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-   -   forces(pressure, viscous)((-778.236 18309.5 -1.17287e-19) (68.6449 6.41497 -2.43225e- (https://www.cfd-online.com/Forums/openfoam-post-processing/118326-forces-pressure-viscous-778-236-18309-5-1-17287e-19-68-6449-6-41497-2-43225e.html)

 s.m May 25, 2013 03:10

forces(pressure, viscous)((-778.236 18309.5 -1.17287e-19) (68.6449 6.41497 -2.43225e-

hi Dear all;
i am working on multi element airfoils.
my Question is: how should we calculate the forcecoefficient "Drag,Lift & Moment coefficient" from
" forces(pressure, viscous)((-778.236 18309.5 -1.17287e-19) (68.6449 6.41497 -2.43225e-19))
moment(pressure, viscous)((-8.28657e-22 1.45304e-19 8222.87) (5.57025e-20 3.72086e-20 0.0120865))
"
that is gained from force function that is written in the controlDict after finishing analysis ?

Thank you very much for any one could give me a little guideline:)

 immortality June 2, 2013 02:01

hi i think there are functions for that to put in controlDict.

 s.m June 2, 2013 04:36

Quote:
 Originally Posted by immortality (Post 431408) hi i think there are functions for that to put in controlDict.
hi Dear ehsan,
i know that we have functions that give us, "forceCoeffs". this function need lRef and ARef. as i said i have a tree element airfoil, and i don't know what should i write for ARef&lRef in this function. i don't know if i should write the total Chord for all of them or i should write the chord of each element, for each of them separately?!

 immortality June 2, 2013 09:12

Hi Saideh
I think you should define different numbers for main part,slat and the flap if you want to calculate each part role in Cl an Cd.
I'll look into your case ASAP.

 colinB June 3, 2013 05:23

Hi,

to explain what to do you first have to understand what is written in your
force.dat file:

forces(pressure, viscous)((-778.236 18309.5 -1.17287e-19) (68.6449
6.41497 -2.43225e-19))
moment(pressure, viscous)((-8.28657e-22 1.45304e-19 8222.87)
(5.57025e-20 3.72086e-20 0.0120865))

what you basically get is viscous and pressure forces as well as
a viscous and pressure moment.

This set of numbers: (-778.236 18309.5 -1.17287e-19) e.g
represent the pressure forces in x y and z direction
so if z is your altitude z=-1.7e-19 is representing the pressure part

In order to get the coefficients you can use a formula like this:
for pressure e.g.
cp = Fp / (0.5 * rho * Uinf^2 * Aref)

with Fp being the pressure force in z direction
rho being the density of your surrounding fluid
Uinf being the velocity
and Aref being a reference area.

Now that you've asked for Aref. this one depends on what you have
For ships Aref is the wetted surface area and Lref is the length of the
waterline.
For airfoils you can do the same but as far as I remember
(please feel free to correct me if I'm wrong) for Aref the crossectional
area and Lref the chord length is commonly used.

I hope my explanations are of assistance

regards

 s.m June 3, 2013 07:47

1 Attachment(s)
Quote:
 Originally Posted by colinB (Post 431596) Hi, to explain what to do you first have to understand what is written in your force.dat file: forces(pressure, viscous)((-778.236 18309.5 -1.17287e-19) (68.6449 6.41497 -2.43225e-19)) moment(pressure, viscous)((-8.28657e-22 1.45304e-19 8222.87) (5.57025e-20 3.72086e-20 0.0120865)) what you basically get is viscous and pressure forces as well as a viscous and pressure moment. This set of numbers: (-778.236 18309.5 -1.17287e-19) e.g represent the pressure forces in x y and z direction so if z is your altitude z=-1.7e-19 is representing the pressure part of your lift force. In order to get the coefficients you can use a formula like this: for pressure e.g. cp = Fp / (0.5 * rho * Uinf^2 * Aref) with Fp being the pressure force in z direction rho being the density of your surrounding fluid Uinf being the velocity and Aref being a reference area. Now that you've asked for Aref. this one depends on what you have specified in your controlDict. For ships Aref is the wetted surface area and Lref is the length of the waterline. For airfoils you can do the same but as far as I remember (please feel free to correct me if I'm wrong) for Aref the crossectional area and Lref the chord length is commonly used. I hope my explanations are of assistance regards
hi Dear colinB.
thank you very much, your explanation will help me a lot.
yes you are right, for the one element airfoil, we set lRef=chord length
and ARef= chord length*depth of the mesh.
i have tree element airfoil. i write three function for each element separately. in your opinion, i should write lRef and Aref for all of them equal or i should write the chord of each part for it's lRef?

 colinB June 3, 2013 08:15

Hi,
this is a tricky question and I actually don't know the answer,
however using some common sense I would suggest to use
the geometry properties from each part you are investigating
so for the first part ARef1 and lRef1 for the second part Aref2 and lRef2
and so on.
With this method you will gain the contributions of each wing part to
the total drag/lift whatever.

