# Passive Control of Base Pressure: A Review
## 3 Findings Discussion  
Based on the above discussion, most of the passive techniques used for the base pressure control and hence base drag are summarized here below:  
* All the passive control methods to regulate base pressure are useful in creating a favourable pressure gradient. These techniques are also inadequately effective in the presence of an adverse pressure gradient (i.e., when the jets are over-expanded). In passive control, the level of expansion (i.e., \(\mathrm{P_{e}}/\mathrm{P_{a}}\)) plays a vital role in fixing the base pressure values. It is seen that whenever the area ratio is five and above, the control effectiveness is minimal/not useful even though the flow from the nozzles is under the influence of a favourable pressure gradient. The physics behind this trend maybe when the relief effect due to an area ratio increase is beyond some limit. The flow from the converging or converging-diverging nozzle discharged into the enlarged duct tends to attach with reattachment length other than the optimum for a strong vortex at the base. This process makes the NPR effect on base pressure become insignificant after a critical area ratio. The review shows that when the jets are under-expanded for a lower area ratio, the passive control is significant, and base pressure is equal to the atmospheric pressure, or is more than the ambient pressure. Figure 14: Ahmed body with 35\({}^{\circ}\)slant angle detailed view with rectangular flap (dimensions in mm) [38]. Results indicate that for certain combinations of the parameters, an under-expansion level of 1.5 (i.e., \(\rm{P_{e}/P_{a}=1.5}\)) is the optimum value resulting in a maximum increase in the base pressure. * A new passive control device was considered for a form-drag reduction in the flow over a two-dimensional bluff body with a blunt trailing edge. The system consists of small tabs on the top and bottom edges of a bluff trail to effectively interrupt a two-dimensional wake. A wind tunnel experiment and a large-scale simulation were conducted to test its drag reduction efficiency. Extensive parametrical research is conducted experimentally by changing the passive control's height and width in the form of a tab and the spacing along the span between the adjacent tabs. The Reynolds numbers considered for the study are 20,000, 40,000, and 80,000. The base pressure increases for a wide range of parameters (i.e., drag reduces) at all three Reynolds numbers. * Similarly, the splitter plate's effect at various length locations is useful in the base drag reduction at various NPR, Mach numbers, area ratios, and length-to-diameter ratios. The passive control in the form of a splitter plate is significant, resulting in reducing the drag. Drag is reduced from 9% to 57% by attaching the splitter plate to the cylinder. With vertical splitter plates positioned downstream of a straight base, drag reductions of nearly 12% were obtained. Finally, the results presented herein confirm the advantage of splitter plates in reducing aerodynamic drag and demonstrate the need to develop systems capable of adapting their position and angular orientation to external conditions. Triangular splitters that divide the near wake into one-half, one-third, and one-fourth cylinder regions were designed to exploit this flow's specific stability characteristics. The impact of the immediate wake flow changes the base pressure and eventually affects the base drag. A base drag reduction of as high as 39% is achieved by employing the splitter plate, whereas, with passive control with spike, the base pressure starts to increase right from the beginning and becomes more than atmospheric pressure at L/W = 6 to 10, and acquired base pressure values are 5% more than atmospheric pressure. The effect of spike and grooved spikes play an essential role in fixing the base pressure values. The impact of cavities is to reduce the oscillations in the enlarged duct. * A computational fluid dynamics simulation over a payload fairing of the satellite launch vehicle with aerospike presence is carried out.