Change the application pole of the moment of a force

[Marco9518]

Good morning everyone!

I am an aerospace engineering working on my thesis and i am trying to solve a little problem.

In the picture you can see an "aeordynamic" body. The CFD analysis gives me the forces and the moments acting on this body. How can i calculate the momentum acting on the body with respect to O'? In particular, how do i find "R"? I want to point out that the moments that the CFD gives me are just values!

Thank you very much to everyone!

 

[Lnewqban]

Is it possible that you are confusing momentum and moment?

 

[Marco9518]

Is it possible that you are confusing momentum and moment?

Oh yes for sure! I am just not very good with the language but I was talking about moment!

 

[Lnewqban]

If I understand the diagram correctly, there are no forces acting on the body, only moment Mc about point O.

 

[Marco9518]

If I understand the diagram correctly, there are no forces acting on the body, only moment Mc about point O.

I did not draw forces but only because i wanted to focus on the moments.
What i have is basically a chart that says for, every angle of attack, the value of the forces and the moments acting on the 3 axis, in the Oxyz frame of reference.

 

[Lnewqban]

I see.
Next question: Could you describe the meaning of the equation shown in the diagram?

 

[Marco9518]

I see.
Next question: Could you describe the meaning of the equation shown in the diagram?

It is the law used to change the pole of the moment. The problem i think is just R which is defined as the summation for "i" that goes from 0 to N of F(i). But i am not very sure if i can use the forces coming from the CFD.

 

[Lnewqban]

Sorry, Marco, I can't understand this problem.

 

[jrmichler]

I have no idea what the "pole of a moment" is, but do have some experience at interpreting computer software results. There are some basic principles that apply to all computer simulation programs, whether CFD, CAD, or motion analysis. You need to learn how to interpret the software results.

Start by simplifying your problem, which seems to be related to 2D flow around an ellipse. If you are, in fact, studying 2D flow around an ellipse, start by simulating 2D flow around a cylinder. Then you have simulation results that can be directly compared to published data. The process of comparing to published data is where you learn to interpret the results, and also learn if you properly simulated the problem.

The next step is 2D flow around an ellipse, where the flow is aligned with the ellipse major axis. These results can also be compared to published data.

Your mention of moments implies that your flow is not aligned with either the ellipse major or minor axis. If so, there is published data for the case of finite major axis, and minor axis of length zero. It's called a flat plate. Run the simulation for a range of angles, and compare to published results.

You should now be able to run the case of an ellipse with 2D flow not aligned with either the major or minor axis.

CFD is completely different from linear static FEA, and the learning curve is much longer. A person learning CFD can easily take 2 or 3 months of full time work to properly perform the above verification tests.

 

[hutchphd]

Has R been defined somewhere?

 

[Marco9518]

Has R been defined somewhere?

R is defined as the summation for "i" that goes from 0 to N of F(i).
But i am not very sure if i can use the forces coming from the CFD.
The law that i wrote in the picture is one of the basis in mathematical physics but it seems like nobody is able to apply it, including me obviously.

 

[Marco9518]

I have no idea what the "pole of a moment" is, but do have some experience at interpreting computer software results. There are some basic principles that apply to all computer simulation programs, whether CFD, CAD, or motion analysis. You need to learn how to interpret the software results.

Start by simplifying your problem, which seems to be related to 2D flow around an ellipse. If you are, in fact, studying 2D flow around an ellipse, start by simulating 2D flow around a cylinder. Then you have simulation results that can be directly compared to published data. The process of comparing to published data is where you learn to interpret the results, and also learn if you properly simulated the problem.

The next step is 2D flow around an ellipse, where the flow is aligned with the ellipse major axis. These results can also be compared to published data.

Your mention of moments implies that your flow is not aligned with either the ellipse major or minor axis. If so, there is published data for the case of finite major axis, and minor axis of length zero. It's called a flat plate. Run the simulation for a range of angles, and compare to published results.

You should now be able to run the case of an ellipse with 2D flow not aligned with either the major or minor axis.

CFD is completely different from linear static FEA, and the learning curve is much longer. A person learning CFD can easily take 2 or 3 months of full time work to properly perform the above verification tests.

It is not possible to do any other analysis. The picture is already a semplification of a real helicopter fuselage. The thing is i need forces and moments measured with reference to the mast but I jusy can't do it because the analysis has been made by the manufacturer

 

[hutchphd]

R is defined as the summation for "i" that goes from 0 to N of F(i).
But i am not very sure if i can use the forces coming from the CFD.
The law that i wrote in the picture is one of the basis in mathematical physics but it seems like nobody is able to apply it, including me obviously.

I think this is off the edge of the picture when I view it...is it just me who has this issue?

 

[Marco9518]

I think this is off the edge of the picture when I view it...is it just me who has this issue?

Here is the theory. It is in italian but math is still valid..

 

[hutchphd]

How can i calculate the momentum acting on the body with respect to O'? In particular, how do i find "R"?

I am confused. What explicitly is the output you get from CFD ?

 

[Marco9518]

I am confused. What explicitly is the output you get from CFD ?

Unfortunately i cannot give informations but this is what it looks like. I have one of these charts for Fx,Fy,Fz,Mx,My,Mz

 

[hutchphd]

Is F the same as R? Is M the same as $M_O$ ? Then can't you use the cross product equation in #14 ?

 

[Marco9518]

Is F the same as R? Is M the same as $M_O$ ? Then can't you use the cross product equation in #14 ?

That was my idea. The problem now is this. I need to move from the nose (0 0 0) to the mast of the helo, which is located at (2 0 2.4) meters. Assumig i want to keep my problem 2 dimensional so i can use just the charts of Fx and Mx. What distance do i calculate as O' - O? 2 meters (the projection of the mast on the x axis) or 3.2 meters (the real distance)?

Thank anyway for the help!

 

[hutchphd]

The 2D problem (on a flat plate in x-z plane) is still embedded in 3D. The moments are axial vectors which point along the y-axis (out of page) as you draw them. Use the cartesian components to do the vector math in the vector equations you showed in #14. You never need to calculate the length OO' explicitly