Only if you want to compare similar geometries you would use the same ARef
as well as the same lRef e.g. to evaluate the best profile to achieve a certain
task (best lift/ best drag), however probably only if this really comes close
to the reality.

regards

 s.m June 3, 2013 08:49

Quote:
 Originally Posted by colinB (Post 431619) Hi, this is a tricky question and I actually don't know the answer, however using some common sense I would suggest to use the geometry properties from each part you are investigating so for the first part ARef1 and lRef1 for the second part Aref2 and lRef2 and so on. With this method you will gain the contributions of each wing part to the total drag/lift whatever. Only if you want to compare similar geometries you would use the same ARef as well as the same lRef e.g. to evaluate the best profile to achieve a certain task (best lift/ best drag), however probably only if this really comes close to the reality. regards
thank you :)

 s.m June 3, 2013 09:06

1 Attachment(s)
Quote:
 Originally Posted by colinB (Post 431596) Hi, to explain what to do you first have to understand what is written in your force.dat file: forces(pressure, viscous)((-778.236 18309.5 -1.17287e-19) (68.6449 6.41497 -2.43225e-19)) moment(pressure, viscous)((-8.28657e-22 1.45304e-19 8222.87) (5.57025e-20 3.72086e-20 0.0120865)) what you basically get is viscous and pressure forces as well as a viscous and pressure moment. This set of numbers: (-778.236 18309.5 -1.17287e-19) e.g represent the pressure forces in x y and z direction so if z is your altitude z=-1.7e-19 is representing the pressure part of your lift force. In order to get the coefficients you can use a formula like this: for pressure e.g. cp = Fp / (0.5 * rho * Uinf^2 * Aref) with Fp being the pressure force in z direction rho being the density of your surrounding fluid Uinf being the velocity and Aref being a reference area. Now that you've asked for Aref. this one depends on what you have specified in your controlDict. For ships Aref is the wetted surface area and Lref is the length of the waterline. For airfoils you can do the same but as far as I remember (please feel free to correct me if I'm wrong) for Aref the crossectional area and Lref the chord length is commonly used. I hope my explanations are of assistance regards
Dear colinB,
i want to know that i get your meaning right or not,
as you see my airfoil is in the "x-y" plane, and it's chord is along the x axis.
as we now, "total lift=viscous lift+pressure lift "
so, for computing the "cl of viscous lift" i should choose "6.41497" from the force result above.and for computing the "cl of pressure lift" i should choose "18309.5" from the force result above. now i should divided these two forces " "6.41497" and "18309.5" " to "(0.5 * rho * Uinf^2 * Aref) " separately, and at the end, the sum of clviscous and clpressure is the lift coefficient that i want.

 colinB June 3, 2013 10:27

yes you got me right

However if you are only interested in ct you can also add up all forces in y
direction and then divide them by (0.5*rho.......)
for the term you are dividing by is constant

regards

 s.m June 4, 2013 01:41

1 Attachment(s)
Quote:
 Originally Posted by colinB (Post 431673) yes you got me right However if you are only interested in ct you can also add up all forces in y direction and then divide them by (0.5*rho.......) for the term you are dividing by is constant regards
i don't understand this sentence "if you are only interested in ct you can also add up all forces in y
direction"
wold you please explain more on the file in the attachment?

