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-   -   How to calculate torque from the simulation result (http://www.cfd-online.com/Forums/openfoam-post-processing/109063-how-calculate-torque-simulation-result.html)

 sivakumar November 8, 2012 09:19

How to calculate torque from the simulation result

Hi Foamers,
I want to calculate torque from the simulation, can you guys help me?
I have used MRFSimpleFoam for my simulation. The fan RPM is 750.

Thanks and regards,
Sivakumar

 laurentb November 15, 2012 10:35

Hi
Have a look to the thread :
Monitoring Torque on rotating body in MRFSimpleFoam
Laurent

 sivakumar February 11, 2013 07:41

Hi Guys,
After a long struggle, I came to the conclusion regarding torque.
To calculate torque we need to use this function in the controlDict file.

functions ( forces { type forces; functionObjectLibs ("libforces.so"); outputControl timeStep; outputInterval 1; patches (wallBlade); // pname p; // Uname U; rhoName rhoInf; log true; rhoInf 1.205; CofR (0 0 0); } );
results will be as follows:
forces(pressure, viscous)((2162.2 -313.192 35.8823) (12.5865 3.1166 3.97532)) moment(pressure, viscous)((1162.4 6862.37 100.944) (-12.5067 35.4299 -50.9462))
now the the output pressure force is not exact pressure (unit m2/s2)
so to get exact torque = obtained Torque (moment) / density

Thanks,
Sivakumar

 laurentb February 15, 2013 06:04

Hi,

Does somebody can explain in details how to calculate torque with "forces" library data ?

I try to compare experimental and simulation results but there is a big difference.

I calculate the torque as follow : Torque = |moment(pressure) + moment(viscous)| and i obtain twice the experimental value...

Thanks
Laurent

 sivakumar February 15, 2013 07:08

hi,
Which solver you are using?
for any simpleFoam you need to divide it with the density what you are using.

Thanks,
Sivakumar

 laurentb February 15, 2013 07:24

I'm using MRFSimpleFoam. The density is 1300 kg/m3. If i divide by 1300 the difference is bigger...

 sivakumar February 15, 2013 07:36

1 Attachment(s)
hi,
What kind of simulation you are doing, I am also using MRFSimpleFoam for axial flow fan.

In my case MRFSimpleFoam torque results keep on changing for each case
Actually, I tried two cases one is with passage (suction side and pressure side of the fan)
secondly two blade and quarter of the cylinder see the fig.

for passage case I am getting 45 N-m (it should be 70-80 N-m)
for 2 blade case I am getting 80 N-m

I am totally confused, is there any problem with MRFSimpleFoam or in my settings.
I am totally gone.

Thanks,
Sivakumar

 sivakumar February 20, 2013 07:09

Hi Guys and Laurentb,
I got the right way to calculate torque.

I was doing some wrong calculation, that is why my results were wrong.
now I got the right results, with 2% error.

To Laurentb, I am sorry man, I think I have miss guided you.

Actually we need to multiply the pressure with density to get the exact pressure.

to calculate torque: take the z component of the pressure force and viscous.

torque = F*r

F = ((P *A) *density+ tau *A)

A --> area of the patch which you want to calculate torque.

I hope it is clear now

Thanks,
Sivakumar

 laurentb February 22, 2013 10:10

Hi,

Finally the torque computed by the forces library fits my experimental data with 10% error.

My error comes from that i turn off the turbulence in RASProperties file for laminar flow conditions.

If i turn on the turbulence option the force computation works well.

Laurent

 wes1204 April 9, 2013 11:49

direction

Hi all

My case is 2D on x-y plane.

Object look like airfoil and it is in wind tunnel.

I obtain moment using force function added controldic file.

Center of moment is z-axis.

From result, moment has minus value.

In this case, Is object rotated to clockwise or countclockwise??

 sharonyue May 6, 2013 20:15

Quote:
 Originally Posted by sivakumar (Post 408965) Hi Guys and Laurentb, I got the right way to calculate torque. I was doing some wrong calculation, that is why my results were wrong. now I got the right results, with 2% error. To Laurentb, I am sorry man, I think I have miss guided you. Actually we need to multiply the pressure with density to get the exact pressure. to calculate torque: take the z component of the pressure force and viscous. torque = F*r F = ((P *A) *density+ tau *A) A --> area of the patch which you want to calculate torque. I hope it is clear now Thanks, Sivakumar
Hi Sivakumar,

Thats cool,Thanks very much.But I still have some problems.

