# An Analysis of the Factors Influencing Dual Separation Zones on a Plate
## 4 Result and Discussion
### Effect of Expansion Waves
#### 4.2.1 Variation in Parameters
The change in the length of the separation zones indicates that the dual-separation-zone flow field enters a merging/re-separating process. Taking the \(l_{1}\) model as an example, during the merging process, as \(d_{s-r}\) increases, the spacing between the dual separation zones gradually decreases. The corresponding curve of L\({}_{S1}\) begins to rise, signifying that the flow field enters the merging stage. When the relative distance between the dual separation zones increases to zero, the two separation zones are completely merged, and the merging segment of the L\({}_{S1}\) curve terminates at this point. After the complete mergence, the upstream movement of the shock generator away from the wedge causes  
Figure 8: Wall pressure curves of different lengths of G under the same T.  
the flow field to enter the re-separating process. Comparing the merging and re-separating processes, it is observed that along the positive direction of the x-coordinate, the terminal coordinate of the curve corresponding to the blue hollow triangles exceeds that of the blue solid squares. Specifically, under the \(l_{1}\) condition, the \(d_{s-r}\) corresponding to the initial point of the re-separation process is greater than the terminating point of the mergence. This indicates that the re-separation point of the merged separation zone emerges earlier than the mergence point of the dual separation zones, suggesting a stronger tendency for the merged separation zone to undergo re-separation. Furthermore, the length of the newly formed first separation zone after re-separation is greater than that of the original first separation zone, with its value being essentially consistent with the L\({}_{\text{S1}}\) observed in the other two models. A distinct hysteresis in scale variation between dual separation zones exists during the mergence and re-separation. With the increase in \(l\), the hysteresis phenomenon gradually diminishes. In the model of \(l_{2}\), the merging termination point of the dual separation zones coincides with the re-separating initiation point. In Figure 9, although the mid-section of the re-separating curve slightly deviates from the merging process, the final re-separated flow field ultimately maintains dimensions in separation zones that are identical to the fully separated flow field. When the length of SG increases to \(l_{3}\), the merging and re-separating curves nearly overlap, and the hysteresis loop almost disappears. Analyzing the Mach number contour of the flow field, it is clear that when the shock generator length is 90 mm, the influence region of the trailing expansion waves is located between the dual separation zones and closer to the reattachment point of the first separation zone. This configuration reduces the pressure rise in the first separation zone while diminishing its scale. During the merging process, the boundary layer between the two zones has a longer recovery distance, thereby demonstrating enhanced resistance to adverse pressure gradients [19]. Meanwhile, the favorable pressure gradient induced by the expansion waves directly impedes the forward propagation of backpressure from the second separation zone, delaying the merging termination point of the dual separation zones. For the merged flow field, the merged separation zone is a bimodal distribution. The first peak corresponds to the top of SZI, whose morphology is maintained by the incident shock wave. The second peak is located at the apex of the windward wedge, corresponding to SZII, whose existence depends on the presence of the wedge. The subsonic region between the two peaks develops during the merging process, gradually expanding through the interaction between the dual separation zones. However, this intermediate region lacks favorable support prese  
Figure 9: Lengths of separation zone I in different lengths of G under the same T.  
to strongly sustain its existence during the re-separating process.