# Passive Control of Base Pressure: A Review
## 2 Various Types of Passive Controls
### Other Types of Passive Control Technique  
[PERSON] et al. [31] did an experimental and numerical investigation on a passive technique as ventilation for drag reduction, which is performed by connecting a high-pressure region formed in front of the body with a ventilated duct to a low-pressure region in the wake region. This establishes the net mass flux in the wake zone. It is particularly suitable for aerodynamic bodies to achieve a net total drag reduction in the body by globally modifying the body's wake region. PIV measured the velocity, and Reynolds averaged numerical code was employed at a slightly high Reynolds number. This study aims to assess the effectiveness of reduction in drag on a vented bluff body. In this research, the numerical and experimental results are in good agreement. The drag reduction is accomplished for both smooth and rough (single strip) models. It is important to note that the rough models have a rough surface because an adhesive strip in the stripped model is stuck to the end part of the spherical model, whereas, in the clean model, there is no such strip. A typical drag reduction in the model of about 7% to 8% was achieved, which is moderately more significant than the unpredictability (which is almost 5%) and must be verified by velocity field measurement. It is noted that a 20% average reduction in drag is achieved when the model is changed from a non-vented to vented model. The main difference observed is that the drag value is almost the same in the vented model for stripped and clean models. On the other hand, for the non-vented model, different values are seen. For the non-vented strip model, a drag reduction 10% greater than the non-vented clean model is seen. The main reason behind this phenomenon is the flow transition into a fully developed turbulent boundary layer, which causes an increase in the skin friction drag. This is taken into consideration as there is always some roughness on the external surface. The roughness is usually found in aerodynamic vehicles, due to which there is a boundary layer transition. In any situation above, the venting jet with strip configuration overcomes the boundary layer disturbance due to the strip position [31]. [PERSON] et al. experimentally studied [30] the afterbody flow field and the circular cylinder's base region. They considered an Axial Length (L) of afterbody to the Base Radius (R) of the afterbody (L/R) = 3.0. They aligned the model at an angle of attack of 10 degrees with Mach 2.5. [PERSON]'s dispersal visualizations were obtained to distinguish governing features and picture the chaotic systems of this large-scale turbulent structure of the separated flow. Surface oil-streak flashes were used to determine the afterbody surface and three dimensionalities for base surface flow study. Pressure-sensitive measurements have been performed to determine the surface pressure evolution along the cylindrical body at an attack angle and change the body tilt base pressure. The results show that the typical, expected flow characteristics include corner expansion, shear layer formation separation, recompression of shocks, and a turbulent wake. However, no evidence of flow separation was found along the afterbody. A strong secondary circumferential flow is detected due to the pressure gradients on the surface along the afterbody. There is entrainment of fluid into the base zone from the smooth part of the fluid. For the angle of attack scenario, the average base pressure ratio is 48.4% lower when compared to the calculated zero angles of attack, as a result of which there is a substantial increase in the base drag for cylindrical structures at the angle of incidence by [PERSON] et al. [32]. [PERSON] et al. [33] researched tangent ogive after body with a cylindrical afterbody whose L/R = 14.4 is studied. In contrast, the research done by [PERSON] et al. [32] consists of a 10-deg turn of a constant-diameter sting with L/R = 3.0. As a result, [PERSON] et al. [32] data fall perfectly on the Mach 2.5 curve.