 colinB June 4, 2013 05:25

total Liftforce = pressure Liftforce + viscous Liftforce

total cl - pressure cp + viscous cp

Code:

```press.Force+visc.Force        press.Force                visc.Force ----------------------- = --------------------- + --------------------- 0.5*rho*Uinf^2* Aref        0.5*rho*Uinf^2*Aref    0.5*rho*Uinf^2* Aref```
if you know what I mean ;-)

 s.m June 4, 2013 05:56

Quote:
 Originally Posted by colinB (Post 431853) total Liftforce = pressure Liftforce + viscous Liftforce total cl - pressure cp + viscous cp Code: ```press.Force+visc.Force        press.Force                visc.Force ----------------------- = --------------------- + --------------------- 0.5*rho*Uinf^2* Aref        0.5*rho*Uinf^2*Aref    0.5*rho*Uinf^2* Aref``` if you know what I mean ;-)
thank you, you mean "total cl = pressure cl+viscous cl" from equation above?

my question is : how i should obtain the press.Force and visc.Force,
if my simulation is converged after e.g 300 iteration, i should add the second term of pressure force "Fy" for all iteration; to obtain press.Force that you write above? or i should pick the the second term of pressure force "Fy" from the last iteration only, to obtain the cl that is resulted from pressure force.

 colinB June 4, 2013 06:32

Quote:
 thank you, you mean "total cl = pressure cl+viscous cl" from equation above?
sry for the typos, but you are right.
with pressure and viscous Force I was referring to the pressure and
viscous part of the lift forces, again sorry for my incomplete description.

Well to get the values you need to average them over the last iterations
when you see the calculation has stabilized. (I'm referring here to the y
values which represent the lift forces in your calc.)
e.g. I'm running a calculation with 10000 Iterations and I can observe that
the the forces oscillate around a certain value from Iteration 7000 on
I'm going to take the average over the last 3000 Iterations.
If the values are approaching a certain value you have to be little bit more
careful.

In order to get the absolute pressure/viscous forces you need to calculate
the vector with 3D Pythagoras with all three values being within the

I hope I got your question right
regards

 s.m October 15, 2013 04:50

Quote:
 Originally Posted by colinB (Post 431873) sry for the typos, but you are right. with pressure and viscous Force I was referring to the pressure and viscous part of the lift forces, again sorry for my incomplete description. Well to get the values you need to average them over the last iterations when you see the calculation has stabilized. (I'm referring here to the y values which represent the lift forces in your calc.) e.g. I'm running a calculation with 10000 Iterations and I can observe that the the forces oscillate around a certain value from Iteration 7000 on I'm going to take the average over the last 3000 Iterations. If the values are approaching a certain value you have to be little bit more careful. In order to get the absolute pressure/viscous forces you need to calculate the vector with 3D Pythagoras with all three values being within the brackets in your force.dat file. I hope I got your question right regards

Dear colinB

"In order to get the absolute pressure/viscous forces you need to calculate
the vector with 3D Pythagoras with all three values being within the
brackets in your force.dat file. "

how should i calculate the vector with 3D Pythagoras ?
i don't know what 3D Pythagoras is?
Thank you very much:)

 colinB October 15, 2013 07:34

Hi,
sorry for my non-scientific language ;-)

I was referring to the formula:

Code:

``` |a| = sqrt(a1^2 + a2^2 + a3^2)```
to calculate the length of a vector.
Since this is a further development of Pythagoras' famous
c^2 = a^2 + b^2
I simply called it 3D Pythagoras.

I hope that clarifys this matter
regards
Colin

 sharonyue March 18, 2014 22:45

Quote:
 Originally Posted by colinB (Post 431596) Hi, to explain what to do you first have to understand what is written in your force.dat file: forces(pressure, viscous)((-778.236 18309.5 -1.17287e-19) (68.6449 6.41497 -2.43225e-19)) moment(pressure, viscous)((-8.28657e-22 1.45304e-19 8222.87) (5.57025e-20 3.72086e-20 0.0120865)) what you basically get is viscous and pressure forces as well as a viscous and pressure moment. This set of numbers: (-778.236 18309.5 -1.17287e-19) e.g represent the pressure forces in x y and z direction so if z is your altitude z=-1.7e-19 is representing the pressure part of your lift force. In order to get the coefficients you can use a formula like this: for pressure e.g. cp = Fp / (0.5 * rho * Uinf^2 * Aref) with Fp being the pressure force in z direction rho being the density of your surrounding fluid Uinf being the velocity and Aref being a reference area. Now that you've asked for Aref. this one depends on what you have specified in your controlDict. For ships Aref is the wetted surface area and Lref is the length of the waterline. For airfoils you can do the same but as far as I remember (please feel free to correct me if I'm wrong) for Aref the crossectional area and Lref the chord length is commonly used. I hope my explanations are of assistance regards
Hi bro,

I know that pressure in CFD is scalar, you can tell it from document 0/p. But this pressure force:
Code:

```forces(pressure,viscous,porous) moment(pressure,viscous,porous) 0.01        ((122.847 3.11355 1.99776e-16),(2.35502 0.0580908 -6.78944e-19),(0 0 0)) ((-0.0778387 3.07116 -647.967),(-0.00145227 0.0588756 -3.40765),(0 0 0))```
It is a vector. So Im confused. Is this the pressure in some point?

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