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

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 May 25, 2013, 03:10 forces(pressure, viscous)((-778.236 18309.5 -1.17287e-19) (68.6449 6.41497 -2.43225e- #1 Senior Member   saeideh mohamadi Join Date: Aug 2012 Posts: 229 Rep Power: 7 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 likes this.

 June 2, 2013, 02:01 #2 Senior Member     Ehsan Join Date: Oct 2012 Location: Iran Posts: 2,209 Rep Power: 18 hi i think there are functions for that to put in controlDict. __________________ Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King. To Be or Not To Be,Thats the Question! The Only Stupid Question Is the One that Goes Unasked.

June 2, 2013, 04:36
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 Originally Posted by immortality 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?!

 June 2, 2013, 09:12 #4 Senior Member     Ehsan Join Date: Oct 2012 Location: Iran Posts: 2,209 Rep Power: 18 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. __________________ Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King. To Be or Not To Be,Thats the Question! The Only Stupid Question Is the One that Goes Unasked. Last edited by immortality; June 3, 2013 at 12:41.

 June 3, 2013, 05:23 #5 Senior Member   Join Date: Aug 2010 Location: Groningen, The Netherlands Posts: 216 Rep Power: 11 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 Mojtaba.a and sharonyue like this.

June 3, 2013, 07:47
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 Originally Posted by colinB 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?
Attached Files
 controlDict.txt (3.4 KB, 22 views)

 June 3, 2013, 08:15 #7 Senior Member   Join Date: Aug 2010 Location: Groningen, The Netherlands Posts: 216 Rep Power: 11 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 likes this.

June 3, 2013, 08:49
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Quote:
 Originally Posted by colinB 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

June 3, 2013, 09:06
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 Originally Posted by colinB 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.
Attached Images
 stl.jpg (25.1 KB, 35 views)

 June 3, 2013, 10:27 #10 Senior Member   Join Date: Aug 2010 Location: Groningen, The Netherlands Posts: 216 Rep Power: 11 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

June 4, 2013, 01:41
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Quote:
 Originally Posted by colinB 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?
Attached Files
 forces-main.tar.gz (24.0 KB, 10 views)

 June 4, 2013, 05:25 #12 Senior Member   Join Date: Aug 2010 Location: Groningen, The Netherlands Posts: 216 Rep Power: 11 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 ;-)

June 4, 2013, 05:56
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Quote:
 Originally Posted by colinB 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.

June 4, 2013, 06:32
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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

October 15, 2013, 04:50
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Quote:
 Originally Posted by colinB 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

 October 15, 2013, 07:34 #16 Senior Member   Join Date: Aug 2010 Location: Groningen, The Netherlands Posts: 216 Rep Power: 11 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

March 18, 2014, 22:45
#17
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Dongyue Li
Join Date: Jun 2012
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Quote:
 Originally Posted by colinB 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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