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Sign convention on force and force components

Question asked by David Lipke on Jan 4, 2016
Latest reply on Jan 7, 2016 by Boyko Tchavdarov

I am performing a lift calculation wherein a spherical ball is sitting atop a gas stream. The gas is flowing in the positive Y direction and gravity is in the negative Y direction.


When I perform my Flow Simulation and look at the results for the surface force on the spherical ball, I get the following:


Value of Force [N]: 0.200

X-component of Force [N]: 1.434e-005

Y-component of Force [N]: -0.200

Z-component of Force [N]: 8.170e-005


In other words, there is no net force in the X or Z directions (as expected); however, the Y-component of Force is negative when it should be positive. The Value of Force reported has the correct sign in this case, which is different from the Y-component (the only non-zero component).


I find this strange. Shouldn't both of these values have the same sign convention?


I then setup a scenario in which I expected the sign of the lift force to in the negative direction and got the following results:


Value of Force [N]: 0.300

X-component of Force [N]: 3.397e-005

Y-component of Force [N]: 0.300

Z-component of Force [N]: -2.561e-005


In this case, the sign convention for both the Y-component of Force and Value of Force are the same, but both are wrong and should be negative.


I have double checked that indeed the directions of flow and gravity are properly defined and that the model is fully defined in the proper orientation. Does anyone have any idea as to what might be going on? Any help would be greatly appreciated!