Modelling an accelerating propeller

siw · September 21, 2011, 06:06

Hi,

I need to model an external aerodynamics propeller on a vehicle that is locked in a stationary position and then at some time whilst the vehicle is in forward motion the propeller becomes free to rotate and after some time it reaches a constant angular velocity, whose time to constant rotation is determined by the airflow in motion past the propeller and vehicle. Also, the propeller axis of rotation is not aligned with any of the 3 global Cartesian axis.

I've not currently got the MFR capability in CFX as I only have CFX-Flo (the add-on will be bought for this task) and so I have not been able to do the similar tutorials as practice. Although I have read the tutorials.

Can CFX model a situation whereby a propeller is rotationally acclerated from rest without having to write any CELs? Are they any useful tips I should consider for this?

Thanks.

ghorrocks · September 21, 2011, 07:44

Firstly, consider whether your proposal is the best way. It would be a lot easier to model the propeller at various rotational speeds and get a speed versus torque curve. Then you can do a simple integration of the prop speed taking into account inertia and anything else. Much simpler.

If you insist on doing this using an accelerating propeller I think you can link the rotational speed to the torque and inertia of the prop. There have been a few posts on the forum of how to do this.

But again, I strongly recommend you do the psuedo-transient approach if it is appropriate. It will be MUCH easier.

siw · September 21, 2011, 10:53

Thanks for the reply Glenn.

One of the objectives of the work is to determine the time it takes for the propeller to reach its constant rotational velocity, so hopefully it'll come out of the integration.

Just had another thought. Now that CFX v13 has the 6DoF rigid body solver in it - could that be used for the motion of the propeller? I've read the other posts about the angular speed vs. torque curve that Glenn mentioned but I'm not sure about extracting the propeller motion information from that because wouldn't there be no torque when the propeller was locked at time = 0 seconds and also when it's at its constant speed - since in both cases there'll be zero angular acceleration.

September 21, 2011, 06:06	Modelling an accelerating propeller	#1
siw Senior Member Stuart Join Date: Jul 2009 Location: Portsmouth, England Posts: 742 Rep Power: 26	Hi, I need to model an external aerodynamics propeller on a vehicle that is locked in a stationary position and then at some time whilst the vehicle is in forward motion the propeller becomes free to rotate and after some time it reaches a constant angular velocity, whose time to constant rotation is determined by the airflow in motion past the propeller and vehicle. Also, the propeller axis of rotation is not aligned with any of the 3 global Cartesian axis. I've not currently got the MFR capability in CFX as I only have CFX-Flo (the add-on will be bought for this task) and so I have not been able to do the similar tutorials as practice. Although I have read the tutorials. Can CFX model a situation whereby a propeller is rotationally acclerated from rest without having to write any CELs? Are they any useful tips I should consider for this? Thanks. Last edited by siw; September 21, 2011 at 07:10. Reason: Extra info

September 21, 2011, 10:53		#3
siw Senior Member Stuart Join Date: Jul 2009 Location: Portsmouth, England Posts: 742 Rep Power: 26	Thanks for the reply Glenn. One of the objectives of the work is to determine the time it takes for the propeller to reach its constant rotational velocity, so hopefully it'll come out of the integration. Just had another thought. Now that CFX v13 has the 6DoF rigid body solver in it - could that be used for the motion of the propeller? I've read the other posts about the angular speed vs. torque curve that Glenn mentioned but I'm not sure about extracting the propeller motion information from that because wouldn't there be no torque when the propeller was locked at time = 0 seconds and also when it's at its constant speed - since in both cases there'll be zero angular acceleration. Last edited by siw; September 22, 2011 at 04:33.

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September 21, 2011, 07:44		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Firstly, consider whether your proposal is the best way. It would be a lot easier to model the propeller at various rotational speeds and get a speed versus torque curve. Then you can do a simple integration of the prop speed taking into account inertia and anything else. Much simpler. If you insist on doing this using an accelerating propeller I think you can link the rotational speed to the torque and inertia of the prop. There have been a few posts on the forum of how to do this. But again, I strongly recommend you do the psuedo-transient approach if it is appropriate. It will be MUCH easier.