Applying thurst force to a face

krihamm · February 12, 2018, 11:15

Hi,

I am simulating drag over a rocket for different speeds. I need to apply a force representing the thrust of the rocket at the exhaust in order to determine correct pressure drag. I have tried searching for a way to do this, but with no luck. Does anyone know how this could be done?

Thanks!

ghorrocks · February 12, 2018, 16:52

Are you modelling the rocket as a moving body (so the force is required to get the rocket to move) or are you modelling the rocket fixed in its frame of reference and you want the rocket exhaust to be included so the entrained flow and drag effects are included?

I hope it is the fixed in its frame of reference case as that is the best way to do this simulation for most cases.

krihamm · February 13, 2018, 00:07

Quote:

Originally Posted by ghorrocks

Are you modelling the rocket as a moving body (so the force is required to get the rocket to move) or are you modelling the rocket fixed in its frame of reference and you want the rocket exhaust to be included so the entrained flow and drag effects are included?

I hope it is the fixed in its frame of reference case as that is the best way to do this simulation for most cases.

Yep, fixed in its frame of reference.

ghorrocks · February 13, 2018, 00:11

Excellent.

In that case the simplest approach for the rocket exhaust would seem to be an inlet boundary with the flow conditions of the rocket exhaust specified, presumably supersonic flow.

krihamm · February 13, 2018, 01:55

Quote:

Originally Posted by ghorrocks

Excellent.

In that case the simplest approach for the rocket exhaust would seem to be an inlet boundary with the flow conditions of the rocket exhaust specified, presumably supersonic flow.

I will try that

If I have thrust data for the rocket and I want it to be specified as an inlet - would the correct approach be to use normal speed and pressure as mass and momentum option, and then calculate the pressure equivalent to that of the exhaust from P=F/A, where F in this case would be the thrust force and A the area of the exhaust?

ghorrocks · February 13, 2018, 04:24

This web page seems to be rockets 101: https://www.grc.nasa.gov/www/k-12/airplane/rockth.html

In the thrust equation, if you know the force then you have m(dot), V and p(e) as unknowns. This assumes you know the geometry for the exit geometry. If you know the throat geometry then you can probably have a reasonable guess at the exit velocity from compressible flow equations. With any luck you know the fuel/oxidiser mass flow rate, then you have m(dot) and then you can solve for p(e).

krihamm · February 13, 2018, 04:30

Quote:

Originally Posted by ghorrocks

This web page seems to be rockets 101: https://www.grc.nasa.gov/www/k-12/airplane/rockth.html

In the thrust equation, if you know the force then you have m(dot), V and p(e) as unknowns. This assumes you know the geometry for the exit geometry. If you know the throat geometry then you can probably have a reasonable guess at the exit velocity from compressible flow equations. With any luck you know the fuel/oxidiser mass flow rate, then you have m(dot) and then you can solve for p(e).

Great, I will have a look at it! Thank you, Glenn!

ghorrocks · February 13, 2018, 04:34

The page appears to be from NASA, at their Glenn Research Centre. Any place with an excellent name like that has to be good.

February 12, 2018, 11:15	Applying thurst force to a face	#1
krihamm New Member Join Date: Nov 2017 Posts: 27 Rep Power: 8	Hi, I am simulating drag over a rocket for different speeds. I need to apply a force representing the thrust of the rocket at the exhaust in order to determine correct pressure drag. I have tried searching for a way to do this, but with no luck. Does anyone know how this could be done? Thanks!

February 12, 2018, 16:52		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,723 Rep Power: 143	Are you modelling the rocket as a moving body (so the force is required to get the rocket to move) or are you modelling the rocket fixed in its frame of reference and you want the rocket exhaust to be included so the entrained flow and drag effects are included? I hope it is the fixed in its frame of reference case as that is the best way to do this simulation for most cases. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

February 13, 2018, 00:11		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,723 Rep Power: 143	Excellent. In that case the simplest approach for the rocket exhaust would seem to be an inlet boundary with the flow conditions of the rocket exhaust specified, presumably supersonic flow. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

February 13, 2018, 04:24		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,723 Rep Power: 143	This web page seems to be rockets 101: https://www.grc.nasa.gov/www/k-12/airplane/rockth.html In the thrust equation, if you know the force then you have m(dot), V and p(e) as unknowns. This assumes you know the geometry for the exit geometry. If you know the throat geometry then you can probably have a reasonable guess at the exit velocity from compressible flow equations. With any luck you know the fuel/oxidiser mass flow rate, then you have m(dot) and then you can solve for p(e). __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

February 13, 2018, 04:34		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,723 Rep Power: 143	The page appears to be from NASA, at their Glenn Research Centre. Any place with an excellent name like that has to be good. -Maxim- and krihamm like this. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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