# Passive Control of Base Pressure: A Review
## 2 Various Types of Passive Controls
### Passive Control in the Form of Cavity
[9] reported assessing a control process's performance to postpone the boundary layer's separation. This process consisted of incorporating the surface of contoured, transverse grooves (i.e., small cavities with an acceptable cross-sectional shape). The grooves' shape and depth need to be significantly smaller than the incoming boundary layer's width; fluid recirculation within the grooves is smooth and stable. This strategy is applied to an axisymmetric bluff body with different rear boat ends, distinguished by numerous flow-separating degrees. Variation of large multiscale eddy simulations was examined experimentally [9]. Before boat tails, the boundary layer conditions are fully developed turbulent in experiments and intermediate in simulations due to the different input flow turbulence levels. Every time a single axisymmetric groove is added on the boattails' lateral surface, the boundary layer's separation is postponed considerably, and the pressure drag is reduced. The wake-dynamic structure remains qualitatively similar. It was analogous to an asymmetrical bluff body with quantitative variations that correlate with the wake width of a reduction caused by boattails and grooves. A few additional simulations show that the groove effect is also resistant to change the geometric parameters that define its shape. Both evidences support the notion that relaxing the no-slip boundary condition for the flow around the recirculation regions at an appreciable pace on their boundaries is the physical mechanism responsible for the new separation control method's efficacy. Several techniques are explored to reduce the aerodynamic drag of bluff bodies through base pressure recovery. These include boat tailing and base cavities. [PERSON] et al. [10] changed [PERSON]'s body to have a base cavity of variable depth that may be ventilated via a slot in a square-back configuration considered for investigation. The overall body drag diminished with a single cavity for numerous cavity lengths, but a distinct minimum drag state was achieved. A relative drag reduction is achieved by the incorporation of ventilation slots and at a much-reduced cavity size. The pressure data in the cavity was used to assess the dependence of the base drag. Variations in the slot structure were analyzed to decrease the drag and the slot distribution effects. PIV is used for various cavity setups. Figure 8 shows [PERSON]'s body with vented slots [10]. The base pressure was assessed on the cavity backplate. There was a total of 48 pressure tubes spread over one half of the backplate. Numerous cavity configurations were tested at the ground level [10]. The cavities were the plane cavity, the original vented cavity with square edge slots, the vented cavity with tapered slots on all surface areas, and the same cavity with undesirable slots covered. For both earlier settings, the standard pressure measurements were made for the ideal cavity and a much deeper cavity. The pressure measurements were undertaken for various cavity depths in the two ventilated cavities with tapered holes. In each case, comparisons were made with the base pressures calculated without a cavity on the quarterback scale. Seeing the growing need for energy conservation, [PERSON] et al. [11] researched the reduction of base drag. Here the author has studied the [PERSON] body [12] as the bluff body for the real-time road vehicle. Experiments were conducted to investigate the bifurcation of the wake controlled by the clearance and impact of the base cavity before and after bifurcation that can be used when making road vehicles. It is observed that regardless of ground clearance, above the threshold \(5c^{*}=0.08\), an eight percent reduction is achieved. [PERSON] et al. [13] did a numerical investigation of the asymmetric wake mode of a square back [PERSON]'s body, focusing on a base cavity effect. Both distributions indicate an almost identical pressure field for the baseline and shallow cavity for \(x>0.5\). While they are horizontally and vertically shifted from each other for \(x<0.5\).