the similar result. The 3D streamline seems incorrect. The wake behind propeller is not like screw surface( because of the high Mach number?) .And near the blade, streamline looks like discontinuous. I am confused by this phenomena. Please tell me if more information should be provided.

These pictures are about 3D streamline of the new result. 500 points were chosen on the blade surface.

Thank you
Liu
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Old   May 24, 2013, 09:52
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Glenn Horrocks
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The discontinuous streamlines are because you are plotting velocity. If you plot velocity in the stationary frame they will be continuous.
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Old   May 24, 2013, 10:08
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Hi ghorrocks,

Thank you for your attention.

In this simulation, the radial length of rotation domain is about 1.2 times of the blade. The length of rotation domain about half of the blade before and after the propeller.

I noticed that your design has a turned up tip. I am curious about it since this is usually used on a wing to diminish the induced drag and wonder if it could have a good performance used on a rotation blade. I referred to Adkins's method ( in Design of Optimum Propellers) when I was programming. Sine the result seems deviate from what I got from the code even used date the paper provided, I hope you could give me some advices about aerodynamic design of prop at a high Mach number if convenient.

Thank you
Liu
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Old   May 24, 2013, 10:19
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Thank you ghorrocks

I am not sure why the streamline in reference frame near the blade looks like discontinuous. I search the help document of ANSYS, but could not find any useful information.

Regards
Liu
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Old   May 24, 2013, 19:13
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I should write an FAQ on this, been asked a million times...

The variable "Velocity" is relative to the local frame of reference. If the frame of reference is rotating then it is relative to the rotating frame - so it will be different to the stationary frame by radius x omega. So when streamlines go from a stationary frame to a rotating frame the velocity jumps by radius x omega.

To see continuous streamlines plot the variable "Velocity in stationary frame". This is referenced to the stationary frame regardless of whether the frame is rotating or not, so will be continuous.

UPDATE: Just added this as an FAQ: http://www.cfd-online.com/Wiki/Ansys...f_reference.3F
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Old   May 25, 2013, 07:01
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Hi,ghorrocks

Thank you for your attention.

I understand the discontinuous on the stationary and rotation interface. What I mean is that the streamline near the balde surface looks like short lines or points. In the picture, their color is blue and seems on the blade surface.

Thank you,
Liu
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Old   May 25, 2013, 07:29
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Glenn Horrocks
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But the velocity is zero at a surface. So you can't start streamlines at a surface because they don't go anywhere! So all you are seeing is a bit of numerical noise. You have to seed your streamlines somewhere the flow actually moves.
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Old   July 2, 2014, 23:38
Default cfd help
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Mohsin
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hi
sir i am doing same cfd of propeller using MRF approach. i am also getting very low thrust how can i get rid of it ????
my email is
twinklingbeacon@gmail.com
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Old   July 2, 2014, 23:40
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That's an FAQ as well: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F
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Old   July 3, 2014, 00:50
Default cfd help
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sir FAQ page is not proving helpful right now.
can u pin point what expected error would be due to which thrust is too low at least 4 times. . . . . .
its 4 bladed propeller
inlet velocity = 100 m/s
rpm = 1020
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