1.How can I get the area of my patch? cuz its not a regular geometry.

2.If my impeller is rotating around y axiz.Should I pick the y component forces?

3.I have many impellers,How can I get the sum torque?

5.I am using MRFInterFoam, and I add this code into controldict.
Code:

```outputControl timeStep;         outputInterval 1000;         patches (intermigOpposite1); //        pname p; //        Uname U;         rhoName rhoInf;         log true;         rhoInf 1000;         CofR (0 0 0);     }```
cuz the liquid is water.isnt it be 1000?

and I get the forces:

Code:

```forces output:     forces(pressure,viscous)((-0.111013 9.4727 -0.09283),(0.00142333 -0.00100197 0.0121232))     moment(pressure,viscous)((-0.0196299 0.941766 0.19644),(0.00167602 0.0451228 0.000146112)) forces output:     forces(pressure,viscous)((0.447146 12.7018 -0.170442),(-0.00129838 0.00185303 -0.00024443))     moment(pressure,viscous)((-0.0262434 0.928373 -0.127495),(1.8784e-05 0.0418585 0.000395807)) forces output:     forces(pressure,viscous)((-0.0942824 4.63998 0.16988),(-0.00217494 -0.0359383 0.00219822))     moment(pressure,viscous)((0.0744167 1.1975 0.0769798),(0.00104654 0.0454346 0.00164999)) forces output:     forces(pressure,viscous)((-0.226807 5.13595 -0.159452),(-0.0119485 -0.0779935 -0.00055085))     moment(pressure,viscous)((0.015837 1.36635 0.137174),(4.43451e-05 0.0472514 0.00681036)) forces output:     forces(pressure,viscous)((0.300891 1.99927 0.0561256),(-0.00293573 0.00419335 0.000610194))     moment(pressure,viscous)((0.0233319 9.04241e-06 -0.05736),(0.000348815 0.000324186 0.000962111))```
Now just leave the calculation alone. Does the forces looks normal...
If I want to calculate,for example the first one.
Force=(9.4727-0.0010017)*0.033555(my patch area)=0.3178n

Well, 0.046Nm is too small.

There are many problems,Thanks again for your patience.

Regards.

 sivakumar May 7, 2013 08:33

Hi,

To calculate torque, just add the components (moment and viscous) which belongs to the axis of rotation. You no need to think of the area.

In your case for the first output

y component of moment + viscous = 0.941766+0.0451228

if your torque is small, how small it is? what is the experimental torque value?

Thanks,
Sivakumar

 sharonyue May 7, 2013 09:25

Quote:
 Originally Posted by sivakumar (Post 408965) Hi Guys and Laurentb, I got the right way to calculate torque. I was doing some wrong calculation, that is why my results were wrong. now I got the right results, with 2% error. To Laurentb, I am sorry man, I think I have miss guided you. Actually we need to multiply the pressure with density to get the exact pressure. to calculate torque: take the z component of the pressure force and viscous. torque = F*r F = ((P *A) *density+ tau *A) A --> area of the patch which you want to calculate torque. I hope it is clear now Thanks, Sivakumar
Hi sivakumar,

If I add them all.and is this the torque?
y component of moment + viscous = 0.941766+0.0451228 +0.928373 +0.0418585 +.....=4.6

Well , my experiment is a little different with my CFD case. but the torque should not differ much. if 4.6's unit is Nm, I think its normal.

But in you last post,
torque = F*r

F = ((P *A) *density+ tau *A)

A --> area of the patch which you want to calculate torque.

Why is this using area?

Anyway,you give me lots of hints,Thanks very much!!

 sivakumar May 8, 2013 03:39

Hi,
In the previous post I used force to calculate torque, so OF out put is interns of pressure force and viscous force.

Pressure = F/A, so I used area.

but, in your case just add the Y component of the pressure moment and viscous moment (this is the normal way).

how many % error you are getting in torque?
how good is your results with experimental results?

