# Passive Control of Base Pressure: A Review
###### Abstract  
In the present world, passive control finds application in various areas like flow over blunt projectiles, missiles, supersonic parallel diffusers (for cruise correction), the engine of jets, static testbeds of rockets, the ports of internal combustion engines, vernier rockets, and single expansion ramp nozzle (SERN) rockets. In this review, various passive control techniques to control the base pressure and regulate the drag force are discussed. In the study, papers ranging from subsonic, sonic, and supersonic flow are discussed. Different types of passive control management techniques like cavity, ribs, dimple, static cylinder, spikes, etc., are discussed in this review article. This study found that the passive control device can control the base pressure, resulting in an enhancement in the base pressure and reducing the base drag. Also, passive control is very efficient whenever there is a favorable pressure gradient at the nozzle exit.  
base pressure; drag; recirculation zone; mach number; passive control +
Footnote †: journal: Medical  
flow gets separated and later reattached at a point in the enlarged duct. The distance from the nozzle exit to the reattachment point is known as the reattachment length. This point on the expanded duct wall is significantly affected by the nozzle pressure ratio, the Mach number, and the area ratio.  
Hence, this manuscript reviews some of the latest techniques based on passive control methods to optimize the base pressure and drag. There is an urgent requirement to control this low-pressure in the base region and bring it close to atmospheric pressure to accomplish near zero base drag. The base drag reduction can be achieved by increasing the base pressure by breaking the powerful vortex located in the recirculation zone. There are two ways to regulate the base pressure: (1) active control and (2) passive control.  
Dynamic control of base flows is realized through blowing or suction methods and hence needs an external source of energy. This type of control's main advantage is that it can be switched on and off as needed. However, the main disadvantage of this type of control is the unavailability of such a high source of energy and the extra weight added to aerospace vehicles like rockets, launch vehicles, etc., where the vehicle's weight plays a key role.  
In some cases, it is not feasible to get an additional external source of energy. However, in passive control, the base pressure is controlled by employing the structures' geometric changes using ribs, cavities, boattails, splitter plates, and locked vortex mechanisms. Unlike active control, passive control does not require an external source of energy; hence, it is neither expensive nor leads to design complexity and is simple to fabricate.  
Figure 1 shows the complex flow field of sudden expansion. The boundary layer separation, formation of primary recirculation zone at the base corner, and reattachment location at the wall can be observed. The passive control is used to disturb this flow field, and hence it increases the base pressure and reduces the base drag.  
Figures 2 and 3 show the schematic diagram of over-expanded and under-expanded suddenly expanded flow coming out from a convergent-divergent nozzle.  
In the above Figure 4, the flow is coming from an upstream condition (P\({}_{1}\) M\({}_{1}\) d\({}_{1}\)) to a downstream state (P\({}_{2}\) M\({}_{2}\) d\({}_{2}\)). P\({}_{\text{b}}\) is the pressure at the base of the duct.  
Figure 5A shows the external flow over an artillery shell. Here shock waves are formed at the end of the shell, and expansion waves are formed at its base. Here P\({}_{\text{b}}\) is the pressure at the base of the shell, and P\({}_{1}\)M\({}_{1}\) is the flow conditions. Figure 5B shows the schematic diagram of external flow over a blunt base. Figure 5C shows the schematic diagram of coherent vortex structures.  
Figure 4: Schematic diagram of internal flow.  
Figure 3: Schematic diagram of the under-expanded flow field.