# An Analysis of the Factors Influencing Dual Separation Zones on a Plate
## 4 Result and Discussion  
### Effect of Internal Contraction Ratio  
The internal contraction ratio (ICR), an essential parameter in the design of air-breathing vehicles, determines the compression level of flow within the inlet, directly influencing the starting performance and total pressure recovery coefficient of the inlet. The present model features a quasi-internal compression inlet structure formed between the lower sidewall of the shock generator and the flat plate. As the distance between the dual separation zones decreases, the size of the separation zones gradually increases, which reduces the outlet area of the flow channel. This may lead to a coupling effect between the first separation zone and the inlet starting performance, which is contrary to the original intention of simplifying the research object in this model. Therefore, it is necessary to conduct an analysis on the range of internal contraction ratios in the so-called inlet.  
By adjusting the height of the shock generator to increase the internal contraction ratio, while keeping the shock strength and the inviscid incident point position of the shock wave (highlighted by the red dashed line and red circle, respectively) unchanged during the adjustment process, we ensured that flow field parameters other than ICR remain unaffected. Figure 5 illustrates the variation in dual separation zones during the gradually increasing process of ICR. In the diagram, the black solid line depicts the sonic line, outlining the contours of the dual separation zones under different ICRs. Comparing Figure 5a-c, it is revealed that the separation regions exhibit no significant changes. The sonic lines in the connecting section of the dual separation zones gradually become smoother, indicating a slight expansion of the subsonic region between the dual separation zones. Further increasing the internal contraction ratio to 2.26 induces remarkable changes in the flow field. The first separation zone rapidly expands, forming an open-type unstart flow field beneath the shock generator.  
Figure 5: Mach contour under different ICRs.  
To quantify the changes in flow field parameters during the increase in ICR, the x component wall shear stress curves of the plate are extracted, as shown in Figure 6. According to the definition, the sign change in shear stress values indicates flow reversal, meaning each zero point on the stress curve corresponds to a boundary location of a separation zone. On the red dashed line, four zero points are marked by dark red circles to facilitate understanding. The four zero points represent, from left to right, the separation point and reattachment point of the first separation zone, as well as the separation point and reattachment point of the second separation zone, denoted as S\({}_{1}\), R\({}_{1}\) and S\({}_{2}\), R\({}_{2}\), respectively. Based on these points, the length L\({}_{i}\) of the separation zones can be calculated.  
\[\rm{L}_{i}=X_{Ri}-X_{Si},\ i=1,2\]  
It can be observed that when the internal contraction ratio is less than 1.59, both the position and dimensions of the first separation zone remain unchanged, while the length of the second separation zone slightly decreases with increasing ICR. This is attributed to the reduced propagation loss of expansion waves at higher ICR values, where the enhanced influence of expansion waves leads to a reduction in the size of the second separation zone. Given that wall parameters show minimal variation before \"unstart\", it is concluded that for the model in this study, the internal contraction ratio has a negligible impact on flow field characteristics when the ICR is below 1.59.