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direction vectors for lift and drag in fluent

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Old   September 10, 2012, 05:35
Default direction vectors for lift and drag in fluent
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nauman hashmi
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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.
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Old   September 10, 2012, 09:54
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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
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Old   September 10, 2012, 10:02
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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.
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Old   September 10, 2012, 10:14
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Whatever is parallel to your freestream
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Old   September 10, 2012, 10:17
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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.
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Old   September 10, 2012, 10:28
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You can just take the drag vector in the positive Y direction. Try:

/report/forces/wall-forces yes 0 1 0 no
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Old   September 10, 2012, 10:34
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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.
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