# Passive Control of Base Pressure: A Review
## 2 Various Types of Passive Controls
### Passive Control in the Form of Cavity  
A cavity can be in the form of a rectangular shape. The vortices generated due to the flow past the cavity are studied by [PERSON] and [PERSON] [4]. This study's main focus is instant development and time mean of turbulent flow structures in the cavity's shear layer. PIV is conducted at 32 m/s, 37 m/s, and 42 m/s. The data's main characteristics are creating the shear layer at a distance downstream of the leading edge and reverse flow at the cavity base. The growth of cavity shear layers was studied by vorticity thickness as a measurement of shear thickness to quantify the time-mean development of shear layers. [PERSON] and [PERSON] [5] did an experimental study on Mach 1.74 convergent-divergent axisymmetric nozzle sudden spread to circular pipelines with a full transverse area, provided with rectangular cavities. Experimental studies focused on base pressure and the flow field in the extended duct. Depending on the expansion level, the area ratio of the pipe, and the ratio length-to-diameter ratio (L/D) of the enlarged duct, it is found that the base pressure is affected significantly. The cavities increase the base pressure for a lower area ratio. Therefore, for small area ratios, the cavity aspect ratio substantially influences the base pressure. The duct length in the range from 0 to about 3 plays a significant role in fixing the base pressure level with and without cavities. There is a minimum duct length for the flow to remain attached to the wall. The base pressure for L/D rises steeply from 0 to 3. For L/D more than 3, the base pressure remains almost constant. This shows the minimum extended duct length needed for the flow to stay attached to the duct. It also implies that the effect of increasing oblique shock intensity at the nozzle exit, due to the ample free space available for the flow at the base, is offset by expansion for the area ratio 10 [5]. The isentropic nozzle pressure ratio required for the ideal expansion at the Mach 1.74 nozzle is 5.24. Hence, the nozzle exit flow is over-expanded for the nozzle pressure 2.10 to 3.48 of the present study. Therefore, under these circumstances, an oblique shock wave is positioned at the nozzle exit. After moving through the shock wave in the shear layer, the flow finds ample space at the base to relax. Thus, the base pressure is determined by the combined effect of the oblique shock wave and expansion level. Due to the change in the expansion level and NPR from 2.10 to 3.48, the shock wave pattern also changes. The results show that the flow to remain attached with the duct wall requires a minimum L/D of 4, a powerful vortex located at the base, and a related suction resulting in low pressure in the base region. The flow field downstream of the reattachment point does not affect base flows for L/D more than 4. [PERSON] et al. [6] studied the efficacy of passive control in the form of cavities. They experimentally investigated compressible flow exhausted from a nozzle that can attain a sonic Mach number into a square duct. The measured flow parameters included the Mach number, the pressure ratio, and the Length to Width ratio (L/W) of the duct. For numerous geometries of the cavity, the experiments were done with and without a cavity. Experimental results show that the multiple cavities work very well to decrease the base drag by reducing the base suction, thus increasing the base pressure. The effect of various cavities is found to control the base pressure for all the L/W ratios. The duct length increase is also beneficial for the more significant duct length since the backpressure's influence is not there. The cavity's presence does not adversely influence the wall pressure and the enlarged duct flow quality. The Mach number has a significant influence on the base pressure and control effectiveness. Figure 6 shows how the multiple cavities are mounted on the duct with nozzle and duct dimensions [6].