direction vectors for lift and drag in fluent

nauman55 · September 10, 2012, 05:35

hi i have one query regarding direction vectors of drag and lift forces in fluent. in many tutorial the direction vectors at any angle of attack lets say 4 degree is 0.99576 for x axis and 0.06976 for y axis. and for lift the direction vectors are -0.06976 and 0.99576. however the under mention link says that drag direction vector should be negative also.
http://www.engineeringarchives.com/les_fm_draglift.html
what is the correct way to calculate lift and drag in fluent need guidance thanks in advance.

stuart23 · September 10, 2012, 09:54

It doesn't matter. You can use the following command to get the force in the positive x-direction

Code:

/report/forces/wall-forces yes 1 0 0 no

or in the negative x-direction

Code:

/report/forces/wall-forces yes -1 0 0 no

Drag is usually quoted as a positive coordinate even though it acts in the negative x-direction on the common aircraft coordinate system. Same for lift, it is in the negative y-direction

nauman55 · September 10, 2012, 10:02

then what is the correct drag force vector direction in fluent. my geometry is in yz plane while flow direction is in y direction and it originates from -162 meters. for a 4 degree aoa what should be the direction vectors i should put in.
regards

stuart23 · September 10, 2012, 10:14

Whatever is parallel to your freestream

nauman55 · September 10, 2012, 10:17

as far as my humble understanding my free stream is flowing from -ve y direction to positive y direction so i should be taking a -ve direction vector of drag. correct me sir if i am wrong.
regards

stuart23 · September 10, 2012, 10:28

You can just take the drag vector in the positive Y direction. Try:

/report/forces/wall-forces yes 0 1 0 no

nauman55 · September 10, 2012, 10:34

sir my geometry is at an angle of attack not at zero degree. it would be better to use the cosine and sine of aoa as direction of force vectors while owing to -ve direction of flow i can use the -ve sign in force vector. with positive value i am getting higher values then expected. and they stayed the same at different aoa which is not correct.
regards

Twenty_one · September 23, 2020, 14:47

Drag is defined the component of force parallel to the free stream velocity and lift perpendicular to free stream velocity.
Fluent documentation states, "The total force component along the specified force vector ( $\overrightarrow{a}$ ) on a wall zone is computed by summing the dot product of the pressure and viscous forces ( $\overrightarrow{F_p}$ and $\overrightarrow{F_v}$ ) on each face with the specified force vector."

$\overrightarrow{U\infty}$ is free stream velocity. It makes an angle of $\theta$ with x-axis.
$\overrightarrow{U\infty}=U\infty\cos\theta\hat{i}+U\infty\sin\theta\hat{j}$
Direction of drag vector is going to be same as that of free stream velocity
$\overrightarrow{a}=\cos\theta\hat{i}+\sin\theta\hat{j}$
Direction of lift vector is perpendicular to direction of free stream velocity
$\cos(90+\theta)\hat{i}+\sin(90+\theta)\hat{j}$
$-\sin\theta\hat{i}+\cos\theta\hat{j}$
Let the angle of force vector is $\phi$ with x-axis, then, $\overrightarrow{F}=F\cos\phi\hat{i}+F\sin\phi\hat{j}$
Then, $\overrightarrow{F}.\overrightarrow{a}=F\cos\phi\cos\theta+F\sin\phi\sin\theta=F\cos(\phi-\theta)$ ,is the drag.

September 10, 2012, 05:35	direction vectors for lift and drag in fluent	#1
nauman55 Member nauman hashmi Join Date: Jun 2012 Posts: 54 Rep Power: 13	hi i have one query regarding direction vectors of drag and lift forces in fluent. in many tutorial the direction vectors at any angle of attack lets say 4 degree is 0.99576 for x axis and 0.06976 for y axis. and for lift the direction vectors are -0.06976 and 0.99576. however the under mention link says that drag direction vector should be negative also. http://www.engineeringarchives.com/les_fm_draglift.html what is the correct way to calculate lift and drag in fluent need guidance thanks in advance.

September 10, 2012, 09:54		#2
stuart23 Senior Member Stuart Buckingham Join Date: May 2010 Location: United Kingdom Posts: 267 Rep Power: 25	It doesn't matter. You can use the following command to get the force in the positive x-direction Code: /report/forces/wall-forces yes 1 0 0 no or in the negative x-direction Code: /report/forces/wall-forces yes -1 0 0 no Drag is usually quoted as a positive coordinate even though it acts in the negative x-direction on the common aircraft coordinate system. Same for lift, it is in the negative y-direction

September 10, 2012, 10:02	RE	#3
nauman55 Member nauman hashmi Join Date: Jun 2012 Posts: 54 Rep Power: 13	then what is the correct drag force vector direction in fluent. my geometry is in yz plane while flow direction is in y direction and it originates from -162 meters. for a 4 degree aoa what should be the direction vectors i should put in. regards

September 10, 2012, 10:17	Re	#5
nauman55 Member nauman hashmi Join Date: Jun 2012 Posts: 54 Rep Power: 13	as far as my humble understanding my free stream is flowing from -ve y direction to positive y direction so i should be taking a -ve direction vector of drag. correct me sir if i am wrong. regards

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September 10, 2012, 10:14		#4
stuart23 Senior Member Stuart Buckingham Join Date: May 2010 Location: United Kingdom Posts: 267 Rep Power: 25	Whatever is parallel to your freestream

September 10, 2012, 10:28		#6
stuart23 Senior Member Stuart Buckingham Join Date: May 2010 Location: United Kingdom Posts: 267 Rep Power: 25	You can just take the drag vector in the positive Y direction. Try: /report/forces/wall-forces yes 0 1 0 no

September 10, 2012, 10:34	re	#7
nauman55 Member nauman hashmi Join Date: Jun 2012 Posts: 54 Rep Power: 13	sir my geometry is at an angle of attack not at zero degree. it would be better to use the cosine and sine of aoa as direction of force vectors while owing to -ve direction of flow i can use the -ve sign in force vector. with positive value i am getting higher values then expected. and they stayed the same at different aoa which is not correct. regards

September 23, 2020, 14:47		#8
Twenty_one New Member Join Date: Jul 2020 Posts: 1 Rep Power: 0	Drag is defined the component of force parallel to the free stream velocity and lift perpendicular to free stream velocity. Fluent documentation states, "The total force component along the specified force vector ( $\overrightarrow{a}$ ) on a wall zone is computed by summing the dot product of the pressure and viscous forces ( $\overrightarrow{F_p}$ and $\overrightarrow{F_v}$ ) on each face with the specified force vector." $\overrightarrow{U\infty}$ is free stream velocity. It makes an angle of $\theta$ with x-axis. $\overrightarrow{U\infty}=U\infty\cos\theta\hat{i}+U\infty\sin\theta\hat{j}$ Direction of drag vector is going to be same as that of free stream velocity $\overrightarrow{a}=\cos\theta\hat{i}+\sin\theta\hat{j}$ Direction of lift vector is perpendicular to direction of free stream velocity $\cos(90+\theta)\hat{i}+\sin(90+\theta)\hat{j}$ $-\sin\theta\hat{i}+\cos\theta\hat{j}$ Let the angle of force vector is $\phi$ with x-axis, then, $\overrightarrow{F}=F\cos\phi\hat{i}+F\sin\phi\hat{j}$ Then, $\overrightarrow{F}.\overrightarrow{a}=F\cos\phi\cos\theta+F\sin\phi\sin\theta=F\cos(\phi-\theta)$ ,is the drag.