2.3.6. Aerodynamic Analysis of Connection Area of Negative Pressure Aperture and Air Chamber

The connection area of the negative pressure aperture and air chamber is shown in Figure 6. The geometry of the airway in this area is transformed from an arc-shaped air chamber to a cylindrical negative pressure aperture. The aerodynamic change that occurs is in the form of a sudden reduction in the cross-sectional area, and the reported energy loss is mainly local pressure loss [34].

$$p\_s = 0.5(1 - \frac{A\_2}{A\_1})\frac{\rho v^2}{2} \tag{6}$$

where *ps* is the local pressure loss in the connection area of the negative pressure aperture and air chamber, Pa.

**Figure 6.** Connection area of negative pressure aperture and air chamber. (**a**) Diameter of negative pressure aperture is 34 mm; (**b**) diameter of negative pressure aperture is 21.5 mm.

It can be observed from the formula that the local pressure loss in this area is mainly related to the cross-sectional area of the negative pressure aperture, and the larger the negative pressure aperture, the lower the pressure loss. However, the space of the pneumatic distribution cover is limited, and it is necessary to leave space for the connection with the frame, so the maximum inner diameter of the negative pressure aperture is 34 mm. In addition, the maximum diameter of the fluid domain of the negative pressure aperture in order to maintain a complete circle is 21.5 mm under the structural limitations, as shown in Figure 6b. If it increases continually, the rubber sealing rings will block the two sides, and the bottom section of the negative pressure aperture will change from a circle to a rectanglelike shape, as shown in Figure 6a. It is difficult to determine the optimal inner diameter of the negative pressure aperture only by theoretical analysis, and further experimental research is needed.
