# Passive Control of Base Pressure: A Review
## 4 Conclusions
\begin{table}
\begin{tabular}{c c} \hline \hline
**Type of Passive Control** & **Effect on Flow Field/Effect on Base Pressure Control** \\ \hline \multicolumn{3}{c}{For Internal Flow Conditions} \\ \hline Rectangular shaped cavity & Shear layer development \& increase in vorticity thickness increases the base pressure. \\ \hline Semi-circular grooves & Reduces the base drag \\ \hline Rectangular Rib & Reverse flow creates a low-pressure zone by the vortex \\ \hline Dimples & Passive control highly effective at higher NPR \\ \hline Static cylinder & Increases base pressure up to 59\% \\ \hline Aerospikes & Reduces the suction at the base and increases base pressure up to 5\% more than atmospheric pressure.When the aerospikes are located at the nose, they decrease the fore-body drag. \\ \hline Threaded spikes & Due to the threads, it develops vortices and manipulates the flow field \& ultimately increases base pressure. \\ \hline \multicolumn{3}{c}{For External Flow Conditions} \\ \hline Base Cavity & Increases the recirculation region size \\ \hline Vertilated cavity & Increases base pressure by a natural bleeding process \\ \hline Transverse cavity & Reduces pressure drag \\ \hline Boattailing & Reduces the base drag depends on the ratio of D\({}_{b}\)/D. Where D\({}_{b}\) is the boattail base diameter and D is the maximum diameter of the projectile. \\ \hline Baseline and shallow cavity & Creates viscous vortex flow, which is a source of low pressure. \\ \hline Forward-facing Aerospike & Reduces fore-body aerodynamic drag by dividing the stagnation region of the nose area along the length of the projectile and modifying the flow field. \\ \hline \multicolumn{3}{c}{A splitter plate is also used to divide the stagnation region in front of the nose} \\ Splitter plate & along the width. With an increase in Reynolds number, the drag reduction \\ \multicolumn{3}{c}{increases up to 6.7\%} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of type of Passive control and their effect. **Author Contributions:** Conceptualization, A.K. and P.R. ; Formal analysis, A.K. ; Funding acquisition, P.R. ; Methodology, A.K., J.S.S.S. ; Project administration, P.R. ; Supervision, P.R. and J.S.S.S. ; Writing--original draft, A.K., P.R. ; Writing--review & editing, A.K., P.R. and J.S.S.S. All authors have read and agreed to the published version of the manuscript. **Funding:** The authors would like to acknowledge the Universiti Sains Malaysia RUI Grant No. 1001/PAERO/8014120 to support this research. **Acknowledgments:** The authors would like to acknowledge the technical staff of Aerospace School, USM, in furnishing the research inputs. **Conflicts of Interest:** The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. **Abbreviations**  
X Axial distance along the duct  
D Diameter of the duct  
L Length of the duct  
G Distance between the rear base point of the cylinder and the leading edge of the splitter plates  
d Cylinder diameter. NPR Nozzle Pressure Ratio  
x Mean velocity field in the plane  
c Adjustable distance to the ground called ground clearance  
BFS Backward Facing Step  
CDnet Net drag coefficient  
Cpb Base pressure coefficient  
BB Blunt Base Model  
SVC Small Length Ventilated Model  
SC Solid Model  
L/D Length-to-Diameter Ratio of the enlarged duct (pipe)  
PIV Particle Image Velocimetry  
L/W Length-to-Width Ratio  
PCD Pitch Circle Diameter  
Stronthal Number  
f Vortex shedding frequency  
D Height of the backward-facing step  
U Freestream velocity  
P Pressure  
M Mach number