# Passive Control of Base Pressure: A Review
## 2 Various Types of Passive Controls
### Other Types of Passive Control Technique
There is a weak dependence of base pressure on L/R rather than on \(\alpha\) and the Mach number. The incidence angle plays a critical role in reducing base pressure and increasing total drag on aerodynamic bodies. The effect of a thin slit at the base edge of a blunt asymmetrical body that communicates an inner cavity with the external flow is studied by [PERSON] et al. [34]. A parametric study is performed on changes in slit size and cavity depth based on pressure. The base pressure initially increases with a deeper cavity but saturates at a slit depth. The base pressure increases monotonically up to 5% with increasing slit size for the geometries tested. An upper limit of base pressure recovery of 20% is extrapolated from the data. It is observed that the main effect of the slit is to reduce the instantaneous pressure asymmetry, which is linked to the total base pressure for all the slit sizes. With the slit size for the geometries calculated, the base pressure increases monotonously up to 5%. In highly asymmetrical pressure distributions, the slit creates a base pressure increase not related to the base pressure's asymmetry. The results indicate the wake's effect due to the diametric flow inside the cavity driven by pressure differences in the slit. The most significant pressure regulates the drops between the slit and the cavity. The slit also decreases periodical variations in base pressure, which primarily relates to the vortex shedding, and raises the wake rotational speed. The analysis reveals that for all slit sizes, the rest of the data is equally independent of h for h/D \(\geq\) 2.8%. [PERSON] and [PERSON] [35] studied a circular cylinder's wake regulation is numerically examined within the laminar flow regime by two symmetric plates attached to the rear surface. The two flat plates are arranged in parallel, and the angle of attachment varies between 0 deg and 90 deg. The dual plate decreases the drag and allows more significant wake suppression at a relatively short plate length compared to the use of a single splitter plate. The attachment angle is shown to have a considerable impact on the control efficiency, and the most effective range associated with the highest drag reduction is 40 to 50 degrees. The free shear layers are attached to the outside surface of the controllers within this system. A comparison with other passive control configurations is suggested for the potential mechanism responsible for wake control. The findings state that the Reynolds' steady flow field between 20 and 40 is considered to increase attachment angle, \(\theta_{\mathrm{f}}\). The total mean drag coefficient is more than that of the plain duct for the attachment angle 40 and 60 degrees at Reynolds numbers 20 and 40. The results are due to the plates' viscous effect, which is significant in the flow field's steady regime. When the flow becomes unsteady (60 \(\leq\) Re \(\leq\) 160), a depression can be found in the total mean drag coefficient curve at around 40 and 50 degrees. The drag reduction of the dual plates is placed at a suitable location that is higher than the single splitter plate. And this drag reduction is more significant at a higher Reynolds number than at a lower Reynolds number. At Re = 160, a total drag reduction of 13.1% for cylinder only and 13.4% for cylinder and plates is obtained, compared to a single plate (7.38% and 7.16%) double. A typical planar space launcher fitted with two passive flow controls simulates a turbulent wake with a zonal RANS-LES system and is studied by dynamic mode decomposition (DMD) [36]. The first approach examines the effect of a conventional boat tail on the surface. In the second concept, a semi-circular lobe flow control unit is used, inserted at the base shoulder of the main body. The concepts' objective is to reduce the reattachment length and, thus, the forces' height and stabilize the separated shear layer. The reattachment length can be reduced by 50% by using a boat tail.