Thanks,
Sivakumar

 sharonyue May 8, 2013 23:36

Quote:
 Originally Posted by sivakumar (Post 426025) Hi, In the previous post I used force to calculate torque, so OF out put is interns of pressure force and viscous force. Pressure = F/A, so I used area. but, in your case just add the Y component of the pressure moment and viscous moment (this is the normal way). how many % error you are getting in torque? how good is your results with experimental results? Thanks, Sivakumar
Hi Sivakumar,
My experiment is a little different with my case,so I cannot make a comparision between them.But it should not differ much. If my case's torque is about 4 Nm. its normal. If its below 1 or 2Nm its not normal.

And in my experiment, its two phase.Thats a gas-liquid mixer.Now I dont know how to calculate two phase's torque in FOAM.So...

Well,looks like there are two ways calculating torque,1.use the forces to calculate which would need to know the area.2.use the moment just add them all.But in my case I think the first method is not working...

 sharonyue May 14, 2013 02:48

Quote:
 Originally Posted by sivakumar (Post 426025) Hi, In the previous post I used force to calculate torque, so OF out put is interns of pressure force and viscous force. Pressure = F/A, so I used area. but, in your case just add the Y component of the pressure moment and viscous moment (this is the normal way). how many % error you are getting in torque? how good is your results with experimental results? Thanks, Sivakumar
Hi Sivakumar,

Recently I use simpleFoam to make a test with the paper.There is only one patch in my rotating zone.I get the force like this:
Code:

```forces output:     forces(pressure,viscous)((-2.76427e-06 -2.02655e-05 1.45751e-06),(-1.75831e-08 -9.23311e-08 1.98527e-08))     moment(pressure,viscous)((8.64124e-08 -0.00012881 2.0096e-07),(7.20421e-10 -8.69029e-07 1.63561e-09))```
So according to what you have said,my torque is -0.00012881Nm. So my power is 2*3.14*5(rps)*0.00012881=4.04e-3.
and my power number is 4.04e-3/(1000(density)*5^3*(0.0957diameter)^5)=4.03e-3
which is too small. In paper the power number is 4-5.which is one thousand times bigger.Do you know why.please..

Code:

```forces     {         type forces;         functionObjectLibs ("libforces.so");         outputControl timeStep;         outputInterval 1;         patches (impeller); //        pname p; //        Uname U;         rhoName rhoInf;         log true;         rhoInf 1;         CofR (0 0 0);     }```

 niloogh May 27, 2013 16:42

Quote:
 Originally Posted by sivakumar (Post 407103) Hi Guys, After a long struggle, I came to the conclusion regarding torque. To calculate torque we need to use this function in the controlDict file. functions ( forces { type forces; functionObjectLibs ("libforces.so"); outputControl timeStep; outputInterval 1; patches (wallBlade); // pname p; // Uname U; rhoName rhoInf; log true; rhoInf 1.205; CofR (0 0 0); } ); results will be as follows: forces(pressure, viscous)((2162.2 -313.192 35.8823) (12.5865 3.1166 3.97532)) moment(pressure, viscous)((1162.4 6862.37 100.944) (-12.5067 35.4299 -50.9462)) now the the output pressure force is not exact pressure (unit m2/s2) so to get exact torque = obtained Torque (moment) / density Thanks, Sivakumar

hi dear sivakumar
i had simulated a wind turbine using fluent and i have problem with big amounts of torques in fluent.i don't know what to do exactly to get the exact torque amount.
you had mentioned some formula in these posts but i don't know how to calculate the suitable area and totally what to do:confused:
would you plz help me???

 sharonyue January 29, 2015 06:38

Update,

Oh, time flies...

I think I figure it out, put it simple, if we want calculate the torque on the blade, this is quite simple, please see the output below:

Code:

```forces forces output:     sum of forces:         pressure : (70.3478 2.53417 -0.129928)         viscous  : (-8.35929e-07 -3.77215e-07 -9.01343e-07)         porous  : (0 0 0)     sum of moments:         pressure : (-0.01138 -0.00344619 -9.2415)         viscous  : (-8.14101e-08 -1.81033e-08 7.56807e-08)         porous  : (0 0 0)```
U can neglect all the other things if u just need the value of torque. The value on the red is torque. From this torque we can use this to calculate power.

power = 2 * 3.14 * rps * torque

By this, I got almost the rite power with experiment.

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