Questions about FAST results

Wind & Water Computer-Aided Engineering Software Tools
1

Hi every one,

I have worked for some weeks with FAST. I model a small 3-bladed downwind turbine and I would like to investigate more on the stress analysis of mechanical components such as main shaft and blade roof.

When I apply a constant yaw speed of 45 degree/s to the nacelle and rotor turns at 34 RPM, the total vertical bending moment on shaft (LSShftMys) is caused by rotor weight, wind shear and gyroscopic load which is close to my hand calculations. But FAST results show also that a bending moment exists in the horizontal plane (LSShftMzs) which is about half of LSShftMys.

The same phenomenon also occur when I reduce wind speed to a very low value or I apply a negligible air density ; LSShftMzs is always greater than zero.

Is someone can explain me why LSShftMzs is not equal to zero even is yaw speed is constant ?

Thank you

2

Dear Jean-Philippe,

The bending moments in the low-speed shaft depend on where in the shaft the moments are calculated. “LSShftMys” and “LSShftMzs” are not actually outputs available from FAST. Instead, for a 3-bladed wind turbine, the bending moments in the low-speed shaft at the hub center are available as outputs “LSSTipMys” and “LSSTipMzs” and the bending moments in the low-speed shaft at the shaft strain gage (located by input ShftGagL) are available as outputs “LSSGagMys” and “LSSGagMzs”. Which bending moments are you refering to?