# Passive Control of Base Pressure: A Review
## 2 Various Types of Passive Controls
### Passive Control in the Form of Cavity
The base pressure assumes higher values for higher duct length and Mach numbers (i.e., the drag is reduced, and the control effectiveness is significant). The duct length plays a vital role in the base pressure control, and base pressure assumes high values for low subsonic Mach numbers. Another study [7] on the cavity found that semi-circular grooves reduces the base drag in the backward-facing step. Here two semi-circular grooved cavities are placed at 45\({}^{\circ}\) across the PCD of 23 mm at the base. For the experiment, the square nozzle is used of 10 mm side, with a duct length of 150 mm of side 25 mm. Three duct lengths are considered for investigation, and they are 4 W, 6 W, and 8 W.  
It is apparent that the base pressure has substantially increased, and its reliance on flow parameters like Mach number and L/W ratio is exhibited. Therefore, the base pressure is slightly higher than the ambient pressure. Grooved cavities have a significant effect on the increase of base pressure. Thus, all the Mach numbers considered in the study [7] have reduced drag with lower pressure and minimized the recirculation zone. Below, Figure 7 shows a square nozzle and grooved control plate. Pressure variations in the base of a standard freestream missile configuration in the presence and absence of passive control in the form of the base cavity at a Mach number of 0.7 are investigated experimentally by [PERSON], [PERSON] et al. [8]. The aim was to identify base pressure variations and explain the impact on base cavities' behavior. Experiments  
Figure 6: Dimensions of the nozzle and the duct [6]. Figure 7: Square nozzle and grooved control plate [7]. involve unstable pressure measurements in six azimuthal areas. Due to the model's asymmetry, there is considerable variation in the pressure fluctuations in the azimuthal direction. The base cavities increase the base pressure and reduce the average root square of the base pressure. When the base cavity is enhanced, the higher-order moments show a decreasing trend. High amplitude, low-frequency oscillations, autocorrelation, and cross-correlation plots are observed for cavities of significant length. Virtually no fluctuations are noticeable in the model with a cavity with minimal size. The mean azimuthal pressure variation is recorded and analyzed for various cavity structures on the projectile base. In contrast with the BB model, higher values of the foundation pressure coefficient are noted for the SC model except at an azimuthal position of 270 deg. That is because the base cavity increases the recirculation region's size, and flow also travels downstream of the base separation point. This reduces the interaction between the shear layer and the recirculation field. However, the separation point's relocation results in less contact between the base and the low pressure associated with the shedding vortex. The average base pressure coefficient at all azimuthal locations is \(-0.08\). Due to the pressure difference between the freestream and the recirculation region, the ventilated cavities act as natural bleeding tools. The fluid mass is injected into the base area by bleed devices, thus increasing the base pressure. There is no significant increase in the SVC model's base pressure than the SC model (at the corresponding sensor sites) except the 270 deg azimuthal position. The probability may well be that the normal component of the momentum fluid flow in the base region through the vent holes is negligible. It means that the area of the recirculation will be insignificantly massed. All the vent holes on the cavities are usually directed to the freestream flow in the present study. Thus, the effect of increasing base pressure due to the natural bleeding process is negligible. In general, the base pressure increases due to the lower interaction of base and the low pressure associated with the vortex deposits, as explained earlier, when the ventilated cavities size is changed from small to large. The findings of a numerical and experimental analysis studied by [PERSON] et al.