# Passive Control of Base Pressure: A Review
## 2 Various Types of Passive Controls
### Passive Control with Rib  
As cavities introduce secondary circulation in the flow, it reduces the oscillatory nature of flow and increases the base pressure. This phenomenon is more prominent in longer ducts than in shorter ducts. However, the cavity's main disadvantage is when it starts behaving like a closed cavity, thereby becoming ineffective as a passive control. As a result,  
Figure 8: Ventilation slots details (**A**) with square-edged (**B**) tapered [10]. in 2001, [PERSON] [14] introduced ribs as a passive control device. He employed five ribs in the circular duct diameter of 25 mm of aspect ratio 3:1, 3:2, and 3:3 at 1D (D = diameter of the tube) location. From his results, he found out that the 3:1 is the most effective in reducing the base drag. For pressure ratio 1.141, the base pressure attains the minimum value for L/D = 4 due to the shear layer extending from the nozzle outlet connects downstream of the base to the extended duct wall [14]. The primary vortex formed at the base region affects the base pressure according to the distance of the reattachment point from the base region and the Mach number at the nozzle exit. The primary vortex intensity determines the low-pressure level in the base region. There exists a reverse flow due to the low-pressure zone created by the vortex. The passive control in the form of the ribs comes into effect in this separated flow. Later [PERSON] et al. [15] introduced ribs with aspect ratios 0.45, 0.64, 0.84, and 1.25 at X/D = 0.5, 1, and 1.5, placing a single rib at three different locations in a circular pipe of 22 mm diameter at subsonic to sonic Mach numbers (0.2 to 1). The results show that the rib aspect ratio between 0.45 and 0.64 reduces the backflow and reduces the base pressure. From wall pressure distribution results, it is found that ribs with the smallest aspect ratio, i.e., 0.45 at X/D = 1, gives the minimum disturbance along the pipe while gaining desired base pressure results. [PERSON] et al. [16] studied the effect of the annular ribs of aspect ratio 3:3 at supersonic Mach number 1.8, varying the NPR from 2 to 6 at locations 0.5D, 1D, 1.5D, 2D, 3D, 4D, and 5D in a circular pipe. It is noticed that the base pressure increases with an increase in the NPR with rib location. [PERSON] et al. [17] also studied the annular ribs experimentally with aspect ratio from 0.45 to 1.25 placed in enlarged duct by varying the length to diameter ratio (L/D) 0.5D to 6D, and NPR is varied from 1 to 7 at subsonic to sonic Mach numbers. It is realized that L/D = 0.5 for NPR 3, the base pressure increases with an increase in NPR. [PERSON] et al. [18] did the numerical simulation and studied the flow at the sonic Mach number for a duct diameter of 20 mm. A single rib of three different aspect ratios (3:1, 3:2, and 3:3) is placed one at a time at various locations (1D, 2D, 3D, and 4D) along the circumferential direction of the duct. It is concluded that the increase in NPR is from 1.5 to 5; the base pressure decreases continuously with no rib case. In contrast, with the rib case, the base pressure decreases steadily with aspect ratio 3:1, while for aspect ratio 3:2 and 3:3, it raises the base pressure at NPR 2.5 and 2, respectively. It was also clear that for no rib case, the base pressure decreases continuously, and for rib case 3:1, there is continual decrement of base pressure; however, it is above the base pressure of the smooth pipe. For 3:2 and 3:3, the base pressure starts to increase from NPR 2.5 compared to 3:1, where the base pressure starts to grow right from the beginning, i.e., NPR 1.5. Figure 9 shows the same. Figure 9: Base pressure changes when NPR is increased from 1.5 to 5, and the aspect ratio is changed from 3:1 to 3:3 [18]. The main reason behind this phenomenon is that with an increase in NPR, the expansion level increases. As the reattachment point has shifted as the flow is highly under expanded, the primary vortex's effectiveness decrease.