# Research on the Aerodynamic-Propulsion Coupling Characteristics of a Distributed Propulsion System
## 4 Results and Discussion
### Influence of the Internal and External Flow Coupling Effect on the Performance of Ducted Fans
As a result, the local static pressure drops, the adverse pressure gradient decreases, and the degree of airflow separation weakens. As shown in Figure 14, the separation zone is only concentrated in the lower half of the duct. It can also be seen from Figure 15 that for the coupled configuration, the axial velocity of the airflow at the duct inlet is significantly greater than that of the clean wing, which is the result of the airflow acceleration in the duct. On the other hand, the influence of the airflow acceleration process in the duct on the flow field propagates upstream against the direction of the incoming flow, thus affecting the airflow on the upper surface of the wing leading edge and improving the flow state near the leading edge. Figures 16 and 17 show the two-dimensional and three-dimensional streamline distributions on the upper surface of the wing of the coupled configuration, respectively. These distributions are obtained under the influence of the fan suction effect, at an angle of attack of \(26^{\circ}\) and Mach numbers of 0.2 and 0.4. As shown in Figures 16a and 17a, the flow field on the upper surface of the wing is significantly improved after considering the suction effect of the ducted fans. The air intake of the ducted fan is uniform, and its aerodynamic performance is evenly distributed in the wingspan direction. This indicates that the suction of the ducted fan can accelerate the low-energy fluid in the boundary layer of the wing surface and effectively suppress the airflow separation. However, when the Mach number is 0.4, the inlet flow tube area of the fans shrinks due to the increase in the air velocity. Subsequently, the suction ability of the fans on the airflow on the wing surface  
Figure 15: Axial velocity contours at the x = \(-0.15\) m section at \(M\omega\) = 0.2 and AOA = \(26^{\circ}\): (**a**) NACA 643-618 clean wing; (**b**) coupled configuration without fan suction effect. weakens, while the drag of the fan casing increases, accompanied by a more pronounced adverse pressure gradient and frictional resistance. At the same time, the angle of attack of the incoming flow attains a critical value corresponding to a Mach number of 0.4. Under such circumstances, obvious separation of the airflow occurs in some areas on the upper surface of the wing, and even backflow is generated. As illustrated in Figure 17b, a distinct three-dimensional separated region emerges upstream of the inlet of fan #3. This gives rise to a significant blockage effect at the inlet of fan #3, severely affecting the aerodynamic performance. Moreover, it also has a partial impact on the adjacent fans #2 and #4. From the view of the distribution of the limiting streamlines on the wing surface, a saddle spiral node-type separation structure is observed. The saddle point \(S\) and the spiral nodes \(S1\) and \(S2\) form two separation spiral vortices of comparable size, which are approximately symmetrically distributed about the center position of the wing. Their spanwise span covers the upstream areas of fans #2 to #4. There are two main reasons for the transformation from saddle node-type separation to saddle spiral node-type separation. Firstly, the Mach number of the incoming flow increases. With the increase in the Mach number, the adverse pressure gradient and friction resistance in the center area of the wing rise significantly, and the airflow velocity decreases sharply, which makes the airflow in this area extremely prone to separation. This, in turn, triggers a blockage phenomenon in the upstream area of the ducted fan. Secondly, the suction effect of the ducted fan has an influence. In the ducted fan array, the airflow upstream of fan #3 separates first.