What's the standard unit for forces?

Archoncomando · January 21, 2019, 12:11

Hello there,

So i imagine this question will be very silly and easy for some of you but i haven't found a post that helped me here.

So i'm simulating a Wing travelling through water and air at 12.5m/s. Another student is simulating it in x-foil since the professor wants to compare the two.

However his results are around 10kN+ whereas my openfoam log gives me a value of 900 (which i assume is in Newton). That's after 500 timesteps and looking at the values they seem to have converged.

Does anyone have an idea why such a large difference could occur? i've checked the values i input for velocity, densities etc and so far haven't come up with any faults.

I've seen something about forcecoeffs but i can't really wrap my head around what it does.

Any help would be greatly appreciated.

yambanshee · January 22, 2019, 08:36

I assume you're making use of the forces function in contorlDict? Have you made sure to input your density (rho) here? This is over top of the kinematic viscosity specified in transport properties

Archoncomando · January 22, 2019, 08:42

Yes that's the function I'm using. I've got the Rho specified both in the function in controldict and the transport properties etc.

Bloerb · January 23, 2019, 04:09

If you are simulating a 2D geometry your forces are still in N. So you need to figure out how deep your mesh is in the third dimension and might have to multiply it by a factor to get the actual force for the entire length of the wing. Aside from that you have to make sure the density is included. Every OpenFOAM input is in SI units. So density in kg/m³ etc.

forceCoeffs calculates the drag and lift coefficient from your forces. It basically devides them by a factor.

Archoncomando · January 23, 2019, 04:22

My Mesh is a 3D domain around 10 meters long and 9 meters wide so it's definitely not a 2D Mesh. I'll try around with the coeffs a bit.

yambanshee · January 23, 2019, 06:28

You've made sure your reference area is the blade chord*width?

January 21, 2019, 12:11	What's the standard unit for forces?	#1
Archoncomando New Member Florian Join Date: Dec 2018 Posts: 20 Rep Power: 8	Hello there, So i imagine this question will be very silly and easy for some of you but i haven't found a post that helped me here. So i'm simulating a Wing travelling through water and air at 12.5m/s. Another student is simulating it in x-foil since the professor wants to compare the two. However his results are around 10kN+ whereas my openfoam log gives me a value of 900 (which i assume is in Newton). That's after 500 timesteps and looking at the values they seem to have converged. Does anyone have an idea why such a large difference could occur? i've checked the values i input for velocity, densities etc and so far haven't come up with any faults. I've seen something about forcecoeffs but i can't really wrap my head around what it does. Any help would be greatly appreciated.

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January 22, 2019, 08:36		#2
yambanshee Senior Member Zander Meiring Join Date: Jul 2018 Posts: 125 Rep Power: 8	I assume you're making use of the forces function in contorlDict? Have you made sure to input your density (rho) here? This is over top of the kinematic viscosity specified in transport properties

January 22, 2019, 08:42		#3
Archoncomando New Member Florian Join Date: Dec 2018 Posts: 20 Rep Power: 8	Yes that's the function I'm using. I've got the Rho specified both in the function in controldict and the transport properties etc.

January 23, 2019, 04:09		#4
Bloerb Senior Member Join Date: Sep 2013 Posts: 353 Rep Power: 21	If you are simulating a 2D geometry your forces are still in N. So you need to figure out how deep your mesh is in the third dimension and might have to multiply it by a factor to get the actual force for the entire length of the wing. Aside from that you have to make sure the density is included. Every OpenFOAM input is in SI units. So density in kg/m³ etc. forceCoeffs calculates the drag and lift coefficient from your forces. It basically devides them by a factor.

January 23, 2019, 04:22		#5
Archoncomando New Member Florian Join Date: Dec 2018 Posts: 20 Rep Power: 8	My Mesh is a 3D domain around 10 meters long and 9 meters wide so it's definitely not a 2D Mesh. I'll try around with the coeffs a bit.

January 23, 2019, 06:28		#6
yambanshee Senior Member Zander Meiring Join Date: Jul 2018 Posts: 125 Rep Power: 8	You've made sure your reference area is the blade chord*width?