## 4 Results and Discussion
### Fold Angle  
To investigate the effect of the fold angle, the instantaneous forces and overall performance of the wind turbine with different fold angles from \(-2^{\circ}\) to \(8.5^{\circ}\) were calculated, given an incline angle of \(60^{\circ}\). The range of fold angles was determined in consideration of the power performance of the VAWT. The results are discussed below, which are similar to those of VAWTs with conventional pitch axes [6], as well as having some distinct characteristics. First, the AoA, \(C_{Fn}\) and \(C_{FI}\) of a blade element on the equator were assessed (Figure 6). The variations of the AoA versus azimuth showed that variations of the AoA were approximately proportional to the fold angles all around the revolution. With the increase of the fold angle, the AoA in the fore half of the revolution decreased (in absolute terms), while that in the aft half increased (in absolute terms). The opposite effect occurred with the decrease of the fold angle. The variations of the \(C_{Fn}\) versus azimuth with different fold angles were similar to those of the AoA. The slight decrease of the \(C_{Fn}\) in the second quartile--with a fold angle of \(-2^{\circ}\) and, in the aft half, a fold angle of \(8.5^{\circ}\)--suggests that the blade experienced a stall for these azimuthal angles, which could be seen more apparently by the variations of the \(C_{FI}\). As shown in Figure 6c, a sharp drop of the \(C_{FI}\) in the fore half after the peaks occurred for fold angles from \(2^{\circ}\) to \(-2^{\circ}\). A similar drop was observed in the aft half for large fold angles (\(8^{\circ}\) and \(8.5^{\circ}\)). It is clear that the appearance of a stall happened earlier in the azimuth as it grew deeper. Besides the stall phenomena, variations of the  
Figure 5: Comparison of simulated instantaneous force coefficients against experimental data [36]: (a) normal and (b) tangential. versus azimuth were a bit complicated. For fold angles from \(-2^{\circ}\) to \(8.5^{\circ}\), the climbing of the \(C_{Fi}\) in the first quartile was delayed backward, and lower peaks of the \(C_{Fi}\) occurred at later azimuthal angles. A positive \(C_{Fi}\) was seen in a narrower azimuthal range. On the contrary, the climbing of the \(C_{Fi}\) in the third quartile was advanced forward, and the lowest peaks of the \(C_{Fi}\) occurred with a fold angle of \(4^{\circ}\). Furthermore, for a fold angle of \(4^{\circ}\), two peaks of \(C_{Fi}\) existed in the aft fore, with one in the third quartile and the other in the fourth quartile. When the blade was folded in the positive direction--namely, increasing the fold angles--two peaks grew simultaneously. However, when the blade was folded in the negative direction--namely, decreasing the fold angles--the first peak grew but the second declined. This is reasonable, since the performance in the aft half is influenced by that in the fore half for a VAWT. Next, the instantaneous aerodynamic forces, namely the \(C_{Fx}\), \(C_{Fy}\) and \(C_{Fz}\), of a blade were examined with fold angles from \(-2^{\circ}\) to \(8^{\circ}\) (Figure 7). The \(C_{Fx}\) generally stayed positive in most of the azimuth range. With increasing fold angles, the peaks of the \(C_{Fx}\) in the fore half reduced, and the climbing of the \(C_{Fx}\) was delayed. Opposite changes could be seen in the aft half, making the variations of the \(C_{Fx}\) flatter with larger fold angles. In addition, changes of the \(C_{Fx}\) in the fourth quartile were greater when compared with those in the third quartile because of the performance in the fore half, similar to \(C_{Fi}\). As for the \(C_{Fy}\), its direction changed between quartiles. Different from the \(C_{Fi}\) and \(C_{Fx}\), the effect of the fold angle on the \(C_{Fy}\) was reversed at azimuthal angles of \(90^{\circ}\) and \(270^{\circ}\).